Geochemical Evaluation Of The Hydrocarbon Potential Of Source Rocks In The Anza Basin

Abstract

Geochemical analysis of source rocks is a method used to estimate and understand the formation of hydrocarbons (if any) in the rock samples. Over time, technology applied has improved and it is now possible to quantitatively and qualitatively determine the percentage of organic content, their maturity or immaturity as well as the migration paths. This study involved the use of Total Organic Content (TOC) and Rock-Eval pyrolysis methods to measure organic content and thermal maturity of hydrocarbon present in the source rock respectively. Geochemical analysis of sub-surface Cretaceous sedimentary rocks was undertaken in order to understand oil and gas potentiality of source rocks in Anza basin, based on a case study on the four wells namely; Chalbi-3, Siruis-1, Ndovu-1 and Kaisut-1. Anza basin is located on a fault block within a Paleocene–Cretaceous rift basin. The methodological approach used involved evaluation of source rocks through petrographical study of lithological features and geochemical evaluation of source rocks. The physical interpretation of depositional environments from lithologies and sedimentary structures as seen from the lithologs helped in elucidating the characteristics of the source rocks. Additionally, petrographical data provided valuable information such as provenance and depositional environments. Well sections with higher shale-volume ratio were selected for geochemical screening methods which included, TOC and Rock-Eval pyrolysis of well-logs to determine the quantity of organic matter and quality (kerogen type), and the thermal maturity of potential source rocks. TOC determination was used as a screening tool to identify intervals of organic richness. Those samples with satisfactory source rock richness (≥ 0.5%) were evaluated further for their petroleum potential using petroleum indices (PI). Samples with TOC values greater than 0.5% for shales and 0.3% for carbonates were subjected to Rock-Eval pyrolysis to determine the level of thermal maturity of potential source rocks, migrated hydrocarbons and in-situ generated hydrocarbons present in sedimentary facies. The geochemical evaluation of rock cutting samples from the drilled wells’ sections of Anza Basin confirmed both oil and gas potential. Sirius-1 proved to be the most oil-rich well. Gas Chromatography and Mass Spectrometry (GCMS) were used to characterize the biomarker signatures and oil-oil correlation of Sirius-1 samples. The results from this research-related work provided further understanding of hydrocarbon potentiality in Anza basin’s four wells, namely; Chalbi-3, Siruis-1, Ndovu-1 and Kaisut-1. A predictive model was developed to further map the regional basin extension and thus exhaustively attempt to decipher the depths with high TOC and hydrocarbons’ potential in the Anza basin; thus triggering future researches on oil and gas exploration rationale.