The complexity, secondary geochemical process, genetic mechanism and distribution prediction of deep marine oil and gas in the Tarim Basin, China

Most Paleozoic marine craton basins in China are rich in natural gas except the Tarim Basin, where both oil and gas are prolific. Various hydrocarbon types have been found in this basin, including deep ultra-heavy oil, heavy oil, normal oil, volatile oil, condensate and natural gas. Petroleums with diverse types even coexist within a single reservoir. Generally, the Ordovician and Cambrian reservoirs are deeply buried (5500-8000 m) and intensively altered after initial oil and gas accumulations, bringing challenges to physical property prediction before drilling and evaluation of exploration targets. Based on recent exploration discoveries and experimental data, we systematically analyzed the origin of oil and gas, demonstrated the mechanisms and processes of secondary geochemical alteration (e.g. biodegradation, gas washing fractionation, TSR and high-temperature cracking). Moreover, the preservation mechanism of ultra-deep normal oil was also clarified, and the distribution patterns of oil and gas were predicted. Tarim Basin has been uplifted and shallowly buried in the early accumulation stage, then rapidly and deeply buried in the late stage, resulting in various geochemical alteration processes at different burial stages. Through the reconstruction of complex deep-strata oil and gas accumulation histories, the spatial distribution of deep petroleum types were predicted. This review has provided detailed assessment with case studies in the Tarim Basin, and shed light on the preservation mechanisms and the alteration processes of deep liquid petroleums. In addition to massive gas accumulations, liquid oils still have great exploration potential in strata deeper than 8000 m in the Tarim Basin.