Almon, William A.1, William C. Dawson1
(1) ChevronTexaco, Bellaire, TX
ABSTRACT: Seal Character and Variability Within Deep-Marine Depositional Systems: Part II -Seal Quantification and Prediction
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 end-member has distinctive textures and fabrics, which record variations in depositional conditions. Textural and compositional variations of 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 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 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 high frequency stratigraphic cycles.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004