Petroleum Source Rock Potential Assessment of the Oligocene - Miocene Ogwashi Asaba Formation, Southern Anambra Basin, Nigeria

Abstract

The Oligocene - Miocene Ogwashi Asaba Formation of the Anambra Basin is the outcropping equivalent of the Agbada Formation in the Niger Delta, subsurface. Exposed rocks of this formation consist of interbedded successions of lignite, shale, claystone, siltstone, sandstone, and conglomerate facies of a paralic environment. The present study has investigated the petroleum potentials of the outcrop samples exposed at Azagba Ogwashi, Oba and Ihioma areas from the organic petrologic and geochemical viewpoints. Maceral analysis revealed that the Azagba Ogwashi lignites have an average composition of huminite : liptinite : inertinite (H: L : I) ratio of 60:31:9, 79:18:3 for the Oba lignites, and 71:21:8 for the Ihioma lignites. Maceral constituent ratio in the shales range from 72:22:6 in Azagba Ogwashi, 83:7:10 for Oba, and 69:30:1 in the Ihioma area. Total Organic Carbon (TOC) of the lignite samples range from 13.28 to 68.04wt% (mean = 46.56 wt%) for twenty samples whereas those of the shales range from 4.98 to 12.38wt% (mean = 8.52wt%) for thirteen samples. Petroleum source potential (S1 + S2) for the lignites range from 13.05 – 460.63 kg HC/t (mean = 133.59 kg HC/t) and HI range from 53 to 639 mgHC/gTOC (mean = 234.85 mgHC/gTOC). In the shale samples, S1 + S2 range from 1.25 – 35.19 kg HC/t (mean = 14.59 kg HC/t) and HI range from 17 to 260 mgHC/gTOC (mean = 129.52 mgHC/gTOC). The Tmax values range from 394°C - 421°C (mean = 411°C) in the lignites while those of the shales range from 384 to 426°C (mean = 411°C). The relatively high proportion of liptinite (Type II kerogen) in the lignite especially in Azagba Ogwashi suggests its potential for oil generation, while the predominance of huminite (Type III kerogen) in Oba and Ihioma suggest their potential for gas generation in the deeper section of the basin where the appropriate thermal maturity would have been attained. The source rock facies investigated have good to excellent organic matter content and can therefore be considered as potential petroleum source rocks which upon thermal maturity will generate oil and gas. This conclusion suggests that equivalent lithologic units in the Agbada formation are candidate petroleum source rocks in the subsurface Niger Delta.