海相含气页岩水平层理类型、成因及其页岩气意义——以川南地区古生界五峰组-龙马溪组为例

Types and genesis of horizontal bedding of marine gas-bearing shale and its significance for shale gas:A case study of the Wufeng-Longmaxi shale in southern Sichuan Basin,China

页岩发育多种成因类型水平层理,不同类型水平层理渗透性存在差异。综合岩心观察、大薄片全直径成像和氩离子抛光片扫描电镜分析发现,川南地区古生界五峰组-龙马溪组含气页岩发育递变型、砂-泥递变型、砂-泥互层型和书页型4种水平层理。递变型水平层理由多个粉砂层水平叠置而成,粉砂和泥混杂堆积,粒序发育。砂-泥递变型水平层理中粉砂纹层和泥纹层水平互层,粉砂纹层呈颗粒支撑结构,整体正递变,其底部为突变界面,顶部与上覆泥纹层渐变接触。砂-泥互层型水平层理由粉砂纹层和泥纹层水平互层构成,层和纹层界面均突变接触,粒序不发育。书页型水平层理由极薄层状泥纹层构成,发育微弱的正粒序。4种水平层理具有不同的成因机制。递变型和砂-泥递变型水平层理均为相对低能的泥质浊流成因,递变型水平层理形成的水动力更弱;砂-泥互层型水平层理为陆棚相等深流成因;书页型水平层理为远洋悬浮沉降成因。水平层理类型直接影响页岩渗透性。书页型水平层理页岩富含有机质和有机孔,页岩渗透性最好;砂-泥互层型水平层理页岩颗粒分选性较好,页岩渗透性次之;递变型和砂-泥递变型水平层理页岩颗粒分选性差、有机质含量低,渗透性最差。

Shale has horizontal bedding of diverse origins in differential permeability.An integrated analysis of core data,full-diameter images of enlarged thin section and scanning electron microscope(SEM)images of argon-ion-milled samples,shows that the gas-bearing Wufeng-Longmaxi shale in southern Sichuan Basin develops four types of horizontal bedding,that is,grading type composed of claystone,grading type composed of siltstone and claystone,alternating siltstone and claystone type,and page type.The grading type of claystone constitutes the multi-layer superimposed siltstones in parallel,with siltstone and claystone depositing in graded bedding.The grading type composed of siltstone and claystone is composed of parallelly alternating silty and clayey beds,where the silty beds mainly in clast-supported texture,has abrupt boundary at the bottom and gradual contact with the clayey bed on top,featuring normal grading as a whole.The alternating siltstone and claystone type features abrupt contact between both beds and lamina with no grading.The page type constitutes parallel bedding of very thin clayey lamina with weakly normal grading.The four types of horizontal bedding are of different genesis.The grading type of claystone and grading type of siltstone and claystone are of relatively low-energy turbidite origin,with the former derived from even weaker hydrodynamic conditions;the alternating siltstone and claystone is of contourite origin such as shelf facies;and the page type is of pelagic origin from suspended sediment deposition.The horizontal bedding serves to directly affect shale permeability.The page type is characterized by the abundance of organic matter and organic pores,ranking top in permeability;the alternating siltstone and claystone type takes the second place in permeability with better sorting;while the two grading types come at last in permeability with poor sorting and low organic matter content.