Study On Isotopic Geochemistry And Origins Of Coalbed Methane In The Northern Qinshui Basin

The isotopic geochemistry characters and origins of coalbed methane?CBM?in the northern Qinshui basin,associated with how karstic collapse columns?KCC?impact on coal seam production water?CSPW?and CBM,were studied using the theoretical knowledge of coal and CBM geochemistry,coal petrology,isotopic geochemistry,hydrology chemistry,hydrology geology and mineralogy.The research set up a model for the assessment of origins and contents of CBM based on isotopic geochemistry.Coal composition,diagenetic process and carbon and nitrogen isotopic compositions in Carboniferous-Permian coal-bearing strata in the northern Qinshui basin were studied to predict the variation of carbon isotopic composition of atmospheric CO₂ of the Late Carboniferous to the Early Permian,and then paleoclimate and paleoenvironment during those periods.The genesis and relationship among CH₄,N₂ and CO₂ of CBM were studied,which was used to evaluate the origins and enrichment mechanism of CBM.The analyses of KCC impact on groundwater quality,CBM geochemistry and enrichment pointed out that the presence of KCC significantly influences CSPW quality and its hydrogen and oxygen isotopic compositions.A series of contour maps?coal depth,hydrocarbon isotopes,nitrogen isotopic composition,and initial gas content?,associated with tectonic characteristics and evolution history,were used to investigate the CBM enrichment area.First,the maturation,components,as well as carbon and nitrogen isotopic compositions of bulk coals from Qinshui basin were assessed.?1?Coal is the source rock of CBM and also the main component of CBM reservoirs.The average mean random vitrinite reflectance?Rr?of the #15 coal of the Taiyuan formation in Yangquan mining area of the northern Qinshui basin is 2.4%?n=23?,Taiyuan Xishan #8 coal Rr averages 1.4%?n=3?,and Shanxi #2 coal average Rr is 1.2%?n=5?.By contrast,the average Rr of the #21 coal in Shanxi formation of Hebi mining area in North China basin is 1.6%.Therefore,the order of the thermal maturity of coal is: #15> #21> #8> #2.?2?Coal macerals are the main carbon sources of CBM.Vitrinite on a dry-ash-free dry basis dominates the Carboniferous #15 coal from Yangquan mining area,with an average content of 79%?n=23?,but there is no liptinite in the #15 coal due to its higher maturation?Rr 2.4%?.The #8 and #2 coals from Taiyuan Xishan mining area are also dominated by vitrinite whose average contents are 61% and 60% respectively;however,inertinite dominates in the #8 coal sample TL82-1 from Tunlan mine and the #8 coal sample XJS8-C of Xingjiashe drillhore and makes up 53% and 63% respectively in whole macerals;no liptinite exists in the #8 coals except for the #2 coal sample from Malan mine with 2% liptinite and 1.1% Rr.The #21 coal from Hebi coal mine of North China basin has 62% vitrinite and 38% inertinite,excluding liptinite.?3?The ?13C of Carboniferous-Permian coals?coal gangues?from North China averages-23.7‰?n=41?.Among them,the average ?13C of Taiyuan Fm.#15 coal from Yangquan mining area is-23.7‰?n=27?,12.2‰ heavier than ?13C-CH₄ of CBM from the Taiyuan Fm.?-36.6‰,n=46%?.The #8 coal??13C-23.4%?and the #2 coal??13C-23.5%?of Taiyuan Xishan mining area have similar carbon isotopes with the #15 coal.However,the Shanxi Fm.#21 coal from Hebi coal mine North China basin has a more negative ?13C value?-25.2%?.This may record the negative carbon isotopic composition of atmospheric CO₂ during the Early Permian when the ice age dominated the world.?4??15N of the Carboniferous-Permian coals averages 3.7‰?n=33?.The average ?15N of the Taiyuan Fm.#15 coal from Yangquan mining area is 3.8‰?n=23?,while the #15 coal gangue has ?15N of 8‰?Zheng et al.,2016?.Higher ?15N in coal gangue than coal may infer that microbial activity in coals is stronger than in coal gangues at the early peat-forming stage.Second,molecular and isotopic compositions of CBM from Qinshui basin were studied,and the origins and relationships among CH₄,CO₂ and N₂ were discussed.?1?Methane dominates Taiyuan Fm.CBM in Sijiazhuang block of northern Qinshui basin with an average content of 98.5%?n=32?,but ethane is rare?0.026%?,and there are no hydrocarbons higher than ethane.The average content of N₂ is 1.12%,and CO₂ 0.32%;coal seam gases also contain a small amount of Ar?0.023%?.The relative amount of N₂ from organic matter pyrolysis is 16%,whereas from the atmosphere is up to 84%.?2?The carbon isotope ratios of the production CH₄ have an average of-36.6‰?n=46?,lighter than ?13C-CH₄ of Zhengzhuang-Panzhuang block in southern Qinshui basin?average-32.6‰,n=10?.The corresponding hydrogen isotope ratios of CH₄ have an average of-182.0‰?n=38?.The ?13C-CO₂ ranges from-17.5‰ to +8.6‰,with an average of-9.4‰?n=32?,which infers that the CO₂ was generated during initial coal forming or organic matter pyrolysis after the most recent uplift.The positive ?13C in residual CO₂ in shallower areas is due to the increasing microbial CO₂ reduction process in Sijiazhuang block.The ?15N-N₂ averages-1.6‰.?3?Thermogenic CBM is the primary source of CBM from Taiyuan Fm.of Sijiazhuang block,with a minor amount of microbial CO₂ reduction origin CH₄.Therefore,the CBM in Sijiazhuang block is of mixed origins.Third,the influence of KCC on CSPW salinity,chemical and isotopic compositions,and enrichment of CBM were assessed.?1?The type of Taiyuan Fm.production water in Sijiazhuang block of northern Qinshui basin is HCO3-Na,belonging to brackish water,whose total dissolved solids?TDS?averages 1492.9 mg/L.The TDS of production water from southern Qinshui basin averages 2252.8 mg/L in Panzhuang block,and 2375.3 mg/L in Zhengzhuang.Therefore,the average TDS of CSPW from northern Qinshui basin is lower than the southern Qinshui basin.?2?The distribution of KCC in Sijiazhuang CBM block in northern Qinshui Basin significantly affected Na+ concentration,hydrogen and oxygen isotopic composition and TDS of CSPW.KCC occur more often in the northern part of Sijiazhuang block than the south,and the TDS of northern CSPW with an average of 1531.1 mg/L is higher than in the south?average 1453.3 mg/L?.This study,using the single factor variance analysis method,proved that the KCC significantly increases the Na+ concentration and TDS of CSPW in the north due to enhancing evaporation effects in the north.Evidence from hydrogen and oxygen isotope compositions of CSPW that plot below and to the right of the global meteoric water line also proved that the ground water evaporation dominated northern Sijiazhuang block.?3?The desorption-diffusion-migration effect plays a leading role in carbon isotopic fractionation of CH₄ in the northern Sijiazhuang block.The CH₄ carbon isotopic value decreases with increasing depth.The reservoir pressure is smaller in shallower area so that 12CH₄ preferentially undergoes desorption-diffusion-migration,that is,escapes into the upper strata or even the atmosphere.This process also results in lower CBM gas contents in the northern Sijiazhuang block.?4?The ratio of CH₄ with organic origin N₂???CH₄?/??N₂org??was set up as the basis to judge the origins of CBM which were divided into three categories: primary biogenic CBM???CH₄?/??N₂org?<7?,degradation origin CBM?7< ??CH₄?/??N₂org?<111?,and thermal cracking origin of CBM???CH₄?/??N₂org?>111?.The CBM degradation origin contains secondary biogenic CBM and thermal degradation which can be shown by dryness data.Fourth,a comprehensive exploration and development method of CBM was established based on the isotopic geochemistry of coal seam gases.?1?The method used a series of contour maps,and a CBM origin discrimination diagram.A coal seam depth contour map,methane carbon isotopic contour map,methane hydrogen isotopic contour map,N₂ nitrogen isotopic contour map,and production waters' TDS contour map were produced,referred to as the "1 plot & 5 contour maps".?2?Considering the regional geological structure characteristics of Sijiazhuang block,the origin and enrichment of CBM in the Taiyuan Fm.in this area are determined and predicted accurately.Based on the prediction model above,it can be predicted that the areas with higher gas content of CBM should be in the central and western regions of Sijiazhuang block.