黔西北地区龙潭组海陆过渡相泥页岩孔隙分形特征

Fractal characteristics of the Longtan formation transitional shale in northwest Guizhou

在黔西北地区海陆过渡相龙潭组页岩低温氮气吸附实验基础上,利用FHH模型研究了其孔隙分形维数,得到其分形维数D_1和D_2,整体数值较大,平均值分别为2.718 6和2.782 4,反映龙潭组泥页岩孔隙非均质较强,结构较为复杂。将分形维数与页岩其他地质参数进行相关性分析,结果表明:分形维数与泥页岩的比表面积、TOC呈正相关关系,与平均孔径、石英含量、石英/黏土呈负相关关系,与镜质体反射率、黏土矿物含量呈"先正后负"的相关关系,拐点分别出现在Ro为2.5%以及黏土矿物含量为50%处。综合研究认为:D_1能够较为综合反映泥页岩孔隙的大小、体积、比表面等特征,可以综合表征泥页岩孔隙的结构特征,数值越大越有利于页岩气富集,而D_2对于孔隙结构复杂程度的表征更为明显,可以专用于研究泥页岩孔隙结构的复杂程度,数值需在一定范围内,不宜过大,并认为黔西北地区龙潭组海陆过渡相泥页岩孔隙分形维数D_1的有利分布范围为大于2.60,而D_2的有利分布范围为2.50~2.80,当D_2>2.80时需要系统进行可采性评价及压裂开采设计。

Based on the low temperature Nitrogen adsorption experiments of transitional facies of Longtan Formation shale in the northwest Guizhou,the pore fractal dimension was studied by using the FHH model,and the fractal dimension of D_1 and D_2 were obtained. The average values were 2.718 6 and 2.782 4,reflecting the complexity of Longtan Formation shale pore. The correlation analysis of fractal dimension and other geological parameters showed that the fractal dimension with shale surface area,TOC are positively correlated,with the average pore size,the content of quartz,quartz/clay are negatively correlated,with Roand the content of clay minerals are related positively first and negatively later at the inflection point of Ro= 2.5% and the content of clay minerals of 50%.The study results show that D1 can be more comprehensive response to the shale pore size,volume,and surface characteristics.Also,the structural features can be integrated to characterize shale pore. The numerical value is more conducive to the accumulation of shale gas.D2 is more obvious for complex pore structure characterization,and can be used in the research of shale pore structure.The value should be in a certain range,should not be too large.In the northwest of Guizhou Longtan Forma-tion,the transitional facies shale pore fractal dimension of D1 is greater than 2.60,and the distribution of the favorable range of D2 is 2.50-2.80,when D2 is higher than 2.80,then the recoverability evaluation and the fracturing design are necessary.The significance of this research is to provide new ideas and means for the exploration and development of shale gas in Guizhou area.