Petroleum accumulation history of deeply buried carbonate reservoirs in the northern Tarim Basin, northwestern China

The Tabei uplift in the Tarim Basin is one of the deepest and most important petroleum-producing areas in China, with more than 3 billion t (21 billion bbl) of oil equivalent discovered in the Paleozoic carbonate reservoirs. Further petroleum exploration and development in the Tabei and neighboring areas will greatly benefit from an in-depth understanding of the hydrocarbon charge and accumulation history of these deeply buried carbonate reservoirs. The molecular correlation of reservoir oils indicates that oils from major accumulations in the area share similar geochemical characteristics and were presumably derived from the same source rocks deposited in a marine environment. The Shunbei reservoir oil has the highest thermal maturity, followed by the Yuecan reservoir oil, whereas the Tahe reservoir oil has the lowest thermal maturity. Six generations of calcite cementation spanning over 130 m.y. have been delineated in calcite veins, with U-Pb ages ranging from ca. 446 Ma to ca. 316 Ma. The second and fifth generations of calcite cementation were accompanied by oil charge events, as indicated by the occurrence of bitumen and primary oil inclusions. Fluid inclusion analysis coupled with basin modeling results reveal that the Tabei area experienced two major oil charges, with the first charge occurring during the late Caledonian Orogeny, at 426 to 415 Ma, and the second charge during the middle−late Hercynian Orogeny, at 339 to 278 Ma. The Shunbei and Yuecan reservoirs contain well-preserved oils accumulated during the two charge events, whereas the Tahe reservoir oil has been partially biodegraded.