Geological Features And Nature Gas Accumulation In Xujiaweizi Fault Depression, Songliao Basin

The generation, migration, accumulation and gas are associated with the formation, evolution and reformation of the basin, on which the prediction of resource prospect, distribution law and abundance zone depends mostly. In order to point out the exploration direction and the potential exploration targets it is attempted to combine the sedimentary basin analysis with the oil and gas geology study, and to apply the modern geological theory and the new exploration techniques to recognize the conditions of generation, migration and preservation of the oil and gas based on the basic geology study in this dissertation.So we selected Xujiaweizi depression with good data and slowly exploration pace as the object to study the stratum,sedimentary and pretroleum system and so on,and we have obtained the completely new knowledge about the geological features and the gas accumulations,as well as many qther creative achievements in the basin.Geological Features of Xujiaweizi Fault Depression There are 5 levels sequence boundaries recognized from the integrated seismic and well data below the Quantou Formation in Xujiaweizi fault depression.The lower Cretaceous are separated on the level of member and bed by 9 regional higher level sequence boundaries and 2 significant flooding surfaces, including Huoshiling Formation (Member 1and 2 recognized again), Shahezi Formation (Member 1and 2), Yingcheng Formation (Member 1,2,3,4 recognized again), Denglouku (Member 1,2,3,4),and Lower Quantou Formation (Member 1,2).The Yingcheng Formation is also divided into 4 members.And the intermediate volcanic rock bed is overlain the coal-bearing clastic rock bed in the Huoshiling Formation.The formations of Xujiaweizi fault depression are consisted of three sets of volcanic rocks and four sets of clastic rocks.The spatial distribution of the formations is complex in the depression.The Xujiaweizi fault depression is a halfgraben-like fault depression controlled by two boundary faults of Xuxi and Songxi, which extend to NNW and control the subsiding center in the depression, furthermore, to form the three depression centers: eastern Shengping, Xingshan and Bohetai. The turning point of the downchasm-dominated faults to the northeast, which resulted in the formation of the Songzhan low uplift and the Fengle low uplift to separate the subsiding center. The tectonic movements caused the formation of Shengping-Xingcheng fault extend fold belt, Yuxi fault bend fold and reverse structures in the late depositing periods of the Shahezi and the Member 1 of Yingcheng.The Xujiaweizi fault depression has been subjected to three tectonic evolution stages:the fault-subsiding stage whichis the Huoshiling Formation depositing period,the down-wraping stage which is the Shahezi Formation depositing period,the shrinking stage which is the Yingcheng Formation depositing period.The large scale expanding and faulting activities occur in the Early Cretaceous when the Shahezi Formation is depositing under the background of left-lateral strike-slip fault.The deep depression is obliquely located to NNE from the horizontal view which is constituted by the southern segment of the Songxi fault,the south of the northern segment of the Xuxi fault and the southern segment of the Xuxi fault.The two regional compressing and reforming tectonic movements make the initial tectonic framework of the expanding basin complex in the late depositing period of the Shahezi Formation and the late depositing period of the Member 1 of Yingcheng Formation.Xujiaweizi fault depression contains the formation bearing volcanic rocks and coals of the fault-subsiding stage and the formation bearing clastic rocks characterized by fluvial facies and lacustrine facies in the early down-wrapping stage.The conglomerates of the braided plain fan and braided delta are the strata characteristics of the faulting-subsiding stage,which are formed in the wet climate condition by the traction on the edge of the low-angle slope.But the sediments of various fluvial facies are more common in the str...