Syn-Sedimentary Structural Growth in a Deep-Water Reservoir, Alba Field, Equatorial Guinea

Jeannette M. Wolak and Michael H. Gardner
Department of Earth Science, Montana State University, Bozeman, MT

Alba Field is a locally conglomeratic deep-marine sandstone reservoir located 30 km northwest of Bioko Island, Equatorial Guinea. Gas and lesser oil production in the Alba Field are sourced from the Late Miocene Isongo Formation, which records deep-water sedimentation associated with uplift of the Cameroon Volcanic Line to the northeast and growth of the Niger Delta to the west.

Detailed sedimentological study of over 400m of conventional core from the Alba Field has established fourteen lithofacies which characterize six sedimentary body types: erosional slope channelform, distributary channelform, lobeform, channel-flank wedgeform, mudstone drape, and mass transport deposits. Incorporation of core descriptions with wireline logs and low-resolution seismic demonstrate that sedimentation from a variety of subaqueous flow types deposited a 30 km-long, fan-shaped feature that thickens and widens (20 km) to the southwest and thins and pinches out to the east. Syn-sedimentary phases in Alba Field include: (1) sediment bypass, incision and formation of a slope valley on the crest of a seafloor high during active structural growth which produced northward onlap of distributive channel and lobe deposits along the southern flank of the structure; (2) depositional back-filling of the slope valley in the east during a period of waning structural growth; and (3) a westward shift in deposition of the youngest distributive system in response to depositional back-filling of the valley during structural quiescence. Collectively, these sedimentation patterns are consistent with a southwestern decrease in gradient which may have been due to: (1) growth of the Alba anticlinal structure; (2) an inherited base-of-slope related to submarine channels crossing the contractional belt of the Niger delta toe; or (3) some combination of these different scale but related structural elements.

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