Acoustic And Petrophysical Properties Of A Clastic Deepwater Depositional System From Lithofacies To Architectural Elements Scales
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Acoustic And Petrophysical Properties Of A Clastic Deepwater Depositional System From Lithofacies To Architectural Elements Scales

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An analysis of acoustic, petrophysical, and stratigraphic heterogeneities has been completed at three scales for an outcropping/subcropping deep-water stratigraphic sequence: lithofacies, core/plug, ...

An analysis of acoustic, petrophysical, and stratigraphic heterogeneities has been completed at three scales for an outcropping/subcropping deep-water stratigraphic sequence: lithofacies, core/plug, lithostratigraphic unit, well log, and architectural element seismic. Measurement techniques/instruments included
outcrop measured sections; behind-outcrop drilling/logging/
coring and subsequent core and log analysis; ground-penetrating
radar; shallow seismic reflection profile; and electromagnetic induction.

At the lithofacies scale, four rock types are defined: 1)heterogeneous sandstones and; 2) uniform sandstones, which differ in their grain composition and sedimentary structures, but do not differ significantly in average porosity, permeability, and acoustic impedance; and 3) organic-rich shales and; 4) organic-poor shales, which exhibit significantly higher acoustic impedance than either sandstone type. There is an inverse relation between porosity and permeability versus acoustic impedance of the lithofacies at this scale. At the lithostratigraphic unit scale, three units of interbedded lithofacies are defined: 1) uniform sandstone prone; 2) heterogeneous sandstone prone, and; 3) shale prone. Successive merging of thinner beds with thicker beds results in clear differences in average rock properties between the lithostratigraphic units, but there is insufficient variation about the averages to preclude statistically significant differentiation of the sandstones. Lithostratigraphic unit properties also vary laterally. At the architectural element scale, two architectural
elements are channel element and lobe element. Only wellbore acoustic impedance differs significantly between these
two elements. However, the internal lateral architecture of these two elements is quite different. The results highlight the difficulty in evaluating stratigraphic patterns away from the wellbore. More research in this area is warranted. Attempts to quantify lateral variability of properties in a geologically realistic manner are encouraged because lateral variability is as important to reservoir characterization and performance as is vertical variability.

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Acoustic And Petrophysical Properties Of A Clastic Deepwater Depositional System From Lithofacies To Architectural Elements Scales Acoustic And Petrophysical Properties Of A Clastic Deepwater Depositional System From Lithofacies To Architectural Elements Scales Document Transcript

  • GEOPHYSICS, VOL. 74, NO. 2 MARCH-APRIL 2009 ; P. WA35–WA50, 19 FIGS., 4 TABLES. 10.1190/1.3073760 Acoustic and petrophysical properties of a clastic deepwater depositional system from lithofacies to architectural elements’ scales Roger M. Slatt1, Eric V. Eslinger2, and Staffan K. Van Dyke3 the lithofacies at this scale. At the lithostratigraphic unit scale, ABSTRACT three units of interbedded lithofacies are defined: 1 uniform An analysis of acoustic, petrophysical, and stratigraphic heter- sandstone prone, 2 heterogeneous sandstone prone, and 3 ogeneities has been completed at three scales for an outcropping/ shale prone. Successive merging of thinner beds with thicker subcropping deep-water stratigraphic sequence: lithofacies beds results in clear differences in average rock properties be- core/plug , lithostratigraphic unit well log , and architectural tween the lithostratigraphic units, but there is insufficient varia- element seismic . Measurement techniques/instruments includ- tion about the averages to preclude statistically significant differ- ed outcrop measured sections; behind-outcrop drilling/logging/ entiation of the sandstones. Lithostratigraphic unit properties coring and subsequent core and log analysis ; ground-penetrat- also vary laterally. At the architectural element scale, two archi- ing radar; shallow seismic reflection; and electromagnetic induc- tectural elements are channel element and lobe element. Only tion. At the lithofacies scale, four rock types are defined: 1 wellbore acoustic impedance differs significantly between these heterogeneous sandstones and 2 uniform sandstones, which two elements. However, the internal lateral architecture of these differ in their grain composition and sedimentary structures, but two elements is quite different. The results highlight the difficul- do not differ significantly in average porosity, permeability, and ty in evaluating stratigraphic patterns away from the wellbore. acoustic impedance; and 3 organic-rich shales and 4 organic- More research in this area is warranted. Attempts to quantify lat- poor shales, which exhibit significantly higher acoustic imped- eral variability of properties in a geologically realistic manner are ance than either sandstone type. There is an inverse relation be- encouraged because lateral variability is as important to reservoir tween porosity and permeability versus acoustic impedance of characterization and performance as is vertical variability. cally significant attributes seismic attributes that capture changes in INTRODUCTION waveform shape that are caused by changes in stratigraphy . The senior author of this paper was invited to the 2007 Rainbow Because stratigraphic properties, such as thickness, interlayering conference to present an overview of scales and types of stratigraph- of strata, porosity, and permeability, change laterally as well as verti- ic heterogeneities that might occur in oil and gas reservoirs. To do cally, so will the seismic response. For example, Hart 2008 cites this for a largely geophysical audience, an attempt was made at the the wide range of values of net to gross and other characteristics of conference, as well as for this paper, to describe the heterogeneities architectural elements of a submarine fan that can occur laterally and in terms of geophysical characteristics rather than stratigraphic de- vertically, thus affecting the seismic waveform in ways that can be scriptors, such as upward-fining and upward-coarsening sequences, difficult to predict or interpret. Conversely, changes in seismic fre- Bouma beds, stratigraphic pinchouts, etc. In a geophysical sense, quency and other variables will affect the seismic signature of a this is similar to Hart’s 2008 differentiation of physically signifi- stratigraphic feature Zeng and Kerans, 2003; Hart, 2008 . Geosci- cant seismic attributes i.e., attributes thought to respond to or direct- entists and reservoir engineers often resort to geostatistical tech- ly image changes in acoustic or elastic properties from stratigraphi- niques variograms, etc. to accommodate the difficulty of character- Manuscript received by the Editor 1 April 2008; revised manuscript received 3 November 2008; published online 11 March 2009. 1 University of Oklahoma, Institute of Reservoir Characterization, Norman, Oklahoma, U.S.A. E-mail: rslatt@ou.edu. 2 Eric Geoscience, Inc. and the College of St. Rose, Albany, New York, U.S.A. E-mail: e.eslinger@gmail.com. 3 Nexen Petroleum U.S.A., Inc., Dallas, Texas, U.S.A. E-mail: staffan vandyke@nexeninc.com. © 2009 Society of Exploration Geophysicists. All rights reserved. WA35
  • WA36 Slatt et al. izing or quantifying lateral variations in stratigraphic properties for consistent throughout the literature. For instance, lithofacies has been used to represent the well-log scale with a finer scale existing at geological and reservoir modeling, Thus, it is important to be able to the core-plug scale Curtis, 2002 . identify lateral variability of stratigraphic sequences and their seis- mic response, in addition to vertical heterogeneity or homogene- ity , which is a common geophysical focus i.e., anisotropy: “… lay- STUDY AREA AND FIELD DATA COLLECTION ered bedding in sedimentary formations…” McGraw-Hill Dictio- nary of Scientific and Technical Terms, 2003 . The data set was compiled from spine 1 outcrop and the immedi- An outcrop stratigraphic interval, which has been studied in detail ately adjacent shallow subsurface. Spine 1 is an outcrop of the Creta- for more than 10 years, was chosen to provide examples of strati- ceous Dad Sandstone member of the Lewis Shale in Wyoming, graphic variability and accompanying variability in acoustic and U.S.A Slatt et al., 2008 Figure 1 . The data set has been compiled petrophysical properties. To compare rock properties according to over 10 years by obtaining more than 120 outcrop stratigraphic sec- their scale and measurement technique , the stratigraphic interval tions, a 550-m long behind-outcrop well that was logged and cored, was subdivided into three hierarchical scales: lithofacies, lithostrati- ground-penetrating radar GPR surveys, a hammer-seismic-reflec- graphic unit, and architectural element scale. In terms of remote de- tion survey, and an electromagnetic-induction survey summarized tection i.e., identification by some means other than whole core de- in Slatt et al., 2008 . The outcrops provide a degree of ground truth in scription , these three scales roughly translate into core-plug scale, defining vertical and lateral heterogeneities at a variety of scales that well-log scale, and seismic-reflection scale. The scales are defined is not usually possible with a subsurface data set consisting only of more fully later in this paper, but it is important to mention that the well logs because of a lack of bed form, bed style, and other deposi- terminology of this threefold subdivision of stratigraphic units is not tional features, as well as limited vertical resolution and seismic-re- a) b) c) d) Figure 1. a Topographic map of the spine 1 and spine 2 area in Wyoming and location of the CSM strat test #61 well. b Spine 1 outcrop area. c View looking south of the eastern part of spine 1. North-facing side of Channel Sandstone I is labeled. d Geological model, developed for the spine 1 and spine 2 area, shows two master channels with external levees modified from Slatt et al., 2008 . Within each channel are sinuous channel sandstones and associated, smaller internal levees. The channel deposits are underlain by two lobe splay sandstones.
  • Clastic heterogeneities WA37 flection lines or volumes because of limited vertical resolution A critical boundary between the leveed-channel complex and the Slatt, 2000 . underlying lobe bodies occurs at 137 m in the wellbore Figure 2 Spine 1, a ridge that stands above an adjacent valley floor Figure and at 140 ms on the seismic line at the location of the wellbore 1 , is composed of 10 lenticular sandstones, separated by shales, Figure 3 . This boundary is the base of a master channel that con- which are sufficiently resistant to modern erosion processes to form fines the 10 internal leveed channels within a 0.5-km-wide channel a backbone Witton, 1999 Figures 1b and c and 2a . The sandstones complex approximately perpendicular to the interpreted channel are interpreted as deep-water turbidite channel sandstones encased axes Slatt et al., 2008 Figure 1d . A second spine — 1 km south of in thin-bedded turbidite levee/overbank deposits that form what is spine 1, termed spine 2—also is present but is not part of this study commonly termed a leveed-channel complex. In general, channel Figure 1d . bodies comprise one architectural element and levees comprise a Spines 1 and 2 are separated by external levee-overbank strata separate architectural element; both are readily imaged seismically characterized by alternating fine-grained, thinly laminated sand- in the subsurface and form many important reservoirs worldwide stones, siltstones, and claystones. In addition to the external levee- Weimer and Slatt, 2006 Figure 1d . Here, net sandstone of the lev- overbank deposits, internal levees associated with each of the 10 eed-channel complex measured along the spine averages 60%, but channel sandstones are also present. Levee-overbank deposits are varies laterally within the confines of the channel complex owing to less resistant to erosion than channel sandstones so that they form the lenticular geometry of the outcropping channel sandstones Van valleys on both sides of spine 1 Figure 1a and enhance topographic Dyke et al., 2006 Figure 2c . relief between the channel complexes Figure 1a-d . A third architectural element underlies the leveed-channel com- plex. This element is laterally continuous, deep-water lobes Figures HIERARCHICAL SUBDIVISION 1d and 2c that can be traced in outcrop for up to 6.0 km and correlat- AND DEFINITION OF UNITS ed with subsurface wells for many more kilometers Witton, 1999 . Lobe bodies also termed sheets, frontal splays, and lowstand fans Lithofacies are also readily imaged on the seismic reflection record and form Lithofacies is defined as “a lateral, mappable subdivision of a des- many important subsurface reservoirs worldwide Weimer and Slatt, ignated stratigraphic unit, distinguished from adjacent subdivisions 2006 . on the basis of lithology, including all mineralogic and petrographic A northwest-southeast 2D hammer-seismic line, shot at the top of characters, and those paleontologic characteristics that influence the spine 1 cf. spine 1 in the northwest part of Figure 1a and OU seismic line in Figure 2a provides an excellent image of the internally discontinuous nature of the internal a) b) c) channels individual discontinuous seismic re- Depth Vertical exaggeration 4x CSM strat test flections comprising the leveed-channel deposit Ft (m) Core #61 OU seismic line and the underlying, more laterally continuous gamma 100 Channel- ray lobe sandstones lobe splay in Figure 1 and the fill continuous seismic reflections below the yellow Sandstone 200 bodies channel deposits in Figure 3 . Acquisition of the line was designed so that it crossed the 550-m 300 well termed the Colorado School of Mines (91) ChS-1 CSM strat test #61 well , which was drilled 400 from the top of spine 1 Figures 1a, 2a and b, and 3a . A suite of conventional logs and a borehole 500 image log were obtained from the well, along with continuous core from 50- to 180-m depth, and a smaller cored interval of 265–275-m depth Lithofacies 600 Heterogeneous SS (HS) Figure 2b . This well was used to calibrate the (183) Uniform SS (US) seismic line and also to correlate the seismic line Outcrop measured 700 Shale (Sh); both OrgP and OrgR to 10 outcropping channel sandstones details of stratigraphic sections and correlations of the seismic acquisition and well-log calibration are 800 channel and lobe CSM strat test #61 gamma log provided by Van Dyke et al., 2006 Figures 2b sandstones and 3 . Core gamma scan 900 From the outcrop, 121 stratigraphic sections (274) were measured and described at the cm scale for sedimentary textures, structures, and stratifica- Figure 2. a Digital elevation map showing the distribution in 3D space of the 10 channel sandstones. Channel Sandstone I ChS-1 is labeled. Estimated location of the master tion style Van Dyke et al., 2006 . These sections channel is shown by the dashed line. Locations of the CSM strat test #61 and the hammer- form the basis for characterizing each of the 10 seismic-reflection line are also shown. Width of the channel complex is approximately sandstone bodies and the entire leveed-channel 500 m. b Gamma-ray log and core gamma scan gray of the CSM strat test #61 well. complex. Witton 1999 also measured six, long, Colored dots refer to the locations of the lithofacies sampled for petrophysical and acous- outcrop, stratigraphic sections that included the tical properties. c Three measured stratigraphic sections along spine 1 along the track shown in a by the brown line and correlations of the channel sandstones modified underlying lobe bodies and the leveed-channel from Witton, 1999 . complex Figure 2c .
  • WA38 Slatt et al. appearance, composition, or texture of the rock” Bates and Jackson, are genetically related to each other and generated in a common dep- 1980 . Lithofacies comprise laminae 1 cm thick or beds 1 cm ositional setting Brookfield, 1977; Allen, 1983; Miall, 1985; Pick- thick , which typically are deposited during a single event. Though ering et al., 1989; Sprague et al., 2002; Sprague et al., 2003 . They small in scale, lithofacies and their contained laminae and beds are are the larger features of a stratigraphic sequence with discrete sizes, the fundamental building blocks of reservoirs that control the stor- geometries, trends, volumes, and internal arrangement of lithostrati- age and flow capacity of fluids by the arrangement and distribution graphic units. In this paper, we have imaged and described architec- of pores, pore throats, and grain-scale barriers. Properties measured tural elements using standard outcrop stratigraphic sections, GPR at the core-plug scale, such as porosity and permeability and some- obtained at a frequency of 100 MHz, electromagnetic induction times acoustic properties , are considered here to be representative EMI obtained at a frequency of 1–20 kHz, and a 2D hammer-seis- of a given lithofacies. mic-reflection line obtained at a frequency of 28 Hz. Although standard well logs i.e., excluding micrologs generally have a vertical resolution of about a meter, the clustering procedures Subseismic- and seismic-scale subdivisions that we use to discriminate differences in rock types break out beds as small as the digitizing interval, which for this study was 0.03 m. The various scales of measurement described above are subdivid- Thus, herein we consider these thin beds to be at the lithofacies scale. ed into those considered to be subseismic lithofacies and lithostrati- In this context, the core-plug data and the thin beds that are generated graphic units beneath normal marine acquisition of 2 to 60 Hz , and from the clustering runs are at the lithofacies scale. those that can be seismically resolved, i.e., architectural elements. We realize that there is not a clear boundary between these two scales, that subsurface features that might not be seismically resolv- Lithostratigraphic unit able under one set of conditions e.g., depth of burial, acquisition pa- A lithostratigraphic unit is a rock body composed of a certain rameters, etc. might be resolvable under another set, and that vast lithologic type or combination of types, deposited under a given set improvements continue to be made in geophysically resolving of environmental conditions Bates and Jackson, 1980 . In the con- smaller and smaller stratigraphic heterogeneities Chopra and Mar- text of this paper, it is used to describe thicknesses of interbedded furt, 2007 . 3D seismic-reflection volumes, coupled with well data, lithofacies, specifically interbedded sandstone-shale intervals, usu- are the common tools for describing architectural elements in the ally several meters thick. subsurface. Lithostratigraphic units also exert control on reservoir perfor- mance, such as by the presence or absence of stratigraphic seals PROPERTIES OF SANDSTONE e.g., shales atop sandstones , lateral pinchouts of sandstones into AND SHALE LITHOFACIES shale, etc. Lithostratigraphic units are normally measured and de- scribed from whole core or well logs. There are four basic lithofacies comprising the Dad Sandstone at spine 1, two sandstones and two shales. A description of the mea- surements made on representative samples from these lithofacies is Architectural element made first, and then the lithofacies are discussed along with a com- Architectural element is defined as a distinct body or assemblage parison of their properties. of sediment bodies with lower and upper confining boundaries that Core-plug porosity and permeability measure- ments were made from the sandstones by routine a) methods and at atmospheric pressure. Results are summarized in Table 1. In addition, four core plugs from each of the two sandstone lithofacies were analyzed for acoustic properties. Bulk den- sity was measured directly on the core plugs by determining the weight mass and volume of each plug. Back calculation of grain densities based on the independently measured bulk densi- ty and porosity values for each sample gave a range of 2.68–2.72 g/cm3, with an average of 2.70 b) g/cm3; these values are typical of sandstones with the mineralogic composition described by Thyne et al. 2003 . Velocity measurements were made by direct traveltime measurement for the given length and density of each plug; berea sandstone and aluminum were used as standards. Acoustic impedance was derived from these measured P-wave velocities and bulk densities. Results are summarized in Table 2. Figure 3. a Inverted relative acoustic impedance profile across spine 1 showing the lo- Measurements of properties from 12 shale core cation of the CSM strat test #61 well. Numbers refer to the individual channel sandstones. plugs from the CSM strat test well, reported by b Interpreted seismic relative impedance profile. Numbers refer to channel sandstones. Castelblanco-Torres 2003 , are used in this Yellow is sandstone and light blue is thin-bedded internal levees. Dotted arrows illustrate gradational and lateral migration trends of channel sandstones. study. Clay minerals dominantly illite, with less-
  • Clastic heterogeneities WA39 er smectite, kaolinite, and chlorite generally comprise 50% of the sorted, angular to subangular, subarkose to arkosic arenites Thyne shales, with 25–50% quartz plus feldspar, and 10% carbonate. et al., 2003 . Framework composition consists of 50–70% detrital, Two types of shale lithofacies were defined for this paper on the basis monocrystalline quartz, with lesser amounts of plagioclase, sedi- of total organic carbon TOC content. Seven of the plugs have TOC mentary lithic grains, micas, and organic matter. Clay matrix values of 0.5–0.7 wt.% and five plugs have values of 1.4–1.7%. 2–12% is formed mainly by disaggregation of shale lithic grains These two types of shales are termed organic-poor shales OrgP-sh during early burial compaction i.e., pseudomatrix . Cements con- 1% TOC and organic-rich shales OrgR-sh 1% TOC , al- sist of calcite, quartz, and clay. though we recognize that such shales are not truly organic rich in The two types of sandstones differ primarily in the presence or ab- terms of hydrocarbon source rock potential. sence of sand to pebble-sized shale lithic clasts within the beds Fig- For the shale plugs, porosities were measured at ambient pres- ure 4 . When present, the shale clasts occur within a sandstone ma- sures and permeabilities were measured at 500 psi Castelblanco- trix. Torres, 2003 . Bulk density and velocity were measured at 500 psi. A secondary, significant difference is that sandstones that do not To directly compare the shales with the sandstones measured at am- contain lithic clasts are often uniformly bedded i.e., massive with- bient pressures, Castelblanco-Torres 2003 measurements were in- out significant cross-bedded or slumped strata, in contrast to the creased by 6%, a value determined by two additional stressed mea- clast-bearing sandstones Figure 3 . In this paper, we refer to the surements. Results are summarized in Table 2. clast- and sedimentary structure-bearing sandstones as heteroge- neous sandstones HS and the clast-free, uniformly bedded sand- Sandstone lithofacies stones as uniform sandstones US . Lewis Shale sandstones, of which the Dad Sandstone is represen- tative, are generally fine to medium grained, poorly to moderately US lithofacies Table 1. Average and standard deviations of porosity and US exhibit little variability in sedimentary textures, structures, permeability of US and HS lithofacies based upon 109 and stratification at the single-bed scale. Individual sandstone beds core-plug measurements. Measurements were made at are in direct contact with other sandstone beds across amalgamation atmospheric pressure. surfaces Figure 4 . Average properties are given in Table 1, based upon 94 plug measurements of two groups of US, labeled as lithofa- Lithofacies # n Porosity % Permeability mD cies #3 and #15 the reason for this subdivision is explained in a later section . Uniform Sandstone Lithofacies US The four plugs from which acoustic properties were measured are 3 64 28 1 188 124 from core depths of 138–140.5 m log depth 137–139 m and 266–268 m log depth 264.5–267 m Figure 2b Table 2 . 15 30 28 4 421 281 Heterogeneous Sandstone Lithofacies HS HS lithofacies 12 13 29 4 396 25 HS exhibit interval variability in textures and sedimentary struc- 8 2 25 8 497 702 tures as described above Figure 4 . Average properties of HS are Table 2. Averages and standard deviations of properties measured on selected core plugs representative of the two sandstone lithofacies. Values for shale lithofacies were obtained by Catleblanco-Torres (2003). All porosity measurements were made at atmospheric pressure. Shale permeabilities were measured at 500 psi and corrected to atmospheric pressures for comparison with sandstone properties. Porosity Permeability Bulk Density Al VP VS g/cm3 m/s % md gm/cc m/s m/s VP/VS Uniform Sandstone Lithofacies US Average 29.5 415 1.91 1964 1156 1.7 3747 Standard deviation 1.1 293 26 54 77 Heterogeneous Sandstone Lithofacies HS 4 core plugs Average 29 304 1.91 2129 1206 1.77 4087 Standard deviation 2.3 301 0.49 294 193 Organic l l Poor Shale Lithofacies OrgP-sh 8 core plugs Average 18.4 0.162 2.69 3735 1896 1.94 9444 Standard deviation 2 0.08 91 183 Organic l l Rich Shale Lithofacies OrgR-sh 4 core plugs Average 14.6 0.003 2.67 3788 2066 1.95 9485 Standard deviation 0.7 0.01 233 92
  • WA40 Slatt et al. given in Table 1, based upon 15 plug measurements. The four plugs Comparison of OrgP-sh and OrgR-sh lithofacies at the from which acoustic properties were measured are from core depths core-plug scale of 48–50 m log depth 46.5–48.5 m and 125–126.5 m log depth Comparing the two shales at the core-plug scale, OrgP-sh exhibits 123.5–125 m Figure 2 Table 2 . higher average porosity, two orders of magnitude higher average permeability, and a greater range of permeability than OrgR-sh Ta- Comparison of US and HS lithofacies at the core-plug ble 2 . Bulk density, sonic velocity, and acoustic impedance values scale are equivalent between the two Table 2 . It is apparent from the larger group Table 1 and smaller group Comparisons of core-plug properties of sandstones and Table 2 of porosity and permeability measurements that porosity shales and permeability are about equivalent between the two sandstones. Bulk densities are equivalent for the two sandstone types, but At the core-plug scale, sandstones exhibit higher average porosity P-wave velocities, and thus acoustic impedance values, are slightly and permeability values than shales Table 2 . Bulk density and son- higher for HS than for US Table 2 because of shale clasts in the ic velocity values, and consequently acoustic impedance values, are sandstones. much higher for shales than for sandstones. Because porosity and permeability are desirable properties to be Shale lithofacies determined from seismic-reflection data, comparisons were made between core-plug porosity and acoustic impedance and from core- plug permeability and acoustic impedance, both from the eight sand- OrgP-sh lithofacies stone and 12 shale core plugs Table 2 . The resulting trend lines and The core-plug interval of OrgP-sh lithofacies chosen for analysis correlation coefficients Figure 6 show that at least at the core-plug 135.4–137.5-m core depth; 134–136-m log depth comprises the scale, porosity and permeability are inversely related to acoustic im- laminated shale immediately above the base of the master channel pedance. Figures 2b and 5 . Measured parameters for eight core plugs are list- ed in Table 2. LITHOSTRATIGRAPHIC UNITS Identifying lithostratigraphic units OrgR-sh lithofacies Lithostratigraphic units represent the next higher scale within the The core-plug interval of OrgR-sh lithofacies chosen for analysis drilled stratigraphic sequence Figure 2a and b . To evaluate vertical comprises the shale deeper within the stratigraphic section 279– 285-m core depth; 280–282.5-m log depth Figure 5 . Core exami- nation reveals occasional vague lamination. Measured parameters 282 m 135 m for the four core plugs are listed in Table 2. 48 m 138 m 140 m 50 m 284 m Unstratified Sandstone Heterogeneous Sandstone 138 m (US) (HS) Figure 4. Core of the US and HS sandstones. Black dots on core are locations of core plugs for porosity and permeability measurements Organic-rich shale (OrgR-sh) Organic-poor shale (OrgP-sh) Table 2 . Core depths are in meters. Features in the core of HS in- clude slump, shale-clast conglomerates, erosional scour surfaces, and cross-beds. Compare this with the featureless characteristics of Figure 5. Typical core of the OrgR-sh and OrgP-sh. Core depths are the US core. in meters.
  • Clastic heterogeneities WA41 stratigraphic variations in properties at this and higher scales, a tabulated include an assigned lithofacies number, the lithofacies type either sandstone, siltstone, or shale based on gamma-ray log probabilistic-clustering procedure PCP was used to combine dif- response , the percentage of digitized samples assigned to each ferent lithofacies into successively thicker groups of strata based lithofacies, the number of core plugs from each lithofacies, and the upon similarity in properties. Clustering procedures within the PCP mean gamma-ray API well-log response for each lithofacies. Four of can be used to divide samples into probabilistically defined groups the lithofacies were also assigned to a lithostratigraphic unit based within which samples have similar properties. Specifics of the clus- on prior study of the whole core. Specifically, lithofacies #3 and #15 tering methods and ancillary supporting routines for single- and belong to the lithostratigraphic unit that contains mainly US mixed multiwell studies are being prepared for publication E. V. Eslinger, with shale lithofacies and lithofacies #8 and #12 contain mostly HS personal communication, . Clustering variables can include well- lithofacies, also combined with shale. log data and core data plus any other depth-related parameter that A bed-thickness filter BTF routine within the PCP Eslinger, can be digitized Eslinger et al., 2004a, 2004b; Eslinger and Everett, 2007 was then used to merge lithofacies beds so any bed thinner 2005 . than a user-defined thickness was eliminated. Beds thinner than the The work flow used for this study is described here. First, a clus- minimum permitted thickness were then added to either the overly- tering run was made using three well logs as clustering variables — ing or the underlying lithofacies bed, with the decision based on a bulk density RHOB , gamma ray GR , and inverted sonic transit probability matrix argument. The probability distribution of each time 1/DT, converted to m/s . Fifteen groups clusters, or modes sample in the thin bed is compared with the probability distributions were requested. Because only well-log curves were used as vari- in a user-defined interval at the bottom of the overlying bed and at the ables, the resulting 15 clusters could be termed electrofacies, but top of the underlying bed. Specifically, two dot products are comput- here we consider them to be equivalent to lithofacies. Contiguous ed using the probability distributions as vectors and the resulting dot samples assigned to the same lithofacies define a bed, and the thin- products are treated as a similarity index that dictates whether the nest bed possible is the digitizing interval 0.03 m . The choice of 15 thin bed is assigned to the overlying or the underlying bed Eslinger, lithofacies was somewhat arbitrary; more or fewer could have been 2007 . Each BTF computation produces a merged or upscaled real- used. However, the intention was to generate a sufficiently detailed ization of the original beds. A given BTF run can be made on the classification that would result in the definition of beds that were ap- original, unfiltered stack of beds, or on a previously filtered stack. In proximately the same scale that might be defined in a typical core de- this exercise, each run was made on the original unfiltered stack. scription. Using more lithofacies results in a more detailed classifi- During a series of runs in which the minimum bed thickness is suc- cation more thin beds than use of fewer lithofacies. cessively increased, the scale gradually changes from the lithofacies The 15 lithofacies are listed in Table 3, arranged according to their scale to the lithostratigraphic unit scale and then to the architectural well-log gamma-ray response increasing down the column . Data element scale. As BTF thickness is increased with successive BTF computations: 3 Log10 permeability (md) Table 3. Fifteen arbitrarily defined lithofacies for the 2 GAMLS clustering run. Shown are the defined lithofacies, the percentage of each lithofacies of the total stratigraphic 1 interval, the number of core plugs from which porosity and permeability data were obtained, the mean well-log 0 y = –0.0008x + 5.6038 gamma-ray API value for the lithofacies, and the color code 2 R = 0.9672 in Figures 8 and 17. –1 –2 Lithofacies # Lithofacies % # Plugs GR mean Color –3 15 US 4.25 30 70.2 yellow 3000 4000 5000 6000 7000 8000 9000 10000 11000 Core-derived acoustic impedance [(g/cm3)(m/s)] 1 SS 1.2 0 71.5 gold 35 3 US 10.98 64 77.5 orange 13 Sltstn? 6.8 0 97.5 lt grn 30 2 Sltstn? 11.86 30 88.3 med grn Porosity (%) 10 Sltstn? 0.66 0 89.3 dk grn 25 y = –0.0022x + 37.876 12 HS 5.01 13 94.4 white 2 R = 0.9139 20 9 Sltstn? 0.8 0 101.1 gray 8 HS 12.73 2 111 purple 15 6 Sh 10.55 0 119.1 gray 10 11 Sh 6.08 0 123.8 gray 3000 4000 5000 6000 7000 8000 9000 10000 11000 Core-derived acoustic impedance [(g/cm3)(m/s)] 4 Sh 7.17 0 128.3 gray 14 Sh 8.18 0 123 gray Figure 6. Crossplots showing inverse the relation between core- 7 Sh 2.27 0 137.2 gray plug-measured permeability logarithmic scale and porosity versus core-plug-derived acoustic impedance. Linear equations and corre- 5 Sh 11.44 0 142.5 black lation coefficients are shown.
  • WA42 Slatt et al. a) • Fewer beds remain Figure 7a . • The average bed thickness for each lithofacies increases Figure 7b . • Some lithofacies disappear. • The original properties of any remaining lithofacies might change as the original litho- facies are merged. For example, by adding beds of shale lithofa- cies to a stratigraphic interval that was originally composed entirely of US beds, the thickness of the lithostratigraphic unit increases while the b) added properties of the shales progressively alter the average properties of the sandstones. In the same manner, as more sandstone beds are added to a lithostratigraphic unit originally composed of shale, then the thickness of the lithostratigraphic unit will increase and the average properties of the shale will more closely approach those of a sandstone unit. Identified lithostratigraphic units Figure 7. a Crossplot showing the reduction in numbers of beds of the three lithofacies As mentioned in the previous section, for the US, HS, and Sh with successive BTF runs. b Crossplot showing the increase in aver- age bed thickness with successive BTF runs. CSM strat test number #61 data set, the BTF pro- cess was applied to the 1D stack of 524 beds de- fined in the initial clustering run. A first BTF run after the clustering run, beginning with 15 litho- facies column A in Figure 8 was designed to eliminate beds thinner than 1.5 m thick column B in Figure 8 . This BTF run reduced the total num- ber of beds from the original 524 to 115 Figure 7a . Additional BTF runs were made with the minimum bed thickness permitted increasing with each run, and with each run operating on the original stack of 524 beds. With each successive run, the average bed thickness of each lithofacies tended to increase as the number of lithofacies de- creased Figure 7b . Additional BTF runs were completed to suc- cessively eliminate beds thinner than 3 m, 6.1 m, 12.2 m, 24.4 m, 61 m, and 122 m columns C, D, E, F, G, and H in Figure 8, respectively . The final BTF run column H in Figure 8 resulted in three remaining intervals, which basically represent lithostratigraphic units that are amalgamations of the original 15 lithofacies. These three remaining intervals, from top to bottom, originated prior to BTF as HS lithofacies #8 purple in Figure 8 , US lithofacies #3 orange in Figure 8 , and Sh lithofacies #5 black in Figure 8 . Figures 7 and 9–14 show what happens to the properties of these three original lithofacies during successive stages of BTF. Similar plots could be made for the other 12 original lithofacies, but here we just Figure 8. a The initial 15-mode PCP run and results of applying the BTF by eliminating track changes in the three main lithofacies, US, beds thinner than b 1.5 m thick, c 3 m thick, d 6.1 m thick, e 12.2 m thick, f 24.4 m HS, and Sh combined OrgP-sh and OrgR-sh to thick, g 61 m thick, and h 122 m thick. The horizontal line at 137 m is the base of the illustrate trends that occur during the successive master channel Figure 2 .
  • Clastic heterogeneities WA43 slowness DT values. Average velocity of the Sh-prone lithostrati- BTF runs. Below, we apply the terminology US prone, HS prone, and Sh prone in the discussion of the three lithostratigraphic units graphic unit is greater than those of the sandstone-prone units. Aver- that we track. age velocities of US-prone units are higher than those of HS-prone Prior to filtering, the sum thickness of lithofacies #3, #5, #8, #12, units, and the spread between values at each BTF thickness above and #15 Table 3 , identified as belonging to these three lithostrati- 12.2 m is large, even though the standard deviation values suggest a graphic units US prone, HS prone, and Sh prone , was only 44% of chance of overlap in this property. the total stratigraphic thickness in the CSM strat test #61 well. The The end result of this bulk density-velocity combination is that, other 56% of the total interval was initially composed of the other 10 for any BTF thickness, the acoustic impedance of the Sh-prone lithofacies. These 10 lithofacies were merged into the three final lithostratigraphic unit is highest, followed by the US-prone unit, lithofacies US prone, HS prone, and Sh prone during the succes- then HS-prone lithostratigraphic unit Figure 11 . The spread of val- sive BTF runs because of similarities in well-log properties among ues between the two sandstones at any BTF thickness above 12.2 m sets of lithofacies. is large, but the standard deviations suggest a chance of overlap in The gamma-ray API trends provide the simplest example to show this property. Shale-prone and HS-prone lithostratigraphic units are the effects of merging of beds during the BTF runs Figure 9 . The statistically different, but that is not the case for shale-prone and US- Sh-prone lithostratigraphic unit exhibits the highest average gam- prone lithostratigraphic units. ma-ray API values, and the US-prone lithostratigraphic unit exhibits Because the PCP subdivides and combines lithofacies based upon the lowest average gamma-ray API values Figure 9 . The HS-prone the combination of well logs, similar computations can also be made unit lies between these two lithostratigraphic units because of a combination of the presence of shale clasts in individual HS beds termed dis- persed shale by geophysicists and because of shale lithofacies interbedded with the sandstone lithofacies termed laminar shale by geophysi- cists . For the same reason, with each successive BTF run, average API values of the Sh-prone lithostratigraphic unit decrease as more beds of sandstone are incorporated into them, and API values of the US-prone lithostratigraphic units in- crease slightly as more beds of shale are incorpo- rated into them Figure 9 . Although there is a good separation of API values at all BTF runs Figure 9. Crossplot showing the variation in gamma-ray log response of the three lithos- among the three lithostratigraphic units, statisti- tratigraphic units with successive BTF runs. The numbers at each data point are the cal variability standard deviation indicates the standard deviation about that average data point. possibility of some potential overlap in API val- ues among the units Figure 9 . a) A plot of BTF bed thickness versus well-log average bulk density Figure 10a shows that at all BTF thicknesses, the Sh-prone lithostrati- graphic unit has a significantly higher density than the two sandstone-prone units, as is the case at the core-plug, lithofacies scale Table 2 . How- ever, there is a bulk density crossover at 12.2-m BTF thickness where HS-prone average bulk density goes from being higher than that of the US-prone unit to being lower. Figure 8 helps ex- plain how this can happen. The HS lithofacies is b) colored purple mode 3 of the PCP run in Figure 8. Prior to any BTF, only about one-third of the section above the horizontal orange line at 137 m is assigned to the HS lithofacies. But, with suc- cessive BTF runs, increasing amounts of other lithofacies are merged with the original HS litho- facies so the resulting bulk density of the amal- gamated lithofacies changes. It is apparent that increasing amounts of lithofacies with lower bulk densities were added to the original BTF facies through the first five BTF runs. Figure 10. Crossplot showing the variation in well-log a bulk density and b sonic A somewhat similar trend is apparent for sonic velocity response of the three lithostratigraphic units with successive BTF runs. The velocity Figure 10b . The velocities were com- numbers at each data point are the standard deviation about that average data point. The puted as the arithmetic mean of the inverse of the sonic velocity was computed as the inverse of the slowness DT log .
  • WA44 Slatt et al. with the core-plug porosity and permeability data because they were Sh-prone lithostratigraphic unit, at the 122-m BTF run. This is be- included in the log ASCII standard files associated with depths, just cause there are no plug samples within the shales except for those as were the well-log depths. Figure 12a and b shows that HS-prone listed in Table 2, which were not used in the BTF runs below a depth units have lower porosity and permeability averages at thin BTF of 274 m Figure 8 , and it is not until the last BTF run that the evolv- thicknesses, but reverse trends with increasing BTF. This change ap- ing Sh-prone lithostratigraphic unit incorporated a depth interval pears to be associated with the crossover from higher to lower bulk within which core plugs were taken. Specifically, at the last 122-m density for HS-prone lithostratigraphic units with increasing BTF BTF run, a sequence of lithofacies #15, belonging to the US-prone thickness i.e., lower bulk density equates to higher porosity and per- lithostratigraphic unit yellow in Figure 8 , became incorporated meability Figure 10a . Even though there is a good spread of values into the Sh-prone lithostratigraphic unit. Between 262 and 267 m, at each BTF thickness for porosity, and to a lesser extent for perme- this lithofacies #15 consists of homogeneous sandstone with perme- ability, there is sufficient overlap in standard deviation values such abilities approaching 1000 mD. that the differences are not statistically significant. The inclusion of these high-reservoir quality sandstone samples In the plots of porosity and logarithm of permeability versus BTF into the Sh-prone lithostratigraphic unit caused its arithmetic mean bed thickness Figure 12a and b , there is only one data point for the permeability at the 122-m BTF run to actually be higher at log 10 perm 2.5 than the mean permeabilities of ei- ther the US-prone log 10 perm 2.1 or the HS-prone log 10 perm 2.2 lithostratigraphic units Figure 12b . This is perhaps a prime exam- ple of how a thin, high-reservoir quality unit that is resolvable at the lithostratigraphic unit well- log scale might be missed at the architectural ele- ment seismic scale. As noted, the lithostratigraphic units are most variable up to a BTF thickness of 12.2 m. Above this BTF thickness, the properties usually vary within 10% with increasing BTF thickness. In other words, above this critical thickness, which Figure 11. Crossplot showing the variation in well-log acoustic impedance response of coincidentally is about the same thickness as a the three lithostratigraphic units with successive BTF runs. The numbers at each data seismically resolvable stratigraphic interval, point are the standard deviation about that average data point. there is much less change in rock properties than at BTF thicknesses below 12.2 m. a) ARCHITECTURAL ELEMENTS The three architectural elements comprising spine 1 are channel-fill deposits, levee deposits not analyzed in this paper , and lobe deposits Figure 1d . The base of the master channel, at a depth of 137 m 450 ft in the CSM strat test #61 well Figures 2a and b and 3 , corresponds to the boundary between the HS-prone lithostrati- graphic unit above and the US-prone lithostrati- graphic interval below as defined by the 122-m b) BTF run column H, Figure 8 . Shale-prone lithostratigraphic units occur from a depth of 279 m to the base of the core. Thus, the boundary between the two major intervals defined by the 122-m BTF run is coincident with the boundary between the two main architectural elements. At this scale, the two architectural elements can be defined by the set of properties that are based on the well control Table 4 . Of the properties com- pared, the only one that has a wide spread of val- ues at this scale is acoustic impedance US HS, Figure 12. Crossplots showing the variation in a porosity and b permeability logarith- which is the opposite of the relationship at the mic scale of the three lithostratigraphic units with successive BTF runs. The numbers lithofacies scale Table 2 . Yet as discussed in the at each data point are the standard deviation about that average data point. The logarithm summary, the architectural arrangement of beds of the horizontal-plug permeabilities was computed, and then the arithmetic mean of and lateral properties is quite different. these logarithmic values was used.
  • Clastic heterogeneities WA45 abundance toward the south end of the outcrop Figure 13b . There- Summary fore, lithostratigraphic properties are also going to vary laterally. The above analysis of wellbore data shows variations in vertical To illustrate the internal lateral variability of this lithostrati- properties at the architectural element scale; however, in the statisti- graphic unit, two behind-outcrop GPR lines Figure 14a-c , and 12 cal sense, most differences are not significant because of overlap in measured stratigraphic sections Van Dyke, 2003 were obtained standard deviation values. along the outcrop. These data provided average estimates of 80% US and 20% HS. Various characteristics of sinuous channel deposits also were imaged by the GPR lines; features include lateral accretion LATERAL PROPERTIES OF surfaces, slumps, and onlap surfaces Figure 14b . Such features are LITHOSTRATIGRAPHIC UNITS found in larger subsurface leveed-channel reservoirs Abreu et al., AND ARCHITECTURAL ELEMENTS 2003 . Lateral characterization of a single channel sandstone Two boreholes labeled 1 and 5 in Figure 15 were also drilled and gamma-ray logged to depths of 3-7 m in front of the east-facing out- Based upon field observations, differences in the lateral as well as crop. These and other boreholes in the same area were used to cali- vertical arrangement and variability in lithofacies and lithostrati- brate the GPR lines and lithofacies by 1D convolutional modeling graphic units are quite noticeable. These differences will affect res- Young et al., 2007 . These boreholes were drilled after an EMI sur- ervoir performance, so in an analog reservoir, it is essential to define vey had been run and had detected additional sandstone stratigraphi- the lateral attributes in 2D, or preferably 3D, space as best as possi- ble, and preferably with hard data. Accurately de- fining lateral attributes can be a difficult task if Table 4. Averages and standard deviations of properties of the channel and lobe architectural elements. data, including seismic data, are sparse. In this re- gard, outcrop data may be useful, as demonstrat- ed by Sullivan et al. 2000 for a field in the Gulf Channel architectural Lobe architectural of Mexico. Property element element For our study, sufficient 3D data have been ob- Gamma ray API 106 16 90 20 tained from only one of the channel sandstones Bulk density g/cm3 that comprise spine 1. This particular sandstone is 2.27 0.12 2.27 0.06 named Channel Sandstone I, the lowermost chan- Sonic velocity m/s 2327 252 2674 131 nel sandstone of the 10 comprising spine 1 Fig- 3 acoustic impedance g/cm m/s 5292 267 6070 347 ure 2a and c . This sandstone is composed of US Core plug porosity % 28.5 4.2 27.6 2.3 and HS lithofacies, so it can be considered as a Core plug permeability md 175 310 135 115 lithostratigraphic unit according to our definition. Channel Sandstone I has been characterized in 3D space using a variety of geological and a) Southwest geophysical techniques. It is exposed in three Northeast dimensions as east-facing north-south trend and north-facing west-east trend outcrops Figure 1a-d; Figure 14 . The east-facing outcrop is 150-m wide and oriented in a direction that is ap- proximately perpendicular to its depositional dip orientation Figures 1d and 13a . The longitudi- 150 m b) nal outcrop extends for approximately 300 m in a Massive sands w/fluid escape structures north-facing, east-west parallel to depositional dip orientation Figures 1c and 13a and c . The X-bedded sands east-facing outcrop is particularly significant be- Red = shale-clast cong. Brown = sandy debrites cause it exhibits a laterally asymmetric distribu- c) tion of lithofacies Figure 15b . Along the Cross-bedded sandstone Massive sandstone 150-m-wide face, HS lithofacies occurs on its Interbedded turbidites and north side and near the base of the outcrop Figure shale-clast 13b and c . US lithofacies occurs across the top of conglomerates the outcrop Figure 13b and c .Athird lithofacies, crossbedded sandstone, occurs on the south side of the outcrop Figure 13b and c . Because it has Figure 13. a Photograph of the east-facing outcrop of Channel Sandstone I. b Lithofa- core-derived properties similar to US lithofacies, cies distribution along the rock outcrop: to the right, the rocks are interbedded turbidite for the purpose of this paper, it is considered US and debris flow shale-clast conglomerate beds, here classed as HS sandstone lithofa- cies. To the left, the rocks are massive to cross-bedded sandstones, here classed as US lithofacies. The proportion of US to HS averages lithofacies. This asymmetry of lithofacies is typical of sinuous deep-water channel sand- 60% and 40%, respectively, along this outcrop. stones with an outer cutbank-like side and an inner point-bar-like side. c Pictures of the However, HS is not uniformly distributed lateral- three sandstone types: cross-bedded sandstone, massive sandstone, and interbedded tur- ly across the outcrop, but rather decreases in bidites and shale-clast conglomerates.
  • WA46 Slatt et al. a) cally beneath Channel Sandstone I, which was a covered by thin soil Stepler et al., 2004 Figure Top surface of Channel Sandstone 1 15b . Gamma-ray logs and cuttings from bore- holes 1 and 5 provided estimated sandstone/shale proportions of 67/23% and 83/17%, respectively. A second EMI survey to the west Figure 15a c shows the location of the survey revealed a bend GPR lines of the channel sandstone into the outcrop, further supporting the sinuous character of this channel b sandstone Stepler et al., 2004; Slatt et al., 2008 . Base of channel Based upon core and outcrop features, the complexity of HS lithofacies is interpreted as a 20 ns result of slumping and erosional processes that b) are common to such channel systems Weimer Shale-clast conglomerate and Slatt, 2006 . Specifically, shale clasts that are a b 10 m contained within HS sandstone in Channel Sand- c) 0 b stone I are generally thinly laminated. They ap- c pear to be blocks of associated levee beds that had slumped into the channel where they probably 100 ns Slump (4.6 m) then mixed with sandy-sediment gravity flows Lateral accertion surfaces 100 m moving down the channel to form the slumped and shale-clast-bearing sandstones comprising HS Figure 4 . This observation supports the seis- Figure 14. a Aerial photograph of Channel Sandstone I showing the location of GPR lines a-b and b-c that show the key architectural features of Channel Sandstone I. Scale mic-reflection images of slump scars along outer bars for a-b and b-c are shown in those figures. b Base of channel, overlain by shale- bends of sinuous channels, reported by Posamen- clast conglomerate beds encased in red , slumped beds, and dipping lateral accretion tier and Kolla 2003 , and demonstrates a similar- beds. c Base of channel and overlying radar-transparent shale-clast conglomerate beds ity in deep-water sinuous channels and fluvial encased in red , and bedded sandstones. meandering channels, whereby the outer bend is a point-bar-like area. By contrast, the US across the channel from the HS Figure 13 represents depo- sition along a point-bar-like inside bend of the a) sinuous channel. Top surface of Channel Summarizing all of this information leads to Sandstone 1 the conclusion that there is significant lateral vari- ability in US and HS lithofacies, and between the sandstones and shales along and across Channel Sandstone I. Most important though is the com- GPR lines plexity of the stratification as revealed by the GPR lines Figure 15a-c . It is these variations that will play a significant role in fluid flow be- havior in a subsurface reservoir analog, yet may Eroded gully not be considered important if the properties mea- Base of channel Sandstone sured at the wellbore are mainly considered in Channel Sandstone 1 fluid-flow modeling. c) b) Shalier 1400 00 Comparison with a seismically defined 5 1 1000 00 Shale 900 00 10 m leveed-channel deposit 800 00 700 00 600 00 500 00 A cross-sectional seismic image of a Gulf of 400 00 Mexico leveed channel Figure 16b , which is of 300 00 0 API 60 200 00 EMI Survey 1 a similar width as Channel Sandstone I Figure 125 00 Gamma ray logs of shallow 75 00 boreholes 1 and 5 16a , provides an example of variability within a 25 00 60 0 –100 00 API channel fill that is similar to that of Channel Sand- –400 00 –800 00 stone I. This line shows an asymmetric leveed Sandier channel with thicker levee wedge and steeper Figure 15. a Same aerial photograph as in Figure 14, but with the addition of the location levee margin on its right side than on its left side, of two EMI surveys and the positions of boreholes 1 and 5 labeled dots . Distance be- suggesting the right side is the outer cutbank-like tween wells 1 and 5 is approximately 150 m. b EMI survey image beneath the ground side of a sinuous channel Weimer and Slatt, surface in front of Channel Sandstone I showing the termination of that sandstone at the 2006; Slatt et al., 2008 . Thus, the more steeply gray color, and a second sandstone in the lower left bright colors . c Gamma-ray logs of dipping seismic reflections that downlap toward shallow boreholes 1 and 5, the locations of which are shown in a and b .
  • Clastic heterogeneities WA47 the channel floor on the steep, outer side of the channel are interpret- DT logs. Because there were six intervals from the core used for ed to be slumped HS beds and the more gently dipping beds to the the model, the clustering results generated six lithofacies two each left are point-bar-like sandstones. It is anticipated that reservoir fluid for US, HS, and Sh and this was done throughout the depth range flow would vary across this channel because of these differences. used in the clustering run. That is, the model developed between This example illustrates the point made by Hart 2008 that when well-log signal and the six short-cored intervals permitted lithofa- evaluating potential fluid-flow properties of a reservoir, attention cies to be predicted for the noncored intervals. This supervised i.e., should be paid to its seismic-reflection stratigraphic patterns as calibrated using core data initialization contrasts with the unsuper- well as its rock properties, measured at a wellbore. A basic under- vised i.e., core data not used for calibration initialization used for standing of the type of deposit comprising the reservoir, and its for- the clustering run discussed above in which 15 lithofacies were de- mative processes, is quite useful for interpreting such significant fined prior to the BTF filtering. stratigraphic features. For the CSM strat test well, the resulting cluster run over the depth interval 11–275 m identified the stratigraphic vertical distribution of the lithofacies throughout the well Figure 17 . It is worth noting that the PCP independently verified outcrop observations Slatt et Characterization of leveed-channel and lobe al., 2008 that HS is the predominant sandstone lithofacies above the architectural elements 137-m depth of the master channel i.e., the leveed-channel architec- tural element , and US is the predominant sandstone lithofacies be- General neath the master channel i.e., lobe architectural element . The 2D seismic line across the top of spine 1 indicates consider- Kve of leveed-channel architectural element able lateral stratigraphic variability within the entire leveed-channel complex Figures 1d, 2c, and 3 . The 10 channel sandstones appear For the leveed-channel architectural element, the equation was to be at least partially offset-stacked and they have associated inter- used to calculate Kve for a variety of shale lengths L from 10 m up nal levee beds alongside, above, and below them. Without more than to 200 m, which is the maximum cross-sectional width of the lev- one 2D seismic line, it is not possible to quantify the lateral variabili- eed-channel architectural element i.e., at its top, flattened on 0 ms ty of the entire leveed-channel complex, although the approximate TWT Figure 3 . The measured parameters that were held constant distribution of sandstone and levee deposits is interpreted in Figure are Kss 175 mD, A 0.64, and Nsh 0.53 73 shale beds in 3b. However, one way to partially quantify the combined effect of 137-m total thickness of leveed-channel complex; from Figure 17 . lateral and vertical attributes of a leveed-channel complex is by cal- The lengths can be considered possible lengths of shale within the culating its effective vertical permeability, as described below. master channel that might affect fluid flow in an analog reservoir of equivalent size such as shown in Figure 18 . Results of the calcula- Calculating effective vertical permeability Kve tions Figure 18 show that as L increases, Kve decreases exponen- tially, which illustrates the effect of shale lengths on the internal ar- Schuppers 1993 published the following equation, which relates chitecture of this element and potential effects on reservoir perfor- lateral and vertical attributes of an architectural element: mance. Kss A / 1 2 Kve Nsh L/4 , a) Shale-clast conglomerates where Kve effective vertical permeability, A (cutbank side) Cross-bedded sandstone net/gross, Kss standstone horizontal per- (point-bar side) meability, Nsh number of shales per meter of strata, and L average shale length or width m Note: all shales are assumed to have K 0. Kss, A, and Nsh are values that can be deter- mined at the wellbore. For the leveed-channel 150 m and lobe architectural elements in the CSM strat test #61 well, Kss used is the respective values at b) the BTF of 122 m Table 4 and A was estimated from the gamma-ray log Figure 4 . To obtain estimates of Nsh it was necessary to 100 ms determine the number of shale beds along cored and uncored portions of the CSM strat test well. To do this, the PCP was used to determine the stratigraphic distribution of US, HS, and Sh litho- 250 m facies in the well for the leveed-channel and lobe architectural elements. This PCP analysis was initialized calibrated using six short intervals Figure 16. Comparison of a Channel Sandstone I and b subsurface leveed-channel de- within the overall cored interval of the well by re- posit as imaged on a vertical seismic line from the Gulf of Mexico. The seismic channel is lating the lithofacies to their respective well-log bounded on both sides by levee wedges. The steeply dipping reflectors on the right side of signals, using the bulk density RHOB , neutron the channel are interpreted as slumped, shale-clast conglomerates and the shallow dip- NPHI , gamma-ray GR , and sonic transit time ping reflections on the left side of the channel are interpreted as bedded sandstones.
  • WA48 Slatt et al. Six lithofacies GAMLS-predicted GRD (Raw) for calibration lithofacies 0 (GAPI) 200 0 ft 0 ft Lithofacies US-2 (91 m) US-1 Figure 19. Schematic illustration of the method of characterization HS-2 and upscaling used in this paper. Lithofacies scale data are obtained from core plugs. Lithostratigraphic unit data are defined by a cluster- ing procedure that divides samples into probabilistically defined HS-1 500 ft 500 ft groups within which samples have similar well-log properties, then successively adds thinner beds to thicker beds and calculates their OrgP-sh (183 m) average properties. Architectural element data are obtained by the fi- nal merging of beds into the dominant types within the thicker strati- graphic sequence. In terms of scales, the lithofacies scale covers a OrgR-sh very small range 1 m , the lithostratigraphic unit covers a range of meters to tens of meters, and the architectural element covers a (274 m) range of 100 m. Kve of lobe architectural element 1000 ft 1000 ft For the lobe architectural element, the measured parameters that were held constant are Kss 135 mD, A 0.90, and Nsh 0.56 Figure 17. Gamma-ray log from the CSM strat test #61 well and the 79 beds in 140 m from Figure 17 . Separate Kve calculations were stratigraphic positions of the lithofacies US, HS, OrgP-sh, and made for the same L values as for the channel sandstone architectural OrgR-sh . Based upon calibration of well-log patterns to core data element. Lobe sandstones near spine 2 extend for lengths up to using the geologic analysis via maximum likelihood system 320 m Minken, 2004 , but the Kve calculated for this L is only GAMLS , the lithofacies were identified in uncored portions of the well. The HS lithofacies dominates in the channel interval above the slightly smaller than that for L 200 m so is not included in Figure master channel and the US lithofacies dominates below the master 18. Results show a similar relation between Kve and shale length as channel. These trends were verified by outcrop observations Van was computed for the leveed-channel architectural element; Kve de- Dyke et al., 2006; Slatt et al., 2008 . creases exponentially with an increase in shale length Figure 18 , again demonstrating the potential importance of lateral attributes in affecting reservoir performance. CONCLUSIONS In this paper, we have documented the variability in important properties at three different scales within a deep-water leveed-chan- nel/lobe outcrop analog of a common deep-water reservoir type. The three scales — lithofacies, lithostratigraphic units, and architectural elements — correspond to core-plug, well log, and seismic scales in terms of properties measured using different techniques and instru- ments . A generalized plot showing the process used to describe the three scales is shown in Figure 19. The concepts developed from the results reported here can be directly applied to better understand deep-water reservoir characteristics from seismic-reflection vol- umes. 1 Lithofacies scale. Two sandstones comprise the lithofacies in this stratigraphic sequence, one that is uniformly bedded termed US and one that is internally complex and heteroge- neous termed HS . At this scale, porosity and permeability are about equivalent between the two sandstones. Acoustic imped- ance values are also equivalent for HS and US Table 2 . Two shale lithofacies are also present within the stratigraphic sequence, one OrgR-sh that contains more organic matter and is more massively bedded than the other OrgP-sh that is finely laminated and probably slightly coarser grained. The OrgP-sh lithofacies exhibits higher porosity, two orders of magnitude Figure 18. Calculated effective vertical permeability Kve for differ- higher average permeability, and a greater range of permeabili- ent values of shale length L for channel and lobe architectural ele- ments. The two trends are virtually identical. ty than the OrgR-sh lithofacies.
  • Clastic heterogeneities WA49 Sandstones are more porous and permeable than shales as ex- tempts to quantify lateral variability of properties in a geologi- cally realistic manner are encouraged because this variability is pected . Shales exhibit much higher acoustic impedance values as important to reservoir fluid flow and performance as vertical than sandstones. Good inverse correlations are established be- variability. tween acoustic impedance, and core-plug permeability and porosity. ACKNOWLEDGMENTS 2 Lithostratigraphic units. Lithostratigraphic units are combina- tions of lithofacies. Three lithostratigraphic units are defined, The authors extend their sincere appreciation to David R. Pyles, termed US prone, HS prone, and Sh prone, because of the type Neil F. Hurley and the many other people and supporting companies of lithofacies that dominate each unit. A cluster analysis pro- who R. M. Slatt has worked with for several years on the Lewis Shale gram was used to evaluate the effects on properties of the project. Much has been learned about this formation since the lithostratigraphic units by combining different lithofacies, be- project began in the late 1990s while Slatt was at Colorado School of ginning with 15 different lithofacies defined by specific re- Mines. Slatt would also like to thank Andrew M. Slatt for compiling sponses of a combination of gamma-ray, sonic velocity, and all of the statistical and other information into informative graphics bulk density logs, and ending with three lithofacies also defined for this paper. S. Van Dyke would like to thank John DeLaughter for by combined log responses. This merging or upscaling process his critical review of geophysical material and his continued mentor- results in the following trends: 1 average bed thickness in- ing over the years. E. V. Eslinger would like to thank Alan Curtis creases while the number of beds decrease because of sequen- BHP-Billiton for numerous discussions concerning upscaling. tial merging of beds of given thicknesses; 2 gamma-ray API Seismic interpretation was accomplished on Seismic Micro Tech- nologies Kingdom 8.2 software. Core-plug measurements were values follow the pattern: Sh HS US, the relation between completed in the Poro-Mechanics facility at the University of Okla- the two sandstone types being a result of the presence of shale homa by Younane Abousleiman and John Brumley. clasts within HS sandstone beds dispersed shale and merging of shale beds with sandstone beds laminated shale , which in- creases the average API value of sand-prone lithostratigraphic REFERENCES units; 3 acoustic impedance follows the pattern Sh US HS owing to systematic variations in bulk density and sonic ve- Abreu, V., M. Sullivan, C. Pirmez, and D. 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