Quantitative Characterization Of Shale Lithofacies Of Shanxi Formation In Yan’an Area

The typical continental shale series developed in the upper Paleozoic Shanxi Formation in the southeast of Ordos Basin（Yan’an area）has become the key layer of shale gas exploration in this area.The open flow rate of gas tests in many wells has reached 60,000 cubic meters/day,which fully shows that this area has rich shale gas exploration prospects.To further clarify the geological characteristics and distribution law of shale gas in the Shanxi Formation,the basic characteristics of shale in Shanxi Formation were determined by geochemical analysis of shale samples,X-ray diffraction experiment,and logging data,and the lithofacies division scheme of target strata was formulated.Through core observation,thin section observation,argon ion polishing scanning electron microscope,N₂ adsorption experiment,and high-pressure mercury injection experiment,the different lithofacies types and pore structure characteristics of Shanxi Formation shale in Yan’an area were studied.The gas generation capacity,reservoir capacity,fracability,gas content,and related characteristics were discussed,and the dominant lithofacies were optimized by comparative analysis.It provides the geological basis for reservoir sweet sections identification and favorable area prediction.Mainly achieved the following research results and understanding:（1）The shale of Shanxi Formation in Yan’an area is widely developed,with large single-layer thickness and good lateral continuity.The lithology is the frequent interaction of shale,sandstone,carbonaceous shale,and coal seam.The cumulative thickness of shale is up to 40m～110m,which is widely distributed,and the mineral composition has the characteristics of high clay.The mud shale mainly develops three kinds of sedimentary structures:layered,layered,and massive.The ancient sedimentary environment is mainly dominated by a warm and humid climate,and the water body is shallow.The whole is in a freshwater anoxic reduction environment,and the local development is in an oxidizing environment.（2）The TOC content of shale is low,mainly in1.0%～2.0%,and the maturity of organic matter is more than 2.0%,which is in the stage of high maturity-over maturity,and the type of organic matter is type III.（3）The division scheme of mineral content+sedimentary structure+organic matter content is established.The shale of the Shanxi Formation develops 144 lithofacies.It is determined that the Shanxi Formation in Yan’an area develops 6 main lithofacies:medium organic matter laminated clay siliceous shale facies,rich organic matter massive siliceous clay shale facies,medium organic matter layered siliceous clay shale facies,organic matter massive clay siliceous mixed shale facies,medium organic matter laminated clay siliceous mixed shale facies,rich organic matter massive clay shale facies;（4）Among the six main lithofacies,two organic-rich shale facies have higher TOC content and are identified as favorable hydrocarbon-generating lithofacies.The medium-organic layered siliceous clay shale phase is recognized as the most favorable reservoir facies due to its high pore-specific surface area and pore volume,rich pore types,and layered structure.After calculating the brittleness index according to the mineral composition,it is found that the brittleness index of the medium organic matter laminated clay siliceous shale,the organic matter containing massive clay siliceous mixed shale phase,and the medium organic matter laminated clay siliceous mixed shale phase is higher,which is established as a favorable fracturing lithofacies.Combined with the gas-bearing data,it is considered that the medium organic matter layered siliceous clay shale phase is widely distributed in the target layer,and has good gas generation capacity and reservoir capacity,which can be used as the dominant lithofacies for key exploration and development.