陆相页岩孔隙结构特征研究——以鄂尔多斯盆地延长组页岩为例

Characterization of lacustrine shale pore structure:An example from the Upper-Triassic Yanchang Formation,Ordos Basin

鄂尔多斯盆地三叠系延长组张家滩陆相页岩层系中发育大量粉砂质纹层,与相邻纯页岩呈薄互层状产出。为了更全面地研究页岩层系的孔隙结构发育特征,采用物理方法分离页岩岩心中的粉砂质纹层和纯页岩层,综合采用二氧化碳吸附法、氮气吸附法和压汞法获得全孔径范围内的孔隙结构特征。结果表明：粉砂质纹层孔径分布特征呈多峰分布,发育微米级孔隙,孔径较大,总孔体积平均为2.02mL/g,孔隙度平均为5.40%,大孔（〉50nm）是其孔体积的主要贡献者;纯页岩孔径分布特征呈双峰分布,主要分布在100nm以下孔径区间,孔径较小,总孔体积平均为1.41mL/g,孔隙度平均为3.67%,中孔（2～50nm）是其孔体积的主要贡献者。粉砂质纹层的实测孔隙度是纯页岩孔隙度的1.2～1.8倍,估算渗透率比纯页岩大2～4个数量级。因此,页岩层系中的粉砂质纹层相对纯页岩具有较大的孔径和孔体积,有利于提高页岩层系整体的储渗能力。

Amounts of silty laminae in continental shale gas reservoir were investigated for the Zhangjiatan shale of Yanchang Formation, Ordos Basin.The purpose of this study is to provide awareness of the nature and discrepancies in pore structure between the silty laminae and clayey laminae. By separating the silty laminae from the shale core mechanically,a combination measurement series of mercury injection capillary pressure, N2 adsorption and carbon dioxide adsorption were performed on the two parts.An integrated pore size distribution covering pore diameter range of 0.1nm-100μm was obtained by using appropriate sample particle size and calculation model.The comparative analysis of pore structure shows that the clayey laminae are dominated by mesopore and micropore while the silty laminae are dominated by macropore. The pore volume distribution in clayey laminae can be sorted as mesopore〉micropore〉macropore while in silty laminae it is macropore〉 mesopore〉micropore. The average total pore volume is 2.02m3/100g for silty laminae and 1.41m3/100g for clayey laminae The porosity of silty laminae is 5.40% ,which is larger than that of clayey laminae 3.67%Since the silty laminae have larger pore width and pore space, they are more permeable and porous than the clayey laminae and can act as favorable conduits and reservoirs for shale gas.