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Shale Facies Variability in Deep Marine Depositional Settings: Seal Quantification and Prediction

Previous HitWilliamNext Hit R. Almon and Previous HitWilliamTop C. Dawson
ChevronTexaco, Bellaire, TX

Fine-grained lithofacies are dominant components of deep-marine (middle to upper slope) depositional systems. Failure to study the sedimentology and architecture of fine-grained siliciclastic lithofacies results in an incomplete characterization of marine petroleum systems. Most significantly, these fine-grained strata are baffles and barriers to fluid flow. Petrologic analyses of Tertiary-aged samples from wells in South Atlantic basins reveal the common occurrence of 6 shale types in marine stratal packages: 1) well-laminated organically-enriched shales; 2) slightly silty, weakly laminated shales; 3) silty shales weakly laminated shales; 4) distinctly mottled silty shales; 5) very silty shales and argillaceous siltstones; and 6) calcareous shales and claystones. Each shale type represents a limited range of depositional and geochemical conditions. Shale types 1, 2 and 6 have significantly greater 10% MICP pressures relative to shale types 3 and 4. Shale type 5 has the lowest sealing potential. Shale facies vary systematically in terms of sequence stratigraphy and exhibit a strong correlation with seal capacity suggesting that at least some textural parameters have a direct effect on seal capacity. Silt-poor transgressive shales typically have excellent to exceptional sealing behavior. Increased percentages of silt-sized detrital grains (> 25%) can inhibit mechanical compaction and allow preservation of relatively large-diameter pore throats, which typify highstand and lowstand shales. Well-developed laminar fabrics and early marine carbonate cementation appear to enhance seal character. Bioturbation generally degrades seal behavior. Stacking patterns inherent to deep-marine depositional systems can result in considerable (several hundred feet) vertical separation of lowstand reservoirs and overlying transgressive seals.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005