Major period and process of hydrocarbon accumulation of deep reservoirs — A case study of Tuoputai area in Tarim Basin, China

Abstract

Deep hydrocarbon reservoirs in China's Tarim Basin are characterized by multistage accumulation, but the major period and the process of hydrocarbon accumulation are still ambiguous. By integrating analyses of fluid inclusions, light hydrocarbon and diamondoid, and reservoir bitumen, we determined the contribution of oils charged in different periods and reconstructed the hydrocarbon accumulation process of Ordovician reservoirs in the Tuoputai area, which is significant to expand deep petroleum exploration in Tarim Basin. The results show that the abundance of yellow to yellowish-green (34.6%–42.1%) and bluish-green oil inclusions (42.1%–46.2%) is relatively high, followed by the blue oil inclusions (15.4%–23.1%). The Th values of the corresponding coexisting aqueous inclusions are mainly in 70–80 °C, 90–100 °C, and 120–130 °C, respectively. The hydrocarbon charging is mainly in the late Caledonian and the late Hercynian, followed by the Himalayan. However, oils charged in the late Caledonian experienced strong biodegradation with the tectonic uplift in the early Hercynian and were transformed into heavy oils and carbonaceous bitumen. The recharged oils have not been altered by biodegradation, thermal cracking, gas washing, and thermochemical sulfate reduction, the amount of oils charged in the Himalayan was relatively small. And the calculated maturity of oils, determined by vitrinite reflectance (Rc = 1.3%–1.5%), is similar to the thermal evolution degree of the Є1y source rock in late Hercynian (1.3%–1.6%) and far less than that in Himalayan (1.8%–2.0%). Large amounts of oils accumulated in the late Hercynian made a major contribution to the current reservoirs. Deep hydrocarbon reservoirs in Tuoputai area were mainly got accumulated when the formation was shallowly buried and then preserved with the deep burial.