Occurrence And Enrichment Characteristics Of Shanxi Formation Tight Sandstone Gas And Shale Gas In Yan’an Exploration Area

The Permian Shanxi Formation in Ordos Basin develops coal-bearing source rocks under the background of land-sea transition.There are a lot of tight sandstone gas and shale gas in the strata,which has high exploration and development potential.In this paper,the plane distribution characteristics and lithologic superposition characteristics of sandstone and shale in the study area are analyzed by logging data.The formation mineral composition,porosity and permeability characteristics and pore development are studied by thin section observation,reservoir physical property research and scanning electron microscope analysis.The basin simulation and fluid inclusion method are used to determine the accumulation time of gas reservoirs,and the main control factors affecting the enrichment of tight sandstone gas and shale gas are analyzed.The overall sedimentary facies of Shanxi Formation in the study area is delta facies,and the strata in the sedimentary period are not underwater as a whole,which is a warm and humid freshwater reduction environment.Shan 2 sandstone thickness is 5～15m,shale thickness is20～45m,Shan 1 sandstone thickness is 5～25m,shale thickness is 15～40m.The lithology of Shanxi Formation is mainly tight sandstone,shale and a small amount of coal line,and the lithology changes rapidly vertically.There are many different gas-bearing lithologic combinations,including coal-sand-mud,coal-mud-sand,coal-sand,coal-mud,mud-sand-mudstone.The sandstone rock types are mainly quartz sandstone,quartz lithic sandstone and lithic sandstone.The porosity of sandstone reservoir is mainly distributed between 2%and 6%,and the permeability distribution range is 0.01×10^-3μm²～30.05×10^-3μm²,most of which is less than 0.01×10^-3μm².It belongs to the tight sandstone reservoir with low porosity and low permeability,and mainly develops secondary dissolution pores.The main component of shale is clay minerals,accounting for about 56%,followed by quartz,accounting for about 35%.Shale reservoir porosity is mainly distributed in the range of 0～2%,permeability is mainly distributed in the range of 0.0013×10^-3μm²～0.1×10^-3μm²,poor physical properties,mainly developed micro fractures,intergranular pores,intragranular pores and organic pores and other microscopic pores.The nitrogen adsorption results show that the pore system in the shale of Shanxi Formation is complete and the structure is complex.The pore morphology is mostly semi-closed and open at both ends,and the most developed pore is the middle pore of 15～30nm.The inclusions developed in the shale interlayer were selected for the measurement of uniform temperature combined with the simulation results of hydrocarbon generation and expulsion.It is considered that the shale gas of Shanxi Formation has two stages of accumulation,and the first stage is 195～180 Ma.This stage is the early generation of liquid hydrocarbon and a small amount of gaseous hydrocarbon of source rock type III kerogen,with a short duration.The second stage is 175～125 Ma,which is a large hydrocarbon generation stage of coal-bearing source rocks.The tight sandstone gas of Shanxi Formation belongs to near-source accumulation,which is accumulated in a large number of hydrocarbon generation stages of source rocks,and the accumulation period is 175～97.5 Ma.The enrichment of gas reservoirs in Shanxi Formation is mainly controlled by hydrocarbon generation conditions,reservoir conditions,sedimentary facies conditions,cap rocks and preservation conditions.The shale gas reservoir belongs to the self-generation and self-storage type,and the organic matter type is III.The organic matter abundance of source rock is high,which is in the stage of overmature evolution,and is beneficial to the enrichment of shale gas.The pore structure in the reservoir is complex.Macropores larger than 50 nm are less developed,and the physical properties are poor,which is not conducive to the enrichment of free gas,and the clay mineral content is high,which is conducive to the occurrence of adsorbed gas.Shale gas self sealing is good,not easy to spread.Tight sandstone gas belongs to near source reservoir with good source rock conditions.The reservoir belongs to tight sandstone reservoir with low porosity and permeability,but its physical properties are better than shale reservoir.Shale gas in Shanxi Formation migrates to sandstone reservoir through abnormal high pressure and diffusion.Sedimentary facies control the development of lithofacies.The lithofacies with high organic matter content such as black carbonaceous mudstone are favorable lithofacies types for forming high-quality source rocks.The lithofacies such as siltstone and fine sandstone are favorable lithofacies types for forming tight sandstone reservoirs.Since the uplift of the strata in the late Early Cretaceous,there has been no large tectonic movement in the area,the gas reservoir has not been destroyed and the preservation conditions are good.