Organic geochemical characteristics of C28 R-sterane dominated Cretaceous Nkporo shales of the Anambra Basin: insights from organic petrology, biomarkers, and isotopes

Abstract

The present study investigates the Upper Cretaceous source rocks collected from well-x of the Anambra Basin in the southern Nigeria. Organic petrological analysis, bulk and molecular geochemistry, bulk carbon isotope, and compound specific isotope analysis of n-alkane (CSIA) were carried out with the aim of providing detailed information on the geochemical characteristics of the recently discovered crude oil and its potential source rocks, petroleum system, and a better understanding of the reasons behind abrupt seizure in the liquid hydrocarbon production from the ANAR X-oilfield of the basin. Based on petrographic and bulk geochemical results, the source rocks under study are primarily composed of type III organic matter and are in immature to marginally mature hydrocarbon generation stages. Molecular marker parameters also revealed that source rock deposition occurred in sub-oxic to oxic settings. While the major source input is the lacustrine algae in the source rocks, there were also minor contributions from terrigenous land plants. The studied source rocks are currently in the late oil to wet-gas generation windows based on basin modeling results. In addition, the previously postulated Upper Cretaceous (Mamu-Nkporo/Ajali) petroleum system (!) in the basin is supported by oil-source correlation. Two main reasons led to the abrupt seizure in liquid hydrocarbon production from the ANAR X-oilfield of the Anambra Basin: Higher thermal maturity attained by the producing source rock intervals, and the absence of oil producing liptinitic macerals. While vitrinites in the source rocks continue to produce gas, the liptinite macerals have been completely transformed to liquid hydrocarbons, and eventually to gas due to their instability at higher temperatures. In the Anambra Basin and other sedimentary basins in the southern Nigeria, the study will lower exploration risks and provide new exploration directions.