四川盆地龙马溪组页岩孔隙度控制因素及演化规律

为查明四川盆地龙马溪组页岩储层孔隙控制因素及演化规律,利用场发射扫描电子显微镜和氮气吸附实验等方法对页岩储层孔隙类型及孔隙结构进行研究。页岩储层孔隙类型包括有机孔、晶间孔、晶内溶孔和粒间孔,有机孔是主要孔隙类型之一,且有机孔中微孔所占的孔体积和比表面积大,是页岩气储集的主要空间。通过对有机质丰度(TOC)、有机质成熟度(Ro)、成岩作用和构造作用对页岩孔隙度的影响研究,结果表明:1有机质丰度对孔隙度的影响可以分为4个阶段:快速增大(TOC为0%~2%)、缓慢减小(TOC为2%~3%)、快速增大(TOC为3%~4%或6%)、快速减小(TOC4%或6%);2四川盆地页岩成熟度对孔隙度的影响可分为3个阶段:快速减小(Ro为1.5%~2.2%)、快速增大(Ro为2.2%~2.7%)、快速减小(Ro2.7%);3高热演化阶段有机成岩作用强于无机成岩作用;4构造作用对孔隙度影响较大,构造作用越强烈的区域孔隙度越小。四川盆地龙马溪组页岩孔隙度演化经历5个阶段:未熟快速压实阶段(Ro0.7%)、成熟生烃溶蚀阶段(Ro为0.7%~1.3%)、高成熟孔隙封闭阶段(Ro为1.3%~2.2%)、过成熟二次裂解阶段(Ro为2.2%~2.7%)、过成熟缓慢压实阶段(Ro2.7%),其中成熟生烃溶蚀阶段和过成熟二次裂解阶段是最有利的页岩孔隙发育阶段。

Controlling factors and evolution laws of shale porosity in Longmaxi Formation,Sichuan Basin

To find out the controlling factors and evolution laws of shale porosity in Longmaxi Formation, Sichuan Basin, the pore types and structures of shale reservoir were studied using the field emission scanning electron microscopy (FE-SEM), N₂ adsorption experiment and other methods. The results show that the pore types of shale reservoir include organic pore, inter-crystalline pore, intra-crystalline dissolved pore and intergranular pore. Organic pores are a dominant type, among which the micro-pore has large pore volume and specific surface area, providing the main reserving space for shale gas. Through studying the influences of organic matter abundance (TOC), organic matter maturity (R_o), diagenesis and tectonism on shale porosity, the results have proved that the influence of organic matter abundance on porosity can be divided into four stages, i.e., rapid increase (TOC of 0%-2%), slow decrease (TOC of 2%-3%), rapid increase (TOC of 3%-4% or 6%) and rapid decrease (TOC>4% or 6%); the influence of shale maturity on porosity in Sichuan Basin can be divided into three stages, i.e., rapid decrease (R_o of 1.5%-2.2%), rapid increase (R_o of 2.2%-2.7%) and rapid decrease (R_o>2.7%); the organic diagenesis is stronger than the inorganic diagenesis in the high thermal evolution stage; the tectonism has a great impact on porosity, and the more intense tectonism corresponds to the smaller porosity. The shale porosity evolution in Longmaxi Formation, Sichuan Basin experienced five stages, i.e., immature rapid compaction stage (R_o<0.7%), mature hydrocarbon generation-dissolution stage (R_o of 0.7%-1.3%), high-mature pore sealing stage (R_o of 1.3%-2.2%), overmature secondary cracking stage (R_o of 2.2%-2.7%) and overmature slow compaction stage (R_o>2.7%). Among them, the mature hydrocarbon generation-dissolution stage and overmature secondary cracking stage are the most favorable stages for the development of shale pores.