Conditions Of Petroleum Accumulation In Wushi-Shenmu Area Of Kuqa Depression

Based on gas chromatography-mass spectrometery of saturated hydrocarbon,grain fluorescence,fluid inclusion,casting lamella,high pressure mercury and nitrogen adsorption methods,the source of hydrocarbon,reservoir properties,petroleum accumulated period and process in the Wushi-Shenmu area of Kuqa subbasin were studied,and the main factors that controlling the hydrocarbon distribution were determined.The results showed that three source rocks developed in the Wushi depression,including Permo-Carboniferous,Triassic and Jurassic formations,while no effective source rocks existed in the Wushi-Shenmu area.The results of oil-source correlation showed that the hydrocarbon in the studied area mainly originated from Jurassic lacustrine source rocks in the northeast Takeli area.Both oil and source rock were characterized by high maturation,high content of diasteranes and diahopanes,showed the genetic relationship between the oil and Jurassic source rock.Hydrocarbon in the Wushi-Shenmu area mainly distributed in the Shushanhe formation of Cretaceous,in which the oil-gas-water correlation was complex,and the distribution of hydrocarbon was mainly controlled by sedimentary facies and lithologies.The sedimentary facies in the Shushanhe formation consisted of fan delta front,braided delta front and shallow lake subfacies,in which the reservoir properties differ greatly in different depositional environments,and the best occur in mouth bar and distributary channel microfacies of fan delta front subfacies and distributary channel microfacies of braided delta front facies.The lithologies in the reservoir were mainly coarse and medium sandstones.The oil saturation of the reservoirs were mainly controlled by two factors,one is the source-related faults that controllong hydrocarbon migration,and hydrocarbon mainly accumulated in the local high positions near faults;the other one is the physical properties of reservoirs,and hydrocarbon mainly distributed in areas with good reservoir properties.Condensate gas and oil reservoirs were distributed in the Wushi-Shenmu area,with the characteristics of long distance migration and accumulation.The accumulation process of condensate gas was complex,and existed two accumulation periods since 5 Ma.The key factor for the formation of condensate gas reservoirs was the weak tectonic deformation after condensate gas accumulated.The accumulation process of Shenmu-1 oil reservoir was relatively simple,and hydrocarbon mainly accumulated after 2Ma.The oil reservoir was damaged by later tectonic deformation.Comprehensive studies showed that the distribution of hydrocarbon was mainly controlled by three factors,(1)the abundant hydrocarbon generated by Jurassic source rocks was the material basis for hydrocarbon accumulation,(2)The perfect transport system make the oil and gas migrate into Wushi sag.