Characteristics Of Mesozoic Faults And Their Effects On Chang 8 Oil Reservoir In The Southern Part Of Tianhuan Depression,Ordos Basin

Fault structures control the process of hydrocarbon accumulation,so clarifying the characteristics of fault development is the key to promote the increase of oil storage and production.The southern part of Tianhuan Depression is located in the transition zone between strong deformation in the basin margin and weak deformation towards central basin,and the Meso-Cenozoic fault structures are very complex.In order to further clarify the laws of Ch8 oil accumulation in this area,characteristics of Mesozoic faults are investigated,types of faults and structural units are divided,and the evolution mechanism of faults since the Mesozoic has been analyzed in this paper,which are based on the investigation of geological backgrounds,and mainly focused on the combination of logging and high-precision 3D seismic.Combined with the analysis of typical well areas,the dynamic formation process of Ch8 reservoir is analyzed,and various configuration models among faults,source rocks and reservoirs are proposed.Based on the above,the geological significances of major tectonic movements since Mesozoic are discussed,the influences of different scale fault structures on oil accumulation are clarified,and the main controlling factors of enrichment and high yield of Ch8 reservoir are clearly pointed out.Through the combination of geological analysis and various 3D seismic interpretation techniques,it is clear that the Mesozoic fault system is developed in the southern part of Tianhuan Depression.In addition to a large number of high-angle fractures,three groups of NW,NEE and nearly EW faults are well developed,which are difficult to identify with linear extension in plane and steep occurrence in profile.Since the Mesozoic,the faults have successively undergone the cyclical transformation and influence of the Indosinian movement,the Yanshanian movement,and the Himalayan movement,forming zonal faults,trap-level faults and interstratified faults,which divided the study area into two secondary structural units:Zhenyuan slope zone in the north and Pingliang-Jingchuan steep slope zone in the south.Comprehensive geochemical analysis and basin simulation results,it is clear that the Ch8reservoir is mainly supplied by the overlying Ch7 semi-deep lacustrine oil shale,which began to generate hydrocarbons in the late Mid-Jurassic,and reached the peak of thermal evolution at the end of the Early Cretaceous.According to the statistics of logging lithologic data and the observation of drilling cores,it is clarified that the NE-SW braided river delta deposits are developed in the key oil-producing zone of Ch8₁.Although the sand body is produced in thin layers,its geometric connectivity is better.Summarizing the oil-bearing statistical results of four key well areas,the configuration types among the faults,source rocks and reservoirs are constructed,according to the relative relationship of fault combination style,fault distance and source-reservoir distance.By means of the fluorescence measurement of hydrocarbon inclusions,temperature measurement of hydrocarbon associated brine inclusions,and laser Raman spectroscopy analysis,it is concluded that the hydrocarbon charging of Ch8 began in the Late Jurassic.In succession,according to the corresponding relationship between physical property and oil bearing,the physical property condition of oil charging in Ch8₁ in is determined.Then,with the help of core and mineral thin section analysis and quantitative X-ray diffraction detection,it is found that Ch8₁ reservoir is mainly composed of debris-arkosic sandstone and feldspar lithic sandstone,with low porosity and low to ultra-low permeability,and is currently in the A period of middle diagenesis.Among them,the continuous compaction and multiple cementation are the key to the accelerated densification of the reservoir,for which the diagenesis effect simulation method is used to restore the sandstone palaeo-porosity.And finally,based on the matching analysis of structural evolution,reservoir diagenesis and crude oil charging,the formation process of Ch8 reservoir are summarized as follows:1)the formation of initial sedimentation,2)the formation of low abundance primary lithologic reservoirs in the early stage,3)the development of structure-lithologic complex reservoirs in the middle stage,and 4)the accumulation of dispersed secondary reservoirs with high abundance along the fault zone in the late stage,indicating the end of the Early Cretaceous is the key to the Ch8 reservoir.In conclusion,tectonic movements in different periods and fault structures of different scales have different effects on the formation of Yanchang Formation reservoirs.The Indosinian movement established the basic structural framework of the study area,the Mid-late Yanshanian movement determined the key of hydrocarbon accumulation,and the Himalayan movement determined the final distribution of the reservoir.The zonal faults control the sedimentary characteristics and structural framework of the study area.The trap-level faults affect the types of oil reservoirs and determine the effective hydrocarbon transportation.And the interstratified faults mainly improve the physical properties of the reservoirs.Only when the oil-rich faults,networked channel sandbodies and source rocks are effectively matched in space,can they jointly determine the enrichment location and production degree of the reservoir.As for Ch8₁ in the study area,the networked channel sandstone with sand ratio greater than 50%matches the medium-scale horst pattern,which makes it easier to enrich high-yield reservoir.