沁水盆地陷落柱特征及其对气藏的影响

Characteristics of collapsed columns in Qinshui Basin and its impact on gas reservoir

沁水盆地广泛发育岩溶陷落柱,对气藏形成与分布产生了较大影响.为分析其对气藏的影响,通过对陷落柱的形成条件、发育特征及分布规律等方面的研究发现,沁水盆地陷落柱主要形成于喜马拉雅期;在平面上成群集中分布,多为半截柱,直径为100～300 m,大多切穿主煤层的3号煤层;陷落柱内发育了大量破碎带及松散堆积,为煤层及砂岩中的天然气向外运移提供了通道;陷落柱形成并切穿煤层以后,使煤层压力降低,为柱内及附近的煤层气解析提供了条件;陷落柱还沟通了煤系地层上、下相邻的含水层,致使不同地层之间水动力发生联系,使得解析出来的天然气以游离态或伴随地下水循环运移至上覆地层中.因此,除个别情况起到输导作用在浅层形成新的气藏外,陷落柱对煤系地层中的气藏常起到破坏作用,在其周围形成煤系地层含气量低值区,致使天然气中甲烷含量降低,氮气和二氧化碳含量增高,但陷落柱的影响范围较局限.

Karst collapsed columns are widely developed in Qinshui basin,which has great influence on formation and distribution of gas accumulation. In order to analyze its influence on gas accumulation,by studying the column's formation conditions,characteristics and distribution,it is found that the columns mainly formed during the time of Himalayan movement;they distribute in groups in the plane and most of them are half columns with a general diameter range of 100-300 m,cut through the upper main coal bed,namely the 3rdcoal bed. Lots of broken belts and decompaction belts developed in the collapsed columns,which provide pathways for the gas accumulating in coal and sandstone to migrate outside. The seam pressure declines after the formed columns cut through the coal bed,which provides conditions for the coal seam gas desorption. The collapsed columns also communicate the seam vertically with the upper and the lower adjacent aquifers,which results in the hydrodynamic connectivity of different strata and the migration of desorbed gas into the upper stratum in free state or accompanying with underwater circulation. So the collapsed columns would play a role in destroying the gas accumulation in the coal measure strata,which leads to a low coal seam gas concentration around the columns and a decline methane concentration and an increasing concentration of N2,CO2. Additionally,several collapsed columns play a role in migrating seam gas into shallow strata to form new gas reservoirs. But most of the cases show that the areas influenced by the columns are always limited.