Foraminiferal biostratigraphy, depositional environments and hydrocarbon source rock potential of sediments from five wells in the western offshore region of the Niger delta, Nigeria

The Tertiary lithostratigraphy of the Niger delta consists of three units: the Akata, Agbada and Benin Formations. The Akata formation is characterized by a uniform development of grey, silty, under-compacted marine shales, containing lenses of abnormally high pressured fine grained sandstones. These pro-delta shales were deposited predominantly under anoxic conditions, on the continental slope. The Akata Formation is of Paleocene to Early Miocene age.;The Agbada Formation is of Miocene-Pliocene age, and consists of a paralic sequence of alternating sandstones and shales. These were deposited on the delta front, distributary channels and lower delta plain environments.;The Benin Formation consists predominantly of massive, porous, freshwater-bearing sands, with local thin shale interbeds. This formation was deposited in alluvial environments, including the braided stream and meander belt systems of the upper delta plain.;Foraminiferal biostratigraphic analyses were made of sediments from the Akata and Agbada Formations in the Okan-74, Mesan-B, Meji-29, ET-1, and EA-5 wells. Index planktonic foraminifera enabled the zonation of the individual stratigraphic sequences. Five benthonic foraminiferal zones and ten biofacies/biofacies complexes are defined and correlated across the study area.;Depositional environmental interpretations suggest deposition in proximal fluviomarine through bathyal-abyssal environments. A river dominated deltaic sedimentation model is proposed.;Rock evaluation pyrolysis results indicate that the sediments contain adequate organic matter for hydrocarbon generation, while the Tmax data confirm burial to depths corresponding to the oil generation window. The vitrinite reflectance data confirm that some of the sediments are mature, and hydrocarbons (predominantly gas) generation has taken place from the predominant type III kerogens present in the organic matter.;At least 2500 feet of mature Agbada source rocks are present within the studied wells. Despite evidence of hydrocarbon generation from both shale formations, the Agbada shales are shown to be the dominant source rocks within the western part of the delta.