鄂尔多斯盆地兴县地区煤层气地球化学特征及成因

煤层气化学组分、甲烷碳氢同位素特征对煤层气成因、分布规律和煤层气资源评价具有重要意义。为了查明河东煤田北部兴县地区山西组、太原组煤层甲烷及二氧化碳成因,采集研究区煤层气井解吸气样,通过组分分析、CH_4碳氢同位素和CO_2碳同位素测试,根据煤层气成因图版,分析了煤层气稳定同位素的地质影响因素,揭示了研究区煤层气成因。结果表明,区内主力煤层的甲烷碳同位素存在明显差异:8煤甲烷δ¹³C_1值介于–55.1‰~–44.2‰,平均为–49.2‰;13煤δ¹³C_1值介于–65.7‰~–55.7‰,平均为–59.8‰。同一煤层内甲烷碳同位素呈现出随煤层埋深增加而变重、随水动力条件增强变轻的特点;甲烷碳同位素偏轻,重烃组分偏少,表明受到一定因素或次生作用的影响。8煤以热成因气为主,13煤以次生生物成因气为主。研究区8煤δ¹³C(CO_2)介于–17.3‰~–4.8‰,13煤δ¹³C(CO_2)介于–26.3‰~–6.9‰,二氧化碳为煤热演化初期或最近一次煤层抬升再沉降后煤中有机质热裂解产生。研究成果为明确该区煤层气勘探开发方向提供了理论依据。

Geochemical characteristics and their origin of CBM in Xingxian area,Ordos basin

The distribution of carbon and hydrogen isotope of methane is of signification in revealing the origin, the distribution of CBM reservoir, and the source evaluation of CBM. To study the origins of coalbed methane and carbon dioxide, the data of the components、carbon and hydrogen isotopes of CBM from the desorption gas of coal core from Xingxian area were analyzed. With the CBM origins chart board, and the geological environment, the geological effect of the formation and the evolution of CBM were analyzed. the ratios of CBM with different origins were estimated. The results show that the δ13C, values of CH₄ in coal seams 8 are between -55.1‰ and -44.2‰, with an average of -49.2‰; the δ13C₁ values of CH₄ in coal seams 13 are between -65.7‰ and -55.7‰, with an average of -59.8‰. The carbon isotopes of CH₄ of the same coal seam were characterized by increase with the buried depth and decrease with the hydrodynamic conditions. The methane carbon isotope values were obviously lighter and the heavy hydrocarbon component were less, which show the gas was influenced by some factors and secondary transformation. Tectonic-thermal history and geological conditions of coal seams show that the forming conditions of secondary biogenic gas were found, and the biogas was generated by the reduction of CO₂. The δ13C values of CO₂ in coal seams 8 are between -17.3‰ and -4.8‰, the δ13C values of CO₂ in coal seams 13 are between -26.3‰ and -6.9‰, the CO₂ could be generated from thermal degration or thermal cracking or both at the early stage of coalification or after the latest coal seam uplift. This study provides a theoretical basis for the direction of coalbed methane exploration and development in this area.