Faults-Fractures Characteristics In The Central Part Of The Western Margin Of Ordos Basin,and Its Reservoir-Forming Effects

The central part of the western margin of Ordos Basin is located in the western part of the middle section of Tianhuan Depression and the Eastern margin of the thrust belt on the western margin,with a special structural position.Due to the scarcity of drilling Wells and the lack of high-quality seismic data,it is difficult to identify the structure in the study area,and the knowledge of fault-fracture characteristics and active periods is still insufficient.At present,there is a lack of unified understanding.With the deepening of oil and gas exploration in the western margin of Ordos Basin,especially the implementation of 3D seismic profile,this area has become a great potential area for oil and gas exploration in the basin.It is of great significance to study the characteristics of faults and fractures and their reservoir effects.On the basis of systematically collecting previous research results about the western margin of Ordos Basin and Tianhuan depressio,this paper analyzes the characteristics and formation stages of faults and fractures in the middle of the western margin of Ordos Basin by comprehensive analysis of field outcrops,drilling data,logging data,3D seismic profiles,3D slices,equilibrium profiles,C-O isotopes,inclusions and calcite vein U-Pb dating,and discusses the role of faults in hydrocarbon accumulation.Natural fractures are developed in Mesozoic in the central part of the western margin of Ordos Basin,and fracture characteristics is different in time and space.The fractures are most developed in Triassic Yanchang Formation,and the Chang 7 Member is the most developed in Yanchang Formation,followed by Chang 8 and Chang 6.The fractures are mainly high-angle and vertical fractures,and the fracture surface is relatively flat.The strike is mainly in NEE direction,followed by NE direction,and a few NW、NWW and near SN directions.The formation of fractures has multi-stage activities,which can be divided into three stages:Indosinian,Yanshanian and Himalayan.The main active periods are the Mid-Late Yanshanian and Himalayan.The Mesozoic in the central part of the western margin of Ordos Basin mainly develops three groups of faults with typical characteristics : NW-trending,near-NS-trending and NEE-trending.The NW-trending faults are small in scale,mainly developed in Yanchang Formation,mostly normal faults,distributed in en-echelon,with left strike-slip nature,and formed mainly in Late Triassic(Indosinian).The near-north-south faults are mainly reverse faults,with the largest scale and complex plane-profile characteristics,and was formed in the late Jurassic(Yanshan period);The NEE-trending normal faults developed late,distributed in parallel at nearly equal intervals,with certain strike-slip characteristics,and formed since the early Cretaceous(Himalayan period).Some faults are characterized by multi-stage activities.According to the relationship between the occurrence of faults and fractures and the period of fault activity,the development relationship between faults and fractures in the study area is discussed.Late Triassic NW-trending faults were formed,accompanied by a few NW-trending fractures.Late Jurassic thrust belt in the western margin was strongly active,accompanied by nearly SN,NWW and some NEE-trending fractures.Since the early Cretaceous,NEE and NE fractures are related to NEE faults.Faults-fractures have different controlling effects on reservoirs in different periods.Late Triassic NW-trending faults and late Jurassic near-N-S faults occurred earlier than or at the same time as the main accumulation period of Mesozoic reservoirs,provide traps,and they communicated with Chang 7 source rocks and upper and lower sandstone reservoirs,providing a good channel for upward and downward oil migration.The NEE fault formed since the early Cretaceous occurred later than or at the same time as the main accumulation period of the reservoirs in this area,and cut through many horizons upward,mainly showing the damage and adjustment of the early reservoirs.Fracture-Fracture is the main controlling factor of reservoir formation in multi-layer high-yield reservoirs.The number of oil wells distributed near Indosinian NW fault is the largest,which further proves that Indosinian fault has more obvious control effect on Yanchang Formation reservoir.The reservoir-forming model in Yanchi area is established.