De-convoluting crude oil mixtures from Palaeozoic reservoirs in the Tabei Uplift, Tarim Basin, China

Abstract

Geochemical characteristics of 61 crude oil samples collected from Palaeozoic reservoirs in the Tarim Basin (60 from the Tabei Uplift and one from the Tazhong Uplift) were analyzed. The samples proved to be mixed oils of different maturity from diverse source rocks. Concentrations of 40 biomarkers and carbon isotopic compositions for the whole oils were analyzed by alternating least squares (ALS) regression to de-convolute the mixtures. Three endmember (EM) oils were identified. EM1 is the minimum contributor to the mixed oils, accounting for less than 10% of most oils. EM1 originated from Cambrian–Lower Ordovician source rocks in the early to peak oil window stage and experienced two phases of mixing and biodegradation. EM2 is the secondary contributor with proportions ranging from 10% to 40% in most oil samples. EM2 originated from Middle–Upper Ordovician source rocks at the early oil generation stage and underwent two phases of mixing and one stage biodegradation in the reservoirs. EM3 is the major contributor to most samples with proportions ranging from 13% to 95%. EM3 was generated from Middle–Upper Ordovician source rocks at the late oil generation stage and mixed with earlier emplaced mixtures in the reservoirs. The final mixtures that were not biodegraded are currently exploited from Palaeozoic reservoirs in the Tabei Uplift. Biomarkers in the crude oils reveal mixed characteristics, including evidence for two phases of oil charge and severe biodegradation.