川西坳陷中段上三叠统须二段致密砂岩孔隙度演化史

川西坳陷深层上三叠统须家河组经历的地史时间长,构造期次多,成岩作用改造普遍,储层致密化严重,导致致密砂岩气藏成因机制研究困难,其中孔隙度演化史的研究是该区致密气藏成因类型的判识和天然气勘探的关键。在利用显微镜观察、扫描电镜分析、阴极发光分析等认识了储层成岩作用特征的基础上,结合自生矿物中流体包裹体的均一化温度和古地温梯度等,建立了须二段储层的成岩作用序列,并确定了成岩矿物形成时间。在充分考虑沉积过程中的粒度、分选性等岩相特征对原生孔隙的影响,及压实作用、胶结和溶蚀等成岩作用对孔隙度演化的影响前提下,分成岩作用阶段建立了孔隙度演化的定量模型。研究结果表明,川西坳陷须二段储层致密化时间较早,一般在晚成岩作用早期,即晚侏罗世的早期已经基本致密化,但不同的气藏由于储层的岩相特征和成岩作用特征存在差异,导致储层致密化时间存在一定差异。

Porosity Evolution of Tight Gas Sand of the Second Member of Xujiahe Formation of Upper Triassic,Western Sichuan Depression

The formation of tight gas sand of the second member of Xujiahe Formation of Upper Triassic,western Sichuan depression is controlled by multiple-stage structural movements,strong diagenetic modification,which leads to the porosity evolution being the key to define the genetic types of gas reservoir and to improve gas exploration.Based on the studying of diagenesis by observing under microscope and analyzing under scanning electron microprobe and cathodeluminescence,the diagenetic series and diagenetic stages are constructed by combination of measure of homogenization temperatures of inclusions,and paleotemperature grades.Porosity evolution models of each diagenetic stage are quantitatively established by fully consideration of the effect of lithofacies like granularity and sorting on the primary porosity in sedimentary process,and the effect of diagenesis such as compaction,cementation and corrosion on porosity evolution.The quantitative research shows that the tighten time of the tight gas sand of the second member of Xujiahe Formation is about the early stage of late diagenesis corresponding to the early stage of Late Jurassic.The analysis also indicates that differential lithofacies and diagenesis of gas reservoir result in the difference of tighten time of sand in gas reservoir.