煤层气成藏机理及气藏类型划分——以鄂尔多斯盆地东缘为例

随着我国煤层气勘探开发工作的不断深入,定义和归纳赋存状态多样化的煤层气藏,并据此指导煤层气有利勘探目标优选就显得愈发重要。为此,在系统梳理国内外煤层气富集成藏理论的基础上,结合鄂尔多斯盆地东缘煤层气勘探开发的地质规律,借鉴常规油气藏按照圈闭成因进行分类的方法,提出了煤层气圈闭的概念和煤层气藏类型的划分方案。研究成果包括:①结合煤层、砂岩、泥岩和石灰岩在垂向上的组合关系,刻画了"泥裹藏、砂通煤、水包气"的气体封闭样式,"泥裹藏"即以泥岩等致密岩层封闭煤储层而成藏,"砂通煤"即上覆砂岩与煤层连通而与煤层统一形成气藏,"水包气"即在水力封闭和水力封堵作用下,气体在煤层中富集而成藏;②阐释了物性封闭、气压封闭、水压封闭和吸附封闭在煤层气藏形成过程中的作用,指出气藏开发的上、下限分别为风氧化带和地应力转折带;③提出了以构造、岩性、水动力和复合型圈闭为主导的气藏类型划分方案,并分别解剖了背斜、断层、向斜、地层—岩性、水力封堵和水力辅助型煤层气藏。

Accumulation mechanisms and classification of CBM reservoir types: A case study from the eastern margin of the Ordos Basin

With the continuous deepening of coalbed methane (CBM) exploration and development in China, it is becoming more and more important to define and summarize CBM reservoirs with diverse occurrence states and accordingly to guide the optimization of favorable CBM exploration targets. In this paper, domestic and foreign CBM enrichment and accumulation theories were systematically analyzed. Then, based on geological laws of CBM exploration in the eastern margin of the Ordos Basin, the concept of CBM traps and the classification scheme of CMB reservoirs were put forward in reference to the classification method of conventional oil and gas reservoirs based upon the genesis of traps. And the research results are as follows. First, the gas sealing pattern of "mudstone-surrounding reservoir, sandstone-connecting coal and water-enclosing gas" is depicted based on the vertical combination relationship of coal beds, sandstones, mudstones and limestones. The pattern of "mudstone-surrounding reservoir" refers to the hydrocarbon accumulation with tight rock beds (e.g. mudstones) to seal coal reservoirs. In the pattern of "sandstone-connecting coal", a gas reservoir is formed jointly by the coalbeds and its overlying sandstones when they are connected. In the pattern of “water-enclosing gas", gas is enriched and accumulated in coal beds under the effect of hydraulic sealing and plugging. Second, the effects of physical sealing, gas pressure sealing, water pressure sealingand adsorption sealing during the formation of CBM reservoirs are illustrated. And it is pointed out that the upper and lower limits of gas reservoir development are the efflorescent oxygenized zone and the in-situ stress transition zone, respectively. And third, the gas reservoirclassification scheme with structural, lithologic, hydrodynamic and compound traps as the leading factors is developed, and various CBM reservoirs are dissected, including anticline, fault, syncline, stratigraphy–lithology, hydraulic sealing and hydraulic assistance CBM gas reservoirs.