Organic geochemical heterogeneity of the Cretaceous Abu Gabra Formation and reassessment of oil sources in the Sufyan sub-basin, Sudan

Abstract

The Lower Cretaceous Abu Gabra Formation (AG) is considered to contain the major source rocks in the Sufyan sub-basin, Sudan. Although prior studies have established the presence of organic-rich lacustrine mudstones, the geochemical heterogeneity of organic facies, thermal maturity, and molecular nature of individual members of the Abu Gabra Formation are insufficiently characterized for oil-source rock correlation. Here, we report on geochemical analysis of 228 cutting samples utilizing Rock-Eval pyrolysis and TOC measurements, from which twenty samples were chosen for investigation of saturated biomarkers. The well cutting samples are from four members (AG1, AG12, AG22 and AG3) comprising the Abu Gabra Formation. These analyses were compared to eleven oils produced from the Abu Gabra sandstones to establish member-specific oil-source correlation. The AG22 and AG3 members have the greatest source potential, containing a higher content of oil-prone organic matter and more favorable maturity than those of the AG12 and AG1 members. Biomarker data for the AG22 and AG3 source rocks indicate that deposition occurred under a moderately stratified and somewhat saline water column with a predominance of algal/bacterial input (Type I kerogen). The AG22 and AG3 source rocks have entered the window of oil generation. In contrast, the AG1 and AG12 source rocks were possibly deposited in fresh to slightly saline water conditions, have a greater input of terrigenous organic matter input, and are immature to marginally mature. Oil-oil and oil-source rock correlations suggest that the crude oils in the Sufyan sub-basin are genetically homogenous, belong to a single oil family, and are mainly derived from the AG22 and AG3 source rocks. Contribution from the AG1 and AG12 source rocks is not apparent from the molecular distributions and is considered to be negligible. The current study provides valuable guidance of source rock evaluation and oil-source rock correlation, which can be a benefit to future petroleum exploration work in the Sufyan sub-basin.