Controls by organic geochemistry trackers of secondary hydrocarbon accumulation during Neotectonic central uplift of the Tarim basin

Abstract Because of the Himalayan Neotectonic uplift, the Tarim Tectonic Block, a cratonic terrane which contain cover rocks of the Lower Paleozoic marine succession (a marginal basin containing huge petroleum paleo-reservoirs) experienced strong extrusion tendency. This extrusion was one of the most important factors to control the migration or accumulation of petroleum, and impact the pattern of paleo-reservoirs. In the Tarim basin, the Neotectonic activity tilted the Carboniferous and younger formation and activated old fractures. The inherited active faults of Tazhong area are mainly distributed in two big groups, respectively around the central uplift and the Tazhong-I faults. Tazhong-4 structure is combined with the two major fault zones, and is strongly affected by Neotectonics. According to the distribution characteristics of triaromatic dinosterane, 4-methyl-cholestanes and other special biomarkers in the oil samples of Tazhong-4 area, it was showed that oils in the different reservoirs were from the same source. And the vertical Pressure–Volume–Temperature (PVT) characteristics of the petroleum reservoirs changed from unsaturated oils into the unsaturated condensates. It was shown that the Tazhong-4 reservoirs had experienced a complicated accumulation process of multi-period charge, mixture and adjustment, which related closely with the multi-stage compressive Neotectonics affecting the Cenozoic sequences of the Tarim Basin and its lower paleozoic source rocks and paleo-reservoirs. The transport parameters of nitrogen compounds indicated the vertical migration of petroleum in the Tazhong-4 reservoirs. The paleozoic and current oil–water contacts (POWC & OWC) could be clearly separated by the analysis of the grains with oil inclusion (GOI). Based on the gas chromatography of oil–sand between POWC and OWC, biodegradation was not obvious. That reflects the formation of OWC and migration of POWC occurred much later. Recent activity of fault led to vertical migration of hydrocarbons and the OWC of paleo-reservoir ascended. In the central uplift of basin platform area, the multi-stratum hydrocarbons migrated and accumulated along the active faults, opening during Neotectonic activity.