Geochemical Characteristics Of Oli And Gas In Eastern Wushi Depression

Wushi Sag is located in the northeast of Beibuwan Basin and is one of the main exploration areas in the basin.In this paper,the hydrocarbon geochemical characteristics of source rocks and crude oils from different Wells in Wushi Sag are analyzed by means of oil and gas geochemistry.By drawing burial history map and combining with inclusions homogenization temperature,the approximate accumulation periods of different Wells in Wushi sag were estimated.The source rocks in Wushi sag were evaluated by studying the types,abundance,maturity and the characteristics of organic matter.The results show that the main hydrocarbon generation strata in Wushi sag are located in the second member of Liushagang Formation,and the quality of source rocks is the highest,followed by the third member of Liushi Formation,which is medium to high quality source rock,and the first member of Liushi sag,which is poor,is medium source rock.The results show that the source material of source rocks in Wushi sag is mainly derived from terrestrial higher plants,but the large amount of 4-methylsterane indicates that the input of aquatic lower plants and algae also contributed to the formation of source rocks.The sedimentary environment is mainly weak reduction-oxidation environment,and the water salinity is low at the time of deposition,which is normal freshwater lake deposition.The comprehensive evaluation of crude oil in Wushi sag was carried out by studying the physical properties,family components and biomarker characteristics of crude oil.Research shows that the study area is normally more crude oil,sulfur content is low,colloid and asphaltene content is higher,mainly formed in the freshwater environment,for normal fresh water lake sediment,parent material source for terrestrial higher plant,aquatic algae also have great contribution to the formation of crude oil,crude oil and wushi sag are already in the mature stage.According to the amount of algae input from parent material,the crude oil in Wushi sag can be divided into three types.The first type of crude oil has less aquatic algae input,which is mainly located in the first section of WS23-X series well flow in the middle-east area of Wushi sag.Secondary crude oil aquatic algae input medium,mainly located in WS16-X,WS17-X series well flow second section in the eastern Wushi area;The aquatic algae input of the three types of crude oil is high,which is mainly located in the third member of WS17-X series well flow in the inversion structural zone.The results of oil source correlation show that the first type of oil is formed from the upper source rock of the second member of the Liuzhou Formation,and the second and third types of oil are formed from the lower source rock of the second member of the Liuzhou Formation.The faults and effective sand bodies in the Wushi sag constitute a complex and variable oil and gas transport system in time and space.The existence of these transport conductors makes the vertical and lateral migration of oil and gas in the Wushi sag exist simultaneously,which not only ensures the migration of oil and gas,but also controls the distribution of oil and gas.The reservoir forming periods of different types of crude oils in Wushi sag are studied and analyzed by means of inclusion temperature measurement.First,using Petromod basin simulation software,combined with the basic geological data of the study well location in Wushi Sag,the sedimentary and burial history map of the well was drawn.Then,combining with the homogenization temperature of the inclusions of the well,the hydrocarbon accumulation period of the well was analyzed.The study shows that the accumulation time of the first type of crude oil is about from Dengloujiao Formation to Wanglougang Formation,and the accumulation time is relatively late.The accumulation time of second type crude oil is from Xiayang Formation to Jiaowei Formation,and the accumulation time is relatively early.The accumulation time of the three types of crude oil is approximately from Jiaowei Formation to Dengloujiao Formation.