低煤阶煤层气成藏模式和勘探方向

中国低煤阶煤层气资源丰富,但勘探和认识程度较低,煤层气成藏的主控因素和富集规律认识不足。在分析国内外不同盆地、不同区块低煤阶煤层气含气量随埋深变化趋势差异的基础上,从现今和古水文地质条件两个方面,探讨了中国低煤阶煤层气成藏的前提条件,分析了成煤环境对含气量的影响。研究结果表明：①低阶煤（R_o<0.65%）主要分布于埋深<1 200 m的中-新生界煤系地层中,低煤阶煤层气主要是次生生物成因气;②低煤阶煤层气含气量随埋深总体呈现先增加后降低的趋势,普遍存在"最佳深度"这一特征,现今淡水入渗或古淡水入渗水文地质条件是次生生物气生成的前提条件;③除淡水入渗水文地质条件控制低煤阶煤层气的生成与富集外,煤岩组分、煤层厚度、煤岩顶底岩性以及煤岩物性等储层特征对含气量也有重要影响;④低煤阶煤层气存在盆缘斜坡带新近成藏和小型断陷古成藏两类成藏模式,指示了低煤阶煤层气勘探的两个方向。

Low coal rank coalbed methane accumulation model and exploration direction

Low coal rank coalbed methane resources are rich in China, but are less known and explored; there is still insufficient understanding of the main controlling factors and enrichment laws of coalbed methane accumulation. Based on analyzing the variation trends in the gas content of low coal rank coalbed methane changing with depths in different basins and blocks worldwide, this paper explores the premise of low coal rank coalbed methane accumulation in terms of the present and ancient hydrogeological conditions, and also analyzes the impact of coal-forming environment on gas content. Research indicates:(1) Low rank coals (R_o<0.65%) are mainly distributed in the Mesozoic and Cenozoic coal measures strata with buried depth<1200 m, and low rank coalbed methane is mainly dominated by secondary biogenic gas. (2) The gas content of low rank coal tend to increase firstly and then decrease with the buried depth, generally showing an optimal depth. The present hydrogeological conditions of fresh water infiltration or paleo-fresh water infiltration are the preconditions for secondary biogas generation. (3) In addition to the formation and enrichment of low coal rank coalbed methane controlled by hydrogeological conditions for freshwater infiltration, the reservoir characteristics such as coal-rock component, coal seam thickness, coal-rock top lithology and coal-rock physical properties also have an important influence on gas content. (4) Low coal rank coalbed methane presents two accumulation modes, i.e., the Neogene hydrocarbon accumulation in marginal slope zone of the basin and the paleo-accumulation in small fault depressions, indicating two directions of low coal rank coalbed methane exploration.