煤层气封闭体系及其与煤层气富集的关系

CBM sealing system and its relationship with CBM enrichment

研究封闭体系对煤层气富集的控制作用对于指导煤层气勘探开发具有重要的意义。为此,分析了煤层气富集与封隔层(包括区域盖层、区域底板和直接顶底板)、后期构造调整、地层产状等地质要素的关系,结果发现具有区域性泥岩盖层及底板分布稳定、成藏期后构造抬升—回返幅度小,地层平缓且处于均势状态的地区煤层气易于富集。进一步结合多年来国内外煤层气的勘探开发实践,提出了封闭体系的概念,即具有阻止气体向上、下运移的封盖层和侧向稳定带组成的地质单元,在封闭体系环境中不仅煤层气富集,而且煤系气也可以富集成藏。最后根据煤层(系)气的封闭体系要素配置关系,划分了"三明治"式煤层气藏、煤层气—砂岩气共生气藏及煤成砂岩气藏等3种气藏类型,提出了煤层气勘探思路应由单纯的煤层气勘探向煤层气与煤系气立体勘探的方向转变,同时建立了煤层(系)气地质选区评价指标体系,指出在准噶尔盆地与鄂尔多斯盆地东缘开展煤层气与煤系气立体勘探有可能取得良好的应用效果。

It is of great significance to study the controlling effect of sealing systems on CBM enrichment in coalbed methane （CBM） ex- ploration and development. In this paper, the relationships between CBM enrichment and geological elements were analyzed. The geolog- ical elements include sealing layers （e.g. regional cap rock, regional floor, and immediate roof and floor）, later structural adjustment and strata production status. It is shown that CBM tends to enrich in the areas where regional mudstone cap rocks and floors are distributed stably, structures are uplifted and inversed slightly after the hydrocarbon accumulation period and the strata is gentle in a balanced state. Then, the concept of sealing system was put forward based on the worldwide CBM exploration and development practices over the years. A sealing system refers to a geological unit composed of a lateral stable zone and cap rock which prevents gas from migrating upward and downward. In a sealing system, CBM can get enriched and coal-measure gas can also be accumulated. Finally, three gas reservoir types （i.e., sandwich-type CBM reservoir, associated CBM-sandstone gas reservoir and coal-derived sandstone gas reservoir） were identified based on the configuration relationships between elements of the CBM （or coal-measure gas） sealing system. It is recommended to change the exploration ideas from simple CBM exploration to 3D CBM and coal-measure gas exploration. In addition, an evaluation index system of CBM （or coal-measure gas） geological selection was established. It is pointed out that good application effects may be realized if the ste- reoscopic CBM and coal-measure gas exploration is applied in the Junggar Basin and the eastern margin of the Ordos Basin.