Study On Oil Source,Maturity And Charge Episodes Of Marine Crude Oil In Cratonic Region Of Tarim Basin

Tarim Basin is rich in oil and gas resources and has been a key area for oil and gas exploration and research for a long time.Tarim Basin has experienced multistage tectonic movement and multistage accumulation process,and the distribution of oil and gas resources in the basin is complex.With the deepening of exploration and development in Tarim Basin,oil and gas exploration has gradually developed from shallow to deep or ultra deep stratum,and has achieved remarkable achievement.The basin still has great potential for oil and gas exploration.Tazhong and Tabei area are important oil producing areas in Tarim Basin.This study carried out oil source study on crude oil samples and source rock samples in Tazhong and Tabei area in the Tarim Basin,to clarify the characteristics of biomarkers, adamantanes and carbon isotope distribution of crude oil samples and source rock samples,and then carried out oil source correlation to reveal the process of oil-gas mixed charging.In this study,the n-alkanes and isoprene alkanes in 138 crude oil samples without biodegradation and 40 source rock samples in the basin were analyzed,and the following conclusions were obtained:(1)the biological sources of crude oil samples in this study were low-grade aquatic organisms such as bacteria and algae;(2)most of the source rock samples in this study suffered from different degrees of biodegradation process.The biogenic sources of source rock samples are lower aquatic organisms such as bacteria and algae,which are typical marine algae sedimentary types;(3)the sedimentary environment of the shallow drilling source rock samples of Shiairike profile,the source rock samples of Shiairike profile and the source rock samples of Yuertusi formation of Sugaitebulake profile are relatively reduced.Based on the analysis of steranes and terpanes in 138 crude oil samples without biodegradation and 40 source rock samples in the basin,the following conclusions are obtained:(1)the distribution characteristics of C₂₇ steranes>C₂₈ steranes<C₂₉steranes are consistent with those of typical marine crude oils in the basin;(2)crude oil samples in Tabei area and most of the crude oil samples in Tazhong area show the typical biomarker distribution characteristics of Ordovician crude oil,and a small part of crude oil samples in Tazhong area show the typical biomarker distribution characteristics of Cambrian crude oil;(3)partly crude oil samples in Tazhong area has high C₃₀ rearranged hopane/C₃₀ hopane ratio;(4)the 40 source rock samples in this study are consistent with the biomarker composition of Cambrian to Lower Ordovician source rocks in Tarim Basin;(5)combined with the experimental result of adamantane isomerization index of crude oil samples.The results show that the maturity of most of the crude oil samples in Tazhong area is higher than that of most of the crude oil samples in Tabei area,the crude oil samples in Tabei area also contain a small amount of high maturity crude oil samples;(6) according to crossplots of ratio of C₂₃ tricyclic terpane/(C₂₃ tricyclic terpane+C₃₀hopane)versus concentrations of steranes and terpanes,steranes and terpenes are divided into three groups:the first group includes pentacyclic triterpene such as Tm,C₂₉ hopane and C₃₀ hopane etc.,with the increase of maturity,the concentration of these compounds decreased rapidly and have relatively low thermal stability.The second group includes not only pentacyclic triterpanes such as Ts,C₂₉Ts and C₃₀ rearranged hopane but also different isomers of C₂₇ and C₂₉ regular sterane,with the increase of maturity,the concentration of these compounds decreased slowly and had moderate thermal stability.The third group includes C₂₇ rearranged sterane,C₂₁sterane and C₂₃ tricyclic terpane,with the increase of maturity,the concentration of these compounds first increased and then decreased,and the decreasing rate was lower than that of the first two kinds of groups,the thermal stability of these compounds was relatively high.Among the commonly used maturity indexes of steranes and terpanes,the ratios of Ts/(Ts+Tm),C₂₉Ts/(C₂₉Ts+C₂₉hopane)and C₃₀ rearranged hopane/(C₃₀ rearranged hopane+C₃₀ hopane)are based on the difference of thermal stability between the first and the second class compounds.The ratio of C₂₇ rearranged sterane/(C₂₇ rearranged sterane+C₂₇ regular sterane)and C₂₁/(C₂₁+?C₂₉)sterane is based on the difference of thermal stability between the second and the third class compounds.The ratio of C₂₃ tricyclic terpane/(C₂₃ tricyclic terpane+C₃₀ hopane)is based on the difference of thermal stability between the first group and the third group,which has better application effect.According to the distribution characteristics of carbon isotope of 138 crude oil samples,it is found that the variation range of carbon isotope of crude oil samples in Tazhong area is relatively wide,while that in Tabei area is relatively narrow,mainly because the sources of crude oil in Tazhong area are relatively diverse or the change of lithofacies of source rocks of crude oil in Tazhong area is relatively a lot;the sources of crude oil in Tabei area are relatively simplex or the source rock facies of crude oil in Tabei area is relatively homogeneous.According to the experimental results of carbon isotope of 40 source rocks and carbon isotope of 34 kerogen samples,there are some differences in sedimentary environments of the four groups of source rock samples.There is little difference in biogenic origin among the source rock samples of shallow drilling in Shiairike profile,Shiairike profile and Yuertusi formation in Sugaitebulake profile.According to the comprehensive study of steranes,terpanes,adamantanes and physical properties of crude oil samples in the study area,this study shows that there are four episodes of oil and gas charging process in Tazhong area:the first episode is the crude oil generate during the peak period(oil generation window);The second episode is the composition of crude oil produced in the late stage of oil generation window.The third episode is the hydrocarbon component generated in the wet gas stage.The fourth episode is mainly charged with high mature natural gas components.The oil reservoirs in Tabei area mainly experienced the first and second episodes of oil and gas charge,and no crude oil component was found generated in the wet gas stage.The fourth episode of high mature natural gas charge had little effect on the crude oil in Tabei area.The forth oil and gas charge is very important in Tazhong area.Adamantane compounds are mainly injected into the reservoir along with the charge of natural gas in the late episode.The natural gas directly generated by high over mature source rock or cracked crude oil carries high content of adamantane into the reservoir.The content and maturity of adamantane have no correlation with the maturity of crude oil in the reservoir...