| Western margin of Ordos Basin is located in the piedmont complex structural belt.There are many types of source and reservoir combination in this area,and the oil and gas reservoir types are the most abundant and have great exploration potential,which has important research value.In piedmont complex structural belt,concealed structure plays an important role in controlling petroleum entrapment,which has become a hot spot for research in various basins in recent years.At present,there are few evaluation research on the characteristics of Permian concealed structure,petroleum entrapment and favorable zones in western margin of Majiatan area.Based on the collection of a large number of geological,seismic,logging and other relevant data,guided by the latest frontier theories of geophysics,sedimentology,tectonic geology,petroleum geology and so on,structural style,sedimentary and structural characteristics,formation and evolution,and petroleum entrapment process of Permian concealed structures in Majiatan area are studied in detail.Firstly,TTI pre-stack depth migration technique is used to identify the concealed structure developed in the piedmont complex structural belt.Secondly,Geoeast seismic data is used to interpret system,and the second and third-dimensional seismic reflection layer and fracture in the study area are explained in detail.Thirdly,evolutionary analysis is carried on by using balanced section technology in research area.Fourthly,on the basis of a large number of core observations,sedimentary facies analysis is carried out through the analysis of lithofacies,log facies and seismic facies,combined with the characteristics of tectonic evolution in the study area and research data result.Fifthly,the prediction of reservoir and fracture in the study area is made by full use of high density 3D seismic.The following points are gained:(1)The Permian concealed structure in Majiatan area was mainly caused by the development of a large area of bedrock Paleouplift,dense basal rock Paleouplift hindered the Yanshanian movement,upper Paleozoic and Mesozoic strata glided along the carboniferous and Permian coal measures under the action of east-west compression stress,wrinkle occurred,and concealed structures was formed.The Paleozoic structure is characterized by "high in the east and low in the west".On the footwall of the Huisha fracture,the paleouplift controls the distribution of the Permian concealed structure.On the paleouplift,three stripe concealed structures are respectively developed from west to east,namely,the Yandunshan concealed structure,the Hui ’anbao concealed structure and the Majiatan concealed structure.It is an important area for gas exploration in the basin with complete concealed structural traps.(2)The Permian Taiyuan,Shanxi and Shihezi formations in the study area are mainly developed into continental deposit system which is mainly alternative sea and river facies of lacustrine and delta.Among them,the Taiyuan formation is mainly developed into alternative sea and river deposit.Meandering river deltas are mainly distributed in Shanxi Formation,among which underwater distributary channels and interdistributary bay microfacies are the most developed,which constitute the main part of meandering river deltas in the study area.The braided river delta is the main source of the 8th section of Shihezi formation.The lithology of the reservoir is manily lithic quartz sandstone,and the lithic dissolution pores and intercrystalline pores are well developed with good physical property.High quality source rock of upper Paleozoic Per-Carboniferous system coal measure is developed in the area with sufficient gas supply.(3)In the high part of the bulge,the fracture has obvious damage effect on the concealed structure reservoir,and the concealed structure of Majiatan is greatly affected.In Hui ’anbao and Yandunshan concealed structure,the degree damaged by fracture is low,which is conducive to accumulate oil-gas reservoir.According to the analysis of tectonic evolution and thermal evolution of source rocks in Majiatan area,the main tectonic period is in late Jurassic-Cretaceous.The main hydrocarbon expulsion stage of source rock is basically synchronous with the main formation stage of the trap.Meanwhile,the mudstone cap rock developed between Permian layers has good plugging ability,and the source-reservoir-seal combination is well configured,which can form lithologic and structure-lithologic gas reservoirs.Reservoir forming model has the characteristics of near supply for source rock,discharging while accumulation,lower generation and upper storage. |
