Geochemical Characteristics Research Of Source Rocks From Median Tectonic Zone Of Lishu Depression In Songliao Basin

Lishu Depression is located in the southeast uplifted zone of the Songliao Basin,which is one of the most important oil and gas regions in the south of Songliao Basin,belonging to be an independent half-graben fault basin with an area of 2346km2.It develops mang sets of source rocks vertically,including Huoshiling Formation,Shahezi Formation,Yingcheng Formation,Denglouku Formation and Quantou Formation in the Early Cretaceous period.The geochemical characteristics of the Early Cretaceous source rocks from the median tectonic zone of Lishu Depression in Songliao Basin was analyzed using convertional organic geochemistry to reveal the depostional environment and organic matter source,and then further investigate the similarities and differences in biomarker compositions of source rocks from different intervals or areas.Combined with the geological setting of Lishu Depression in Songliao Basin,the conventional organic geochemistry methods are used to investigate the organic matter abundance,type and maturity of the source rocks from the median tectonic zone of Lishu Depression in Songliao Basin,and discuss the chanracteristics of saturated and aromatic hydrocarbons.It is to uncover the depostional eenvironment and organic matter sources in the study area.Some main achievements have been made in research as follow:1.The higher abundance of organic matter characterizes the majority samples in the study.These samples are mediunm or excellent source rocks.But,the source rocks in Denglouku Foramtion are probably worse source rocks,which have a characteristic of organic carbon at a range of 0.18-1.86%,hydrocarbon generation potential ranging fron 0.04-1.44 mg/g and chloroform bitumen "A" between 0.0281%and 0.3544%.Whereas the source rocks from Yingcheng Formation and Shahezi Formation are dominated by mediunm or good source rocks.The content of organic carbon and chloroform bitumen"A" in Yingcheng Formation and Shahezi Formation are both higher.Their organic carbon contents are respectively from 0.03%to 5.03%and 0.21%to 24.54%,while their chloroform bitumen "A" contents vary from 0.0171%to 2.3224%and 0.0253%to 2.9219%,respectively.The organic matter of source rocks in Denglouku Formation are probably composed of type ? kerogens,whereas the type ? and ? kerogcns predominate in the source rocks from Yingcheng Formation or Shahezi Formation.Furthermore,the source rocks in the study area have a Ro values ranging from 0.624%to 1.474%,indicating that the most of samples are at mature stage and only several samples are at lower mature or higher mature stages.2.There are some differences in the organic matter abundance of the source rocks from different regions.The source rocks from Houwujiahu region in the study area belong to the worse source rocks,whereas the source rocks in Yingcheng Formation from 1684.7-1701.5m interval of Bawu region and 1109-1333.80m interval from Shuanlong are medium to good source rocks,and the source rocks in Shahezi Formation from 2034.6-2455.00m interval of Bawu region are also medium to good source rocks.It suggest that the good to excellent source rocks are mainly developed in the 1684.7-1701.5m and 2034.6-2455.00m interval of Bawu and 1109-1333.80m interval of Shuanglong.3.The n-alkane in the study area exihibt a uimodal distribution maximuming at C17 and C23 w:ith neither odd nor even carbon predominance.The majority of samples have Pr/Ph values less than 1.0,only the Pr/Ph values in the source rocks from Taipngzhuang and Shuanglong areas in the northeast of study area are greater than 2.0.It suggests that the depistional environment,from southwest to northeast in the study area,is gradually changed from reduced to oxic depostional environemt.4.The compositons of termanes and steranes in most samples are generally similar.They are featured by the lower tricyclic terpanes relative to hopanes with a normal distribution maximuming at C21 or C23 tricyclic terpane,the relative abundance of C24 tetracyclic terpane comparable to that of C26 tricyclic terpane,a certain concentration of long-chain tricyclic terpanes,C27???R,C28???R and C29???R sterane displaying a distribution of asymmetric "V" type,and gammacerane/C30 hopane between 0.2 and 0.4.But the higher abundance of rearranged hopanes and gammacerane exist in some source rocks from Yingcheng Formation in Bawu area.The Ts/Tm values are probably more than 3.0 and gammacerane/C30 hopane values are greater than 0.8.For the source rocks in Suanglong and Taipingzhuang area,their biomarker compositions are different from the characteristics as mentioned above.They have a characteristic of abnormally lower amount of tricyclic terpanes displaying a decreasing distribution from C19 tricyclic terpane to C24 tricyclic terpane with abundant C24 tetracyclic terpane,unusually higher concentration of Tm and gammacerane/C30 hopane less than 0.08,as well as a distribution of C29>C27>C28 existing in the regular steranes.5.The relative proportion of phenanthrene compounds of aromatic hydrocarbons are the highest in study area,while the naphthalenes are in low concentrations.The contents of fluorene,dibenzofuran and dibenzothiophene are variable,the relative proportions of dibenzothiophene,dibenzofuran and fluorene range from 5.59%to62.59%,1.59%to 82.62%and 7.87%to78.78%,respectively.The most of samples in the study area are dominated by dibenzothiophene and fluorene,these samples are mainly deposited in the half deep-deep lacustrine facies.While the dibenzofur-an predominates in the source rocks from Shuanglong and Taipingzhuang area,indicating that these samples are deposited in shallow lacustrine facies.6.Base on the comprehensive evaluation and analysis on the Early Cretaceous source rocks from the median tectonic zone of Lishu Depression in Songliao Basin,it is induced that the source rocks from 1684.7-1701.5m and 2034.6-2455.00m interval of Bawu and 1109-1333.80m interval of Shuanglong are effective source rocks.There are significant differences in the biomarker composition and depostional environment of these source rocks.The abundant pristane and dibenzofuran characterize the source rocks in Yingcheng Formation from 1109-1333.80m interval of Shuanglong area,whereas the phytane predominates in the source rocks from Shahezi Formation in 2034.6-2455.00m interval of Bawu.The tricyclic terpanes in source rocks from Yingcheng Formation from 1109-1333.80m interval of Shuanglong display a generally decreasing distribution with increasing carbon number,whereas the the source rocks from Shahezi Formation in 2034.6-2455.00m interval of Bawu exhibit a normal distribution with maximum at C21 or C23.A predominance of C29 steranes and abundant C24 tetracyclic terpane are present in source rocks from Yingcheng Formation from 1109-1333.80m interval of Shuanglong,but an asymmetric distribution pattern of C27,C28 and C29 steranes exists in the the source rocks from Shahezi Formation in 2034.6-2455.00m interval of Bawu.These samples are respectively deposited in delta environment,shallow lacustrine environment and half deep-deep lacustrine environment.