Mesozoic-Cenozoic Tectonic Evolution Characteristics And Its Controlling Effects On Hydrocarbon Accumulation In Yanchi Area,Ordos Basin

Yanchi area is located in the middle of the western margin of Ordos basin,and its structural position belongs to the combination of western margin thrust belt and Tianhuan depression.The special tectonic location makes it undergo a complex tectonic evolution process,especially since Mesozoic-Cenozoic,it has suffered from multiple tectonic superposition,which makes the fault and fold deformation structure in this area more developed.while it is well known that there is good oil and gas exploration potential of Mesozoic Strata in Ordos Basin,the understanding of Mesozoic oil and gas enrichment and high yield law in the study area is still unclear due to the complex tectonic evolution background of basin margin,which seriously restricts the procedure of oil and gas exploration in this area.Therefore,this paper takes the main oil-bearing strata of Mesozoic in Yanchi area as the research object,This paper focuses on the main oil-bearing strata of Mesozoic in Yanchi area,aiming to elucidate the tectonic evolution influences on the features of hydrocarbon accumulation,which will not only deepen the studying of tectonic evolutionary processes in Yanchi area but also provide a certain geological basis for the subsequent oil and gas exploration in this area.Based on a large number of domestic and foreign related literature and the predecessors’research results,we present the data of fission-track thermochronology,methods of sedimentology,geotectonics and equilibrium profile,together with data from seismic,drilling,logging and field geological section,to research into the tectonic characters,main events and evolutionary processes of Yanchi area in Ordos basin.According to the methods of rock pyrolysis,thin section observation,physical property analysis,high pressure mercury injection analysis,field emission-scanning electron microscopy（FE-SEM）,energy spectrum analysis,cathodoluminescence,X-ray diffraction,carbon and oxygen isotope analysis,fluid inclusion test and fluorescence microscope observation,we studied on the featuress of key hydrocarbon source rock,reservoir,reservoir-caprock combination,trap,migration system and hydrocarbon accumulation.This paper comprehensively analyzes the controlling effect of Mesozoic-Cenozoic tectonic evolution on the static elements and dynamic accumulation process of Mesozoic hydrocarbon accumulation in this area from two aspects of oil and gas geological elements and accumulation process,establishes a pool-forming evolution model of coupling five major tectonic events in the evolution process with the hydrocarbon accumulation process,and sums up the main controlling factors of oil and gas accumulation,reservoir development types and distribution laws.The main studied results and understanding are as follows:（1）There are four stages of faults in Yanchi area,and their formation time is mainly distributed from the late Triassic to early Jurassic and from the late Jurassic to early Cretaceous.The structural styles of faults are mainly"Y"-shaped,imbricate,hedging and graben.Major high-angle structural fractures are more developed,and most of them are open.Low-amplitude structures are mainly anticlines,successive nose-structures and fault-related folds.Their vertical distribution is inherited,and their plane distribution is directional.（2）For the first time,the time limits of five major tectonic events in the Mesozoic and Cenozoic in Yanchi area are determined,namely,Late Trassic-Early Jurassic（from230 Ma to 190Ma）,Late Jurassic-Early Cretaceous（from 165Ma to 138Ma）,late Cretaceous-Early Paleocene（from 98Ma to 63Ma）,Eocene（from 50Ma to 39Ma）and Miocene（from 23 to 10 Ma）.（3）The Mesozoic source rocks in the Yanchi area are mainly mud shale in Chang7 Member with theⅡ₁kerogen type and the high degree of thermal evolution.The main types of reservoir rocks are lithic feldspathic sandstone and feldspathic lithic sandstone,which are mostly in the middle diagenesis B stage.The types of reservoir space are mainly intergranular pores and intragranular dissolved pores.The reservoir property is poor as a whole,mainly developing middle-low porosity-low permeability reservoirs and ultra-low porosity-ultra-low permeability reservoirs,which form multiple sets of reservoir-cap assemblages with lake-delta mudstone and swamp coal measures.Hydrocarbon migrate along various conductors such as unconformities,faults and permeable sandstones into fault block traps,micro-structural traps and lithologic traps.（4）Different tectonic events have different controlling effects on Mesozoic oil and gas accumulation.The tectonic uplift event from the end of Late Triassic to the beginning of Early Jurassic slowed down the thermal evolution process of Chang-7source rock,and developed the unconformity surface of the pre-Jurassic region,which laid the foundation for the lateral migration of oil and gas in the later period.The intense compression and thrust nappe tectonic activity in the late Jurassic-early Cretaceous resulted in the development of a kind of high-angle fracture system and the formation of a good configuration relationship with the micro-amplitude structure.At the same time,influenced by the"tectonic thermal event",the Chang 7 source rock generated a large amount of hydrocarbon and filled it into reservoirs on a large scale at the end of the early Cretaceous.Multi-stage tectonic uplifting events since the Himalayan period caused the thermal evolution of Chang 7 source rock to stagnate,and developed a large number of late second-class faults,which resulted in the destruction,adjustment and reconstruction of the ancient oil reservoir and re-accumulation.（5）The control mode of five major structural change events on Mesozoic hydrocarbon accumulation in Yanchi area is establish,and that main controlling factors of hydrocarbon accumulation are summarized.The first type of fault communicate with the source rock of Chang-7 is the foundation of primary oil reservoir formation,and the adjustment and transformation of ancient oil reservoir by the second type of fault is the key of secondary oil reservoir formation.The high point of paleogeomorphology has laid the foundation for the development of differential compaction low-amplitude structures,which are favorable orientation areas for hydrocarbon migration and accumulation.The favorable target area of Mesozoic hydrocarbon exploration is predicted,namely,the micro-amplitude structure or relatively high permeability reservoir area near the opened fault and the favorable trap area on one side of the blocked fault with good oil-bearing display.