Hydrocarbon Accumulation and Main Controlling Factors of Ordovician Reservoir in Guchengxu Uplift, Tarim Basin

Guchengxu Uplift in which commercial oil and gas have been drilled out in Ordovician marine carbonate rocks in some wells in recent years has become one of the exploration focus areas in Tarim basin. Two series of Ordovician hydrocarbon-bearing reservoirs have b een confirmed to be present in Guchengxu Uplift, one is the limestone at the top of Ordovician Yijianfang Formation and the other is the limy dolostone of internal Ordovician Yingshan Formation. The hydrocarbon is present mostly in dry-gas state with normal pressure and high temperature in Ordovician reservoirs but exceptionally, light oil with low content of sulfur and wax which is characteristic of condensate oil are drilled out in the top of Yijianfang Formation in Well SN-1. Combined with the analysis of hydrocarbon property, biomarkers and hydrocarbon inclusion characteristics, it is suggested that two periods of hydrocarbon emplacement experienced in this uplift. Emplacing oil derived from the Cambrian to lower Ordovician source rocks in the first period of emplacement that occurred during the late Caledonian to Early Hercynian movements, and emplacing over-mature dry gas were dominated in the second period of emplacement that occurred during Himalayan movement. The following gas emplacement resulted in forming pure gas or condensate gas reservoirs. Different ranks of oil / gas shows are present in the drilled wells in Guchengxu Uplift, and specially all of oil and gas yielded are with no water. It is generally considered that high-quality carbonate reservoirs are the key for hydrocarbon accumulation and the developing NE-trending faults play an important role in hydrocarbon accumulation.