Origins and accumulation processes of the Ordovician gas condensate in the Tazhong area, Tarim Basin, northwest China

Abstract

Abundant gas condensates have been proven in the Ordovician carbonate reservoirs in the Tazhong area, the centre of the Tarim Basin, where complicated geological evolution and multiple hydrocarbon accumulations have occurred. Property, geochemistry, and stable carbon isotopes of the Ordovician condensate are characterized to identify the oil and gas origins in the Tazhong area. Fluid inclusion data, combined with numerical modelling methods was used to determine petroleum accumulation processes. Our results suggest that oils are characterized by mixed sources, with 64% of contributions from the Middle‐Upper Ordovician (O2+3) source rocks and 36% of contributions from the Lower‐Middle Cambrian (Є1+2) source rocks. Gases are primarily generated from the thermal cracking of pre‐existing oils in the underlying strata, with a small amount derived from kerogen cracking accompanied with oil generation. Three petroleum filling stages are determined, including the filling of the Є1+2‐derived oils during the Late Hercynian period, filling of the O2+3‐derived oils during the Yanshan period and oil‐cracking‐gas charge during the Himalayan period. The accumulation processes and relative contribution ratios of the two source rocks vary among the reservoirs and are mainly related to the transport system. Due to the lack of faults in the regions away from the No. 1 Fault Belt, the Є1+2‐derived oils are difficult to fill into the Ordovician reservoirs through the gypsolyte, and thus the accumulated oils are mainly from the O2+3 source rocks. The percentage of the O2+3‐derived oils is high in the southeast and northwest segments of the No. 1 Fault Belt, but relatively low in the middle segment and the vicinity of the No. 10 Structure Belt. Likewise, the late gas charge intensity is controlled by regionally varying conduit systems. Gas condensate formed in reservoirs with high gas/oil ratios. Otherwise, light oil retains with respect to low gas/oil ratios.