Characteristics And Microscopic Pore Model Of Longmaxi Shale Gas Reservoir In Northern Yunnan And Guizhou

Dianqianbei area locates at the boundary of the Sichuan,Yunnan and Guizhou province.In tectonical area,it belongs to Qianqianbei depression locating at southwest of Yangtze Plate.Dianqianbei area is a good play for marine shale of South area and sediments several shale Formations.Silurian Longmaxi Formation shale is regarded as source rocks and reservoirs of high quality.Pores of Micron-nanometer size are the main characters of shales,whose permeabilities are extremely low and differ from conventional clastic and carbonate reservoirs.Therefore,it helps to clarify the characters of Longmaxi Formation shale reservoirs in exploration and development.Three-dimension model construction will lay the foundation for later quantitive study in three-dimension.By observation and analysis of coring sections in Wufeng Formation to Longmaxi Formation of Dianqianbei area in Y9 well,based on classification principles of Upper Ordovician to Llandovery proposed by Chen Xu,we classify six graptolite zones from WF2 to LM3 and construct the biostratigraphic system.Charactes of shale gas reservoir mainly includes minerals type and lithofacies,organic matters,quality,pore types,pore structures and gas bearings.By XRD test and analysis,we find that six lithofacies types are classified containing siliceous shale,clayey shale,calcareous shale,clayey and siliceous shale,clayey and calcareous shale,calcareous and siliceous shale,based on quartzs and feldspars,clay minerals and carbonate minerals.TOC in siliceous shale and clayey shale are relatively high,whose average values are over 3%.Type of OM is kerogen of I type.Ro of OM values ranges from 2.0%to 3.0%,indicating they are over-mature and TOC of shales ranges from 0.5%to 8%.By GRI method to test porosiy and permeability of shales,we find that porosity ranges from 1%to 4%with extremely low permeability.By SEM observation,we find pore types are various in shales,including mineral mold pores,intergranular pore,pores in particles,intergranular pores of pyrite,interlayer pores in clay minerals and OM pores and the last two parts are the main pore types.Nitrogen adsorption experiment helps to analyze pore structures of shales and we find that the specific surface area ranges from 4.365 to 23.015 m2·g-1,while pore volume ranges from 0.005 to 0.049 cm3·g-1 and average pore diameter ranges from 1.475 to 4.046 nm.Nitrogen sorption isotherm suggests slot pores and micro pores are the majority in shales,which matches the results of SEM analysis.Obtained by filed test,gas bearing valume ranges from 1.33 to 3.67 m3/t,mainly controlled by five factors including kerogen types,TOC volume,mineral types,degree of themal evolution and mineral structure.On the basis of fractal theory,combining with the fact that illites and illites mixed layers are the main types of the study area and high evolution takes the major factor,we construct shale pore model of molecular scale by building layers of illite molecular model and graphene layer model based on the software named Materials Studio.On the basis of FHH formula,guided by the theory of Mathematical Statistics,model verification problem is turned into a math problem to solve and finally we verifacte the molecular model is reliable to some extent.