Transient fan architecture and depositional controls from near-surface 3-D seismic data, Niger Delta continental slope

Abstract

A near-surface, three-dimensional seismic data set from the Niger Delta continental slope, offshore Nigeria, reveals important stratigraphic and architectural features of channel and fan systems in intraslope basins and permits the development of predictive models for application to deeper reservoir systems. Synsedimentary extensional faulting and mud diapirism control slope gradient, but erosion and deposition from sediment gravity flows tend to smooth the depositional profile and establish an equilibrium profile that adjusts to the changing slope gradient. Architectural features and sediment deposits interpreted from seismic character and seismic stratigraphy, in the absence of borehole data, include mass-transport complexes, distributary channels, submarine fans, and hemipelagic drape complexes. Leveed channel complexes are absent in this study area. These architectural features reflect a combination of active (sediment input from channel systems) and relatively passive (slope failures and slumps) sediment supply systems. Deposition of sandy fans is caused by a hydraulic jump at an abrupt reduction of slope gradient. Channel incision results from knickpoint migration headward from an abrupt increase of slope gradient. Submarine fans that show evidence of channel incision and bypass are termed “transient,” whereas fans without channel incision and bypass are termed “terminal.” This distinction has implications for both exploration and reservoir management. The presence of incised channels in transient fans indicates bypass of significant sand volume to a basinward location. If the transient fan is a hydrocarbon reservoir, the incised channel, which commonly is shale filled, may compartmentalize the reservoir. Dayo Adeogba has had 11 years of experience as a petroleum geologist with ChevronTexaco, mostly in development geology and reservoir management. He earned a B.Sc. degree from the Obafemi Awolowo University (Nigeria) and an M.S. degree from Stanford University. He currently focuses on deepwater depositional systems, seismic geomorphology, seismic stratigraphy, and stratigraphic analysis to solve complex reservoir development and fluid-flow issues.Tim McHargue is a research consultant at ChevronTexaco and a consulting faculty at Stanford University. He received his B.S. and M.A. degrees from the University of Missouri and his Ph.D. from the University of Iowa. His research interests are in sequence stratigraphy, seismic stratigraphy, exploration, and reservoir characterization. Currently, Tim is coordinating geological research on turbidite reservoirs at ChevronTexaco. Steve Graham is a professor in the School of Earth Sciences, Stanford University. He teaches courses in sedimentary geology, seismic interpretation, sedimentary basin analysis, and petroleum reservoir characterization. His current research projects include studies of sedimentary basins in eastern Asia, South America, and the western United States, as well as studies of the sedimentology and stratigraphic architecture of deepwater deposits.