Geochemical characteristics of Ordovician crude oils in the FI17 strike-slip fault zone of the Fuman oilfield, Tarim basin: Implications for ultra-deep hydrocarbon accumulation in the Tarim basin

Abstract

The Ordovician crude oils in the Fuman Oilfield usually show varying physical properties and phase states along a single strike-slip fault. This study collected nineteen crude oil and gas condensate samples from the fault zones (FI17, FI19, and FI20) located in the eastern zone of the Fuman oilfield. Organic geochemistry methods were used to investigate the oil-source correlation, thermal maturity and secondary alternation processes of the samples, to reveal petroleum charge and accumulation models in ultra-deep reservoirs in the Fuman oilfield. The results showed that biomarkers of most crude oils have been modified by thermal stress, and the primitive geochemical information were distorted. Similar light hydrocarbon chromatogram characteristics and parameter ratios indicated that crude oils in the study area were generated from the same source rock. The carbon isotope of the whole oils (−31 ‰ to −33 ‰) and n-alkanes (−31 ‰ to −37 ‰) was comparable to the kerogen of the Lower Cambrian Yurtus Formation source rock (−30 ‰ to −37 ‰) considering both the carbon isotope fractionation between oils and kerogen and the effect of thermal maturity. The calculated equivalent vitrinite reflectance (%VRE) of oil and gas condensate based on adamantane, aromatic hydrocarbon and light hydrocarbon parameters ranged from 0.77 % to 1.44 %, suggesting a mature to highly mature hydrocarbon-generating stage. The lower dibenzothiophene content, 2-methylhexane/3-methylhexane ratios and relatively stable K1 values from light hydrocarbons revealed that the oil and gas condensates fluids were not subject to significant modification by thermochemical sulfate reduction (TSR). The calculated n-alkanes loss based on the whole oil gas chromatography ranging from 8 % to 26 %, suggested that the oil and gas condensate in the study area underwent relatively weak gas washing. Additionally, the concentrations of 3-methyldiamantane (3-MD) and 4-methyldiamantane (4-MD) in the samples ranged from 40.42 μg/g to 121.43 μg/g, showing realtively low oil cracking degree (<40 %) except for the gas condensates from the FI20 fault zone, whose oil cracking degree can reach 61 %. The comprehensive analyses in this study indicate that the differences in oil and gas physical property and phase state in the FI17 fault zone were primarily controlled by the vertical connectivity of the fault and the differences of crude oil charging periods. The hydrocarbon generation occurred during the early Caledonian period, followed by migration and accumulation in the Hercynian period, and subsequent adjustment and transformation in the Himalayan period. This study could provide significant exploration guidance for further ultra-deep oil and gas exploration in the Fuman oilfield.