Deep-Water Depositional Systems of the Gulf of Guinea and the Alba Field, Equatorial Guinea

Rowell, Philip, J. D. Carballo, Marathon oil Company, Houston, TX

The giant Alba gas/condensate field is situated offshore Equatorial Guinea 20 kms north­west of Bioko Island. It lies to the south of the Rio Del Rey Basin on the eastern fringe of the Niger River Delta system. The hydrocarbons are reservoired within the Upper/Middle Miocene deep water depositional system known locally as the Isongo formation. The Isongo formation is a massively developed turbidite sand system and represents a unique deposi­tional formation within the area due to its uncommon mineralogical composition associated with the Cross River drainage system. The depositional environment within the field was predominantly at the base of slope of the up-dip Rio Del Rey prograding deltaic system. Sediment supply was from the north and northeast. Reservoir architecture varies from deeply incised channels to broad meandering channel forms to frontal splays. Thin-bedded overbank facies are also interpreted. Gross sand thickness varies from 100 to 300 metres with net to gross up to 85% in some of the sandy channel systems. Structurally the Alba field is a large anticline 10 by 25 kms lying at the terminal edge of the Niger River / Rio Del Rey toe-thrust system. To the north, intense deformation, uplift and erosion occurred along the main shale-cored toe-thrust front. The Alba structure sits basinal of this deformation front with significant but more subtle growth. This occurred during Late Upper Miocene and Early Pliocene. Migration of hydrocarbons occurred from Late Upper Miocene to the pres­ent generated by underlying Miocene and Oligocene source rocks.