IMPERIAL COLLEGE LONDON                                                     (University of London)                        ...
ABSTRACTErosion magnitude estimation is a vital step in understanding petroleum systems. The MalayBasin offshore Peninsula...
ACKNOWLEDGEMENTSI appreciate the opportunity provided by PETRONAS Research to work on data from the MalayBasin. I would li...
TABLE OF CONTENTS1. Introduction                1  1.1 Aims..................................................................
TABLES, FIGURES AND ENCLOSURESTable 1.1: Dataset ............................................................................
Figure 5.6: I and K maturity points .........................................................................................
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune G...
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)
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Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)

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Transcript of "Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay Basin (2008)"

  1. 1. IMPERIAL COLLEGE LONDON (University of London) Department of Earth Science and Engineering Centre for Petroleum StudiesInvestigation of the amount of erosion at the upper Miocene unconformity in the southeasternpart of the Malay BasinbyLune Gene Yeo A report submitted in partial fulfillment of the requirements for the degree of MSc Petroleum Geoscience September 2008DECLARATION OF OWN WORKI declare that this thesis is entirely my own work and that where any material could be construedas the work of others, this has been fully cited and referenced, and/or with appropriateacknowledgement given.Signature:.....................................................................................................Name of student: LUNE GENE YEOName of supervisors: Professor Howard Johnson, Imperial College, London Dr. Jamaal Hoesni, Petronas Research, Bangi (Malaysia)
  2. 2. ABSTRACTErosion magnitude estimation is a vital step in understanding petroleum systems. The MalayBasin offshore Peninsular Malaysia underwent an inversion period in the late Miocene, whichled to erosion and the subsequent creation of a basin-wide unconformity. The erosion isgreatest in the southeast part of the basin, which forms the area of investigation for this study.The erosion at given wells in the study area was quantified using compaction techniquescomparing compaction trends to a normal trend (of sonic transit time data), and a tectonicmethod comparing backstripped basement depths to theoretically predicted basement depths.There is a good correlation between the results from the compaction and tectonic methods;although it was found that different methods are more suitable under different conditions.The contour maps plotted from these estimates were found to tie in well with palaeo-geographicmaps with high erosion (up to 1400 m) at the basin margin in the southeast. The maps alsosupported the theory that inversion was initiated by the rotation of Borneo.The erosion thicknesses in the southeast Malay Basin were found to have little effect onhydrocarbon generation timings, hastened the onset of thermal maturity, terminated expulsionearly for some wells and induced strata parallel hydrocarbon migration while reducing verticalmigration.The aforementioned results can be further utilized in more wide-scale basin studies combinedwith reservoir modeling in order to identify new petroleum systems or better understand existingones. This can lead to the discovery of new prospects for drilling.
  3. 3. ACKNOWLEDGEMENTSI appreciate the opportunity provided by PETRONAS Research to work on data from the MalayBasin. I would like to thank Dr. Jamaal Hoesni and Prof. Howard Johnson for their invaluableguidance; and my parents for making geoscience a possibility for me. “Our world has had many lives.” Antares, Tanah Tujuh
  4. 4. TABLE OF CONTENTS1. Introduction 1 1.1 Aims.................................................................................................................................. 1 1.2 Geological Setting: Malay Basin ........................................................................................ 1 1.3 Available Dataset .............................................................................................................. 62. Literature Review – The Determination of Erosion and Its Effects 7 2.1 Compaction Based Methods ............................................................................................ 7 2.2 Thermal History Based Methods .................................................................................... 11 2.3 Tectonic Based Methods................................................................................................ 12 2.4 Stratigraphic Based Methods ......................................................................................... 13 2.4 Previous Work................................................................................................................ 133. Methodology 14 3.1 Well Conditioning ............................................................................................................ 14 3.2 Constructing Normal Compaction Trends........................................................................ 16 3.3 Thermal Method .............................................................................................................. 20 3.4 Tectonic Method ............................................................................................................. 20 3.5 1D and 2D Basin Modeling ............................................................................................. 214. Results 24 4.1 Normal Compaction Curves ............................................................................................ 24 4.2 Exhumation Estimations From Compaction Methods ...................................................... 27 4.3 Exhumation Estimations From Thermal and Tectonic Methods ....................................... 28 4.4 Final Erosion Values ....................................................................................................... 29 4.5 Basin Models .................................................................................................................. 305. Discussion 31 5.1 Comparison of Methods .................................................................................................. 31 5.2 Erosion Trends ............................................................................................................... 35 5.3 Possible Causes of Inversion .......................................................................................... 38 5.4 Petroleum System Implications ....................................................................................... 426. Conclusions and Recommendations 527. References 53Appendix 1 – Erosion Estimation iAppendix 2 – Basin Modeling vi
  5. 5. TABLES, FIGURES AND ENCLOSURESTable 1.1: Dataset ...................................................................................................................... 6Table 3.1: Measured SBHT and corrected SBHT ..................................................................... 22Table 4.1: True exhumation estimates for all wells from compaction methods.......................... 28Table 4.2: True exhumation estimates for Well 10 from the tectonic and compaction methods 28Table 4.3: True exhumation estimates for all wells from the tectonic method ........................... 29Table 4.4: Final erosion estimates for all wells ......................................................................... 29Table 4.5: Different erosion magnitudes used in 1D basin modeling ........................................ 30Table 5.1: Formations present in Wells 8 and 13 ...................................................................... 34Table 5.2: Timing of regional tectonic events............................................................................ 41Table 5.3: Change in the timing of the onset of thermal maturity .............................................. 43Figure 1.1: Location of the study area ........................................................................................ 1Figure 1.2: Malay Basin stratigraphic column ............................................................................. 2Figure 1.3: Kinematic evolution shear models ............................................................................ 3Figure 1.4: Cross-sections.......................................................................................................... 5Figure 2.1: Porosity-depth trends for sands and shales .............................................................. 9Figure 2.2: Interval transit time evolution .................................................................................. 10Figure 2.3: Thermal methods ................................................................................................... 12Figure 2.4: Tectonic methods ................................................................................................... 13Figure 3.1: Well conditioning workflow...................................................................................... 15Figure 3.2: Well tops ................................................................................................................ 15Figure 3.3: Well conditioning .................................................................................................... 16Figure 3.4: Least squares regression ....................................................................................... 17Figure 3.5: Windowed averaging .............................................................................................. 18Figure 3.6: Subsidence curves ................................................................................................. 20Figure 4.1: Heasler normal compaction curves ......................................................................... 24Figure 4.2: Final Heasler curve................................................................................................. 25Figure 4.3: Hillis normal compaction curves ............................................................................. 26Figure 4.4: Heasler true exhumation......................................................................................... 27Figure 4.5: Hillis true exhumation ............................................................................................. 27Figure 5.1: Stretch factor versus basement depths................................................................... 32Figure 5.2: Exhumation map overlays ...................................................................................... 36Figure 5.3: Erosion maps ......................................................................................................... 37Figure 5.4: Tectonic models ..................................................................................................... 40Figure 5.5: Inversion mechanisms ............................................................................................ 41
  6. 6. Figure 5.6: I and K maturity points ............................................................................................ 43Figure 5.7: L and M maturity points .......................................................................................... 44Figure 5.8: Burial-maturity: Wells 8 and 10 ............................................................................... 45Figure 5.9: Well 8 event charts ................................................................................................. 46Figure 5.10: Well 11 and 14 event charts ................................................................................. 47Figure 5.11: Well 10 and 13 event charts ................................................................................. 48Figure 5.12: Hydrocarbon saturation: low case erosion ............................................................ 49Figure 5.13: Hydrocarbon saturation: best case erosion ........................................................... 49Figure 5.14: Hydrocarbon saturation: high case erosion ........................................................... 50Enclosure 1.1: Khalid method .......................................................................................................iEnclosure 1.2: Thermal method for Wells 2 and 4 ........................................................................iEnclosure 1.3: McKenzie formula ................................................................................................ iiEnclosure 1.4: Heasler method histograms ................................................................................ iiiEnclosure 1.5: Hillis method histograms ..................................................................................... ivEnclosure 1.6: Erosion estimates from seismic section ...............................................................vEnclosure 2.1: Eustasy curve and SWIT .................................................................................... viEnclosures 2.2 to 2.4: Thermal fits ............................................................................................. viEnclosures 2.5 to 2.11: Burial-maturity ....................................................................................... ixEnclosure 2.12: Event charts .................................................................................................... xviEnclosure 2.13: Change in peak hydrocarbon generation timings ........................................... xviiEnclosure 2.14: Maximum hydrocarbons generated ................................................................ xvii
  7. 7. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering*Locations classified: Following the agreement with PETRONAS, field location names cannot berevealed in this report or the associated presentation or poster.1. INTRODUCTION1.1 AimsThe aims of this study are to quantify and map the amount of erosion at the upper Mioceneunconformity in the southeastern part of the Malay Basin; and determine the effects of theerosion thicknesses on the generation and migration of hydrocarbons.1.2 Geological Setting: Malay BasinThe Malay Basin is an elongate intra-continental pull apart basin, about 250 km long and 250km wide, located at the centre of Sundaland, the cratonic core of Southeast Asia (Figure 1.1), aregion where three converging lithospheric plates interact: the India-Australian, Eurasian andPacific plates (Hall, 1996). Figure 1.1 – Location of the southeastern part of the Malay Basin (study area), wells and cross-sections (modified from USGS, 1999) within Southeast Asia (inset map). 1
  8. 8. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringThe Malay Basin was subjected to three main tectonic events: i) late Eocene to Oligocene (syn-rift) extension and subsidence, ii) early to middle Miocene thermal subsidence (early post-rift)accompanied by middle to late Miocene basin inversion and iii) late Miocene to Recentsubsidence (late post-rift) (Madon and Watts, 1998).The basin fill (over 14 km thick in total) is subdivided via seismic stratigraphy into Units A to M,from top to bottom (Madon and Watts, 1998). See Figure 1.2 for event timings relative to thestratigraphic formations.Figure 1.2 – Stratigraphy, hydrocarbon occurrences, source rocks and structural history of theMalay Basin (after EPIC, 1997)1.2.1 Basin Extension (Syn-rift) – 35 to 25 MaGeophysical studies around the Natuna Islands (Ben-Avraham and Emery, 1973) have shownthat the region is underlain by relatively thin continental crust (about 21 km thick). Most workerstherefore believe that the Malay Basin was formed by crustal or lithospheric extension.White and Wing (1978) suggested that it was formed by the collapse of a regionally thinnedcontinental crust. Some authors suggested a major role for strike slip tectonics in basindevelopment. The occurrence of E-trending en-echelon anticlines in the basin has led Hamilton(1979) to postulate that the Malay Basin may have been formed by right lateral wrenching.Kingston et al. (1983) interpreted the Malay Basin as a wrench or shear basin formed by theoblique subduction of the Indian plate beneath the Southeast Asia lithosphere along theSumatra-Java arc-trench system. Another common view is that the basins are pull-aparts along 2
  9. 9. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringmajor strike –slip faults related to the India-Asia collision during the late Eocene (Tapponnier etal., 1982; Daines, 1985; Polachan and Sattayarak, 1989).More recent studies on the Malay Basin (Ngah et al., 1996; Tjia and Liew, 1996; Madon et al.,1997) have adopted the extrusion tectonics model but emphasized the importance of pre-existing basement faults in controlling basin development. Ngah et. al (1996), in particular, haveproposed that the basins were formed initially as aulacogens above a Late Cretaceous hotspotand later developed into wrench or pull apart basins when their bounding faults were reactivatedas strike-slip faults due to the extrusion tectonics.Heat flow data from the Malay Basin (Halim, 1994) indicate abnormally high heat flows (>100mW m-2) that are characteristic of rift basins. Extensional structures associated with the basinformation include half grabens that are bounded by major normal faults and filled by non-marinesynrift sediments. The onlapping geometry of the postrift strata (Units L and younger) at thebasin margins (Madon and Watts, 1998) is characteristic of basins formed by lithosphericstretching (Dewey, 1982).Madon (1997) suggests kinematic models for the extension and inversion of the Malay Basin(Figure 1.3), where the basin opening and inversion are due to sinistral and dextral shear of abroad NW-trending deformation zone, which Tjia (1994) referred to as the Axial Malay FaultZone. Figure 1.3 – Shear model for the kinematic evolution of the Malay Basin (after Madon, 1997) 3
  10. 10. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringCross sections of the basin, such as in Figure 1.4, show the “steer’s head” geometry of a typicalrift-sag couplet (Dewey, 1982); which is widely attributed to McKenzie’s 1978 uniform stretchingmodel. In such a model, initial rifting of the brittle upper crust results in fault controlledsubsidence; followed by a later sag phase which represents the thermal subsidence as thethermal anomaly associated with lithospheric stretching decays. The gentle inward tilting of thebasin flanks, however, suggests a combination of thermal and flexural subsidence of the basinflanks.During the extensional phase, E-trending half grabens were formed and filled by braided-fluvialand lacustrine sands and shales, with increasing lacustrine influence towards the basin centers;represented by Units M and older (Figure 1.2) (Lovell et al., 1994).1.2.2 Basin Thermal Subsidence Followed by Inversion (Early Post-rift) – 25 to 10.5 MaIn the following period of post-rift thermal subsidence, the depositional environment initiallychanged to marginal marine. Lower Miocene progradational and aggradational fluvial to tidally-dominated estuarine sediments were replaced by shallow marine sediments as relative sealevel continued to rise in the middle Miocene (Figure 1.2).Dextral shear on the Axial Malay Fault Zone during the middle to late Miocene resulted in theinversion of the Malay Basin. Compressional deformation caused the reactivation and hence,transpressional deformation of those earlier basement faults, which resulted in the E-trendingwrench-fault anticlines and pop-up structures (Madon et al., 1999). In the basin center, there arenumerous faults that originate in the basement but propagate up-section into the overlyingstrata: interpreted as basement extensional faults that were reactivated during thetranspression/inversion event (Madon, 2007).The EPIC (1994) study shows that the peak of fold growth is earlier in the south than in thenorth, despite the timing of structural growth being generally synchronous across the wholebasin. Syn-inversion stratigraphic units thin towards the crestal region of the inversionstructures.The basement uplift in the south-eastern (and likely the southwestern) Malay Basin, which led tothe erosion and development of an upper Miocene regional unconformity (Ismail et al., 1994;Madon et al., 1999) is attributed to this regional deformation event (Madon, 1997).Unconformities represent hiatuses in sedimentation caused by changes in relative base leveldue to tectonic uplift (as in this case), or relative sea level fall. This hiatus period which createdthe Upper Miocene Unconformity (UMU) is postulated by Madon (1998) to be between 8.5 to 7Ma. This timing, however, is inconsistent with the timing of the unconformity in the widely usedEPIC (1994) stratigraphic column, which puts it at 10.5 Ma (Figure 1.2).Inversion was greatest in the southeast and center of the basin, where erosion may haveprovided a sediment source for the northern part of the basin. The UMU, which overlies the Dformation truncates folded and uplifted syn-rift to early post-rift strata (Madon and Watts, 1998)(Figure 1.4) and represents a regional marine transgression in the Malay Basin(Watcharanantakul and Morley, 2000). 4
  11. 11. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering 71km B W E 21kmFigure 1.4 – Cross-sections A and B of the Malay Basin. The interpreted seismic cross-sectionwas obtained via personal communication with J. Hoesni, 2008. For locations, see Figure 1.1.The formations deposited during this period are units L to D. Units L to K were deposited underfluvial – lacustrine settings. Formations I and J consist of progradational to aggradational fluvialto tidally dominated estuarine sands.At the start of the inversion, unit H was deposited in deltaic to shallow marine settings andinclude coals and coaly shales. Strata of the overlying middle Miocene H Group through upperMiocene D Group were deposited during alternating marine transgressions and regressions(Tjia and Liew, 1996). Formations H and F are composed of dominantly marine to deltaicsediments with fluvial and estuarine channels, which include coals and coaly shales, depositedduring an overall sea level rise. Groups E and D were deposited by the progradational stackingof dominantly fluvial and estuarine channels; and culminated with the UMU.Deformation was contemporaneous with sedimentation, such that erosion and non-depositionon the crests on inversion structures occurred simultaneously with deposition on the flanks.Although basin inversion caused wholesale uplift and formation of flower structures, on a basinscale the subsidence accelerated.1.2.3 Basin Subsidence (Late Post-rift) – 10.5 Ma to PresentIn the latest Miocene-Pliocene, a tectonically quiescent period, regional subsidence resumed.Fully open marine conditions were established, with undeformed clays and silts (Formations Aand B) deposited over the UMU; in an overall marine transgressive cycle of sedimentation innearshore to shallow marine environments. 5
  12. 12. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering1.3 Available DatasetThe dataset for this study (Table 1.1) was provided courtesy of PETRONAS Research. Well Data Type Names Table 1.1 Available dataset Well 1 Sonic GR SP Calliper VR/SBHT TOC Well 2 Sonic GR Calliper VR/SBHT TOC Well 3 Sonic GR SP Calliper VR/SBHT Well 4 Sonic GR SP Calliper VR Well 5 Sonic GR VR Well 6 Sonic GR Calliper VR Well 8 Sonic GR SP Calliper VR TOC/HI Well 9 Sonic GR SP Calliper Well 10 Sonic GR SP VR Well 11 GR SP Calliper VR Well 12 Sonic GR SP Calliper VR Well 13 Sonic GR SP Calliper Well 14 Sonic GR SP Calliper**Note that the vitrinite reflectance data for the individual wells are a combination of data fromwells in the same fields. 6
  13. 13. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering2. LITERATURE REVIEW – THE DETERMINATION OF EROSION AND ITS EFFECTSExhumation is defined as the displacement of rocks with respect to the surface whereas erosionrefers not only to the process of elevating rocks with respect to the surface, but also to theremoval and transport of weathered material. Only removed rocks are eroded; but the entirerock column is exhumed as a result of erosion (Hillis, 1995).Exhumation can be determined using the techniques outlined below with emphasis is placed oncompaction technique, the main technique used in this study. As published methods ofdifferentiating erosion and exhumation values were not found to be available, exhumationmagnitudes given by the methods below were taken to be erosion magnitudes. Thus, whenreferring to erosion estimates in this study, both terms are used interchangeably.2.1 Compaction Based Methods2.1.1 A Definition of CompactionCompaction is the reduction of sediment volume during burial due to mechanical and thermo-chemical processes (Bulat and Stoker, 1987; Sclater and Christie, 1980; Magara 1976). Theresulting matrix reorganization causes an increase in bulk density and a decrease in intervaltransit time.Mechanical compaction starts immediately after deposition and is caused by the closer packingof grains due to increased vertical stress with depth. Chemical compaction is a result of thedissolution and precipitation of minerals (and hence matrix reorganization) and is mainlycontrolled by temperature (Bjørlykke, 1999). Cement precipitation near grain contacts increasethe rigidity of the grains, and when rock strength exceeds the vertical stress, mechanicalcompaction ceases. At this point the porosity of the clastic unit is zero and the density and sonicinterval transit time measured represent the rock matrix values.Both mechanical and chemical components therefore make up any compaction with depthtrend. The influence of the chemical component is difficult to quantify, although the mechanicalaspect is straightforward to estimate.Since the original volume of a sedimentary unit of a given age is not known, the amount ofcompaction cannot be measure directly. Instead, compaction must be computed from measuresof changes in porosity, or a proxy thereof.If porosity determinations form the basis of the technique, it must be assumed that all reductionin pore space is caused by mechanical, and not chemical, compaction. This can be determinedfrom the measurement of either shale or sand compaction.2.1.2 Compaction Data TypesSonic, density and neutron well logs are proxies for porosity; and as such are used to representcompaction with depth trends. The density tool, however has a shallow depth of penetration(approximately one foot), and is thus greatly influenced by borehole wall conditions and mudcake thickness. Thus, it effectively measures the density of the flushed zone.The neutron porosity tool measures the effect of the formation on fast neutrons emitted by asource, where hydrogen has the biggest effect in slowing down and capturing neutrons. 7
  14. 14. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringHowever, neutron log is calibrated to read the correct porosity assuming the pores are filled withfresh water for a given matrix; and is strongly affected by the presence of clays and gas.The sonic tool measures the time taken for a compressional; wave to travel form the transmitterthrough the formation to the receiver at the either end of the tool. This tool has a greater depthof penetration and thus is not so adversely affected by borehole conditions. It does not respondto secondary porosity (fracture or vuggy porosity) as the emitted wave takes the fastest travelroute from the transmitter to the receiver; thus making it an ideal tool for measuring thecompaction of shales and sands which is chiefly a function of primary porosity. Due to thefactors outlined above, the sonic log was used in this study.2.2.2 Compaction RelationshipsThere are number of porosity-depth functions available, the simplest of which is a lineardecrease in porosity with depth as assumed by authors such as Hillis, 1995; Hillis andMavromatidis 2005 and Storvoll et al., 2005. This trend has been proven to fit porosity-depthdata in units over finite depth intervals (Bulat and Stoker, 1987) and is typically observed insandstones (Magara, 1980). However a linear depth trend does not represent the exponentialtrends observed in shallow depths at shales and implies negative porosities below a certaindepth. Direct porosity measurements of core plugs (sands) from an unexhumed section wouldprovide an empirical porosity depth relationship (Corcoran and Doré, 2005).Most studies, such as those of Heasler and Kharitnova (1996), and Corcoran and Mecklenburgh(2005); use shale only lithologies in their analysis as shales are not as susceptible to chemicalcompaction compared to sands and carbonates (personal communication J. Hoesni, 2008), andthus porosity decrease expressed mainly by mechanical compaction, which is 95% according toTaylor (2007).Athy (1930) developed an exponential function for porosity with depth, which is morerepresentative of measured compaction trends within shale units (Figure 2.1)Athy’s equation is given as Ø = Øoe-bx …….. (2.1) where Ø = porosity at depth x, Øo = porosity of unconsolidated sediments at the depositional surface (mudline) and b = lithology dependent compaction coefficient which determines the slope of the porosity versus depth trend. 8
  15. 15. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering Figure 2.1 – Typical porosity-depth trends for sandstones and shales (after Magara, 1980)This equation proposes that porosity decreases exponentially with depth as do the intervaltransit time. Most authors, such as Heasler and Kharitnova (1996) agree that the transit timetrend must approach a finite value with depth and as a result adopt equation 2.1.In this study, both shale and sand lithologies are used for compaction analysis to maximize theamount of constraints on the results.2.2.3 Determining ExhumationExhumation (and in effect erosion) can be determined from compaction analysis where a plot ofporosity versus depth for a “normally compacted” un-exhumed succession within the basin mustfirst be generated, which requires a significant number of porosity measurements at maximumburial. Where localized exhumation is dominant, this can be approximated from a sonic log of anunexhumed well; but this must be approximated form nearby unexhumed basins if there isregional exhumation as was done by Corcoran and Mecklenburgh, 2005.After this “normal compaction” trend has been established, porosity depth trends can bedetermined for individual well locations and compared with the reference curve. Apparentexhumation refers to the elevation for exhumed sedimentary rocks in the well underinvestigation above their maximum burial depth in the normally compacted well, and is given bythe displacement, along depth axis, of the observed compaction trend from the normalcompaction trend (Figure 2.2).Post-exhumation burial, however, perturbs the compaction trend, creating the facade of lesserosion than in actuality as apparent exhumation is only equal to true exhumation where therehas been no post-exhumation burial (Figure 2.2). 9
  16. 16. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 2.2 – Interval transit time evolution during burial (A), subsequent uplift and exhumation(B) and post-exhumational burial (C, D and E). The apparent exhumation (EA) is the amount ofexhumation not reversed by subsequent burial. It can be measured by displacement along thedepth axis of the transit time relationship of the exhumed sequence (B or C) from that of areference of normally compacted sequence (A, D or E) (after Hillis, 2005)The compaction technique operates on the fundamental assumptions that porosity ‘rebound’magnitudes are negligible, as confirmed by laboratory tests and empirical observations (Giles etal., 1998); and that all relevant stratigraphic units in the basin have experienced equilibriumcompaction with burial.This technique is not applicable to sections with relatively high transit times due to other factors,such as overpressure or hydrocarbon-filled porosities (Hillis, 1995).2.2.4 Limitations of the Compaction MethodThere are a number of limitations to using compaction techniques to determine exhumation:i) The relationship between compressional velocity and porosity depends critically on lithology(Storvoll et al., 2005)ii) Unreliability in establishing the normal compaction trend for a basin or rock unit is a keylimitation, particularly in basins where regional exhumation has occurred, such as in the MalayBasin.iii) In hydrostatically pressured extensional basins, both effective stress and temperatureincrease with burial depth, so it is generally uncertain whether compactional or thermalprocesses are responsible for observed increase in velocity (decrease in transit time). The sonic 10
  17. 17. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringtransit time method assumes that mechanical compaction is the dominant control on porosityloss through burial, although this is not strictly the case.iv) The technique is indecisive when there is a similar amount of post-exhumational burial to thatof the missing section at the UMU. The compaction trend from such a well would appear to benormally compacted.v) This method cannot be applied to overpressured sections, which have higher transit timesthan in normal sections. The combination of the rapid burial of certain stratigraphic units(especially Formations H to D) and uplift of initially normally pressure strata (Groups J to M) isput forth by Singh and Ford (1982) as the common cause for widespread occurrence ofoverpressures in the Malay Basin. In the basin centre, the onset of overpressure occurs instratigraphically younger formations (Formations E and F), but in the basin flanks, this is atstratigraphically older horizons due to increasing sand percentage away from the centre. Thetop of the overpressure usually coincides with the top of the oil window, suggesting a linkbetween hydrocarbon generation and the onset of overpressure.2.2 Thermal History Based MethodsThermal history based techniques for exhumation estimation provide information about themovement of rocks relative to a thermal reference frame by utilizing the principle thatsedimentary rocks are heated as they are buried and cool as they are exhumed (Corcoran andDoré, 2005). In a vertical succession of rocks, a maximum palaeo-temperature profileinterpreted from vitrinite reflectance (VR) or fission track analysis (FTA) (or a combination of thetwo) can be used to estimate the palaeo-geotherm for that succession and, by extrapolation toan assumed palaeo-surface temperature, the magnitude of exhumation at that location can bedetermined (Figure 2.3). Two of the most prominent studies are those of Cavanagh et al. (2006)and Green et al. (2002). This technique was used in this study to compare with results from the 11
  18. 18. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringtectonic and compaction techniques.Figure 2.3 – Thermal history based methods. ∆z refers to exhumation amount (after Corcoranand Doré, 2005)2.3 Tectonic Based MethodsCorcoran and Doré (2005), describe a theoretical technique for assessing the magnitude ofexhumation in offshore basins, in which the uniform lithospheric stretching model of McKenzie(1978) plays a major role. The model predicts an initial syn-rift, fault related subsidence followedby an exponentially decreasing post-rift, thermal subsidence phase. McKenzie’s modelfacilitates the determination of theoretical subsidence curves which can then be compared withobserved tectonic subsidence histories derived from the backstripping of sediment columns inwellbores. The exhumation is determined by identifying locations where the modeled tectonicsubsidence is greater than the observed subsidence (Figure 2.4). This technique was used byRowley and White (1998) to inverse model extension and denudation in the East Irish Basin;and will also be used in this study to compare with results from the thermal and compactiontechniques. 12
  19. 19. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 2.4 – Tectonic subsidence analysis based methods. EG refers to true exhumation whichis a function of uplift (UT, UP) (after Corcoran and Doré, 2005)2.4 Stratigraphic Based TechniquesBasic stratigraphic techniques of unconformity identification, section correlation, extrapolationand restoration can offer a rationale for estimating the timing and magnitude of exhumation.When using this technique, it is, however, hard to quantify exhumation in areas of epeirogenicuplift and denudation; or varying original depositional thickness (Corcoran and Doré, 2005) suchas in the Malay Basin. Thus, this technique is not applicable in the context of this study.2.5 Previous WorkMurphy (1989) estimated around 1200 m of the Miocene strata was eroded away in the MalayBasin whereas Ramly (2004) estimated a range of 10 to 200 m of erosion in the southernmargin, while Ngah (1990) found up to 800m of erosion in the southeast. 13
  20. 20. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering3. MethodologyBased on a review of literature, four chief methods are available to estimate exhumation. Thecompaction method, however, serves the objective best due to reasons discussed in Section5.1.As both sand and shale sections are available from the Malay Basin, separate methods wereused to derive the normal compaction trends for each lithology. The method applied in Heaslerand Kharitnova (1996)’s erosion estimates study on the Bighorn Basin was used for the shalesections; while the method used by Hillis (1995) was used for the sand sections. The reasonsbehind these will be explained in Section 3.2. For the convenience of reference, the formermethod will be referred to in the rest of this report as the Heasler method, while the latter will bereferred to as the Hillis method.A tectonic method based on the description by Corcoran and Doré (2005) was used to furtherconstrain the resultant exhumation values; and calculate exhumation for Wells 4 and 11, whichis not suitable for the compaction method for reasons that will be explained in Section 3.2.A thermal method using vitrinite reflectance is also used to provide values for comparison.The approaches used to approximate exhumation in this study can be summarized as follows 1. Compaction methods a) Shale sections – Heasler method b) Sand sections – Hillis method 2. Tectonic method 3. Thermal method3.1 Well Data ConditioningIn order to prepare each well for the establishment of a normal compaction trend andsubsequent exhumation analysis, conditioning must be applied so that analogous readings areremoved to ensure that the measurement points represent the true log properties of the shale orsand units. Conditioning followed the process chart shown in Figure 3.1.Figure 3.3 illustrates the type of features removed and the effects of conditioning that improvesthe accuracy of exhumation estimates; and is related to the numbered steps in Figure 3.1.As only vertical wells were present, the sub-seabed depths were converted directly from MDKB(Measured Depths from Kelly Bushing). The formation tops in sub-seabed depths and Kellybushing depths for all wells is shown in Figure 3.2.No attempt is made in this study to convert transit time to velocities or porosities, as the majorityof work on compaction trends to date is using interval transit time (Hillis, 1995; Corcoran andMecklenburgh, 2005; Heasler and Kharitnova, 1996 and others), and thus it was deemedunnecessary. 14
  21. 21. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 3.1 - Well conditioning process flowFigure 3.2 - Well Kelly Bushing depths and formation thicknesses 15
  22. 22. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 3.3 – Well conditioning process for Heasler method (Well 10). Conditioning for HIllismethod is similar with the exception of clean sands picked out instead of shales.3.2 Constructing Normal Compaction Trends3.2.1 Shale Sections – Heasler MethodTypical porosity depth trends for shales generally follow an exponential type curve as inscribedby Magara (1980) (Figure 2.1). Thus for the shale sections of the Malay Basin, such a type ofcurve was computed for the normal compaction trend using the Heasler method.Following section 2.2.2, Athy’s (1930) compaction trend for shales was adapted for analysis ofinterval transit time by Magara (1976), to give DT = DToe-bx …….. (3.1) where DT = transit time as measure by the sonic wireline tool, DTo = interval transit time at the mudline, b = shale compaction coefficient and x = depth where the measurement DT was recorded.As equation 4.1 is not constrained at depth and transit time DT will trend to 0, Heasler andKharitnova (1996) have introduced a constant which is in effect the interval transit time of therock matrix. DT = DToe-bx + C…….. (3.2) 16
  23. 23. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering where C is the asymptote of the exponential function, effectively the shift constant applied to the dataset to take account of the points at maximum compaction.The approach taken in this study expands on the fundamental assumption of Heasler andKharitnova (1996) that porosity reduction with depth in a heterolithic sedimentary sequence isdescribed by a single, average shale compaction coefficient, b; and uses different b values fordifferent depth intervals (See Section 4.1 page x)In order to determine the “best fit” equation that describes the normal compaction trend for thesouthwest Malay Basin, equation 3.2 was transformed logarithmically to Ln(DT-C) - Ln(DTo) = -bx…….. (3.3)A least squares regression (Figure 3.4) was used to find b, where regressions with differentvalues of C (C-trials) were run. By varying C and DT a series of b values were determined. Theset of parameters which give the maximum coefficient of determination, R2 (a value representingthe closeness of the fit to the dataset) represent the best-fit curve. Figure 3.4 – Least squares regression fit for Well 2 dataWells 1, 2, 5, 9 and 12 were picked to produce candidate normal compaction curves as thesewell sections relatively unaffected to any erosion (formations A and B are intact), based on theirrespective formation tops (Figure 3.2). Wells 3, 6, 8 and 10 were subjected to erosion as shownby the formation tops (Figure 3.2) (and thus exhumation), Well 11 does not have any sonic logdata whereas Well 4 does not have any units logged below the UMU; and hence these wellswere excluded from the normal compaction curve derivation process.The interval transit times of the (conditioned) shale sections were subjected to windowedaveraging (Figure 3.5) on the scales of every 10, 25, 50 and 100 meters from the seabed. Awindowed average of every 25 meters from the seabed was gauged to be the mostrepresentative sonic log data curve, as it had minimal uncertainty range (noisiness of data) andminimal loss of data trends due to upscaling. 17
  24. 24. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 3.5 – Windowed averaging every 25 m (Well 5)Both data pre- and post-averaging were subjected to least squares regression analyses in orderto preserve a balance between the original and upscaled data. The best fit combination of thetwo yielded suitable parameters C, b and DTo for each well. The final normal compaction curvewas picked out of a cross comparison of all curves generated by the aforementioned wells(Section 4.1.1).Estimates of exhumation would then be approximated by comparing this normal compactiontrend with the conditioned transit time data for each of the exhumed wells within the study area.The difference between the depth of the computed normal compaction trend and the depth ofeach measured transit time point in an exhumed well yielded a single estimate of apparentexhumation given by the equation below: Ln(DT - C N ) - Ln(DToN ) Apparent Exhumation = – xunderinvestigation…….. (3.4) -b where CN and DToN is the C-value and DTo value for the final normal compaction trend, respectively and xunderinvestigation is the current sub-seabed depth where exhumation is being calculated.Exhumation using the Heasler method was calculated for all wells except for Well 11 (which hasno sonic log data) as all of them had clean shale sections.Khalid’s Method:A variation on the Heasler method was also carried out using the steps done by Ngah (1990),which will be referred to Khalid’s method in this report. This method takes into account theamount of exhumation undergone by after the formation of the unconformity (and henceerosion), which is taken away from the true exhumation magnitudes calculated by the Heaslermethod (Enclosure 1.1). 18
  25. 25. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering3.2.2 Sand Sections – Hillis MethodFor sandstones, typical porosity depth trends are linear Magara (1980) and fit porosity depthdata in units over finite depth intervals as proven by Bulat and Stoker (1987). See Figure 2.1 forcomparison with a typical shale porosity depth trend.The linear normal compaction trends over finite depth intervals (one per formation) as proposedby Hillis (1995), which is the method used on the clean sand sections of the study area’s wells.In an area subject to exhumation, the wells with the highest interval transit times (lowestvelocity) for their given burial depth should be taken to be normally compacted, provided theirrelatively high transit times is not due to phenomena which may inhibit normal compaction suchas overpressure or hydrocarbon filled porosity. For a linear decrease with depth, any two ormore data points which can be linked by a straight line that has no points falling to its right lesscompacted side, may potentially define normal compaction. This line is given by a general linearline equation: xunderinvestigation = mDT + c…….. (3.5) where x = sub-seabed depth under investigation and DT = interval transit time for that depth. The gradient and y-intercept of the line is given by m and c, respectively.Eight such lines could be plotted for formations D, F, H, I, J, K, L and M (Section 4.1.2 - Figure4.2) where the transit times for each formation is plotted from every well where a clean sandinterval was available for the formation under investigation. Individual m and c values werederived for each formation.Wells 1, 2, 5, 6, 8, 9 and 10 contain clean sand intervals and were thus exhumation estimatesusing the Hillis method. The normally compacted interval transit time, DTN, can be found foreach well by rearranging equation 3.4 to the equation below: x underinvestigation - c DTN = …….. (3.6) m where m and c differs for each formation which was computed earlier by using equation 3.5.Apparent exhumation is then the difference between DTN and the measured transit time for thedepth under investigation.3.2.3 Estimating Exhumation Using Compaction MethodFor each well under investigation, all the measured apparent exhumation values can becharacterized by standard descriptive statistics by plotting these in a histogram. It is thenpossible to identify the mode. This is the most useful measure of the data as it gives the mostfrequently occurring value of exhumation rather than the mean which also takes into account theextreme high and low values which are not representative of the dataset as a whole.This mode, however, does not represent the total (true) exhumation, as post-exhumation burial,BP must be considered. The apparent exhumation, EA, calculated by the abovementionedHeasler and Hillis methods therefore underestimates the exhumation and so all estimates must 19
  26. 26. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringhave the thickness of post-exhumation burial added on. This was taken as the sub-seabedthickness of sediment fill above the UMU for each well.In error analysis, the most likely exhumation is the mode. However, the likelihood of gettinglarge values of exhumation is just as likely as getting a small value, so the percentiles are notused. Instead, the deviation from the mode, which is well dependent, was computed to identifythe uncertainty range.3.3 Thermal MethodExhumation values for Wells 2 and 4 were calculated by applying the thermal method explainedin Section 2.2 on VR data for the respective wells. This was done using a combination of thetechniques used in Green et al. (2002), where the temperature profile derived from VR data wascompared with corrected SBHT data (see Section 3.4) for Well 2; and that in Gallagher (2008),which involves analyzing discontinuities in the VR data at unconformities for Wells 2 and 4. SeeEnclosure 1.2 for details. As this method does not play any role in determining the finalexhumation values, no detailed description on the thermal method is made.3.4 Tectonic MethodThe tectonic method approximates exhumation by comparing theoretical subsidence curves(which varied with stretch factor) with observed tectonic subsidence histories derived from thebackstripping of sediment columns in wellbores (Figure 2.4).Two approaches were considered for obtaining the theoretical curves. One was to calculateinitial synrift and subsequent thermal subsidence using a formula (Enclosure 1.3) described byAllen and Allen (2005) based on McKenzie’s 1978 uniform stretching model.Another method was to use preexisting subsidence curves at various wells in the Malay Basincalculated by Madon and Watts (1998) (Figure 3.6), who used a constant strain-rate, finite-rifting, uniform stretching model by Cochran (1983). A linear relationship was found between thefinal basement depths of Madon and Watts (1998) and stretch factor (Section 5.1.2).Figure 3.6 – Examples of predicted subsidence curves and stretch factor values from Madonand Watts (1998) 20
  27. 27. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringIn both approaches, timing of rifting was to have started in 35 Ma (Early Oligocene) and endedat 25 Ma based on the age of the oldest sediment (Yakzan et al., 1994).Water loaded sediment backstripping was then done for wells 10 and 11 as both these wellshave sonic log data that penetrate down to the basement. This was done through standardbasin modeling software (Petromod 1D Express) by inputting formation top depths and palaeo-water depths. Backstripped curves were also available from several other locations in theMalay Basin (Madon and Watts, 1998).The difference between the theoretical curve basement depths and the water loaded basementdepths gives apparent exhumation magnitudes. Post-exhumation burial was then added on toprovide true exhumation. The magnitudes of exhumation approximated from comparison withboth the McKenzie and Cochran theoretical curves for Well 10.The basement depths of Wells 10 and 11, as well as those of Madon and Watts (1998) werethen extrapolated to the other wells in the dataset using a map of the pre-Tertiary basementdepths as a rough guide.An uncertainty of 0.1 in stretch factor was assumed for each of the well sections, whichtranslated into the uncertainties in exhumation in Section 4.3.3.5 1D and 2D Basin Modeling3.5.1 1D Modeling1D basin modeling using the PetroMod (Express) software from Integrated Exploration Systems(IES) was carried out for all wells to investigate the difference in petroleum generationproperties between uneroded sections and sections with erosion (values from the bestestimates).Erosion estimates from Khalid’s method for Wells 9 and 12 displays a significant differencecompared to best estimates of erosion for these wells (Section 4.4). The same estimates forother wells, including Well 5, do not display any significant difference (Section 4.4). Thus,additional modeling was carried out for Wells 9 and 12 to investigate the effects of thisdifference on the timing of hydrocarbon generation, and on Well 5 as a control (representing theother wells).As stated in Section 1.2.2, non-deposition on the crests of inversion structures occurredsimultaneously with deposition on the flanks. Thus, the models for Wells 1, 4, 8 and 11, whichare located on the crests are given periods of non deposition during the deposition of themissing sections (8.5 to 7 Ma); whereas for other wells deposition was inputted to continue forthis period of time.Total organic carbon (TOC) values and kerogen types for Wells 1, 2 and 8 were included in thedataset, but those for Wells 9, 11 and 12 were obtained from nearby fields (*locationsclassified). The HI and TOCs (for Pre-I group source rocks) for wells 5, 10 13 and 14 wereobtained from the Epic (1994). The source rock kinetic model used in this study is the Easy Romodel of Sweeney and Burnham (1990).Maturity-burial histories were generated for Wells 1, 2, 5, 8, 9, 10, 11, 12, 13 and 14; but not forWells 3, 4 and 6 as these wells do not have TOC information. 21
  28. 28. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringThe sea level curve for all models was taken from the global eustasy curve as the relative sealevels for the Malay Basin was similar to the global sea levels (personal communication J.Hoesni). This was taken from Miller et al. (2005)’s interpretation of Haq’s eustasy curve(Enclosure 2.1).Vitrinite reflectance (VR) data was available for Wells 1, 2, 5, 8, 9, 10, 11 and 12 and singlebottom hole temperature (SBHT) for Wells 1 and 2. SBHT was corrected using the methodproposed by Waples and Mahadir (2001); given by the formula: Tc = Ts + f X (Tm-Ts) …….. (3.7) - 0.1462Ln(TSC) + 1.699 f= …….. (3.8) 0.572 × Z 0.075 where Tc = corrected temperature, Tm = measured temperature, Ts = surface temperature, f = correction factor, TSC = time since circulation and Z = depth.As Wells 13 and 14 had no VR or SBHT data, they were calibrated using VR and SBHT datafrom Wells 8 and 5 respectively, as these were the nearest wells.A McKenzie crustal stretching model (from PetroMod) was used to approximate the heat flowhistory for the models. Rifting times and stretching factors were kept the same as in theaforementioned tectonic method, but the duration of thermal subsidence was adjusted to fit thecorrected SBHT (Table 3.1) and VR data for each individual well (Enclosures 2.2 to 2.4). TSC SBHT, Corrected Temperature, Well Name Depth, Z (m) (hours) Tm (°C) f Ts (°C) Well 1 985 9 57 1 70 1294 3 73 2 100 1381 3 74 2 100 1594 5 81 1 106 1734 4 79 1 105 1950 5 90 1 119 2212 5 97 1 128 2405 5 110 1 147 2498 5 118 1 157 Well 2 991 4 27 2 27 2395 6 96 1 124 Well 3 1161 7 74 1 96 1773 6 87 1 113 2162 11 118 1 147 2226 11 121 1 152 Surface Temperature, Ts = 27 °CTable 3.1 – Measured SBHT and corrected SBHT (see equation 3.8 above)Regional sediment water interface temperature (SWIT) values were obtained from PetroMod(which uses values from Wygrala (1989) for Southeast Asia, latitude five (study area). Trap 22
  29. 29. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringformation timing was taken from field reports from the Malay Basin from C & C Reservoirs(2008).3.5.1 2D Modeling2D modeling of a cross-section through Well 9 to Well 13 to Well 8 was accomplished using theTemis2D software from Beicip Incorporated to investigate the effects of erosion on hydrocarbonmigration.A cross-section was first digitized from an interpreted seismic cross-section (Figure 1.4B)across the aforementioned wells, sourced via personal communication with J. Hoesni, 2008.This was a laborious process involving manual data entry from interpreted horizons and faults inthe cross-section image into a format (.ext file) readable by Temis2D.The eustasy, crustal model, lithology and source rock parameters were kept the same as in the1D modeling. The bottom (of the basement) temperatures of the basin were kept as a constant1333 °C, the temperature assumed for the Moho.The source rock intervals for the cross-section modeled are formations I, K, L and M. The faultswere digitized from the cross-section (Figure 1.4B).The faults were interpreted as open during the time of rifting and inversion (due to reactivation);and closed at all other times. 23
  30. 30. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering4. RESULTS4.1 Normal Compaction Curves4.1.1 Heasler MethodFrom the normal shale compaction curves derived from Wells 1, 2, 5, 9 and 12 as shown inFigure 4.1, the final normal compaction curve for the Malay Basin (Figure 4.2) was computedusing C (shift constant) and b (shale compaction coefficient) values from that of Well 2 for sub-seabed depths of the interval 0 - 2329 m; and from that of Well 2 for 2330 - 2399 m. This wasdone as the normal compaction curve generated from Well 2 had the highest interval transittimes from depths 0 to 2329 m, but for the interval 2330 - 2399 m, this was true for Well 12.Figure 4.1 – Potential normal compaction curves from the Heasler method 24
  31. 31. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringFigure 4.2 – Final normal compaction curves from the Heasler methodAlthough normal compaction curves from Well 5 and Well 9 are shown to have higher transittimes, they were not used for the computation of the final normal compaction curve as the linesof best fit through the logarithmically transformed data (see Equation 3.3) for these wells had alow maximum coefficients of determination (poor fit).As Figure 4.2 shows, the interval transit time in which the normally compacted curve intersectsthe mudline (where sub-seabed depth = 0) is in the range of 180 to 200µs, in agreement withMagara’s (1980) observations.4.1.2 Hillis MethodAs explained in Section 3.2.2, the normal sand compaction curves, were derived from linkingtwo or more with a straight line that has no points falling to its right less compacted side forformations D, F, H, I, J, K, L and M (Figure 4.3). 25
  32. 32. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringF H IJ K LMFigure 4.3 – Normal compaction curves from the Hillis method for formations D to M 26
  33. 33. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering4.2 Exhumation Estimations from Compaction MethodsThe exhumation magnitudes and associated uncertainties from the Heasler and Hilliscompaction methods are displayed in Figures 4.4 and 4.5, Enclosures 1.4 and 1.5, and Table4.1. Depths with negative (true) exhumation values were inferred to be overpressured andremoved.Figure 4.4 – True exhumation estimate for Well 1 using the Heasler method. The mode of thehistogram is the final estimate and uncertainty is defined by the deviation from the mode.Figure 4.5 – True exhumation estimate for Well 1 using the Hillis method. The mode of thehistogram is the final estimate and uncertainty is defined by the deviation from the mode. 27
  34. 34. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering Heasler Hillis True True Post Well Exhumation Uncertainty Exhumation Uncertainty Exhumational Name (m) ± (m) (m) ± (m) Burial (m) Well 1 600 221 560 6 532.3 Well 2 600 226 680 3 643.4 Well 3 550 0 1083.1 Well 5 750 284 860 9 852.3 Well 6 1110 157 770 5 750 Well 8 1400 129 410 1 383.7 Well 9 250 178 570 1 558.7 Well 10 100 440 540 4 530 Well 12 900 243 830 10 797.36 Well 13 600 216 500 Well 14 800 276 890 7 852.3Table 4.1 - True exhumation estimates for all wells from compaction methods (data for emptycells are not available due to non-suitability of method)4.3 Exhumation Estimations from Thermal and Tectonic MethodsThe exhumation values calculated from the thermal method are 206.7 m for Well 2 and 30 m forWell 4. These values are very low compared to estimates from the compaction methods (Table4.1) and the tectonic method (Table 4.3). They are not valid for reasons outlined in Section5.1.3.From the two aforementioned approaches in approximating theoretical subsidence curves fromSection 3.3, the Cochran curve seems to be a more accurate theoretical comparison curve (asopposed to the McKenzie curve). This was concluded from comparing the (true) exhumationestimates approximated by comparing water loaded backstripped basement depths with thatestimated by the curves for Well 10 with calculated by the compaction methods. As Table 4.2shows, the exhumation estimates from the Cochran method shows a much better match withthe ranges predicted by compaction methods. True Exhumation for Well 10 (m) Tectonic Compaction Cochran McKenzie Heasler Hillis 483.9 3330 100 540Table 4.2 - True exhumation estimates for Well 10 from tectonic and compaction methodsThus, the Cochran technique was used to calculate exhumation values for the remaining wells(Table 4.3). This was particularly important for Wells 4 and 11, as both were not suitable for thecompaction method for exhumation analysis. 28
  35. 35. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering Water Predicted Post True Well Stretch loaded Uncertainty basement Exhumational Exhumation Name factor basement ± (m) depth (m) Burial (m) (m) depth (m) Well 1 1.85 2250 2100 532 682 0 Well 2 2.4 2890 2600 643 933 0 Well 3 2.5 3000 3500 1083 583 5 Well 4 2 2320 3000 1038 358 115 Well 5 1.6 1890 2000 852 742 10 Well 6 1.2 900 500 750 1150 50 Well 8 1.5 2430 1700 384 1114 740 Well 9 1.8 2220 2400 559 379 10 Well 10 1.6 1900 1946 530 484 0 Well 11 1.7 2100 2565 534 69 40 Well 12 2.5 3000 2700 797 1097 5 Well 13 1.7 2100 1900 500 700 40 Well 14 1.7 2100 2100 852 852 40Table 4.3 - True exhumation estimates for all wells from the tectonic method4.4 Final Erosion ValuesThe exhumation values from the Heasler, Khalid, Hillis and tectonic methods were compiled togenerated erosion values for four cases in Table 4.4 – a low case, a high case, a best case anda Khalid method case. The final erosion values were picked based on the best methods foreach well section. Low High Best Method Uncertainty Erosion Well Name (m) (m) (m) Used ± (m) Khalid (m) Literature Well 1 379 821 600 Heasler 221 600 Well 2 374 826 600 Heasler 226 -142 Well 3 550 550 550 Heasler 0 529 Well 4 243 473 358 Tectonic 115 Well 5 466 1034 750 Heasler 284 738 Well 6 953 1267 1110 Heasler 157 1110 Well 8 1271 1529 1400 Heasler 129 1377 Well 9 72 428 250 Heasler 178 134 Well 10 536 544 540 Hillis 4 300 Well 11 29 109 69 Tectonic 40 Well 12 657 1143 900 Heasler 243 162 Well 13 384 816 600 Heasler 216 590 Well 14 524 1076 800 Heasler 276 787 Well 15 800Table 4.4 – Final erosion estimates for all wells for low, high and best cases. The literatureerosion estimates for Wells 10 and 15 were obtained from Ngah (1990). 29
  36. 36. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering4.5 Basin Models4.5.1 1DMaturity-burial histories, and event charts were generated for Wells 1, 2, 5, 8, 9, 10, 11, 12, 13and 14 (Section 5.4; Enclosures 2.5 to 2.10) for different cases of erosion (Table 4.5). Theerosion values not listed in Table 4.4 was picked based on fitting to VR and SBHT data tried outfor different possible erosion values. The best case erosion estimates were found to fit thethermal parameters the best (Enclosures 2.2 to 2.4). Well Erosion Well Erosion Well Erosion Well Erosion Name (m) Name (m) Name (m) Name (m) Well 1 379 Well 2 374 Well 5 466 Well 8 1271 Well 1 600 Well 2 600 Well 5 738 Well 8 1400 Well 1 821 Well 2 826 Well 5 750 Well 8 1529 Well 5 1034 Well 9 72 Well 10 536 Well 11 29 Well 12 162 Well 9 134 Well 10 540 Well 11 69 Well 12 657 Well 9 250 Well 10 544 Well 11 109 Well 12 900 Well 9 428 Well 11 700 Well 12 1143 Well 13 384 Well 14 524 Well 13 600 Well 14 800 Well 13 816 Well 14 1076Table 4.5 – Different erosion magnitudes used in 1D basin modeling for all wells.4.5.2 2DThe hydrocarbon saturations and erosion thickness effects on hydrocarbon migration are shownin Section 5.4.3. 30
  37. 37. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering5. DISCUSSION5.1 Comparison of MethodsThe four methods used in this study which are the (Heasler and Hillis) compaction, thermal andtectonic methods (outlined in Section 3. Methodology), are found to be useful under differentcircumstances based on a methodological point of view.The Heasler method is better suited for sections with high clean shale to clean sand ratios andvice versa of the Hillis method. The reason for this is because Heasler compaction curves aremore similar to shale compaction trends and Hillis compaction curves to sand compactiontrends (Section 3.2). For a carbonate dominated section, a velocity based compaction approachused by Japsen (1998) is recommended as porosities vary greatly in carbonates and thussignificant fluctuations in interval transit time may arise due to porosity changes rather than apast exhumation event. This dilemma is solved in a velocity based approach.A variation on the Heasler method used by Ngah (1990), where the amount of exhumationundergone after the formation of an associated unconformity is taken away from totalexhumation (obtained from the Heasler method) to give the amount of erosion.The tectonic method, in this study, produces exhumation estimates close to that of the mostsuitable compaction methods, but the uncertainty for this method is variable as this does notonly come from uncertainty in stretch factor (this study), but also from uncertainties in themodeling of the theoretical subsidence curve, which has many potential uncertainties due to thehigh number of parameters involved in the modeling (Madon and Watts, 1998; Allen and Allen,2005).Although a faster technique compared to the aforementioned three, the VR thermal method isnot suitable for sections with no vitrinite reflectance above the unconformity concerned (such asin this study) and is subject to limitations as outlined in Section 5.1.3 below. The VR data canalso be combined with other organic thermal indicators such as Fluorescence Alteration ofMultiple Macerals (FAMM) and FT data to give the final thermal history.5.1.1 Uncertainties in the Compaction MethodsAs it can be deduced from Table 4.1, the uncertainty range for results from the Heasler methodis two orders of magnitude greater than that of the Hillis method. This is due to the fact that Hillisnormal compaction curves are computed for every formation whereas only one Heasler normalcompaction curve is used for all formations. For Well 1, where clean sands and shales aredistributed more or less equally throughout the stratigraphy, the estimated amounts ofexhumation from Heasler and Hillis methods are close with only 40m of difference. However,the uncertainty for the Hillis method is much smaller than that of the Heasler method. Thissuggests that the accuracy for the two methods does not differ much but the precision of theHillis method is greater than that of the Heasler method.5.1.2 Compaction versus Tectonic MethodIn the two adaptations of the compaction method used in this study, the Hillis method proved tohave a substantially smaller range of uncertainty compared to the Heasler method. However,the more mathematically rigorous Heasler method involves fewer extrapolations compared tothe Hillis method. Despite this, it is still recommended that the Heasler method be used for 31
  38. 38. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringshale-dominant sections and the Hillis be used for sand-dominant sections (and a velocitybased approach for carbonates) due to the reasons outlined above.The normal compaction trends for basins where regional exhumation has occurred, for examplethe Malay Basin in this study, cannot be established reliably due to the lack of un-exhumedsections and formations. Interval transit time is also sensitive to porosity, particularly if it isderived from density logs (Taylor, 2007).The compaction methods also operate under the assumption that mechanical compaction is thedominant control on porosity loss through burial, although this might not always be the casesuch as in Malay Basin where the low porosities are associated with chemical compaction dueto the high geothermal gradients (Hoesni et al., 2007). Shales, however, are less susceptible tochemical compaction (which operates through the dissolution and re-precipitation of mineralsand is controlled by thermal processes) compared to sands. Chemical compaction alsooperates in the Gulf of Mexico Basin, the Norwegian Shelf and the Baltic Region (Lander andWalderhaug, 1999).If the magnitude of post-exhumational burial is equal to the amount of exhumation, then theeffect of exhumation on porosity loss would be neutralized and thus the compaction trend fromsuch a section would appear to be (falsely) normally compacted (Figure 2.2).Compaction methods cannot be used for overpressured sections, which are dominant in theMalay Basin particularly in the basin center (Madon, 2007). Overpressure is also a commonphenomenon for most North Sea reservoirs (Evans et al., 2003) and the Gulf of Mexico basin(Mello and Karner, 1996). Overpressure is a result of rapid sedimentation or tectoniccompression (which creates uplifted reservoirs).The tectonic method in this study was based on a linear relationship found between the stretchfactor and basement depths predicted by the theoretical curves (Figure 5.1).Figure 5.1 – Stretch factors and the associated predicted basement depths from Madon andWatts (1998)’s subsidence curves 32
  39. 39. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringThis however, may be an oversimplification of the relationship as there seems to be a departurefrom this linearity for stretch factors above 1.7.The version of the tectonic method used in this study estimates exhumation magnitudes fromthe difference between water loaded basement depths and basement depths predicted bytheoretical backstripped curves of various locations in the Malay Basin by Madon and Watts(1998) using a version of the uniform stretching model by Cochran (1983). As evident in Figure3.6, this model has the two phases of McKenzie’s stretching model: an initial syn-rift subsidencephase followed by thermal subsidence phase.Thermal subsidence would begin in the basin center where the geotherms have suffered thegreatest distortion from rifting. The rate of thermal subsidence decreases exponentially with timeand reliance of heat flow stretch factor decreases. Thus the reliance on of heat flow on stretchfactor decreases towards the basin center as thermal subsidence in center started earlier.This explains the departure from the linear relationship between predicted basement depths andstretch factors mentioned above. As stretch factors increase towards the basin center (wherethermal subsidence has been longer), the dependence of thermal subsidence on these higherstretch factors in the Malay Basin is lower.Uncertainties in stretch factor (0.1) may also cause up to 500 m of uncertainty in the predictedbasement depths.Other studies (Rowley and White, 1998; Corcoran and Doré, 2005) used different versions ofthe (theoretical) uniform stretching model in their tectonic method to estimate exhumation. Thisdoes not imply that one model is better over the other. A stretching model that suits the basinunder study most should be used.Despite the fact that the tectonic method can be used in overpressure sections and any lithologyas opposed to the compaction method, it can only be applied to basins which follow McKenzie’suniform stretching model and shows varying ranges of uncertainty (Table 4.3).Due to this varying range of uncertainty and the fact that this version of the tectonic method isbeing used for the first time, the final erosion values were taken from the compaction methodsinstead. However, the tectonic method proved useful in providing erosion values for Wells 4 and11, where the compaction methods could not be applied.5.1.3 Erosion Estimates from Vitrinite ReflectanceThe thermal method is the least time consuming method out of all the methods. The wells in thisstudy, however, mostly only have vitrinite reflectance data below the UMU, which negates theuse of the method in Gallagher (2008), except for Well 4. The Well 4 VR profile, however,shows the discontinuity (due to exhumation) about 200 meters below the measuredunconformity.The Green et al., (2002) method was used on Well 2 but was invalid for all other wells withSBHT data as there is a widespread suppression of vitrinite reflectance (deviations of VR profiletowards lower values) in this area, possibly due to the presence of hydrogen–rich vitrinites.The estimated erosion values from this method are very low and do not correlate well with thosefrom the other methods (Table 4.4). This is a result from the suppression of vitrinite reflectancein the area. Other limitations associated with the thermal method include: 33
  40. 40. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringi) Non-linear palaeo-temperature profiles cannot be used to estimate the magnitude ofexhumation at a given well location (Duddy et al., 1998).ii) Palaeo-thermal indicators such as VR and FT are dominated by maximum palaeo-temperatures and do not preserve information on thermal events that occurred prior to theachievement of peak palaeo-temperatures.iii) Translation of VR values into absolute palaeo-temperatures can introduce systematic errorsin the estimation of palaeo-geothermal gradients and consequently, the magnitude ofexhumation (Green et al., 2002).iv) Variations in the chemical composition of vitrinite, between well locations, may lead to invalidcomparison of VR gradients and associated exhumation estimates.5.14 DiscrepanciesEstimations of erosion from the interpreted seismic cross-section in Figure 1.4B showsconsistency with compaction method estimates for Well 9, but inconsistencies with those ofWells 8 and 13. The erosion value approximated from the seismic section for Well 13 issignificantly higher than the best case estimate by 500 m; and those of Well 8 shows adifference of -177 m (Enclosure 1.6). The erosion values from the seismic section, however, arenot without uncertainty as the original depositional thicknesses vary.As shown in Table 4.4, the relative erosion estimates for Wells 8 is very high whereas theopposite is true for Well 11. The erosion estimate for Well 13 is relatively low considering itsclose proximity to Well 8. The erosion estimates for Well 8 and 13 was calculated using theHeasler compaction method; whereas that of Well 11 was calculated using the tectonic method.The formation tops for Wells 8 and 13 listed in Figure 3.2 show that there is a lot of missingstrata in these wells. This may have led to low representation of strata in the Heaslercompaction method, which may have skewed erosion estimates for these wells.As shown in Figure 1.2, inversion (and hence exhumation of eroded section) occurred duringthe deposition of formations D to H, and would thus affect these formations the greatest. Well 8is comprised only of formations D and L, whereas Well 13 of only formations J, K and L. Hence,any reduction in transit time caused by exhumation can only be measured for formations D andL for Well 8, and formations J, K and L for Well 13: the exhumation effects on the rest of theformations are not measured and this gives rise to statistical misrepresentation (Table 5.1). Well Well Exhumed Table 5.1 - 8 13 Formations D Formations present in Wells 8 and 13; versus E formations affected by exhumation F H I J J K K K L L L M 34
  41. 41. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and EngineeringWell 11, on the other hand, has a near complete set of formations. However, as the tectonicmethod was used to calculate the erosion for this well, other factors are needed to explain therelatively low erosion estimates from this well. As stated in Section 5.1.2, an uncertainty of 0.1 instretch factor may cause up to 500m of uncertainty in the theoretically predicted subsidence,which translates to 500m of uncertainty in exhumation estimates, on top of the uncertainty givenin Table 4.3.5.2 Erosion TrendsErosion values between the wells in the study area can be obtained via the creation ofexhumation contour maps controlled by data points with estimated erosion values (such as thewell sections in this study area). The calculated erosion for a well within the study area fromNgah et. al (1996) was added to the data points to provide better deterministic control over thecontours. Erosion maps based on the calculated exhumation values were computed in twosteps.Firstly, erosion contour maps were computed using 3DField using minimum curvature, blockkriging, block radial basis function, and linear equation algorithms. The minimum curvature mapwas found to be the closest fit to facies changes in the study area.Hence, the minimum curvature contour map was compared with variations in palaeo-environment for the E formation (deposited at the time of peak inversion) and inversion axes asboth were thought to have a strong correlation with erosion trends. The contour map, which wasgenerated based on only erosion values for the well locations, did not correspond well with theinversion axes (Figure 5.2a) but seem to be influenced by variations in facies (Figure 5.2b),although this is not a tight correlation. However, the contours are roughly perpendicular with theinversion axes (Figure 5.2a) which is similar to the relationship between exhumation contoursand inversion axes in the Eromanga basin, Australia (Hillis and Mavromatidis, 2005).The contour maps for the best, low and high cases of erosion had similar erosion trends and sothe aforementioned comparisons were only done for the best estimates of erosion.As shown in Figure 5.2b, there are a lot of lateral changes in palaeo-environment in the moredistal southeast part of the study area during the time of Formation E. In subarea X, a distalfloodplain widens outward to the east-southeast where (probable) palaeo-rivers flow west fromthe basin margin. This corresponds to an area of high erosion. This is also true for a palaeo-river further southeast.Probable palaeo-river locations are marked in Figure 5.2b based on distal floodplain trends. Itbecomes more distal south of that which corresponds to a topographic high at time E (seecross-section in Figure 1.4). The eastern exhumation low in the map is situated away from thepath of the palaeo-rivers and is thus spared from heavy erosion. Erosion decreases towards thewest as the environment goes to more distal swamps (lower energy due to distance fromwaves, rivers and tides).The second step was thus to modify the contour map manually to loosely follow facies changesin parts where there are less control points (Figure 5.2), to give the final exhumation map.These steps were done for the best, low and high case estimates of exhumation to produce 35
  42. 42. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineeringthree exhumation maps for each case (Figure 5.3). As Figure 5.3 shows, the area of highexhumation in the southeast grows as the exhumation estimates increase from the low to thebest to the high case maps. N Figure 5.2 – Exhumation map over a) Inversion anticlinal axes (top) indicated by green lines. Broken black lines indicate the AMFZ b) Palaeo-geography during peak inversion in 12.5 Ma (bottom). Red lines indicate probable locations of a) palaeo-rivers 110km Both maps are modified from Madon et al., 1999. Flow direction Probable palaeo- river locations b) x 110km Erosion contour 36
  43. 43. Investigation of the amount of erosion at the upper Miocene unconformity in the southeastern part of the Malay BasinLune Gene Yeo, MSc Petroleum Geoscience, Department of Earth Science and Engineering Figure 5.3 – Erosion maps for a) low, b) best and c) high cases of erosion x Erosion contour 110km a) Low Case 110km b) Best Case 110km c) High Case 37

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