碱性长石溶蚀微孔发育特征及其对致密砂岩储层物性的改造作用--以鄂尔多斯盆地华庆地区三叠系延长组6段3亚段为例

Dissolved micro-pores in alkali feldspar and their contribution to improved properties of tight sandstone reservoirs:A case study from Triassic Chang-6^(3) sub-member,Huaqing area,Ordos Basin

长石次生溶孔是致密砂岩储层微-纳米级别孔喉网络的重要组成部分。为进一步明确碱性长石溶孔发育特征及其对致密砂岩储层物性的改造作用,以鄂尔多斯盆地中南部华庆地区三叠系延长组6段3亚段(长6^(3)亚段)致密砂岩储层为研究对象,利用能谱分析获取各类长石的钾、钠元素分布特征及组成特点;基于ImageJ软件对场发射环境扫描电镜图像进行二值化处理及参数提取,分别对具有典型溶蚀特征的波状条纹长石和斑块状条纹长石进行图像分析和理论计算,提出了基于溶蚀强度参数评价储层溶孔对储集空间贡献程度的方法;定量评价了长石溶蚀对孔隙发育的影响,精准刻画了长石发育特征、溶蚀率及孔隙度贡献率等参数。图像分析和理论计算结果表明,研究区碱性长石溶蚀微孔主要由弱溶蚀作用形成,形态特征为条带状和蜂窝状,面孔率大小分布在2.06%~35.20%,平均值为13.99%,因溶蚀作用形成的孔隙面积占比与碱性长石中K+富含区域面积大小呈负相关。

Secondary dissolved pores in feldspar account for a major part of the micro-nano pore-throat system in tight sandstone reservoirs.To further clarify the characteristics of these pores and their contribution to improved physical properties of tight sandstone reservoirs,this paper takes the third sub-member of the sixth member of Yanchang Formation(Chang-6^(3))in the Huaqing area,Ordos Basin as the research object.The energy-dispersive spectrometry is used to obtain the distribution and composition characteristics of sodium and potassium of all kinds of feldspar in the member and field emission environmental scanning electron microscope images of feldspar samples are processed binarily to extract parameters with Image J software.Two kinds of feldspar,wavy perthite and mottled perthite,with typical dissolution characteristics are imaged and analyzed to establish a method for evaluating the contribution of the dissolved pores to reservoir space based on dissolution intensity parameters and the impact of feldspar dissolution on pore development.Parameters for characterizing feldspar development,dissolution rate,porosity contribution are determined.The results show that the dissolved micropores in alkali feldspar in the study area are mostly in banded and honeycombed morphology as a result of weak dissolution.The porosity contribution rate is between 2.06%and 35.20%,with an average of 13.99%.Moreover,the proportion of pore area formed by dissolution is negatively correlated with the potassium ion enrichment area size in alkali feldspar.