Study On The Origin And Accumulation Of Deep Oil And Gas In Lishu Fault Depression In Southern Song-Liao Basin

The hydrocarbon resources of Lishu fault depression are rich, but compared with other faulted basins in Songliao basin, the research on oil and gas origin and accumulation is relatively weak. By taking source rocks, oils and gases from deep strata in Lishu fault depression as research objects and integrating utilization of modern organic geochemistry analysis methods and basin model etc technologies, this paper made a further discussion on oil and gas origin on the basis of source rocks evaluation and the geochemical study on source rocks, crude oils and natural gases, through systematical analysis on heterogeneity in reservior hydrocabon and variation characteristics of absolute concentrations and relative ratios of nitrogen compouns and biomarkers especially maturity parameters, put forward the hydrocarbon migration direction and pathway and accumulation mode of Lishu fault depression; carried out study of the burial history of representative single wells in different tectonic regions, meanwhile combining detailed study of fluid inclusions in reserviors and the characteristics of geological structure and petroleum transportation system, explicated the formation stages and accumulation time of hydrocarbon reservoirs in Lishu fault depression, the research result provides a scientific theory basis for exploration of deep reservoirs in Lishu fault depression.The spatial extension characteristcs of source rocks is pointed out in this paper based on systematical evaluation of source rocks from different horizons and regions in Lishu fault depression. The source rocks from Yingchang and Shahezi formations have large thickness and high abundance of organic matter, and Denglouku formation followed by. The major organic matter types of source rocks are Ⅱ2-Ⅲ, the organic matter type of the source rocks in the sag zone is better than that of the slope zone, most source rocks are in mature-high mature stage. In horizontal direction, the thickness of the source rocks in Sangshutai sag is the most thick, and the organic matter content is high, from SW to NE, the thickness and the organic matter content of source rocks decrease gradually. In vertically, the organic matter content range of Yingcheng formation is from0.02%to12.01%, the most thick source rocks are in Gujiazi area, the thickness is731m, the thickness of source rocks in Shuanglong area is200m, good-high quality source rocks are distributed in Gujiazi-Shiwu-Houwujiahu region, and some middle-good quality source rocks are deposited in Sujiatun and Shuanglong areas, Bawu area mainly exists middle quality source rocks, but Qinjiatun area and the northern slope zone basically have no source rocks; the organic matter content of source rocks from Shahezi formation is between0.08%and7.42%, the deposited thickness of source rocks in southwest sag is900m, and the thickness in Shuanglong area is200m, Pijia-Shiwu-Gujiazi-Houwujiahu region exists goog-high qulity source rocks, and some good quality source rocks are deposited in Qinjiatun, Shuanglong and Sujiatun areas; the thickness of source rocks from Denglouku formation in the sag is up to500m, the TOC is3.8%～5.5%, and the thickness of source rocks from Denglouku formation in Shuanglong area is300m, and the mean TOC is2.02%.Based on the geochemical characteristics of biomarkers, source rocks of Lishu fault depression can be divided into three types:the Ⅰ kind of source rocks are from semi-deep to deep lake under strong reduction condition and brackish to semi-saline water, and aquatic organisms and terrigenous higher plants are the major source of organic matter, based on the following characteristics:biomodel state of n-alkanes, the ratio of tricyclic terpanes/hopane values from0.29to1.24, C24tetracyclic diterpane/C26tricyclic terpane ratios between0.35and0.94, high ratios of C27/C29ααα20R sterane (0.58～1.18), low Pr/Ph ratios (0.29～0.70) and gammacerane/C30hopane values between0.19and0.40; the Ⅱ kind of source rocks are believed to originate from semi-saline to saline water under reduction condition and terrigenous higher plants are the major organic matter origin on the basis of following features of biomarkers, n-alkanes are distributed in unimodality, tricyclic terpanes/hopane values are0.75～1.97, C24tetracyclic diterpane/C26tricyclic terpane values are low, between0.24and0.29, C27/C29aaa20R sterane values are0.35～0.50, Pr/Ph ratios are0.88～0.95, gammacerane/C3o hopane values are high, from1.14to2.77; the III kind of source rocks derived from shore-shallow lacustrine with fresh to brankish water and weak reducing to weak oxidizing environment, terrigenous higher plants are the dominated original source:n-alkanes appear a postpeak model and odd carbon number predominace in higher carbon number, tricyclic terpanes/hopane values are low, the range is from0.08to0.41, C24tetracyclic diterpane/C26tricyclic terpane values are high (0.98-5.34), C27/C29aaa20R sterane values are low, between0.07and0.47, Pr/Ph ratios are from1.03to4.27, and gammacerane/C3o hopane values are0.06～0.27.Crude oils from Lishu fault depression can be divided into two origins and three types:the I kind of crude oils are derived from strong reduction environment with brackish-saline water, and terrigenous higher plants are the major source of organic matter:tricyclic terpanes/hopane values range is from0.07to0.42, C24tetracyclic diterpane/C26tricyclic terpane values are0.41～1.29, and the abundance of C28and C29tricyclic terpanes is low, C29steranes are the dominant distribution in steranes, gammacerane/C3o hopane values are0.16～0.60, the abundance of C3odiahopane,C29TS and C29norhopanes increase gradually, and the relative content of Ts is less than Tm; the II kind of crude oils are originated from saline water under reduction condition, and the original source is mainly terrigenous higher plants:tricyclic terpanes/hopane values are high, between0.43and4.17, C24tetracyclic diterpane/C26tricyclic terpane values are low, from0.10to0.29, C28and C29tricyclic terpanes are abundant, C29regular steranes content is high, gammacerane/C3o hopane values are from0.42to1.10, and the abundance of C3odiahopane,C29TS and C29norhaopanes decrease gradually, Ts/Tm ratios are high, up to4.56to10.44; the Ⅲ kind of crude oils are the mixtures of Ⅰ and Ⅱ kinds of oils, the characteristics of biomarkers of the Ⅲ kind are between Ⅰ and Ⅱ kinds of oils.Detail oil and source correlation results show that the source rocks from the middle and lower part of Yingcheng formation in SN202and SN18wells and the Ⅰ kind of crude oils all possess the features of low abundance of C28and C29tricyclic terpanes, and similar abundance of Ts and Tm, and with relative low content of C3odiahopane, C29Ts and gammacerane; the Ⅱ kind of crude oils and source rocks from the middle-upper part of Shahezi formation in SN203well all have the characteristics of high abundance of C28and C29tricyclic terpanes, and high Ts, C29Ts,C30diahopane and gammacerane; and mixed oils are supplied from these two sets of source rocks.The natural gases from Lishu fault depression are organic origin gases with mixed-gases characteristics:①oil-associated gases, are mainly distributed in Pijia,Bawu etc oil and gas fields;②mixtures from gases of different maturity, the gses from Li2well in Sujiatun area and the gases from W162well in Siwujiazi area are mixtures of oil-associated gases and oil cracking gases;③the gases from Gujiazi,Xinli areas and HD20-18and HD26-17wells in Houwujiahu oil and gas field are mixtures of oil type gases and coal formed gases. The oil type gases are mainly originated from Yingcheng and Shahezi formations in the center of the depression, and the coal-bearing source rocks of Shahezi formation in the center of subsidence are the major source of the coal formed gases.In the horizontal direction, the major transportation direction of crude oils in Lishu fault depression is laterally migrated from SW to NE towards periphery slope zones, in the longitudinal direction, migrated vertically along faults:the I kind of crude oils are orignated from the middle or lower-middle part source rocks of Yingcheng formation in the fault terrace zone in the southwest region, migrated from SW to NE along faults, unconformity surfaces and continuous sand bodies towards the northeast region of the depression; the Ⅱ kind of crude oils derived from the middle-upper part source rocks of Shahezi formation in the deep sag in the southwest region, migrated towards northeast region.The formation of hydrocarbon accumulation in Lishu fault depression had three stages:Denglouku formation period, Quantou formation period and the end of Quantou formation to Nenjiang formation. The oil reservoirs of Delouku formation in Qinjiatun area formed mainly in the end of Quantou formation period, and the oil and gas reservoir of Yingcheng formation accumulated mainly in the end of Quantou formation, and the hydrocarbon reserviors of Shahezi formation formed mainly in the early stage of Quantou formation; the major accumulation stage of the oil reserviors of Yingcheng formation of Bawu area is in Quantou formation period, hydrocarbon reserviors of Shahezi formation accumulated in the end of Denglouku formation and the Quantou formation period; the reservoir forming stage of Yingcheng formation in the northern slope zone is mainly in Quantou formation period; and the hydrocarbon reserviors of Denglouku formation in Shuanglong area are mainly formed in the end of Quantou formation to Nenjiang formation period; the main accumulation stage of the oil and gas reserviors of Denglouku formation in Houwujiahu area is Quantou formation period, and reserviors of Yingcheng formation accumulated mainly in Denglouku formation period.