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Analysis of secondary migration of hydrocarbons in the Ordovician Yingshan Formation in the Tazhong uplift, Tarim Basin, China


Understanding hydrocarbon migration mechanisms is one of the key scientific problems that should be solved for effective hydrocarbon exploration in the Ordovician of Tazhong area, Tarim Basin. According to the analysis of the geological setting and the reservoir distribution characteristics, the process and mechanisms of hydrocarbon migration in the Ordovician, Tazhong area, were investigated by integrating geologic, geophysical, and geochemical parameters. Parameters successfully analyzed included the logging reservoir properties, seismic attribute, production outputs, fluid properties, and maturity parameters of hydrocarbons. The results indicated that hydrocarbons were introduced into the Ordovician through a series of charging positions during three accumulation periods, that is, middle Caledonian, late Hercynian, and Himalayan. A total of eleven hydrocarbon charging positions were identified at the intersection zones of the northeast-trending faults and the northwest-trending flower strike faults. There are anomalies of all the parameters on the hydrocarbon charging positions. With increasing distance from the charging positions, the anomalies of oil density, wax content, dry gas coefficient, hydrogen sulfide contents, gas/oil ratio, well production, 4-/1-MDBT ratio of crude oil weakened, and carbon isotope compositions of natural gas mainly became lighter along northwest to southeast. The viewpoint is further supported by the evidence from the theory of main hydrocarbon pathway systems, the formation mechanisms of the fault intersection zones, and obvious improvement of the reservoir properties at the intersection zones. Further exploration activities in the Ordovician of the Tazhong area should be taken to the west area of the North Slope, and the intersection zones in tectonic up-dip direction should be emphasized.