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Sequence stratigraphy and syndepositional deformation of the Agbada Formation, Robertkiri field, Niger Delta, Nigeria

¹ Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843; present address: ChevronTexaco Nigeria Limited, No. 2 Chevron Drive, Lekki Peninsula, Lagos, Nigeria; mknt{at}chevrontexaco.com
² Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843; present address: Chevron Energy Technology Company, 1500 Louisiana St., Houston, Texas 77002; bwillis{at}chevron.com

Olusola A. Magbagbeola has 9 years development geology and reservoir management experience with Chevron. Previously, he was an assistant lecturer at the University of Ibadan. He holds a B.S. degree from the University of Ilorin (Nigeria), an M.S. degree from the University of Ibadan (Nigeria), and an M.S. degree from Texas A&M University. His current research focuses on the influence of mobile substrates on depositional profiles developed above rapidly prograding deltaic systems.

Brian J. Willis is a member of the shallow-marine stratigraphy team of Chevron's Energy Technology Company. Previously, he was an assistant professor at Texas A&M University, a geologist with British Petroleum, a research scientist at the University of Texas, and a research fellow at the Smithsonian Institution. He received a B.S. degree from the University of Minnesota and an M.S. degree and a Ph.D. from Binghamton University. He has published more than 30 articles on fluvial and shallow-marine depositional systems.

The Agbada Formation in Robertkiri field is a 2.7-km (9000-ft)-thick succession of Miocene shallow-marine and nonmarine deposits formed as the continental margin of the Niger Delta structurally collapsed under accumulating sediments. A high-resolution sequence-stratigraphic framework for these strata was constructed by combining data from 20 well logs and a 1400-km² (550-mi²) seismic volume. Syndepositional structural collapse occurred along a succession of major, cuspate, offshore-dipping normal faults and associated antithetic faults and rollover anticlines within downdropped blocks. Six fourth-order sequences, each hundreds of meters thick, formed during episodic progradation and retrogradation of deltaic shorelines. Their development was complicated by the thickening of deposits across major growth faults and away from the crests of adjacent rollover anticlines. Successively younger sequences became thinner, more laterally uniform in thickness, and less structurally deformed and contain less growth strata above downdropped fault blocks. Erosion along successively younger sequence boundaries became shallower and broader as accommodation declined and more sediment was bypassed basinward. Areas of deepest sequence boundary incision and most rapid structural deformation shifted basinward over time with regional delta progradation. Stratigraphic patterns across successive downdropped fault blocks suggest that collapse of this continental margin occurred under prograding deltaic deposits on the shelf instead of within lowstand intraslope basins. Differences in the development of successive sequences reflect gradually slowing rates of structural collapse as underlying mobile shale is depleted, allowing deltaic deposition to shift farther basinward.