Accumulation Conditions And Models Of Permian Shale Gas In Typical Areas Of China

Permian is the key period of transitional shale forming when shale deposits in a great extent in China.Differences obviously appear between shale samples in different area.Shanxi Formation in Liaohe East Uplift and Ordos Basin,Longtan Formation in Guizhou Province are selected as the study targets in the paper after analysis and comparison.Organic geochemical experiments and reservoir experiments on shale samples are operated to figure out the shale condition of gas generation and reservation.The developing models of Permian transitional shale are discussed in this paper,through the study of shale gas content and the analysis on the accumulation characteristics.Because of the instability of sedimentary water,transitional stratigraphy appears diversity,sandstone,argillaceous siltstone,silty mudstone and mudstone are developed alternatively.Coal-bearing beds are associated and carbonate rock are widely developed in Longtan Formation in Guizhou Province.The type of organic matter developed in Permian oceanic-terrestrial transitional shale is mainly type-III kerogen,with a TOC ranging between0%-16.0%and a vitrinite reflectance ranging between 1.26%-3.48%.Different geochemical characteristics exsist between the Permian Shale in Northern China and Southern China,Shales of Longtan Formation in Guizhou Province have higher TOC overall and quite different shale maturity because of its complexity in regional structure.The mineral composition of Permian transitional shale is dominated by clay mineral and quartz,the carbonate mineral content which is greatly lower than the marine gas-producing shale in America.The clay mineral is dominated by I/S mixed-layer mineral,secondly consisted by illite,nearly doesn't contain smecite,which is properly associated with the maturity phase in the thermal evolution of organic matter.The porosity of Permian transitional shale in typical area ranges from 1%to 4%,the permeability ranges from 0.01mD to 0.05mD.The porosity and the permeability in typical area showed positive relationship.The shale reservoir appears abundance in porosity types,which develops all kinds of inorganic and organic pores and micro fractures.The distribution of pore size is mainly mesopore(2-50nm).The Permian transitional shale is lack of organic pores comparing with the Lower Paleozoic marine shale in Southern China which has a widespread development of network-type and favose-type organic pores.Permian transitional shale has a overall high gas content through isothermal adsorption and field desorption experiments.Among the study areas,shale of Longtan Formation in Guizhou Province has the most adsorption capacity,which has some shale samples that its field desorption gas content reached 8 m3/t,which means a good exploration potential.The developing models of transitional shale gas are here divided into two models:The first one has a sedimentary facies of barrier-coastal facies and the shale is developed in braided river deltas,tidal flats,lagoons,barrier islands and coastal facies,in which the peat-flat in tidal flats and lagoon are shale developing areas with high organic matter abundance.The second model has a sedimentary facies of delta facies,underwater distributary bay and interbay marsh in delta front are shale developing areas with high organic matter abundance.