临兴-神府区块深部煤层气地球化学特征及其影响因素

Geochemical characteristics and influencing factors of deep coalbed methane in the Linxing-Shenfu block

查明深部煤层气成因和地球化学特征对其勘探开发具有重要意义,以鄂尔多斯盆地东缘临兴-神府区块深部煤层参数井解吸气为研究对象,系统测试了二叠系下统太原组8+9号煤层气体成分、含量和碳同位素等,结合热演化程度、含气性、煤层埋深、地质构造和水文地质条件等,分析深部煤层气地球化学特征及影响因素。结果表明,煤层气主要成分包括CH4、CO_(2)、N_(2)、乙烷、丙烷、异丁烷、正丁烷、异戊烷和正戊烷,其中CH4含量最高,体积分数在75.98%~96.20%,平均86.49%;其他含量较高的为乙烷(平均5.97%)、CO_(2)(平均5.18%)及丙烷(平均1.24%),气体成分含量与浅部煤层气区块无明显差异。煤层气δ13C1(CH_(4))分布在-51.9‰~-38.2‰,平均-42.84‰;δ13CCO_(2)分布在-8.6‰~-0.63‰,平均-4.66‰;δ13C2分布在-27.36‰~-18.70‰,平均-25.27‰;δ13C3分布在-25.96‰~-14.69‰,平均-22.93‰,且煤层气主要为热成因气体;δ13C1(CH4)较浅部小,δ13CCO_(2)较浅部大。在平面上,从临兴地区东南向西北δ13C1呈增大趋势。从石炭纪起直到三叠纪末,研究区一直处于沉降状态;中侏罗世和早白垩世为主要生烃期,中侏罗世生烃持续时间较长,平均生烃速率较小,生烃产率一般;早白垩世生烃速率较大,生烃产率增加值较高,生气强度达到历史最大,是主要的生烃期。受气体分馏作用、储层孔裂隙发育特征、储层压力以及煤层顶底板密封性的控制,δ13C1随着镜质体最大反射率(Rmax)、含气量和埋深的增加而增加;在向斜核部区、断层应力集中区和褶皱两翼发育区的δ13C1值相对较大;在断层应力释放区和背斜核部δ13C1相对较大,这表明该区域的煤层气封存条件较差,不利于煤层气富集;受地下水溶解的影响,在水动力条件较强的地区δ13C1值较小;在弱流体动力学或滞流区δ13C1没有明显的变轻趋势。综合煤层气地球化学指标认为,临兴和神府地区东南部煤层气富集,且前者更有利,这对后期煤层气选区布井提供参考。

Identifying the origin and geochemical characteristics of deep coalbed methane(CBM)is critical for its ex-ploration and production.This study examined the desorbed gas from deep-CBM parameter wells in the Linxing-Shenfu block on the eastern margin of the Ordos Basin.It systematically tested the composition,content,and carbon isotopes of CBM in coal seams Nos.8 and 9.In combination with the degree of thermal evolution,gas-bearing properties,coal seam burial depths,geological structures,and hydrogeologic conditions,it delved into the geochemical characteristics and in-fluencing factors of deep CBM.The results show that the deep CBM in the study area consists primarily of CH_(4),CO_(2),N_(2),ethane,propane,isobutane,n-butane,isopentane,and n-pentane.Among them,CH4 exhibits the highest content,with volume fractions ranging from 75.98%to 96.20%(average:86.49%),succeeded by ethane(average:5.97%),CO_(2)(aver-age:5.18%),and propane(average:1.24%),exhibiting subtle differences in gas content compared to shallow CBM.The deep CBM manifestsδ13C1(CH4),δ13CCO_(2),δ13C2,andδ13C3 values ranging from-51.9‰to-38.2‰(average:-42.84‰),from-8.6‰to-0.63‰(average:-4.66‰),from-27.36‰to-18.70‰(average:-25.27‰),and from-25.96‰to-14.69‰(average:-22.93‰),respectively.It predominantly proves to be thermogenic gas,displaying lowerδ13C1(CH4)values and higherδ13CCO_(2) values compared to shallow CBM.On a horizontal plane,itsδ13C1 values show a grow-ing trend from southeast to northwest in the Linxing area.The study area kept subsiding from the Carboniferous to the end of the Triassic,with the Middle Jurassic and the Early Cretaceous identified as the primary hydrocarbon-generating phases.Specifically,the Middle Jurassic witnessed a prolonged duration,a low average rate,and moderate hydrocarbon yield of hydrocarbon generation.In contrast,the hydrocarbon generation during the Early Cretaceous featured a higher rate,a significantly increased yield,and a peak intensity,establishing the e Early Cretaceous as a dominant hydrocarbon-generating stage.Due to the controlling effects of gas fractionation,the characteristics of reservoir pores and fractures,reservoir pressure,and the roof and floor tightness of coal seams,theδ13C1 values increase with the increasing maxim-um vitrinite reflectance(Rmax),gas content,and burial depth.They are relatively high in synclinal cores,the stress con-centration zones of faults,and fold flanks.However,they are lower in the stress release zones of faults and anticlinal cores,suggesting inferior CBM storage conditions,unfavorable for gas accumulation,in these zones.Influenced by groundwater dissolution,the δ13C1 values are relatively low in areas with strong hydrodynamic conditions,while show-ing an insignificant downward trend in zones with weak fluid dynamics or fluid stagnation.The comprehensive investig-ation into the geochemical indicators of CBM leads to the conclusion that the Linxing and southeastern Shenfu areas are enriched in CBM,especially the former.This study can serve as a reference for target area selection and well placement in future CBM exploration and exploitation in the study area.