Makowitz et al_2006

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  • 1. GEOLOGIC NOTE AUTHORS A. Makowitz $ Department of GeologicalDiagenetic modeling to assess Sciences, University of Texas at Austin, Austin, Texas 78712; present address: BP America, 501 Westlake Park Blvd., Houston, Texas 77079;the relative timing of quartz Astrid Makowitz joined BP upon completion of hercementation and brittle grain Ph.D. at the University of Texas at Austin (2004). Both M.S. (1999) and B.S. (1997) geology degreesprocesses during compaction were awarded from the Michigan State University. Astrid has enjoyed working as a reservoir quality specialist and is currently in the Onshore NorthA. Makowitz, R. H. Lander, and K. L. Milliken American Gas production setting. Her love for ge- ology remains with studying rocks on a pore to subpore scale. R. H. Lander $ Geocosm LLC, 3311 San MateoABSTRACT Drive, Austin, Texas 78738This study describes porosity reduction by brittle deformation and Robert Lander coinvented Geocosm’s Prism andthe application of Touchstone sandstone diagenesis modeling TM Touchstone models and Geologica’s Exemplar1 model. Rob obtained a Ph.D. in geology from thesoftware to assess the relative timing and interactions between University of Illinois in 1991 and was a seniorgrain fracturing and cement formation during burial compaction. research geologist at Exxon Production ResearchTwo examples from a previous study of compactional fracturing are from 1990 to 1993. He then worked for Rogalandused: the Oligocene Frio Formation, Gulf of Mexico Basin, and the Research and Geologica in Stavanger, Norway.Cambrian Mount Simon Formation, Illinois Basin, United States. Rob cofounded Geocosm in 2000 and is a research fellow at the University of Texas at Austin.Grain fracturing during compaction creates intragranular fracturesurfaces that are favorable sites for quartz nucleation compared to K. L. Milliken $ Department of Geologicalexternal grain surfaces that may bear coatings that inhibit the nu- Sciences, University of Texas at Austin, Austin,cleation and growth of quartz cement. Thus, the progress of brittle Texas 78712fracture processes during diagenesis affects quartz cementation. In Kitty Milliken has degrees in geology from Van-turn, modeling of the quartz cementation process can serve to place derbilt University (B.A.) and the University of Texas at Austin (M.A. degree, Ph.D.). At the University offracturing into its proper context in burial history. Texas at Austin, she currently serves as a research In the Mount Simon Formation, the extent of brittle deforma- scientist in the electron microbeam facility. Togethertion of quartz grains correlates with reconstructed effective stress at with students, she pursues research projects thatthe onset of quartz cementation. For Frio Formation samples, how- apply imaging and analysis to decipher the chem-ever, the extent of brittle deformation does not correlate well with ical histories of low-temperature systems. She isreconstructed effective stress obtained using a one-dimensional basin a coauthor of the recently released interactive teach- ing module Sandstone Petrology: A Tutorial Petro-model that uses compaction disequilibrium as the dominant mecha- graphic Image Atlas.nism for overpressure generation. Judging from the observed degreeof grain fracturing, significant fluid overpressures in the Frio may nothave developed at the shallow depths indicated by our basin models. ACKNOWLEDGEMENTSThe degree of compactional fracturing in sandstones constitutes The authors are grateful to Zyihong He of Zetawareobservable evidence that can be used to decipher the complexities of for generously providing access to the Genesispressure history. Software. We thank Anadarko, BHPBillton, BP, Chev- ronTexaco, ConocoPhillips, ExxonMobil, Kerr-McGee, Petroleos de Venezuela SA, Petrobras, Saudi Aramco, ´ Shell, Total, and Unocal for supporting Touchstone research and development by virtue of their mem- bership in Geocosm’s Consortium for QuantitativeCopyright #2006. The American Association of Petroleum Geologists. All rights reserved. Prediction of Sandstone Reservoir Quality. ReviewersManuscript received March 5, 2005; provisional acceptance June 14, 2005; revised manuscript received Olav Walderhaug, Howard White, and Nick WilsonNovember 15, 2005; final acceptance December 19, 2005. gave constructive suggestions for the improvementDOI:10.1306/12190505044 of our article. AAPG Bulletin, v. 90, no. 6 (June 2006), pp. 873 – 885 873
  • 2. INTRODUCTION Several recent investigations conclude that the sig- nificance of brittle deformation in mechanical compac-Here, we undertake to integrate observations of com- tion is greater than previously thought, especially forpactional grain fracturing with quartz cementation rapidly and deeply buried sandstones (Milliken, 1994;modeling. Because the brittle fracturing process in com- Chuhan et al., 2002; Makowitz and Milliken, 2003).paction creates significant new surfaces for quartz ce- Cathodoluminescence (CL) imaging reveals the ubiq-mentation, it is reasonable to seek linkages between uity of microfractures initiating at quartz grain contacts,these two processes (Makowitz and Milliken, 2003). where the deviatoric stress (condition in which stressModeling adds a vital quantitative perspective to our tensors are not the same in every direction) needed forunderstanding of the timing and depth of quartz ce- brittle failure can be achieved locally, at the grain scale,mentation (Lander and Walderhaug, 1999) and, fur- under conditions that are below the critical conditionsther, into the relative timing of cementation and grain for crack propagation through the sandstone as a wholefracturing in the subsurface. Forecasting brittle grain (e.g., Sippel, 1968; Walker and Burley, 1991; Milliken,deformation influences on reservoir quality can pro- 1994; Dickinson and Milliken, 1995). The fresh micro-vide important insights for hydrocarbon exploration, fracture creates a clean surface that is favorable forespecially in basins where deep sandstones are prolific. quartz cement nucleation (Reed and Laubach, 1996). Quantitative data on fracture aperture, morphology, number of fractures, and volume of cement localizedPREVIOUS WORK within these fractures can be gathered readily using CL imaging (Laubach and Milliken, 1996; Laubach, 1997;Compaction and cementation are the two mechanisms Marrett and Laubach, 1997; Laubach et al., 2004). In-whereby primary porosity is lost in sandstones (e.g., herited fractures are discriminated on the basis of CLLundegard, 1992; Ehrenberg, 1995), and an understand- textures and excluded from measurements of post-ing of the controls on these processes has significant compactional fractures using the criteria of Laubachimplications for predictions of reservoir quality. The (1997).magnitude of mechanical compaction of sandstones Contrasts in the number of fractured grains perduring burial, a process including grain slippage, ro- sample versus maximum burial depth between the Friotation, and deformation, is controlled by the composi- and Mount Simon formations and the differences intion, size, and shape of the constituent grains (Pittman fracture morphology were hypothesized in a previousand Larese, 1991) and the burial history (Lander and study to be dependent on the timing of quartz cemen-Walderhaug, 1999; Paxton et al., 2002). Brittle pro- tation, which, in turn, is governed by burial rate andcesses in compaction are a particularly underestimated geothermal gradient differences between the Frio (Gulfprocess because intragranular fractures in quartz grains of Mexico Basin) and the Mount Simon (Illinois Basin),are typically healed by quartz cement and are therefore together with compositional and textural differencesdifficult to detect and measure and are commonly (e.g., Frio samples have lower quartz grain content andmissed using conventional transmitted light micros- larger grain size) (Makowitz and Milliken, 2002, 2003).copy (e.g., Sippel, 1968; Milliken, 1994; Dickinson and These earlier studies also discuss in detail the evidenceMilliken, 1995; Makowitz and Milliken, 2003). for the postburial timing of the intragranular fracturing Cementation hinders mechanical compaction; thus, and its compactional association, correlations betweeninformation on the timing and physical properties of the degree of fracturing and grain size, and the para-cement phases is necessary for predicting the extent genetic sequence of cements in these sandstones.of mechanical compaction (Ehrenberg, 1989; Pittmanand Larese, 1991; Lundegard, 1992; Wilson and Stanton,1994; Dutton, 1997; Stone and Siever, 1997; Lander GEOLOGIC CONTEXT AND PETROGRAPHYand Walderhaug, 1999; Paxton et al., 2002). Conversely, OF BRITTLE FEATURESthe intergranular volume (‘‘IGV’’ is defined as the sumof the intergranular porosity and cements and matrix Frio Formationthat fill intergranular pores) remaining at a particularstage in the burial history places an upper limit on the The Oligocene Frio Formation sandstone has long servedamount of space that is available for cement emplace- as a natural laboratory for studying burial compactionment at a given depth (e.g., Paxton et al., 2002). because more than 3500 m (11,400 ft) of sediment was874 Geologic Note
  • 3. Sample Location Figure 1. Sample location map. The Frio Formation was sampled from core from various depths in the south Texas Gulf Coast. Samples from the Mount Simon Formation were collected from A core and outcrop localities in the Illinois Basin. Illinois Illinois Basin Basin Aí B Bí Gulf Coastrapidly deposited via subsidence and growth faulting erally confined to individual grains (intragranular frac-during the middle to late Oligocene and early Miocene tures) and do not transect two or more grains (trans-(e.g., Galloway et al., 1982) (Figure 1). Moreover, the granular fracturing).structural history does not involve significant uplift Quartz cementation is expected to stabilize theor compression, the unit is at or near maximum buri- grain framework and thereby inhibit compactional grainal depth, and growth faults impose a wide range of fracturing. Cathodoluminescence textures indicate thatburial depths and temperatures on materials of rela- most fractures precede significant cementation, giventively uniform initial composition. The predominantly that most do not crosscut overgrowths (Figure 3). Thelithic-rich sands of the Frio Formation of the lower minority of fractures that do crosscut overgrowthsGulf Coast were supplied by the ancient Rio Grande (see Makowitz and Milliken, 2003, their figure 10E,draining the volcanic areas of west Texas and northern p. 1015) shows, however, that grain fracturing andMexico (Loucks et al., 1984). Frio sandstones are mod- quartz cementation proceed synchronously, at leasterately sorted, fine to coarse grained, and range from to some degree. Shallowly buried quartz grains exhib-feldspathic litharenites to sublitharenites (Figure 2). iting intragranular grain fractures are generally filledAlthough quartz cement is dominant in most samples, with quartz cement but lack cementation on externalfor any given set of samples, there will be a few that grain surfaces (Figure 4), indicating faster surface area-are dominantly calcite cemented. Zeolite cement is normalized growth rates on fracture surfaces com-abundant at shallow depths (maximum = 10%), asso- pared to outer grain surfaces. The fracture surface isciated with volcanic-derived lithics, whereas quartz fresh and clean, allowing quartz cement to nucleatecement generally increases systematically with depth and grow within the fracture, whereas the external(Land, 1984; Land et al., 1987), as is widely observed grain surface may contain irregularities and detritalin many basins worldwide (e.g., Walderhaug, 1996; particles that slow the rate of quartz precipitation.Giles et al., 2000). Quartz grains in the Frio Formation have a variety Mount Simon Formationof fracture morphologies, including wedge-shaped aper-tures, intense comminution at grain contacts, and grains The Illinois Basin is an intracratonic basin in which upwith exploded fabrics (Makowitz and Milliken, 2002, to 6000 m (19,600 ft) of sediments accumulated dur-2003) (Figure 3A, B). Apparent fracture apertures in ing the Paleozoic (Figure 1). The Mount Simon sand-the Frio grains are slightly wider (average 5 mm) than stones (Late Cambrian) are predominantly of quartzin Mount Simon grains (average measurable aperture arenite composition, medium to coarse grained, andwidth $4 mm). Fractures in both formations are gen- well rounded (Figure 2). Quartz is the most abundant Makowitz et al. 875
  • 4. Figure 2. Ternary plotof sandstone composi-tions according to Folk’s(1980) classificationscheme. Plot shows thevariation of sandstonecomposition between theMount Simon and Frioformations. Averagecompositions of the Frioand Mount Simon for-mations are feldspathicand quartz arenite,respectively.cement, although calcite is locally abundant in shal- in the northerly area. Maximum burial depths of sam-low samples. During the Late Cambrian, the tectonic ples for this study are based on the model results ofsetting of the proto-Illinois Basin was governed by Rowan et al. (2002). Their model considers the tem-thermal subsidence, lasting until the early Mississippi- perature influence of burial (considered the most in-an (Rowan et al., 2002). A second subsidence episode fluential factor for temperature in past models) and(middle Mississippian through Early Permian), in re- advective heat transport from a short period of mag-sponse to the Alleghanian –Hercynian orogeny (Klein matism and is consistent with both vitrinite reflectanceand Hsui, 1987), caused pronounced downwarping in and fluid-inclusion data.the more southerly parts of the basin, leading to thicker Fracture morphologies in the Mount Simon For-sediment accumulation (Sargent, 1991). mation are homogenous and occur as thin straight Other tectonic events that effected Mount Simon traces transecting across the quartz grains. A few wedge-deposition included periodic uplift on bounding arches shaped fractures are also present in some samples(e.g., Wisconsin, Kankawee, and Pascola arches) that (Figure 3).separate the Michigan basin from the Illinois Basin.Coal rank and two-dimensional burial-history modelscalibrated to coal vitrinite reflectance and biomarkerssuggest that maximum burial was attained during the MODELING APPROACHPermian, approximately 1000–1500 m (3300–4900 ft)deeper than present (Rowan et al., 1996; Damberger Basin Modelinget al., 1999). During the Quaternary, glacial outwashwas deposited over most of the Illinois Basin. Amounts Basin modeling was conducted using Genesis1 (devel-of uplift and erosion in the Illinois Basin vary, with up oped by Zetaware) to reconstruct the thermal and ef-to 2000 m (6600 ft) in the south and approximately fective stress histories of the analyzed samples. Data300 m (1000 ft) in the north (Hoholick, 1980). Other for the one-dimensional (1-D) basin models were re-estimates of burial depth provided by Wilson and Sib- trieved from well logs, including mud weights, bottom-ley (1978) indicate nearly 900 m (2900 ft) of erosion hole temperatures, circulation times, stratigraphy, and876 Geologic Note
  • 5. Figure 3. Fracture styles and morphologies characteristic of the Frio (A and B) and Mount Simon quartz grains (C and D). Fractures in the Frio Formation (A and B) are commonly wedge shaped, exhibit spalling, and commonly have small-scale cataclasis as- sociated with grain-grain contacts. In the Mount Si- mon Formation, fractures generally transect the quartz grains as straight traces with fracture apertures more uniform and generally thinner than in the Frio.gross lithology for the Frio Formation. Although vi- derhaug, 1999; de Souza and McBride, 2000; Walder-trinite reflectance data are scarce, when available, they haug, 2000; Bloch et al., 2002; Bonnell and Lander,were used to constrain thermal histories. Where in- 2003; Taylor et al., 2004) or for constraining thermalput data were not available for some of the wells, histories (Awwiller and Summa, 1997, 1998; Landerwe estimated the values by interpolation with nearby et al., 1997a, b; Perez et al., 1999). Such models, how-wells. ever, also have the potential to provide improved tem- Although most of the modeled temperatures match poral constraints on the diagenetic evolution of sand-within ± 5jC of measured temperatures, a substantial stones (Bonnell et al., 1999; Helset et al., 2002). Innumber of measurements fall out of this range. In most this study, we use Touchstone version 6.0 to constraincases, measured temperatures are lower than modeled the history of quartz cementation, so that we can bet-temperatures. Most likely, the true temperatures are ter delineate the precise timing and conditions of brit-higher than the measured values because of the effects tle grain deformation relative to cement emplacement.of drilling. Bottom-hole temperature data retrieved Model inputs include (1) textural and compositionalfrom well logs match other such data from south Texas characteristics of each analyzed sample; (2) thermal(e.g., McKenna and Sharp, 1998). and effective stress histories derived from basin mod- Mount Simon Formation burial history data are eling; and (3) and various model parameters discussedfrom the model of Rowan et al. (2002) for the burial below. We used the same model parameters for allhistory of the intracratonic Illinois Basin (Figure 5). simulations with two important exceptions where pa- rameters were optimized to match measurements: theSimulation of Quartz Cementation History activation energy for quartz precipitation (E a) and the stable packing arrangement (IGVf).Sandstone diagenesis and reservoir quality models Following Walderhaug (1994, 1996), we assume thatsuch as Exemplar (Lander and Walderhaug, 1999) or TM the rate-limiting control on quartz cementation is theTouchstone typically are used for reservoir quality TM rate of crystal growth and not the rate of silica supply. Theprediction (e.g., Bonnell et al., 1999; Lander and Wal- surface area-normalized rate of quartz precipitation, k, Makowitz et al. 877
  • 6. function of time and temperature using thermal re- constructions from basin models. We adjust the E a value for each sample simulation to achieve a match between the calculated and measured quartz cement abundances for each individual sample ( Table 1). The adjusted E a values for a given stratigraphic unit gen- erally fall within a narrow range. An additional important control on quartz cemen- tation is the nucleation surface area and how it changes with diagenetic alteration. We follow an approach sim- ilar to that of Lander and Walderhaug (1999), but as- sume that cements concentrically line spherical pores (Merino et al., 1983; Lichtner, 1988; Canals and Meunier, 1995). The timing of nonquartz cement precipitation is defined by paragenetic rules and burial history re- constructions as shown in Table 2. Compaction reduces intergranular porosity and therefore may reduce surface area for quartz cement nucleation. The compaction state of the sample is de- termined using the function of Lander and Walderhaug (1999): IGV ¼ IGVf þ ðIGVo À IGVf ÞÀbse where IGVf is a stable packing arrangement that rep- resents the minimum likely intergranular volume (%); IGVo is the intergranular volume upon deposition (%), and b is the exponential rate of compaction (MPa À 1) with effective stress se (MPa). The compaction stateFigure 4. Frio sample 3223 (A) scanning electron microscopy- of the sample is determined through geologic time ascathodoluminescence image of grain exhibiting fractures filled the effective stress (from basin modeling) changes, al-with quartz cement. (B) Secondary electron image (SEI) show- though the compaction process is assumed to be ir-ing continuous smooth surface of grain, indicating that frac- reversible should effective stress decline (Lander andtures are filled with quartz. Two possible reason for this pref- Walderhaug, 1999). IGVo is determined using a pro-erential fracture annealing: (1) clays and byproducts from prietary algorithm in Touchstone that is based on thedissolved grains (partially dissolved feldspar in upper left and unpublished experimental work of R. E. Larese and L.corner) adhered to the detrital grain surface and prohibited M. Bonnell, and a constant value of 0.6 MPa À 1 is usedquartz precipitation around the grain and (2) low temperatures for b as suggested by Lander and Walderhaug (1999).at this depth ($50jC) make it difficult for quartz cement to The IGVf value for each sample (Table 1) provides anprecipitate. optimal match between the present-day calculated and measured IGV values. These values vary considerablyis modeled using an Arrhenius kinetic formulation among samples because of differences in the extent of(Walderhaug, 1996): grain deformation and chemical compaction. ÀEa k ¼ Ao e RT MODELING RESULTSwhere E a is the activation energy for quartz precipi-tation (kJ/mol); R is the universal gas law constant To evaluate the potential influence of quartz cemen-(8.31 J/mol K); T is temperature (K); and A o is the tation on fracture characteristics, we used Touch-pre-exponential constant (here taken to be 9 Â 10 À 12 stone simulations to reconstruct the burial conditionsmol/cm2 s). The kinetic equation is integrated as a at which small amounts of quartz cement (0.5, 1, and878 Geologic Note
  • 7. Figure 5. Thermal history for Frio and Mount Simon formations generated from 1-D Genesis basin models. Frio wells are depictedby name and are located in the following south Texas counties: (1) Jack Brown in Live Oak Co.; (2) Slick State in Starr Co.; (3) BaffinState in Kleberg Co.; (4) Hornsby in Brooks Co.; (5) Seeligson and McHaney in Jackson Co.; (6) Gerdts and McCullough in Willacy Co.;(7) Copano State in Aransas Co.; and (8) Pleasant Bayou in Brazoria Co.2%) formed in the analyzed samples ( Table 1). Our of quartz precipitation. Differences in the surface arearesults show wide ranges in conditions. For example, for quartz nucleation are an additional cause of varia-the reconstructed burial depth at which 2% quartz tion in quartz cement abundances. Mount Simon For-cement formed ranges from approximately 1700 to mation sandstones generally would be expected to have2600 m (5500 to 8500 ft) in Mount Simon samples somewhat more quartz cement than Frio Formationcompared to about 2650 – 4400 m (8690 – 14,435 ft) samples of comparable grain size and thermal exposurein Frio Formation samples (Figure 6A). These differ- because of greater nucleation surface associated withences mainly reflect variations in the thermal histories greater quartz grain abundance and lower grain coatingamong the analyzed samples. Thermal history is im- coverage.portant because modeled quartz precipitation rates The percentage of fractured quartz grains corre-increase nearly exponentially with temperature, where- lates strongly with the reconstructed burial depth atas at a given temperature, the amount of quartz cement the time small amounts of quartz cementation formedincreases nearly linearly with time. Sandstones with rap- for samples from both data sets (Figure 6). This cor-id burial rates, therefore, tend to be more deeply buried relation appears to be somewhat stronger for the depthby the time a small amount of quartz cement forms at which 2% quartz formed than it is for 1 or 0.5%because they have lower residence times at shallow (Figure 6A, B). Burial depth is a driving force for com-depths, where temperatures are cooler. Such samples paction, however, only in as much as it relates to effec-also tend to experience significant quartz cementa- tive stress (and temperature when it involves chem-tion at earlier times given that they have earlier ex- ical processes). In the Frio Formation our 1-D basinposure to higher temperatures that lead to faster rates models indicate that those samples with the greatest Makowitz et al. 879
  • 8. 880Geologic Note Table 1. Model Input and Output Parameters Including Modeling Results at 0.5, 1.0, and 2.0% Quartz Cement 2% Quartz 1% Quartz 0.5% Quartz Effective Effective Effective Effective Stress Effective Stress Effective Stress Ea Time Temperature Depth Stress Hydrostatic Time Temperature Depth Stress Hydrostatic Time Temperature Depth Stress Hydrostatic Sample Well Well Unit (kJ/mol) IGVo IGVf (Ma) (jC) (m) (MPa) (MPa) (Ma) (jC) (m) (MPa) (MPa) (Ma) (jC) (m) (MPa) (MPa) 1164 Northern Illinois Mt. Simon 62.8 33.8 22.1 263 103.6 1963 24.5 24.5 338 57.9 1196.9 15.0 15.0 408 55.3 1075.7 13.5 13.5 2166 Northern Illinois Mt. Simon 63.6 35.7 20.3 265 95.3 1931 24.2 24.2 358 65.9 1425.4 17.8 17.8 418 63.8 1308.6 16.4 16.4 2384 Northern Illinois Mt. Simon 65.0 35.1 14.1 272 76.2 1741 21.8 21.8 368 71.4 1583.2 19.8 19.8 426 68.9 1441.5 18.0 18.0 2480 Northern Illinois Mt. Simon 65.2 34.6 19.5 265 108.0 2229 27.9 27.9 354 72.9 1639.8 20.5 20.5 416 71.1 1525.9 19.1 19.1 3177 Northern Illinois Mt. Simon 67.0 38.2 19.5 278 81.1 1893 23.7 23.7 372 76.7 1738.5 21.7 21.7 428 74.4 1594.2 19.9 19.9 3134.5 Northern Illinois Mt. Simon 63.8 34.2 15.9 378 79.6 1818 22.7 22.7 432 77.1 1662.1 20.8 20.8 460.33 76.7 1612.3 20.2 20.2 3225 Northern Illinois Mt. Simon 62.1 33.5 15.9 298 83.9 1990 24.9 24.9 388 80.5 1837.1 23.0 23.0 438 77.7 1662.6 20.8 20.8 3793 Central Illinois Mt. Simon 61.4 33.1 15.4 337 70.3 1749 21.9 21.9 380 64.8 1583.2 19.8 19.8 414 56.7 1345.3 16.8 16.8 3581.5 Central Illinois Mt. Simon 61.2 32.4 21.3 333 69.8 1735 21.1 21.1 378 67.6 1665.0 20.8 20.8 412 59.7 1437.5 18.0 18.0 3619 Central Illinois Mt. Simon 59.9 31.6 12.5 376 68.4 1688 21.1 21.1 414 59.5 1426.3 17.8 17.8 442 50.9 1109.9 13.9 13.9 4038 Central Illinois Mt. Simon 61.7 33.2 14.4 337 78.2 1979 24.8 24.8 386 71.5 1777.5 22.2 22.2 420 64.7 1572.1 19.7 19.7 4119 Central Illinois Mt. Simon 64.2 33.6 19.5 295 84.1 2124 26.6 26.6 354 77.9 1966.7 24.6 24.6 396 70.1 1741.0 21.8 21.8 4469 Central Illinois Mt. Simon 62.0 33.9 15.4 366 77.2 1944 24.3 24.3 406 68.6 1703.4 21.3 21.3 438 59.9 1357.8 17.0 17.0 4477 Central Illinois Mt. Simon 61.8 34.3 10.8 354 78.8 1994 24.9 24.9 398 70.6 1755.5 22.0 22.0 430.5 61.9 1442.6 18.0 18.0 4226 Central Illinois Mt. Simon 62.9 34.1 22.6 308 82.9 2106 26.3 26.3 364 77.9 1963.6 24.6 24.6 404 69.3 1723.9 21.6 21.6 4720 Central Illinois Mt. Simon 63.6 34.7 11.9 332 82.8 2115 26.5 26.5 380 77.1 1938.2 24.2 24.2 420 68.6 1678.9 21.0 21.0 5404 Central Illinois Mt. Simon 63.4 33.0 10.3 366 90.6 2332 29.2 29.2 408 81.4 2078.9 26.0 26.0 442 73.2 1716.0 21.5 21.5 6154 Southern Illinois Mt. Simon 62.0 34.9 13.3 315 104.6 2113 26.4 26.4 340 90.2 1751.0 21.9 21.9 364 78.4 1432.4 17.9 17.9 6235 Southern Illinois Mt. Simon 62.0 34.9 8.9 310 102.9 2092 26.2 26.2 338 92.1 1805.4 22.6 22.6 362 80.2 1480.1 18.5 18.5 6241 Southern Illinois Mt. Simon 62.0 35.2 13.2 310 103.0 2094 26.2 26.2 336 93.1 1834.7 22.9 22.9 358 82.1 1530.2 19.1 19.1 6497 Southern Illinois Mt. Simon 62.0 34.9 17.7 320 103.3 2134 26.7 26.7 346 91.1 1783.1 22.3 22.3 370 80.2 1490.5 18.6 18.6 6500 Southern Illinois Mt. Simon 62.0 34.6 9.4 304 116.7 2394 29.9 29.9 332 98.2 1985.3 24.8 24.8 356 86.5 1656.7 20.7 20.7 8466 Southern Illinois Mt. Simon 62.0 35.1 13.0 340 120.3 2654 33.2 33.2 372 107.7 2282.4 28.5 28.5 400 97.4 2013.6 25.2 25.2 8468 Southern Illinois Mt. Simon 62.0 42.3 8.6 360 111.7 2394 29.9 29.9 390 101.5 2113.8 26.4 26.4 416 89.9 1790.2 22.4 22.4 3223 Jack Brown Frio * * * * * * * * * * * * * * * * * * 4908 Slick State Frio * * * * * * * * * * * * * * * * * * 6105 Seeligson Frio * * * * * * * * * * * * * * * * * * 8910 Baffin State Frio * 38.1 29.2 * * * * * * * * * * * * * * * 9001 Hornsby Frio 58.0 38.2 11.1 5 91.7 2650 30.2 33.1 13.88 84.7 2351.4 26.7 29.4 19.24 80.9 2191.3 24.7 27.4 9547 Gerdts Frio 57.2 38.4 25.5 6 108.5 2795 15.1 34.9 15.05 100.6 2493.2 12.6 31.2 19.85 98.3 2370.0 10.6 29.6
  • 9. Table 2. Depth Constraints for the Paragenetic Sequence37. Used in Modeling for Both the Frio and Mount Simon * Formations 9.9 8.3 7.9 8.134.631.2 * Start (m) End (m)2962.72729.32557.52611.14098.14156.5 Grain coating 0 100 * Calcite 100 1000 Chlorite 200 1000 96.7100.2105.2107.1156.5159.5 Kaolinite 1000 2000 * Pyrite 0 100 K-feldspar 1000 300015.3720.8224.4523.724.6 Dolomite 2000 40000* Iron oxides 100 100036.533.432.754.654.6 * * reconstructed burial depths at the time of significant 9.4 8.5 8.633.611.9 quartz cementation also have the lowest reconstructed * * effective stresses because they experienced faster rates2917.62668.42615.74366.54370.7 of burial and, therefore, greater extents of fluid over- * * pressure development because of compaction disequi- librium (caused by the inability to expel pore fluids100.5106.7107.9170.6172.5 * * in low-permeability shales and clay-rich sediments; hence, most of the overlying sediment’s weight is 16.01 21.78 23.44 supported by the pore fluid instead of the grains) 5.3 23.7* * (Figure 7). Thus, the Frio Formation samples with the greatest degree of quartz grain fracturing also had the lowest reconstructed effective stresses at the time37.033.754.554.5 * * * of significant quartz cementation. Such a result is in- consistent with experimental and theoretical results, *Samples with less than 2% quartz cement that we were not able to model or are insignificant.14.611.1 which indicate that grain fracturing is promoted by9.09.1 * * * greater effective stresses (Chuhan et al., 2002; Chester2956269343574363 et al., 2004; Karner et al., 2005). The extent of grain * * * fracturing correlates much more strongly with effec- tive stress if fluid pressures were near hydrostatic lev-113.7108.3177.2177.6 * * * els at the time that small amounts of quartz cement formed (hydrostatic case in Figure 7). These results 7 18 23 22 suggest that fluid overpressures in the Frio Formation*** may have developed at significantly greater depths (and 7.225.314.126.611.317.516.7 later times) than would be expected in basin models38.038.836.737.243.132.535.1 that rely mainly on compaction disequilibrium. Alter- native mechanisms for fluid overpressure development61.062.558.758.263.463.3 * that could lead to a shift into overpressured conditions late in the burial history include hydrocarbon reac- tions (Luo and Vasseur, 1996; Osborne and Swarbrick,FrioFrioFrioFrioFrioFrioFrio 1997; Hansom and Lee, 2005) and diagenetic reac-Pleasant BayouPleasant Bayou tions (Waples and Kamata, 1993; Bjørkum and Nadeau,McCullough 1996, 1998; Lander 1998; Matthews et al., 2001; HelsetMcHaneyMcHaneyMcHaney et al., 2002).Gerdts As discussed previously, fractures in the Mount Simon Formation samples show thin-straight fracture10169138331562015640971097209744 traces, whereas in Frio Formation samples, fractures Makowitz et al. 881
  • 10. Figure 6. Depths at which quartz cement content reached 0.5% (A) and 2% (B) versus percentage of fractured quartz grains. Apositive correlation exists between the onset of quartz cementation and degree of grain fracturing for both the Mount Simon and Frioformations, and this correlation is best for the 2% level of quartz cement emplacement.have larger wedgelike forms. The difference in the Mount Simon Formation samples had more restrictedeffective stress at the onset of significant quartz ce- dilation because of their greater abundance of rigidmentation may be one factor causing this change in quartz grains or their lower IGV values (average offracture geometry. It is also possible that fractures in 18.6 versus 24.8% for the Frio).Figure 7. Effective stress at low amount of quartz cement, (A) at 0.5% quartz cement and (B) at 2.0% quartz cement, versuspercentage of fractured grains shows a positive correlation in both formations, considering a hydrostatic stress regime at this time inthe burial history. However, if deeper Frio sands are influenced by compaction disequlibrium, which causes overpressure, thus,reducing the effective stress, this trend would not hold true.882 Geologic Note
  • 11. CONCLUSIONS Chuhan, F. A., A. Kjeldstad, K. Bjørlykke, and K. Høeg, 2002, Po- rosity loss in sand by grain crushing — Experimental evidence and relevance to reservoir quality: Marine and Petroleum Geol- Data presented in this article demonstrate that the ogy, v. 19, p. 39 – 53. effective stress at the time of quartz cement Damberger, H. H., I. Demir, and J. Pine, 1999, Age relationships between coalification, deformation, and geothermal events in initiation is an important constraint for predicting the Illinois Basin (abs.): Geological Society of America Annual the degree of grain fracturing in quartz-rich sands. Meeting Program, p. 403. The deeper Frio data support the notion that ef- de Souza, R. S., and E. F. McBride, 2000, Diagenetic modeling and fective stresses were much higher than would be reservoir quality assessment and prediction: An integrated approach (abs.): AAPG Bulletin, v. 84, no. 9, p. 1495. expected from 1-D disequilibrium compaction mod- Dickinson, W. W., and K. L. Milliken, 1995, Diagenetic role of els at the time of quartz cement initiation, suggesting brittle deformation in compaction and pressure solution, Etjo that overpressure began at greater depths (later Sandstone, Namibia: Journal of Geology, v. 103, p. 339 – 347. times) in the burial history. Dutton, S. P., 1997, Timing of compaction and quartz cementa- Differences in degree of fracturing and fracture mor- tion from integrated petrographic and burial history analysis, phologies between the Frio and Mount Simon for- Lower Cretaceous Fall River Formation, Wyoming and South Dakota: Journal of Sedimentary Research, v. 67, p. 186 – mations can be attributed to (1) greater depth to 196. initiation of quartz cementation in the Frio than in Ehrenberg, S. N., 1989, Assessing the relative importance of the Mount Simon, allowing for more and wider frac- compaction processes and cementation to reduction of poros- ity in sandstones: Discussion: AAPG Bulletin, v. 73, p. 1274 – tures and apertures in the Frio; and (2) IGV, whereby 1276. lower IGVs in the Mount Simon resulted in a re- Ehrenberg, S. N., 1995, Measuring sandstone compaction from duced possibility of expansion of grains into the pore modal analysis of thin sections: How do I do it and what do the space and, hence, thinner fracture apertures. results mean: Journal of Sedimentary Research, v. A65, p. 369 – 379. Folk, R., 1980, Petrology of sedimentary rocks: Austin, Texas, Hemphill Publishing Company, 170 p. Galloway, W. E., D. K. Hobday, and K. Magara, 1982, FrioREFERENCES CITED Formation of the Texas Gulf Coast Basin: Depositional sys- tems, structural framework, and hydrocarbon origin, migra-Awwiller, D. N., and L. L. 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