Geochemical Characterization and Correlation of Crude Oils and Potential Source Rocks From the Semliki, Southern Lake Albert and Kaisotonya Basins in the Albertine Graben, Uganda

Abstract

The Albertine Graben, located in the western arm of the East African Rift System is one of the most petroliferous areas in East Africa. Despite its massive commercial quantities of hydrocarbons however, the source(s) of the oils in the graben remain poorly understood. In this study therefore, we have attempted to establish the source of hydrocarbons by undertaking a geochemical characterization and correlation of potential source rocks and crude oils from oil fields and discovery areas in the graben. The study involved evaluation of organic carbon contents, Rock-Eval pyrolysis, assessment of oils and solvent extracts by gas chromatography, biological marker investigations using GC-MS for aliphatic hydrocarbons, and GC-MS analysis of aromatic hydrocarbons. Bulk geochemical analysis of potential source rocks from discovery areas show that they are mature oil-prone source rocks with good potential to generate oil. Source rocks from the sampled wells in the oil fields display very low TOC, S2 and production potential with Tmax and hydrogen index in the range 428–255 0C and 88.2–414.3 (mg HC/gTOC) respectively, consistent with mature oil-prone source rocks with limited or no potential to generate oil. The biomarker characteristics of aliphatic hydrocarbons, including n-alkane and sterane distributions, pr/ph ratios, hopane distribution and isomerisation ratios, alongside GC-MS data from aromatic hydrocarbons show that the oils from the investigated oilfields are from the same family and were derived from a mature mixed algal/terrigenous source rock deposited under anoxic to sub-oxic conditions. Similar biomarker characteristics for source rocks from the investigated oilfields and discovery areas show that the source rocks from the oilfields are mature and of algal source facies deposited under anoxic to sub-oxic conditions while source rocks from the discovery areas are mature and of predominantly terrestrial source facies deposited under oxic conditions. Comparison of biomarker distributions in the oils and source rocks show no correlation between the oils and source rocks therefore, none of the investigated source rocks was responsible for the expulsion of the oils. Keywords: Organic matter, biomarkers, source facies, steranes, terpanes, maturity, oil-oil and oil-source rock correlation, Albertine Graben.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015