| In the process of coal mining, the abnormal emission and injury accidents caused by hydrogen sulfide(H2S) in coal mine often occur in both at home and abroad.Xinjiang Uygur Autonomous Region is a major coal-producing province of China,whose estimated coal reserves account for about 40% that of China. Many coal mines see abnormal accumulation of hydrogen sulfide causing casualties in different regions,which are affected by some factors such as the depositional environment, the geological structure and the spontaneous combustion, etc. Thus, it is urgent and significant to carry out researches on high-hydrogen sulfide in coal mines, from the perspectives of the geological survey, genesis modes and enrichment control factors.In terms of the regional characteristics and organic geochemistry of coal, through data collection, field work, laboratory testing and statistics, this paper conducts researches on the genesis mode and enrichment control factors of hydrogen sulfide in Jurassic coal seam within the midst of southern margin of Junggar basin(hereinafter referred to as Southern Junggar), and comes up with the following discoveries.3 construction evolution stages have experienced since Carboniferous in the midst of Southern Junggar, The depression-uplift structure and structural inversion make the deposit has obvious cyclicity. The coal-bearing strata of Xishanyao Formation formed by continental facies, mainly in lake-type cycle, in which the coal-bearing structure is strongly squeezed to transform, a linear tectonic fault fold belt is formed, which is nearly east-west, en-echelon, in rows and in belts.The climate of Xishanyao Formation is from warm and wet to arid, the organic matter in coal is in the dominant source of higher plants, organic sulfur from plants,which form coal, inorganic sulfur mainly comes from the rock salt and brine evaporation. Regional evolution in the process of gelation of peat swamp was strengthened. Most of the coal formed in low reductive degree belongs to medium or especially low-sulfur coal layers.. A crystalline and cryptocrystalline of pyrite was the main vein in cracks of the coal seams.The carbon isotopic composition of methane among the coal seam gas in the studied region is comparing light, most of which is less than-50.0‰ and due to mixed factors. The natural gas is mainly composed of methane, in which the heavyhydrocarbon generally accounts for lower than 20%, existing as moisture-partial moisture. The values of the carbon isotope of carbon dioxide in the groundwater and among the coal seam gas are negative, manifesting the characteristics of organic origin. Various values of sulfur isotope measured in the area are comparing lower,most has the characteristics of △34S(δ34SSO42--δ34SH2S)=22.5‰>22‰, the hydrogen sulfide with the genesis characteristics of bacterial sulfate reduction(BSR).The homogenization temperature peak of fluid inclusions in coal rock reservoir within Xishanyao Formation is mainly from 80℃ to 115℃ in the studied areas. The average of coal vitrinite reflectance ranges from 0.5% to 0.7%, the relationship between the palaeogeothermal and the vitrinite reflectance shows that the paleogeothermal of coal rock experiences an average of 101.1℃. By means of illite fission track test, the value of tmax obtained is from 105 ℃ to 115 ℃, which experiences a partial annealing process, but does not fully anneal. Currently, the buried depth of the coal rock in the Jurassic Xishanyao Formation is mostly located in400 to 2800 meters, according to this relationship of geothermal gradient, presumably most of the coal rock temperatures in the studied areas are below 70℃. It is visible that the experienced temperature of the regional coal rock strata is mostly less than120℃, and most of the reactions are within the boundaries of BSR.In the current depth of coal mining in the region, the component of hydrogen sulfide in the coal seam is generally less than 3.0%, showing the genesis characteristics of BSR. The berry globular pyrite formed by BSR prevails, which also shows the genesis characteristics of BSR. The natural gas is moisture or partial moisture, having the characteristics of typical humic thermogenic coal seam gas and BSR. Carbon isotope of CO2 is negative, which shows the associated characteristics of BSR into hydrogen sulfide. The checked number of sulfate reducing bacteria of each coal mine is from 100 a/g to 3500 a/g, which is conducive to the formation of hydrogen sulfide. It is concluded that H2 S in coal mine is mainly generated by BSR,but genesis of TSR in partial region cannot be excluded.Through the experimental simulation for the thermal evolution of sulfate in coal,it can be concluded that the rate of heavy hydrocarbon first increases and then decreases, meeting the law of preferential consumption of heavy hydrocarbons in TSRaction. With the increase of temperature, the reaction of TSR increases gradually. The occurrence of TSR is accompanied by heavy hydrocarbon(C2+) gas generation, and the most intense stage of TSR is that at the highest production rate of the heavy hydrocarbon. The generation of H2 S and heavy hydrocarbons gas has the similar trend.TSR is rather complex in the actual geological process, and the possible reaction equation that manifests the way hydrogen sulfide in coal mine is formed by the genesis mode of TSR is shown below.H2S is widely distributed in the studied region, in most cases coexisting with carbon dioxide. The highest gas intensity in hydrocarbon source rocks can exceed100.0×108 m3/km2, the organic-carbon content in coals of Xishanyao Formation is up to 64.49% on average and the rich hydrocarbon source rocks provide a solid material foundation for the genesis of H2 S. The average porosity of the coal rock reservoir is above 10.0%, and its permeability ranges from 0.11×10-3um2 to 122.30×10-3um2, the moderate and better reservoir is widespread, so it has a fair store-permeability condition. The pore structure of each coal seam is the fracture-pore type, and the coal body is for the great part rupture structure, which are conducive to hydrogen sulfide accumulation in coal seam with the growth of structural fracture. The roofs and floors in each coal seam are mainly composed of the fine clastic rocks, together with the low permeability layer, which are conducive to the preservation of H2 S. The regional plain sedimentation basin forms a thick aquifer, and under the effect of hydrodynamic closed air, it forms pore groundwater and fissure water under pressure. In the flow direction, the p H value, salinity, ionic content, etc. of underground water and confined water in the region all have an increasing trend. The enclosed environment of the deep under-pressure water is relatively favorable to the formation of H2 S giving rise to the high density of hydrogen sulfide in water after experiencing BSR or TSR in the reducing environment, under the action of sulfate reducing bacteria and thermodynamic factors. |
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