Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin,NW China and the Cretaceous Qingshankou Formation in the Songliao Basin,NE China as research objects,the characteristics and origins of di...Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin,NW China and the Cretaceous Qingshankou Formation in the Songliao Basin,NE China as research objects,the characteristics and origins of different types of silica in the shales have been studied by means of core observation,thin section identification,cathodoluminescence,X-ray diffraction analysis,scanning electron microscope(SEM),electron probe and rock pyrolysis.The results shows that the origins of silica include felsic mineral dissolution,tuffite devitrification,clay mineral transformation and siliceous mineral metasomatism.The silica formed by feldspar dissolution commonly appears as spots and veins,with low degree of crystallization,and is largely aqueous opal mineral,with an average Si O2 content of 67.2%.Silica formed by devitrification of tuffite mainly occurs in two forms,amorphous silica and authigenic quartz with better crystal shape.The authigenic silica formed during the transformation of clay minerals is embedded in the clay minerals in the form of micron-scale plates and small flakes,or mixed with clay minerals in a dispersed state.The authigenic quartz formed by siliceous mineral metasomatism is in better angular crystal shape,and has an average Si O2 content of 87%.The authigenic siliceous mineral content is positively correlated with the content of terrigenous felsic minerals.The pressure solution of felsic minerals is the main source of authigenic siliceous minerals,followed by the transformation of clay minerals,and the organic matter has some boost on the formation of authigenic silica.The authigenic siliceous materials of different origins have different geological characteristics and occurrence states from terrigenous quartz,which would affect the storage performance,seepage capacity and fracturing effect of continental shale.Although the organic-rich shale has high silica content,different from terrigenous quartz,authigenic silica in this kind of shale mostly floats and disperse in clay minerals,which would have negative effect on the formation of complex fractures in fracturing,fracture support ability after fracturing,and formation of effective seepage channels.Calculating the brittleness index of shale intervals only based on the composition of brittle minerals cannot accurately characterize mechanical characteristics of continental shale oil reservoirs,and would affect comprehensive evaluation and selection of continental shale oil“sweet spots”.展开更多
基金National Natural Science Foundation of China(42072186)Science and Technology Project of Petro China(2021-DJ2203)Scientific Research and Technology Development Project of Petro China(2022KT0302-1)。
文摘Taking lake basin shales of the Triassic Yanchang Formation in the Ordos Basin,NW China and the Cretaceous Qingshankou Formation in the Songliao Basin,NE China as research objects,the characteristics and origins of different types of silica in the shales have been studied by means of core observation,thin section identification,cathodoluminescence,X-ray diffraction analysis,scanning electron microscope(SEM),electron probe and rock pyrolysis.The results shows that the origins of silica include felsic mineral dissolution,tuffite devitrification,clay mineral transformation and siliceous mineral metasomatism.The silica formed by feldspar dissolution commonly appears as spots and veins,with low degree of crystallization,and is largely aqueous opal mineral,with an average Si O2 content of 67.2%.Silica formed by devitrification of tuffite mainly occurs in two forms,amorphous silica and authigenic quartz with better crystal shape.The authigenic silica formed during the transformation of clay minerals is embedded in the clay minerals in the form of micron-scale plates and small flakes,or mixed with clay minerals in a dispersed state.The authigenic quartz formed by siliceous mineral metasomatism is in better angular crystal shape,and has an average Si O2 content of 87%.The authigenic siliceous mineral content is positively correlated with the content of terrigenous felsic minerals.The pressure solution of felsic minerals is the main source of authigenic siliceous minerals,followed by the transformation of clay minerals,and the organic matter has some boost on the formation of authigenic silica.The authigenic siliceous materials of different origins have different geological characteristics and occurrence states from terrigenous quartz,which would affect the storage performance,seepage capacity and fracturing effect of continental shale.Although the organic-rich shale has high silica content,different from terrigenous quartz,authigenic silica in this kind of shale mostly floats and disperse in clay minerals,which would have negative effect on the formation of complex fractures in fracturing,fracture support ability after fracturing,and formation of effective seepage channels.Calculating the brittleness index of shale intervals only based on the composition of brittle minerals cannot accurately characterize mechanical characteristics of continental shale oil reservoirs,and would affect comprehensive evaluation and selection of continental shale oil“sweet spots”.
