Seals are a key element of petroleum systems, yet they have received limited systematic study. Textural and compositional variations permit the recognition of six shale lithofacies in Tertiary deep-marine depositional settings. Each shale type endmember has distinctive textures and fabrics, which record variations in depositional conditions. Textural and compositional variations in shales, considered within the context of sequence stratigraphy, provide a basis for seal risk assessment. As determined from mercury injection capillary pressure (MICP) analysis, the pressure required to attain critical seal pressure (10% non-wetting phase saturation) varies over a considerable range (15 to 20,000 psia). Tertiary shales from offshore Brazil have consistently low critical seal pressures relative to age-equivalent shales from offshore West Africa. Tertiary shales from wells in the Gulf of Mexico have intermediate critical MICP values (mean: 4,700 psia). The organization of shale facies within a sequence stratigraphic framework reveals systematic variations in seal character. Silt-poor shales from uppermost transgressive systems tracts, and some condensed shales, have good to excellent seal potential. In contrast, silt-rich shales from highstand and lowstand systems tracts have moderate to low sealing capacities. Seal quality generally increases as total clay and carbonate content increase; other compositional variables have limited predictive relationship with seal character. Likewise, log-derived parameters lack significant potential to accurately predict critical non-wetting saturation values. Additional seal variability factors include changes in the rate of deposition, early marine cementation, and depositional fabric. Available data provide a compelling argument for textural control of seal character induced by highfrequency stratigraphic cycles.

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