Shale Facies Variability and Sequence Stratigraphy: Application to Top Seal Prediction in Deepwater Depositional Systems
William C. Dawson and William R. Almon
ChevronTexaco, 1500 Louisiana, Houston, Texas 77002
Fine-grained lithofacies are dominant components of deep-marine depositional systems, but limited systematic study of fine-grained siliciclastic lithofacies results in the incomplete understanding of deep-marine petroleum systems. Most significantly, these fine-grained strata are baffles and barriers to fluid flow. Petrologic analyses of Tertiary-aged samples reveal the common occurrence of 6 shale types in deep-marine stratal packages: 1) well-laminated organically-enriched shales; 2) slightly silty, weakly laminated shales; 3) moderate to very silty, weakly laminated shales; 4) distinctly mottled very 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 critical seal pressures (10% non-wetting saturation) relative to shale types 3 and 4. Shale type 5 has the lowest sealing potential. These shale facies vary systematically in terms of sequence stratigraphy and exhibit strong correlations with seal capacity suggesting that textural parameters have a direct effect on seal capacity. Silt-poor (< 10%) transgressive shales typically have excellent (7,000 −10,000 psia) to exceptional (>10,000 psia) critical seal pressures. Increased percentages of silt-sized detrital grains inhibit mechanical compaction and allow preservation of relatively large-diameter pore throats, which typify highstand and lowstand shales. Well-developed laminar fabrics, increased content of organic matter, and/or early marine carbonate cementation can enhance seal capacity. Bioturbation disrupts primary fabrics and thereby degrades seal character. Thin calcareous (condensed) shales are prone to fracturing. Stacking patterns inherent to deep-marine depositional systems can result in considerable (several hundred feet) vertical separation between a lowstand reservoir and the overlying seal resulting in the formation of a thick "waste zone." Log-derived parameters lack significant ability to accurately predict critical leak pressures in deep-water GOM seal samples.
AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana