--> Formation Mechanism of Reforming Hydrocarbon Pools in the Ordovician Carbonate Reservoir of Tazhong Area, Tarim Basin

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Formation Mechanism of Reforming Hydrocarbon Pools in the Ordovician Carbonate Reservoir of Tazhong Area, Tarim Basin

Abstract

The Ordovician formation of Tazhong area in Tarim Basin are carbonate rocks, there were multi-sets of source rocks, multi-periods of hydrocarbon accumulation, suffered multi-periods of tectonic adjustment and destroy. All the discovered hydrocarbon pools were reformed pools, including three types, i.e. fracture, cave and fracture-cave type of hydrocarbon pools. Therefore, it is of important significance to study of formation mechanism of reformed hydrocarbon pools in the Ordovician carbonate reservoirs of Tazhong area for the oil and gas exploration of marine carbonate in the area. Firstly, the origin of hydrocarbons was analyzed by the methods of oil-source correlation and oil-oil correlation. Secondly, the characteristics of hydrocarbon generation were summarized and hydrocarbon reservoir formulation periods were determined by the forward model of hydrocarbon generation and expulsion. Thirdly, the reformation of hydrocarbon pools was analyzed considering the fault evolution in the area. The results indicate that: the carbon isotope of oil is between that of the Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks in Tazhong area, which proves the oil derived from the mixed sources of Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks; the analysis of dry coefficient and carbon isotope of methane, ethane and propane of gas shows the gas was dry gas from oil cracking; there were four periods of hydrocarbon accumulation, i.e. the early Caledonian, late Caledonian, late Hercynian-Indosinian and Yanshan-Himalayan, respectively; the formed hydrocarbon pools experienced three tectonic movements, i.e. early Ordovician, late Ordovician and Silurian-Devonian, respectively. The fault evolution history showed that: in the late of Early Ordovician period, the Tazhong No.1 fault was formed because of weak tectonic movement, only the reservoirs near the fault were adjusted; in Late Ordovician period, the tectonic movement enhanced, and Tazhong No.10 fault was formed; in Silurian-Devonian period, the northeast fault was formed and the entire Ordovician carbonate reservoirs were reformed. Based on the above analysis, the model of the four phases of hydrocarbon formulation and three phases of tectonic reformation was built of Ordovician carbonate reservoirs in the Tazhong area.