Faulting controls on oil and gas composition in the Yingmai 2 Oilfield, Tarim Basin, NW China

Abstract

The accumulation mechanism of hydrocarbons in the Yingmai 2 Oilfield of the Tabei Uplift in the Tarim Basin is not well understood. Here, the characteristics and formation of Paleozoic faults in the Yingmai 2 Oilfield are studied with new 3D seismic data and regional geologic data. Inclusion features and geochemical parameters are used to investigate the faults controlling the oil and gas compositions. The results indicate that three fault systems developed in the Yingmai 2 Oilfield: a NNW-trending strike-slip fault system that formed in the middle Caledonian, a NW-trending strike-slip fault system that formed in the Caledonian–late Hercynian and a NE-trending thrust fault system that formed in the late Hercynian–Indosinian, which was associated with volcanic activity. The two strike-slip fault systems were important early hydrocarbon migration pathways in the late Caledonian–early Hercynian, late Hercynian, and Himalayan accumulation periods. The NE-trending thrust faults were sealed by volcanics, and divided the originally uniform oilfield into two accumulation systems during the Himalayan. This finding is supported by the differences in the compositions of the oils and gases in the eastern (YM area) and the western (YG area) Yingmai 2 Oilfield. The oil density, iC4/nC4, Ph/nC18, and V/Ni ratios of the oils in the reservoirs in the YM area are lower than those in the YG area, whereas the TMNr, TeMNr, 4-1/DBT, Pr/Ph, C29-diasterane/C29-regular sterane ratios, H2S content, and the content of Himalayan hydrocarbon inclusions are higher. The hydrocarbon accumulation in the Yingmai 2 Oilfield is attributed to multiple episodes of hydrocarbon charging and suggests that the YM area has greater exploration potential than the YG area.