沁水盆地南部煤层气地球化学特征及影响因素

Geochemical Characteristics of Coalbed Gas and Influencing Factors in the South of Qinshui Basin

为进一步厘清沁水盆地高阶煤煤层气富集机理,综合运用地球化学分析测试技术,系统刻画了沁水盆地南部煤层含气量和煤层气分子组成特征,初步探讨了其影响因素。研究结果显示沁水盆地南部各区块煤层含气量呈南高北低分布趋势。煤层含气量与煤化程度具有显著正相关性,这可能与煤层内有机孔隙发育关系密切;与煤层厚度,尤其薄煤层厚度(≤2m)具显著正相关性,当煤层厚度大于2m时这种相关性反而不甚明显,表明研究区煤层气赋存状态以吸附态为主,薄煤层中气体饱和度相对较低;含气量与煤层埋深和上覆地层剥蚀量之间在南部区块没有显著相关性,在北部区块呈现出弱相关性,表明煤层气主要以吸附态存在,北部区块保存状况可能相对较好。研究区煤层气属于典型干气,南部区块煤层气甲烷含量和稳定碳同位素值均较北部区块高,非烃气体含量则相对较低。煤化作用程度是控制煤层气分子组成和煤层甲烷稳定碳同位素组成的重要因素。煤层气吸附-解吸-扩散-运移散失可导致煤层气富含CO_2,甲烷稳定碳同位素值偏重。次生生物气的生成对部分煤层甲烷稳定碳同位素组成影响显著。该研究对于寻找沁水盆地煤层气"甜点"区具有重要意义。

In order to decipher the mechanisms of coalbed gas enrichment within high-rank coals, this study concentrates on the detailed characteristics of gas contents, molecular and isotopic compositions of coalbed gas in the south of Qinshui Basin and discusses the possible influencing factors by using geochemical analysis methods. The results demonstrate that gas contents are higher in the southern part of the study area than those in the northern part. A positive correlation can be observed between gas contents and thermal maturity of coalbeds. This is probably attributed to the occurrence of organic pores within these over-matured coals. The correlations between gas contents and the thickness are strongly positive for the coalbeds with the thickness ≤2 m, and are weak for the coalbeds with the thickness 〉2 m. This suggests that ad- sorbed gas plays the major part and gas saturation is comparatively lower within the thin coalbeds ≤2 m）. Gas contents present no correlation with increasing burial depth and the eroded thickness of the overburden formation in the southern part of the study area, and show a slightly weak correlation in the northern area. This result indicates that adsorbed gas still acts as the critical role in this region and a compara- tively better preservation condition of natural gas in the northern area. The produced gas is mainly dry gas in this region. Natural gases from the southern area contain more methane and are ^13C-enriched, but contain less non-hydrocarbon gases than those from the northern area. The coalification level has a controlling effect on molecular and isotopic compositions of coalbed gases. The process of gas adsorption-desorption- diffusive migration-leakage can result in the enrichments of CO2 and isotopically heavier methane. Secondary biogenic gas generation also exerts a significant impact on isotopic composition of methane in several gas samples. This study should be useful in the exploration of sweet spot of coalbed methane in Qinshui Basin.