煤层微观孔隙特征及沉积环境对孔隙结构的控制作用-以鄂尔多斯盆地8号煤层为例

Microscopic pore characteristics of coal seam and the controlling effect of sedimentary environment on pore structure in No.8 coal

寻找深部煤层气资源分布,亟待查明深部煤储层沉积环境和孔隙分布特征。为此,在开展鄂尔多斯盆地煤岩显微组分、核磁共振孔隙和电镜分析的基础上,查明了煤岩显微组分和孔隙分布特征,评价了鄂尔多斯盆地8号煤的煤沉积环境和孔隙结构特征之间的关系。结果表明:①研究区8煤层煤岩有机显微组分以镜质组为主,成煤早期植物类型应主要为木本植物,后期随着覆水程度增大而逐渐转变为木本+草本混生植物为主;②本溪期8号煤由早期为湿地森林沼泽逐步演化为后期开阔水域沼泽,沼泽水体相对滞留,沼泽环境稳定,利于有机质富集和煤层结构稳定;③研究区8煤大孔占比与结构保存指数(TPI)两者呈正相关,而与凝胶化指数(GI)呈负相关;④基质镜质体发育密集气孔群或稀疏带状气孔群,团块镜质体中发育少量气孔,结构镜质体中发育原始组织孔,多为矿物充填,均质镜质体气孔不发育。研究认为榆林地区煤层孔隙分布受到煤沉积环境和显微组分控制,开阔水域沼泽相的煤层孔隙结构呈现三峰,其中微孔和大孔发育最好,湿地森林沼泽相煤层孔隙结构呈现双峰,微孔发育好,大孔发育较差。

To accurately evaluate the distribution and potential of deep coalbed methane resources,it is critical to identify the coal sedimentary and pore distribution characteristics of deep coal reservoirs.Therefore,based on the analysis of coal macerals,nuclear magnetic resonance porosity,and electron microscopy,the characteristics of coal macerals and pore distribution in the Yulin area were identified,as well as the relationship between the coal sedimentary and pore structure characteristics of No.8 coal was evaluated.The results indicated that:①The organic macerals of No.8 coal seam in the study area were predominantly composed of vitrinite,indicating a predominance of woody plants during the early stages of coal formation.As the degree of water cover increases,it gradually transitions into a mixture of woody and herbaceous plants.②During the Benxi period,No.8 coal evolved from an early wetland forest swamp to a later open water swamp with stagnant swamp water,creating a stable environment conducive to organic matter enrichment and coal seam stability.③The proportion of macropores in No.8 coal seam was negatively correlated with the structure preservation index(TPI),while it was positively correlated with the gel index(GI).④Matrix vitrinite developed dense or sparse banded pores,mass vitrinite developed few pores,and the structure vitrinite developed original tissue pores,mostly filled with minerals,while the homogeneous vitrinite did not develop pores.In conclusion,pore distribution in Yulin's coal seams was controlled by sedimentation and macerals.The pore structure of coal seams in the open water swamp sedimentary presented three peaks,with micropores and macropores developing best,which can simultaneously enrich free gas and adsorbed gas.The pore structure of coal seams in the wetland forest swamp sedimentary presented two peaks,with micropores developing well and macropores developing poorly,mainly enriching adsorbed gas.