Analysis Of The Sources And Origin Of Oil And Gas In The Shuixigou Group,taibei Sag,Turpan-hami Basin

The Turpan-Hami basin,located in eastern Xinjiang,is one of the three major oil-bearing basins in Xinjiang and has long been considered a typical basin for coal-derived oil and gas.Four sets of source rocks were developed,including the Middle Permian,Middle Upper Triassic,Middle Lower Jurassic Shuixigou Group,and Middle Jurassic Qiketai Formation,with the Middle Lower Jurassic Shuixigou Group coal-bearing source rocks being the most important source rocks in the basin.This set of source rocks can be divided into Badaowan Formation（J₁b）,Sangonghe Formation（J₁s）and Xishanyao Formation（J₂x）from bottom to top,which are mainly distributed in Taibei Depression.In this paper,we have studied the crude oil,source rock and natural gas of the Shuixigou Group in the Taibei Trench of the Turpan-Hami Basin.Based on the biomarker index,the geochemical properties of the crude oil and the source rock in the Taibei Sag were studied and the oil-source correlation was established.The geochemical properties of the gas were studied based on its composition,carbon and hydrogen isotopes,and light hydrocarbon composition to further determine the origin of the gas and compare it with the source rock.Studies have shown that there are two types of source rocks in the Shuixigou Group of the Taibei Depression:lacustrine mudstone and coal series source rocks.Coal series source rocks developed in freshwater and oxidized environments,and lacustrine mudstones were mostly deposited in brackish and reducing environments.The type of organic material is dominated by kerogen II₂-III,with kerogen I and II₁seen locally.In general,the source rocks are predominantly oil-gas co-existing and gas-tilted,and most of them are in the low-mature phase..Based on the characteristics of the saturated hydrocarbon biomarkers of crude oil,combined with analysis of organic matter sources and sedimentary environments,the genetic types of crude oil in Taibei Sag were classified into three types..The first type,crude oil,forms in the sedimentary environment of partially oxidized river swamps and lake swamps,with a dual parent-source contribution of terrestrial higher plants and aquatic algae;Type II crude oil is formed in the sedimentary environment of strongly oxidized river swamps and lake swamps,with organic matter input dominated by terrestrial,higher plants;Class III hydrocarbons are formed in the sedimentary environment of strongly oxidized river swamps and lacustrine formations,and have a dual parent-source contribution of terrestrial higher plants and aquatic algae,with lake hydrocarbons present in Xiaocaohu and Qiudong Sags.The results of the oil-source correlation show that the Shengbei-I type comes from fluvial-marsh coal-measure mudstone;Shengbei-II is derived from a partially oxidized coal series source rock;Qiudong-I type is derived from mudstone from river and marsh coal beds;Qiudong-II is derived from a typically coal-bearing source rock.Qiudong-III is derived from lacustrine mudstone and coal rock;Xiaocaohu-I is derived from lacustrine-fluvial hydrocarbon-bearing rocks;Xiaocaohu-II comes from coal-bearing mudstone in the fluvial phase.The gas in Taibei Sag is predominantly hydrocarbon gas,with the methane content mainly between 60%and 90%,and the average drying factor is less than 0.95,which is typical of moisture,with some samples having too low a drying factor.Both?¹³C₁and?¹³D₁are<-30‰and<-200‰,respectively,which are organic gas,mainly formed in terrestrial freshwater sedimentary environment.The gas is mainly coal-derived,with a small amount of oil-type gas,and its overall maturity is in the low maturity phase..The gas samples in the Qiudong Sump are relatively mature,and there is also a small amount of cracker gas in the Qiudong Sump and Hongtai Block..The gas source comparison shows that the coal-forming gas in the Shengbei sub-sag is mainly from the J₂x source rocks,and the oil-type gas is mainly from the J₂x mudstone and coal rocks,followed by the J₁b mudstone..The coal-generated gas in the Qiudong Sault is mainly from the J₂x source rock,and the oil-type gas is mainly from the J₁b mudstone..The coal-formed gas in Xiaocaohu Sub-sag mainly comes from J₂x mudstone,and the oil-type gas mainly comes from J₂x mudstone.