Abstract:

The Wufeng and Longmaxi formations are the key shale gas-enriched lithofacies in the Sichuan Basin,drawing significant attention to its sedimentary characteristics and genetic mechanisms. This study,employing MaipSCAN artificial intelligence electron microscopy mineral scanning,thin sections,field emission scanning electron microscopy,and helium ion microscopy techniques,reveals the mineral composition and vertical as well as horizontal distribution characteristics of the siliceous shale in the Zhaotong-Taiyang area of the Wufeng and Longmaxi formations. The research indicates that the siliceous shale in this region exhibits stable and widespread thickness,with major minerals including quartz,feldspar,clay,pyrite,calcite,and dolomite. Vertically,the silicon content decreases gradually,while the calcium minerals increase and clay minerals first decrease and then increase. Horizontally,the mineral distribution trends between different wells are consistent,with high silicon content mainly concentrated in the Long 1-1¹and Long 1-1² sub-layers,where the overall silica content exceeds 50%,representing a concentrated development zone of biogenic siliceous shale. The shale displays two types of laminations,mud laminae and silt laminae. The silt laminae exhibit coarser grains, characterized by an alternation of radiolarian laminae and calcareous. The radiolarian and enriched layers exhibit a lack of grain size sorting,random distribution of radiolarian tests of different sizes,high silicon content,and continuous distribution. In contrast,the calcareous algae-enriched layers appear as cohesive clusters with irregular external morphology,internally showing flocculent characteristics with varying particle sizes,discontinuous,platy,parallel,and rapidly changing thickness features. The study highlights that the formation of siliceous shale is influenced by rapid transgression and volcanic activities,regulating nutrient and seawater chemical property changes,impacting ecological niches and conditions,leading to the formation of different types of laminations. Siliceous shale,characterized by high silicon content,high organic matter content,excellent pore connectivity,good brittleness,and strong fracturability,emerges as a primary target for shale gas exploration.

Key words: biogenic siliceous shale, mineral composition, radiolarian-rich layers, calcimicrobe-rich layers, Wufeng Formation, Longmaxi Formation, Zhaotong area, Sichuan Basin

摘要：

五峰组—龙马溪组硅质页岩作为四川盆地重要的页岩气富集岩相之一,其沉积特征和成因机理一直备受关注。本研究基于MaipSCAN人工智能电镜矿物扫描、大薄片、场发射扫描电镜和氦离子显微技术分析,揭示了昭通地区五峰组—龙马溪组下部硅质页岩的矿物组成及垂向、平面分布特征。研究表明,该区域硅质页岩厚度稳定且广泛分布,主要矿物包括石英、长石、黏土、黄铁矿、方解石和白云石。垂向上,硅质含量逐渐减少、钙质矿物含量增加、黏土矿物先减少后增加。平面上不同井间矿物分布趋势一致,高硅质含量主要分布在龙一1¹和龙一1²小层,整体硅质含量超过50%,为生物硅质页岩集中发育段。硅质页岩呈泥质和粉砂质2类纹层,其中粉砂质纹层颗粒较粗,呈现放射虫纹层和钙质微生物纹层交替特征。其中放射虫富集纹层表现为无粒序特征,不同粒度的放射虫壳体随机分布,硅质含量高且纹层连续性分布。钙质微生物富集纹层表现为凝聚团块,颗粒没有规则的外部形态,内部呈絮凝状,颗粒大小不一,纹层呈断续、板状和平行状,厚度变化快。研究指出,生物硅质页岩形成受到快速海进和火山作用的双重影响,通过调节营养物质和海水化学性质变化,影响生态位和生态条件,进而控制不同类型纹层的形成。硅质页岩具有高有机质含量、孔隙连通性好、良好脆性和强压裂性,因而成为页岩气勘探的首选目标。

关键词: 生物硅质页岩, 矿物组成, 放射虫纹层, 钙藻纹层, 五峰组, 龙马溪组, 昭通地区, 四川盆地

CLC Number: 

• P512.2