A Study On Jurassic Coalbed Methane Accumulation Conditions In Southwestern Ordos Basin

In this paper,through the collection of drilling,logging,logging and analysis of laboratory data combined with previous research results,using sedimentary petrology,logging geology,coal petrology,geochemistry and coalbed methane accumulation theory and method,the Jurassic coalbed methane accumulation conditions in the southwest of Ordos Basin were comprehensively studied.Mainly from the coal-forming environment and coal seam distribution,coal quality characteristics,gas generation potential,coal reservoir and gas-bearing characteristics,coalbed methane preservation conditions were analyzed.The stable tectonic environment of Yan’an Formation in the study area made Yan’an Formation develop a wide range of swampy facies,deposited multiple sets of coal-bearing strata,and formed a large thickness of coal seams and wide distribution.The coal rock is mainly composed of semi-bright and semi-dark coal with low water content,low ash content and medium and high volatile content.The results of the calculation of∑HP and∑OP of maceral indicate that the organic matter type of coal rock is typeⅢkerogen with Ro of0.5%～0.9%,which mainly belongs to the stage of long flame coal and gas coal.The results of thermal simulation experiment show that the gas production of CH₄is 10～25 ml/g,and the cumulative gas production is 10～30 ml/g,indicating that the gas generation potential is good.The coal reservoir has a variety of pore types,with good interpore connectivity,the average porosity is 7.8%,the average permeability is 1.18×10^-3μm²,and the average specific surface area is 13.451 m²/g.The adsorption capacity and pore structure are conducive to coalbed methane development.Gas content is mainly in 1～6 m³/t,and burial depth and thickness is positively correlated,affected by secondary biogas,Bin County area gas content is higher.Coalbed methane is affected by tectonic evolution in the study area.Coalbed methane has both thermal and biological characteristics,and the local low part of the nose structure is conducive to its preservation.The mudstone with wide distribution range,good continuity and large local thickness makes the roof and floor have good sealing conditions.Bin County shallow weak runoff area produces secondary biogas,weak runoff area and the junction of the stagnant area form hydraulic plugging type gas reservoir,Qingyang area water type for Ca Cl₂type,high salinity,the formation of pressure water conservancy in coal bed methane preservation.From the three aspects of"generation,reservoir and preservation",this area is conducive to the accumulation and accumulation of coalbed methane.