Delineation of factors that control hydrocarbon source rock maturity in Anambra and Abakaliki Basins South-Eastern Nigeria

Abstract

Outcrop information from Abakaliki and Anambra were used in this study to characterize the source and reservoir rocks in the two basins in order to give indication(s) for hydrocarbon generation potential in the basins to minimize uncertainty and risk that are allied with exploration and field development of oil and gas. Outrop mapping method was used to carry out geological, stratigraphical, geochemical, structural, petrographical, and sedimentological studies of rock units from outcrop sections within the two basins. Thirty-eight samples of shale were collected fromthese Basins in stratified mode of random sampling, and geochemical analysis (rockeval) was performed on the samples to determine the total organic content (TOC) and to assess the oil generating window. The results were analyzed, to properly characterize the potential source rock(s) and reservoir rock(s) in the basins, and factor(s) that can favour hydrocarbon traps. The results of the geological, stratigraphical, sedimentological, geochemical, and structural mapping were used to develop a new model for hydrocarbon generation in the Basins. The result of the geochemical analysis of shale samples from the Anambra Basin shows that the TOC values are ≥𝟏𝒘𝒕%, Tmax ≥𝟒𝟑𝟏℃, Vitrinite reflectance values are ≥𝟎.𝟔%, and S1+S2 values are >𝟐.𝟓𝒎𝒈/𝒈 for Mamu Formation while shale samples from other formations within Anambra Basin fall out of these ranges. The shale unit in the Mamu Formation is the major source rock for oil generation in the Anambra Basin while others have potential for gas generation with very little oil generation. The shale samples from Abakaliki Basin show that S 1 +S 2 values range from <𝟏−𝟐𝟎𝒎𝒈/𝒈, TOC values range from 0.31-4.55wt%, vitrinite reflectance ranges from 0.41-1.24% and Tmax ranges from 𝟒𝟐𝟑℃−𝟒𝟔𝟔℃. This result also shows that there is no source rock for oil generation in Abakaliki Basin; it is either gas or graphite. Keywords: Source rock, kerogen type, Abakaliki Basin, Anambra Basin, Oil generating window