Organic geochemical characteristics of the pre-uplift Awgu Shale at Okpa River: implication for the petroleum potential of the Anambra Basin, Nigeria

Abstract

The sedimentary pile in the Anambra Basin, Nigeria, was uplifted in the Santonian, and over 2000 m of sediments were reported to have been eroded from the fold-belt before the renewed phase of subsidence and burial. Source-rock characteristics of the top-section of the Awgu Shale, the youngest pre-uplift Formation, exposed at the foot of the Awgu Esarpment along the channel of the Okpa River at Ogugu Village show low petroleum prospects. The section has marginal organic contents: 0.5-1.0% TOC, 17–80 ppm hydrocarbon content, and 1.0 kgt −1 genetic potential. The organofacies, which was rhythmically deposited in a non-anoxic inner-shelf, is dominated by terrestrially-derived organic matter, with a low but persistent admixtures of marine autochthonous organic matter. The current top-section of the Awgu Shale was immature at the onset of the uplift; the more basinal sections entered the oil-window only during the post-uplift reburial and should have generated and expelled mainly gas into the pre- or post-Santonian reservoir rocks or both.