Origin and differential accumulation of hydrocarbons in Cambrian sub-salt dolomite reservoirs in Zhongshen Area, Tarim Basin, NW China

Abstract

The origin and differential accumulation of hydrocarbons in the Cambrian sub-salt dolomite reservoirs in Zhongshen Area were studied based on comprehensive geochemical analysis of core samples, crude oil samples and natural gas samples. Mass spectrometric detection shows the core samples and crude oil samples are characterized by high C28 sterane content, low diasterane content, high gammacerane content and abundant aryl-Isoprenoids, and the associated gas has a low nitrogen content of 0.24%−4.02%, so it is inferred that the oil and gas are derived from Cambrian – Lower Ordovician source rock. The natural gas in the Middle Cambrian has a methane carbon isotope value of −51.4‰ − −44.7‰ and dryness coefficient of 0.65−0.78, representing associated gas, and the natural gas in the Lower Cambrian has a methane carbon isotope value of −41.4‰ − −40.6‰, and dryness coefficient of 0.99, representing cracking gas. The deep formations in the Tarim Basin contain cracking gas with high H2S content produced by thermo-chemical sulfate reduction (TSR). Due to the poorer reservoir properties and undeveloped fracture network system, the Middle Cambrian reservoirs have low charging degree of this kind of gas, so low H2S content (0.003 8%−0.200 0%); in contrast, with good reservoir properties and developed fracture network system, the Lower Cambrian reservoirs have a higher charging degree of this kind of gas, and thus high H2S content of 3.25%−8.20%. In summary, the oil and gas of Cambrian sub-salt dolomite reservoirs in Zhongshen Area are derived from Cambrian – Lower Ordovician source rock, and the differential accumulation of gas is the joint effect of reservoir physical property and development degree of fracture network system. Key words: Tarim Basin, Zhongshen Area, sub-salt dolomite, hydrocarbon origin, hydrocarbon differential accumulation