陆相与过渡相煤系页岩孔隙结构及分形特征对比——以鄂尔多斯盆地东北缘延安组与太原组为例

基于低温氮气吸附分形几何学研究方法,对鄂尔多斯盆地东北缘陆相延安组及海陆过渡相太原组富有机质煤系页岩进行孔隙结构和分形特征研究,运用分形FHH模型计算了大孔隙(4.34~100nm)和小孔隙(4.34nm)对应的分形维数D_1与D_2,对比讨论了孔隙结构参数与分形维数的关系,以及TOC、矿物含量对两者的影响。研究结果表明:(1)延安组孔径分布在1.8~59nm,呈"双峰型",以墨水瓶状、狭缝状和平行板状孔为主;太原组孔径主要分布在3~4.5nm,呈"单峰型",以墨水瓶状孔为主。(2)煤系页岩具有双重分形特征,D_1与D_2正相关,且D_1D_2,表明大孔隙空间结构更加复杂。延安组两类孔隙空间结构均比较复杂,太原组大孔隙空间结构非常复杂,而小孔隙均质性强。(3)煤系页岩平均孔径越小,微小孔隙越多,比表面积越大,总孔体积越大,分形维数越大,即孔隙结构越复杂,孔表面越不规则;延安组D_1、D_2和太原组D_1均可反映各自孔隙结构特征。(4)太原组孔隙结构参数和分形维数受TOC及矿物成分含量影响较延安组明显。(5)延安组页岩储层优于太原组,更利于煤系页岩气的吸附、赋存、扩散和渗流。

Comparison between pore structure and fractal characteristics of continental and transitional coal measures shale:a case study of Yan'an and Taiyuan formations at the northeastern margin of Ordos Basin

Based on the fractal geometry method for studying low-temperature nitrogen adsorption, the pore structures and fractal characteristics of organic-rich coal measures shale in continental Yan'an Formation and sea-land transitional Taiyuan Formation at the northeastern margin of Ordos Basin are studied in this study. The fractal FHH model is used to calculate the fractal dimensions D₁ and D₂ of the large pores (4.34-100 nm) and small pores (less than 4.34 nm), so as to comparatively analyze the relationship between pore structure parameters and fractal dimensions as well as the influences of TOC and mineral compositions on the both. The results are as below:(1) In Yan'an Formation, pore sizes are distributed in the range of 1.8-59 nm, showing a "bimodal" pattern; ink-bottle, slit and plate-like pores are dominantly developed. In Taiyuan Formation, pore sizes are ranged in 3-4.5 nm, showing a "unimodal" pattern; ink-bottle pores are mainly developed. (2) Coal measures shale has double fractal characteristics. D₁ and D₂ are positively correlated, and D₁ is larger than D₂, indicating the more complex structures of large pores. Besides, the spatial structures of two types of pores in Yan'an Formation are all complex, while those of large pores in Taiyuan Formation are very complex, but small pores have strong heterogeneity. (3) The smaller the average pore size of coal measures shale is, the more the small pores will be, the larger the specific surface area will be, the bigger the total pore volume will be, and the higher the fractal dimension will be, i.e., the more complex the pore structure is, the more irregular the pore surface will be. The D₁ and D₂ of Yan'an Formation and D₂ of Taiyuan Formation can reflect the corresponding pore structure characteristics, respectively. (4) The pore structure parameters and fractal dimensions of Taiyuan Formation are more significantly affected by TOC and mineral compositions than those of Yan'an Formation. (5) As compared with Taiyuan Formation, Yan'an Formation has better shale reservoir, and is more favorable for the adsorption, occurrence, diffusion and flow of coal measures shale gas.