川西拗陷须家河组第四段致密砂岩孔隙演化定量研究

Quantitative study on porosity evolution of tight sandstone in the Member 4 of Xujiahe Formation in western Sichuan Depression, China

对川西拗陷须家河组第四段致密砂岩孔隙演化过程进行定量化研究,为油气成藏机理研究提供基础资料。运用岩石薄片、扫描电镜、全岩和黏土X射线衍射分析、包裹体均一温度等测试分析方法,建立孔隙度演化模型,探讨孔隙演化的控制因素。须四段储层孔隙演化过程可划分为:①同沉积阶段的压实作用,使储层孔隙度处于26.2%左右;②早成岩A阶段和早成岩B阶段的第一期溶蚀作用、压实作用与自生矿物填充作用,使孔隙度演化为18.7%左右;③中成岩A阶段和中成岩B阶段的第二期溶蚀作用与自生矿物填充作用等,使孔隙度演化为5.12%～5.71%。定量计算的储层孔隙度为5.71%,接近测试的储层平均孔隙度5.12%。压实作用与强烈的碳酸盐胶结作用是储层致密化的主要因素,多期次的溶蚀作用是砂岩增孔的根本原因。

Methods of observation of ordinary & casting thin sections, scanning electron microscopy, full-rock and clay X-ray diffraction analysis and inclusion homogenization temperature are used to quantitatively study the reservoir porosity evolution of tight sandstone in the Member 4 of Xujiahe Formation, west Sichuan depression, so as to establish the porosity evolution model and discuss the controlling factors of porosity evolution and eventually provide basic data for the research of oil and gas accumulation mechanism. It shows that the pore evolution process in the study area can be divided into 3 stages. At synsedimentary stage, 26.2% of reservoir porosity in the rock is retained after the compaction. At the early diagenetic stage A and early diagenetic stage, after the first-stage dissolution, compaction and authigenic mineral filling, only 18.7% of reservoir porosity reserved. The second-stage dissolution and authigenic mineral filling at the middle diagenetic stage A and the middle diagenetic stage B leave 5.12%~5.71% of reservoir porosity. The quantitative calculation of reservoir porosity is 5.71%, close to the actual test average porosity of the reservoir(5.12%). It is recognized that compaction and strong carbonate cementation are the key factors for densification of reservoirs, while multiple stage dissolutions are the key factor for the incremental porosity in the reservoir. The research results can provide reference for the reservoir evaluation in the area.