柳林区块多煤层含气性差异与煤层气成藏模式

Difference characteristics in gas-bearing between multiple coal seams and coalbed methane accumulation models in Liulin Area

基于柳林地区山西组与太原组煤层含气性差异特征,以保存条件为重点,从煤层顶板岩性和水文地质条件2个方面,剖析了2组煤层含气性差异控制因素,总结了2组煤层气成藏模式。研究表明,煤层顶板岩性和水动力条件共同控制了该区2组煤层含气性的差异。山西组煤层含气量和甲烷浓度总体高于太原组煤层;太原组煤层二氧化碳浓度偏高,甲烷碳同位素偏轻;山西组煤层顶板为弱含水泥岩和砂岩,有利于煤层气富集;太原组煤层顶板为强含水灰岩,不利于煤层气富集。在活跃的水动力条件下,太原组煤层碳同位素分馏作用和甲烷与二氧化碳间的碳同位素交换作用,是造成太原组煤层二氧化碳浓度偏高、甲烷碳同位素偏轻的根本原因。研究认为,山西组煤层气藏为气压封闭型气藏,太原组煤层气藏为水力封闭型气藏。

Based on the difference characteristics in gas-bearing between Shanxi Formation and Taiyuan Formation in Liulin Area,with an emphasis on preservation conditions,the controlling factors of the difference characteristics are analyzed with a view to coal roof lithology and hydrogeological conditions,and accumulation models of the coalbed methane（ CBM） in two formation are summarized.The study shows that the coal roof lithology and hydrodynamic conditions control the difference in gas-bearing between coal seams in two formations comprehensively.The gas content and methane（ CH4） concentration in Shanxi formation coal seam are higher than Taiyuan formation coal seam generally.The carbon dioxide（ CO2） concentration of Taiyuan formation coal seam is higher,and the CH4 carbon isotope is lighter.The coal seam roof in Shanxi formation is mudstone and sandstone,which is a weak aquifer and is favorable to the accumulation of CBM.However,the coal seam roof in Taiyuan formation is limestone,which is a strong aquifer and is not conductive to the accumulation of CBM.With the active hydrodynamic conditions,CH4 carbon isotope fractionation effect and carbon isotopic exchange between CH4 and CO2are the primary causes of higher CO2 concentration and lighter CH4 carbon isotope in Taiyuan formation coal seam.The CBM reservoir in Shanxi formation belongs to pressure closed gas reservoir,and the CBM reservoir in Taiyuan formation belongs to hydraulic closed gas reservoir.