Shale Variability in Deep-Marine Depositional Systems: Implications for Seal Character - Subsurface and Outcrop Analogs

William C. Dawson¹, William R. Almon¹, Kelly Dempster¹, and Sally J. Sutton^2
1Chevron, Houston, TX
²Colorado State University, Fort Collins, CO

Shales are arguably the least understood lithotype causing significant uncertainty in the interpretation of basin modeling results and seal risk. Burial-driven compaction (i.e., systematic reduction of pore throat size during progressive burial) is not the primary control on seal behavior. Rather, variations in depositional conditions, related to high-frequency stratigraphic fluctuations, appear responsible for broad variations in shale properties and seal character. Analyses of samples from deep-water (submarine fan) depositional settings reveal strong relationships between mudstone facies and sealing character. Silt-poor well-laminated shales generally have excellent to exceptional sealing behavior. Increased percentages of silt-sized detrital grains (> 20%) enhance preservation of relatively large-diameter pore throats, thereby lowering sealing capacities. Sub-parallel alignment clay minerals and organic matter, and early marine carbonate cementation can significantly enhance sealing capacity. Bioturbation generally degrades sealing capacity. Sandy injectites can compromise seal effectiveness. Silt-poor well-laminated shales typify more distal parts of submarine fan deposits. In contrast, mudstones associated with proximal channel-levee complexes commonly exhibit highly deformed fabrics and are moderately to very silty (clay-poor) and consequently have relatively low sealing potential. Compartmentalization by shale laminae is common in channel margins. Comparable shale facies patterns are observed in samples from deepwater Gulf of Mexico wells, offshore West Africa wells and outcrop analogs (Arkansas and Wyoming). Because of variations in fabric and texture, deepwater shale types exhibit different compaction rates, which can result in erroneous interpretations of burial history.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas