川东北元坝地区须家河组储层特征与超致密成因探讨

元坝地区须家河组砂岩的致密化程度极高,以超低孔、超低渗储层为主,属于超致密砂岩。本文基于铸体薄片、阴极发光、扫描电镜和物性等资料,在对砂岩的岩石组分、孔隙类型及物性特征分析的基础上,通过与国内外典型致密砂岩对比,揭示研究区砂岩较其他砂岩致密化程度高的原因,分析超致密砂岩的勘探潜能。结果表明,研究区砂岩具有较高的塑性岩屑、碳酸盐岩岩屑和黏土杂基含量;填隙物以黏土杂基和钙质胶结物为主,较强的硅质胶结作用仅发育于石英砂岩中。孔隙多为黏土杂基晶间微孔,喉道呈窄缝状或不发育,孔喉分选差,偏细歪度。砂岩致密化程度高主要有两方面的原因:一是原始组构差,成分成熟度低,含有较高的塑性岩屑、碳酸盐岩岩屑和黏土杂基含量,导致砂岩在早成岩阶段既已经历了很强的压实作用和钙质胶结作用,致密化时间早,溶蚀作用不发育;二是成岩演化阶段高,砂岩经历了中成岩阶段的压溶及进一步的钙质/硅质胶结,使得砂岩超致密化。与国内外典型致密砂岩气藏对比表明,超致密砂岩有效孔隙少,对天然气的富集能力有限。元坝地区须家河组只有孔隙度近于8%(如长石岩屑砂岩)的砂岩才可成为有利的致密气储层。

Characteristics and Densification Causes and of Highly-Tight Sandstone of the Xujiahe Formation (T3x2) in the Yuanba Area, Northeastern Sichuan Basin

The Xujiahe sandstones in the Yuanba area are characterized by ultra low porosity and ultra low permeability and should belong to ultra compacted sandstone. Based on thin sections, cathodeluminescence (CL), scanning electron microscope (SEM) and reservoir property data, densification causes and exploration potential are unraveled through comparison of detrital components, interstitial materials and reservoir properties between the Xujiahe sandstones and other tight sands in the world. The results show that the Xujiahe sandstones contain high contents of ductile rock fragments, clay matrix and carbonate fragments. The interstitial materials are dominantly clay matrix and calcite cements, with strong siliceous cementation occurring in quartz sandstone. Secondary pores are dominated by clay matrix intercrystalline micropores, with pore throats commonly present as narrow, poor sorting and sheetlike slots. There are two reasons for the highly densification of the Xujiahe sandstones. First, the anomalously poor quality of sandstones is attributed to the high compaction and calcite cementation during the early diagenetic stage, which are functions of the primary texture and composition of sandstones. Second, the Xujiahe sandstones were buried deeply and experienced further compaction and calcitic/siliceous cementation to forming highly tight sandstone in mesogenetic stage. Comparison with typical tight sandstone gas abroad indicates that ultra tight sandstone contain less effective porosity, which has limited accumulation capacity to nature gas. Therefore, only when the porosity of the Xujiahe in the Yuanba area reaches to 8% (e.g. feldspathic litharenite) can sandstone be favorable reservoir for highly tight sandstone exploration.