Evolution of a deeply-buried oil reservoir in the north Shuntuoguole Low Uplift, Tarim Basin, western China: Insights from molecular geochemistry and Re–Os geochronology

Abstract

The recently discovered Shunbei Oilfield is perceived as a remarkable breakthrough in deep marine carbonate petroleum exploration in the Tarim Basin, western China. Constraining the absolute timing of petroleum system evolution is crucial for further hydrocarbon exploration and development in the area. Here, we integrated molecular geochemical correlation, stable carbon isotopes of bulk oil (bitumen) and Re–Os geochronology to trace the petroleum sources, to constrain the absolute timing of hydrocarbon generation, and to reconstruct the petroleum system evolution of the deeply-buried oil accumulations in carbonate reservoirs. Biomarker parameters derived from isoprenoids, terpanes, steranes and aromatic compounds, as well as stable carbon isotopic compositions, revealed that the Ordovician reservoir oils and bitumens were all originated from the same source rock, deposited under a highly-reduced environment with organic matters being predominantly contributed by algae and bacteria. The calculated equivalent vitrinite reflectance (Rc) values of reservoir bitumen and oil samples indicate that the Shunbei Oilfield has experienced at least two hydrocarbon charges. Geochemical characteristics and thermal maturity suggest that the bitumen within the Ordovician Yijianfang Formation reservoirs in the north Shuntuoguole Low Uplift is not originated from thermal cracking, thermochemical sulfate reduction, biodegradation, nor gas washing. Re–Os isotopic data from six reservoir bitumen samples yield an isochron age of 399 ± 12 Ma with an initial 187Os/188Os ratio (Osi) of 1.26 ± 0.11. This age is interpreted to represent the timing of oil generation when compared with the thermal evolution of possible source rocks and other radiometric dating results in the region. A comparison of 187Os/188Os of the 399 Ma-old bitumen and that of possible source rocks at the time of oil generation suggests that the reservoir oil was possibly originated from the lower Cambrian Yuertusi source rock. This work highlights that Re–Os geochronology combining with the initial 187Os/188Os ratio can be used to effectively constrain the timing of hydrocarbon generation in petroleum systems with complex tectonic history, multiple oil charges and alterations.