| YitongBasin is a Cenozoic graben basin, located between Changchun and Jilin in Jilin province, which distributes along the northeast direction at 45 ° ~ 55 °. Its south boundary is Liaohe Fault, and the north boundary is Second Songhua River Fault. It belongs to southern part of Jiayi Graben, the north extension of Tan-Lu Fault Zone. The north- south length and north-east widthis160 km and 12-20 km respectively, covering an area of 2200km2.According to the basal properties and geological features of cover regional, Yitong basin can be divided into four second-order tectonic units, Moliqing, Luxiang,ChaluRiver FaultSag and YidanUplift.YitongBasinis filled with Cretaceous,Paleogene and Neogene and Quaternary strata, and the surface is covered by Quaternary sediments.Neogene, Cretaceous and Jurassic outcrops are scattered at the edge of the basin.The interior of the basin is mainly filled withPaleogene, which includes Shuangyang, Shechiling, Yongji,Wanchang andQijiaformation.The studiedlayer section is ShuangyangformationofMoliqing Fault Sagin Yitong basin, in which dark mudstone is widely developed. Using core, well logging and seismic data, together with rock-mineral determination and grading analysis, three sedimentary facies are identified in Shuangyang formation the study area, which are fan delta, lacustrine facies and lacustrine subaqueous fan, and can be further divided into 6 sub-facies and 14 micro-facies. Fan delta developed in every phase when Shuangyang formation was deposited, mostly distributed in the northwest and northeast of Moliqing Fault Sag. Lacustrine facies developed shore-shallow lake and deep lake sub-facies, mostly distributed in the middle of the basin, a minority was developedin edges of the basin. The subaqueous gravitational sediment discovered in the study area is predominant in Shuangyang formation, which is applied in the exploration of Jilin oilfield and proved to be beneficial.Structure of Moliqing Fault Sag is complex. This research made systematic stratigraphiccorrelation of the five sand groups in Shuangyang formation, based on former drilling data. The statistics demonstrate that sand group II in Shuang secondmember has the largest average thickness, with the maximum value of 175 m and minimum of 9.5m.By studying the mudstone characteristics of Shuang second member in Moliqing Fault Sag, and analyzingmudstone hydrocarbon generation conditions, to provide theoretical foundations for identification of hydrocarbon sources in the study area, this research evaluated the source rock characteristics in organic matter abundance, type and maturity. The criteria for abundance evaluation are residual organic carbon content(TOC, %), chloroform asphalt "A"(EOM, %), total hydrocarbon content(HC, %) and rock pyrolysis hydrocarbon potential(S1+S2). Organic carbon content in 25 wells ranges from 0.13%~6.66%, and 1.43% on average. Besides, 40% of mudstones have chloroform asphalt "A" values larger than 0.12%, belonging to the medium-fine source rock. Kerogen in Shuangyang formation is mainly type III, only with a small portion of type II. A large scale of mudstones with vitrinite reflectance(Ro) value higher than 0.5% are developed in deep buried Shuang second member, while those that buried shallow spread in a small area. Compared to other formations, the source rock in Shuangyang formation is better than that in other formations.In order to establish the distribution characteristics of effective hydrocarbon source rocks in the study area, on the basis of plenty mudstone datainYitong basin, distribution of effective source rockin Shuang second memberis summerized.Conclusions draw from the distribution map is that low-mature good source rocks and mature good source rocksare widely developed inShuang second member,followed by muture medium source rocks and low-muture medium source rocks.Among them, low-mature good source rock is the most stable kind in thestudy area, andmature good source rocks distribute in the middle and northeast of the study area. |
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