Geological Characteristics Of The Carboniferous-permian Coal Measure Gas In Lianyuan Depression

In this paper,field exploration,sampling,test analysis,and data compilation are applied to study the Carboniferous-Permian coal-bearing strata in Lianyuan Depression.Coal-bearing strata sedimentary environment,source rock distribution,organic geochemistry characteristics,reservoir characteristics,and favorable areas were analyzed and following conclusions could be drawn:(1)The Ceshui Formation and the Longtan Formation sedimentary environments of the coal-bearing strata in the Lianyuan Depression marine-terrigenous facies.The coal-bearing strata are deposited on the barrier island-lagoon sedimentary system and develop two third-order sequences;the Longtan Formation coal-bearing strata is deposited in the tidal flat-delta sedimentary system,and three third-order sequences are developed,and most parts of Segment SQ1 are missing.(2)The thick coal rocks in Ceshui Depression are mainly distributed in the central region,and the average thickness of coal seams in this area is more than 4m;the thick coal rocks in Longtan Formation are mainly distributed in the eastern region,and the average thickness of coal seams in the area is more than 2.5m.The mud shale thickness of Ceshui Formation and Longtan Formation in the Lianyuan Depression gradually increases from the edge of the depression to the center,relatively thick overall;the mean thickness of mud shale in the Ceshui Formation is greater than that in the Longtan Formation.(3)The coal microscopic components of Ceshui Formation and Longtan Formation in the Lianyuan Depression are dominated by the vitrinite group and anthracite,supplemented by the inertinite group and lean coal.The organic matter types of Ceshui Formation and Longtan Formation mud shale in the Lianyuan Depression are mainly type II2 and type III;the TOC and Ro of the Ceshui Formation increases from north to south generally,and the TOC and Ro of Longtan Formation mudstone shale increases from the east to the west;the mud shale organic matter is in the high mature and post mature stage.(4)The coal structure of Ceshui Formation and Longtan Formation in the Lianyuan Depression is mainly characterized by cataclastic and mylonitic structure,where the porosity is between 8% and 20%,with parallel plate-like holes and open-faced sharp-edged structures.The horizontal permeability is less than 0.1m D,and the gas content is between 5?20 m3/t.The mineral composition of mud shale in Ceshui Formation and Longtan Formation is dominated by quartz,the brittleness index of which is generally greater than 0.4,and the average clay mineral content is32.5%.The average pore size is 4.04 nm,with the maximum adsorption capacity of7.15 m3/t,and the Rankine pressure of 5.57 MPa on average.(5)There are five types of lithofacies combination developed in Ceshui Formation in Lianyuan Depression.Three main coal-gas combinations,two sub-coal-gas combinations,and four lithofacies combinations of Longtan Formation are developed from the north to the south in Lianyuan Depression.A main coal-gas combination and three sub-coal-gas combination models are developed from the west to the east.(6)By analyzing relevant parameters of the gas source conditions,preservation conditions and development conditions of the coal-bearing formations in the Lianyuan Depression,the fuzzy evaluation method is used: the favorable gas-producing zone is located in the central depression ie,the Zhadu mining area,Lengshuijiang mining area,and Jinzhushan mining area;the favorable coal-gas system of the Longtan Formation is located in the eastern part of the depression,that is,the Douli mining area and the Hongshandian mining area.