准南米泉地区低煤阶煤次生生物气藏形成机制研究

Much attention is paid to biogenic coalbed methane (CBM), whose generation is known as longevity, continuity and universality. The exploration practice indicates that low-rank secondary biogenic gases are widely developed in the southern Junggar Basin, but its accumulation characteristics in Miquan region are obviously different from the other coal-bearing basins, which is thought likely to be rare paleo-secondary biogenic gas pools, further experimental verification and the study of its forming mechanism are needed. Low-rank CBM in Miquan region will be taken as research object, on basis of the identification of secondary biogenic gas feature, in-situ simulation experiment of biogenic methanogenesis is firstly performed to verify its supply potential of current secondary biogenic gas. Secondly, combined with basin numerical modelling and paleoclimatic data, multiple testing methods of conventional ion, hyrogen-oxygen isotope, radioisotope dating and fluid-inclusion analysis will be used to explore the history evolution process of coalbed water and gas-water-rock reactions, clarify the critical moment of paleo-secondary biogenic gas accumulation and its corresponding geological event, reveal enrichment mechanisms of paleo-secondary biogenic gas under the controls of ancient freshwater infiltration in the geological history. Thirdly, accumulation mechanisms of CBM migration fractionation and separation will be studied to clarify genesis and the forming process of abnormally high content CO2 in deep coal reservoir. Finally, combined with the current regional geological background, the forming mechanisms of secondary biogenic gas pools in Miquan region will be revealed.

煤层生物气的生成以其长期性、持续性、普遍性而备受人们关注。勘探实践表明，准南广泛赋存了典型的低煤阶煤次生生物气，但米泉地区生物气成藏特征明显区别于其他低煤阶含煤盆地。申请人初步研究认为其很可能是罕见的古次生生物气藏，进一步的实验验证及其形成机制研究亟待开展。项目拟以米泉地区低煤阶煤层气为研究对象，在次生生物气特征判识研究的基础上，开展原位环境下微生物生烃模拟实验，验证该区现今次生生物气补给潜力；应用常规离子、氢-氧同位素、放射性同位素测年及流体包裹体分析等多种测试方法，结合古气候资料与盆地数值模拟手段，探究煤层水历史演化过程和“气-水-岩”相互作用机制，厘清古次生生物气成藏关键时刻及地质事件，揭示地质历史时期古淡水入渗主导下古次生生物气富集机制；开展煤层气的运移分馏/分离聚集机理研究，厘清深部异常高含量CO2成因及形成地质过程；结合现今区域地质背景，最终揭示米泉地区次生生物气藏形成机制。

勘探实践表明，准南地区广泛赋存了典型的低煤阶煤次生生物气，但米泉地区生物气成藏特征与其他低煤阶含煤盆地存在明显区别。本项目对米泉地区煤样、煤层气井产出气样与水样开展了大量的实验分析，明确了煤层气成因及异常高浓度CO2的富集机制；开展了原位条件厌氧发酵产气模拟实验，验证了该区现今水文地质条件下次生生物气的补给潜力；对比水化学特征与微生物群落分析数据，厘清了该区早期微生物产甲烷活动受到抑制的机理与地质过程；最终，结合煤层气勘探资料，揭示了该区封存性次生生物气藏的形成机制。.米泉地区低煤阶煤层气主要由CO2还原途径产生的次生生物气组成，可能含有极少量的热成因气混合；此外，异常高浓度CO2主要为微生物产甲烷活动的副产物，其富集过程受到水体环境、气候变化以及微生物产甲烷活动停止等因素影响，不存在深部幔源CO2的补给。产气模拟实验表明，在更为有利的条件下，煤层水中微生物群落将煤岩有机质转化为甲烷的效率极低，说明米泉地区在现今水文地质条件下不存在或较少存在次生生物气的补给。.米泉地区微生物产甲烷活动的限速机制是：(1)水体环境的逐渐停滞导致盐度增加，微生物可利用的营养物质减少；(2)生态环境的变化决定了微生物群落组成的变化；(3)产酸发酵菌和产氢生乙酸菌的活性受到严重限制；(4)直接利用产甲烷菌(如H2和醋酸盐)的底物产量急剧下降；(5)微生物产甲烷受到限制，直至最终停止。.米泉地区封存性次生生物气藏形成地质过程，可划分为三个阶段，即，（1）埋藏+煤化作用阶段，无大量热成因气形成；（2）抬升+剥蚀阶段，地表淡水补给，微生物产甲烷活动产生大量次生生物气；（3）干旱气候阶段，水体环境逐渐停滞，盐度增加、营养物质减少，菌群组成发生变化，微生物产甲烷活动受到抑制，水动力场逐渐封存次生生物气藏。.上述研究成果进一步完善了我国煤层气成藏地质理论，对于指导低煤阶煤层气勘探开发具有一定的现实意义。