下扬子地区荷塘组细粒沉积岩岩相划分及微观孔隙发育特征

Lithofacies classification and microscopic pore characteristics of fine-grained sedimentary rocks in the Hetang Formation， Lower Yangtze region

张琴 卢东连 王凯 刘畅 郭明强 张梦婕 郭超杰 王颖 胡文忠 朱筱敏

Qin ZHANG Donglian LU Kai WANG Chang LIU Mingqiang GUO Mengjie ZHANG Chaojie GUO Ying WANG Wenzhong HU Xiaomin ZHU

下扬子地区下寒武统荷塘组细粒沉积岩岩相微观孔隙发育特征不清楚制约了页岩气的勘探开发。基于普通薄片观察、氩离子抛光扫描电镜（SEM）分析、物性测试、X射线衍射全岩矿物及黏土矿物含量分析、N及CO等温吸附实验、聚焦离子束扫描电镜（FIB-SEM）分析等实验资料，建立了荷塘组细粒沉积岩岩相划分方案，分析了不同岩相的微观孔隙结构特征。研究表明：①荷塘组细粒沉积岩主要发育块状硅质泥岩相、纹层状硅质页岩相、块状含黏土硅质泥岩相、块状硅/灰混合质泥岩相和块状混合灰质泥岩相5种岩相类型。②荷塘组不同细粒沉积岩岩相孔隙度和渗透率差异较大。孔隙类型主要为粒间孔、晶间孔、粒内孔、有机质孔和微裂缝，以微米-纳米级孔隙为主。介孔孔径为2.0 ~ 10.0 nm，微孔孔径为0.4 ~ 0.9 nm。③块状硅/灰混合质泥岩相有机质和脆性矿物含量高，粒间孔与微裂缝发育，孔隙度和渗透率较大、孔隙连通性较好、比表面积较大，为荷塘组最有利的页岩气勘探开发细粒沉积岩岩相。块状含黏土硅质泥岩相孔隙比表面积和孔体积较高，脆性指数、孔隙度和渗透率低于块状硅/灰混合质泥岩相，为次要的有利岩相。

The lithofacies and microscopic pore characteristics of fine-grained sedimentary rocks in the Lower Cambrian Hetang Formation in the Lower Yangtze region remain unclear， which restricts the exploration and exploitation of shale gas in this region. In this study， we obtain data using methods including ordinary thin section observation， scanning electron microscope （SEM） image analysis of argon-ion milled sample surface， physical property tests， X-ray diffraction （XRD） analysis of whole-rock and clay mineralogy， N and CO isothermal adsorption experiments， and focused ion beam-scanning electron microscope （FIB-SEM） image analysis. Using these experimental data， we develop a lithofacies classification scheme for fine-grained sedimentary rocks in the Hetang Formation and analyze the microscopic pore structure characteristics of various lithofacies. The results indicate the presence of five major lithofacies of the fine-grained sedimentary rocks， namely massive siliceous mudstone， lamellar siliceous shale， massive clay-bearing siliceous mudstone， massive mixed siliceous-calcareous mudstone， and massive mixed calcareous mudstone， which differ greatly in porosity and permeability. Main pores in the Hetang Formation include intergranular， intercrystalline， intragranular， and organic matter-hosted pores and microfractures， with pores of nano-to-micron scale dominating. Of these， mesopores range in size from 2.0 to 10.0 nm， while micropores range from 0.4 to 0.9 nm. The massive mixed siliceous-calcareous mudstone features high organic matter and brittle mineral contents， well-developed intergranular pores and microfractures， high porosity and permeability， excellent pore connectivity， and large specific surface area， which establish itself as the most favorable lithofacies for shale gas exploration and exploitation in the Hetang Formation. The massive clay-bearing siliceous mudstone is characterized by large specific surface area and substantial pore volume but lower brittleness index， porosity， and permeability compared to the massive mixed siliceous-calcareous mudstone， thereby identified as the secondary favorable lithofacies.

10.11743/ogg20240414