Analysis Of Ordovician Geochemical Accumulation In Halahatang Xinken Block, Tarim Basin

With the development of carbonate reservoir in western China,the exploration in Halahatang depression of Tabei uplift had a breakthrough.Some blocks like Jinyue,Xinken in Halahatang depression have found oil successfully,which confirmed that the resource and exploration potential of ancient carbonate reservoir.However,there are still many questions such as the source of petroleum,the accumulation process,which had negative effects on the exploration in Xinken block and surrounding areas.This paper studied the Ordovician carbonate reservoir by geochemistry method,in order to identify the major source rock,the charging history as well as the main controlling factors,and to establish the typical hydrocarbon accumulation model.The crude oil from the Ordovician reservoir in the study area has the characteristics of low density,low wax,low sulfur.The C7 compounds show the dominance of n-heptane,and the isoheptane index suggests that the oil is at high maturity stage.The peak carbon number of light hydrocarbons in crude oils indicates the loss of light hydrocarbons.The baseline of GC and the detection of 25-norhopanoids compounds show that the reservoir has suffered from severe biodegradation.The similar fingerprint features in the middle molecular hydrocarbon indicates that the oil here has the same source.The biomarker characteristics show high abundance of long-chain tricyclic terpane,low gammacerane ratio,and the distribution of regular steranes has a "V" shape,besides,the ratio of diasteranes to steranes is high.These results indicate that few oils originated from the Middle-Upper Ordovician.The existence of aryl isoprenoids indicates a strong reducing environment.Above all,the oil in XinKin block mainly originated from the Cambrian source rocks.The major component of the natural gas is methane with the dry coefficient between 0.65 to 0.85.During the last period,the reservoirs suffered from varying degrees of gas-washing process,and the reservoirs with gas cap were formed.Meanwhile,the high sulfur content in northern Xinken block was due to bacteria sulfate reduction which related to biodegradation.Based on the the whole-hydrocarbon results,the hydrocarbon accumulation model was built.During the middle-late caledonian period,the source rock in Manjiaer depression in the southern part of the Halahatang depression reached the oil window and entered into the stage of massive oil generation.Therefore,Ordovician reservoirs were formed.The northern part of the study area was controlled by structural lifting leading to erosion so that the asphaltene was formed through biodegradation.Secondly,In late hercynian period,the oil distribution was changed by tectonic activities.During the himalayan period,the Cambrian condensate gas migrated upward along the fracture,and it formed the reservoir with gas cap.The thermal evolution history of source rock,multiple tectonic activities and secondary transformation are the controlling factors for petroleum accumulation of Ordovician reservoirs.