黔北正安地区五峰组——龙马溪组页岩硅质矿物成因及意义

Origins and significance of siliceous minerals in the shale reservoirs of the Wufeng-Longmaxi Formation in the Zheng'an area,northern Guizhou

我国南方地区早古生代海相富有机质页岩硅质矿物含量较高,其成因和来源对页岩储层物性和压裂改造效果至关重要。本文以黔北正安地区五峰组—龙马溪组页岩为研究对象,通过岩心和薄片观察、X衍射分析、场发射扫描电镜及主微量元素分析等技术方法,精细识别不同类型硅质矿物的结构,定量计算不同来源硅质矿物的含量,探讨不同成因硅质矿物对页岩孔隙和压裂改造的影响。结果表明:研究区五峰组—龙马溪组页岩硅质矿物含量较高(约为71.5%),主要为石英,多呈棱角状和椭圆状等形态,颗粒大小和结晶程度存在明显差异,反映陆源和生源两种成因,其中生物成因硅含量约为41.4%,陆源碎屑硅含量约为30.1%。相比于焦石坝地区,研究区五峰组—龙马溪组页岩硅质矿物含量较高,主要表现在碎屑硅含量的增加,生物成因硅含量相差不大。生物成因硅在沉积和成岩过程中,可以形成刚性颗粒支撑结构,有利于页岩储层的保存和压裂改造,而陆源碎屑硅对页岩脆性和压裂改造不利。因此,尽管研究区页岩硅质矿物含量总体较高,但在页岩储层物性和压裂改造效果方面不及焦石坝等地区。

Siliceous minerals,highly enriched in the organic shales of the early Paleozoic in South China,play a significant role in the development of shale reservoirs and volume fracturing.This study takes the shales of the Wufeng-Longmaxi Formation in the Zheng'an area of northern Guizhou as the research subject,utilizing a variety of methods and technologies,including rock and thin section observation,x-ray diffraction analysis,scanning electron microscopy(SEM)analysis as well as major and trace element analysis.Therethrough,we have identified the structure of different types of siliceous minerals,quantitatively measured their content,and deeply explored their influence on the development of porosity in the shales and fracturing reconstruction.Results show that siliceous minerals,enriched in shale reservoirs of the research area,are mostly composed of quartz of elliptical and irregular shape.The minerals vary in grain size and degree of crystallization,revealing their origins from two sources of terrigenous and biogenic respectively,in which terrigenous silica accounts for 30.1%,while biogenic silica accounts for 41.4%.Comparatively,siliceous minerals content in the shales of the Wufeng-Longmaxi Formation in the research area is higher than that in the Jiaoshiba of central Chongqing.The increase is predominantly due to a high amount of terrigenous,while the content of biological quartz varies scarcely.Biogenic silica forms a rigid structure during sedimentation and diagenesis,which facilitates the preservation and later fracturing reconstruction of the shale reservoirs.In contrast,the presence of terrigenous silica has the opposite effect.Thus,although the overall content of siliceous minerals in the shale of the research area is high,the conditions for forming optimal reservoir physical properties and subsequent fracturing reconstruction are less favorable than those in the Jiaoshiba area of central Chongqing.