Organic geochemical characterization and hydrocarbon potential of subsurface sediments from Anambra basin, southeastern Nigeria

Abstract

"Subsurface samples of Mamu Formation from Ezimo 1235 and Enugu 1327 wells, located within the Anambra Basin were examined for organic geochemical characterization and biomarker content in order to determine the lithofacies association, hydrocarbon potential as well as the palaeodepositional environment. The sediments are made up of siltstone, dark coal, highly fissile shale and mudstone. Selected samples, made up of shale and coal from both wells were subjected to Rock Eval and gas chromatography/mass spectrometry (GC-MS) analyses. Results of the Total Organic Carbon (TOC) ranges from 5.15 to 69.56wt%, Soluble Organic Matter (SOM) values ranges from 286 to 1700ppm while the Generative Potential (GP) ranges from 10.02 to 311.86kgHC/t. These values suggest that the source rocks are moderately to fairly rich in organic matter. Hydrogen Index (HI) values range from 95-381 mgHC/gTOC and indicate types II and III kerogen. Cross plots of HI against OI signify types II and III kerogen which also indicate oil and gas prone source rocks. Tmax values of 422-437oC and Bitumen Ratio of 6.34 to 101.1 mg ext/gTOC signify low maturation level for the source rock. Ratios of C29 hopane βα/ αβ, C30 βα/αβ, and 22S/22S+22R C32 hopane range from 0.32 to 1.18; 0.21 to 0.57; and 0.13 to 0.52 respectively, and suggest immature organic matter status. Carbon Preference Index (CPI) value ranges from 1.33 to 1.97 suggests immature source rock of terrestrial origin. High values obtained from the C24 tetracyclic/C24 tricyclic terpanes and the C19/ C20 tricyclic terpane ratios, (1.52-2.64) and (0.54- 1.54) respectively, also portray terrigenous organic matter contribution. The presence of C27 to C29 steranes and diasteranes, signify mixed sources (marine and terrigenous) with prospects to generate both oil and gas; however, the dominance of C29 over C28 and C27 amplify higher contribution of terrigenous materials. The terrigenous input is further amplified by the Pr/Ph ratio ranging from 5.44 to 7.81. Cross plots of Pr/nC17 against Ph/nC18 also denote terrigenous sourced organic matter deposited in oxidizing environment. The high ratio of C30/C29Ts, indicates source rocks deposited under suboxic conditions. It can be deduced that the sediments are immature, and were deposited in a suboxic, low Eh environment. They are potential source rocks which are moderately to fairly rich in organic input with substantial amount of terrestrial organic matter, having prospects to generate gas at appropriate maturation. "