Evaluation Of Coal Shale Reservoirs In The Daning-Jixian Area

The Daning-Jixian area on the eastern edge of the Ordos Basin is an old coalbed methane exploration and development area in China,with abundant of coal-measure gas resources.By using sedimentary facies analysis of the Carboniferous Permian including Benxi formation,Taiyuan formation,and Shanxi formation,both distribution characteristics and reservoir evaluation of coal-measure shale were carried out which can provide references for the exploration and development of coal-measure shale gas in the future.The study area is located in the western slope belt of Daning-Jixian where the structure is stable.There are multiple sets of coal-measure shale reservoirs in the Benxi,Taiyuan,and Shanxi formations.Because of the the influence of transgressing and regressing,the depositional combination of Benxi formation is compose of lagoon,tidal-flat,barrier bars,which is thin in thickness.Main rock facies in Benxi formation are limestone,coal and sandstone.The depositional combination of Taiyuan formation is tidal-flat,peat swamp,barrier bars.There is thick limestone in this formation,on the contrary,mudstone and sandstone are mostly in the form of lens or interlayers.The depositional combination of Shanxi formation are underwater distributary channel,interdistributary bay,peat swamp sedimentary,which average thick is 72m.There are multiple sets of coal seams and dark mudstones formed by lagoons,peat swamps and interdistributary bays in Shan 2 formation.Only the gray mudstone deposits formed by the interdistributary bays are developed in Shan 1 formation,and the peat swamps are not developed.The coal-measure shale in Shanxi formation has the widest distribution.The organic matter of the shale is the type?kerogen,and its maturity is relatively high,which is in maturity-over-maturity evolution stage,also known as the late stage of gas generation.The average TOC of shale is 4.93%,the organic matter abundance of coal-measure shale in Shan 2 is higher than that in Shan 1.The test results of the shale mineral composition of the Shanxi formation indicate that the average value of shale brittleness index is 57%,the quartz content is high,and the illite in the clay mineral is well developed.Multiple experiments results including conventional porosity and permeability tests,low-temperature N₂,CO₂ adsorption,and high-pressure mercury injection experiments show that the shale in the Shanxi formation is ultra-low porosity and ultra-low permeability reservoirs,The pore structure of the reservoir is complex,and the surface of pores is rough,.There are various pore types and most of them are open flat slit holes and cylindrical holes.The pore size is mainly micropores and mesopores<50nm and it is relatively concentrated.In addition,cracks are well developed.The analysis of the controlling factors of the pore space in the study area reveals that the pore space is mainly controlled by the content of brittle minerals and the abundance of organic matter.The pore space of micropore is positively correlated with TOC and clay minerals,and negatively correlated with quartz content.According to the evaluation indexes of shale reservoirs both at home and abroad with the investigation,logging,experimental data of the existing study area,the evaluation indexes of the study area are optimize.Using fuzzy mathematics multi-level comprehensive evaluation method and optimal segmentation method,a coal-measures shale evaluation model of Shanxi formation in Daning-Jixian area was established systematically,and favorable areas were selected in Shan 1 and Shan 2.The strata of Shan 2 are divided into?,?,and?favorable areas.The strata of Shan 1 are divided into?and?favorable areas.Type?favorable areas are not developed in Shan 1member,and shale reservoirs in Shan 2 Member are better than Shan 1 member.