川东北元坝地区须家河组钙质交代-胶结致密层分布与成因

方解石是须家河组砂岩中常见的成岩矿物之一，可在局部层段富集形成钙质交代-胶结致密层，对砂岩的物性有明显的破坏作用。基于岩心观察和薄片鉴定，对川东北元坝地区钙质交代-胶结致密砂岩的岩石学特征开展了分析，刻画了致密砂岩的分布规律；结合全岩X射线衍射、稳定碳氧同位素和微量元素等测试资料，分析了钙质交代-胶结致密砂岩的形成期和物质来源，阐明了钙质胶结物和交代物的形成机制。钙质交代-胶结致密层主要发育在元坝地区西部与须家河组三段（须三段）源岩层相邻的须家河组二段和须家河组四段的砂岩中，在东部须三段源岩层附近的砂岩中则较少；通常出现在厚层砂体的中部，厚度为1~2 m；长石含量或白云岩岩屑含量较高，杂基含量较低；经历的主要成岩作用包括压实作用、方解石的胶结作用和交代作用。方解石的胶结物和交代物主要形成于早成岩阶段B期，此时的地层温度为60~80℃，烃源岩处于有机酸生成的高峰期。由于元坝地区西部须三段烃源岩中含有较高的碳酸盐矿物组分，其排出的富有机酸流体可携带较多的Ca²⁺和HCO₃^-离子进入砂岩层，当砂岩富含长石和白云岩岩屑等碱性矿物时，富Ca²⁺的有机酸流体能被有效缓冲，形成有利于方解石胶结物和交代物沉淀的微环境，从而导致方解石大量沉淀。

Distribution and genesis of calcite-replaced and calcite-cemented tight reservoirs in Xujiahe Formation,Yuanba area,Northeast Sichuan

Calcite is one of the most common diagenetic minerals in Xujiahe sandstones. It can accumulate in certain zones and form calcite-replaced and calcite-cemented tight reservoirs, resulting in an obvious damage to the sandstone properties. Based on the core observation and thin section identification, this paper analyzes the petrologic characteristics of calcite-replaced and calcite-cemented tight reservoirs in Yuanba area, Northeast Sichuan and describes the distribution of tight sandstones. In combination with the whole-rock X-ray diffraction, stable carbon and oxygen isotope, trace elements and other tests, this paper also analyzes the formation and material sources of calcite-replaced and calcite-cemented tight sandstones, and describes the genetic mechanism of calcite cements and replacements. The calcite-replaced and calcite-cemented tight reservoirs are mainly developed in the sandstone of Member 2 and Member 4 of Xujiahe Formation adjacent to the source rock of Member 3 of Xujiahe Formation in the western Yuanba area but rare in sandstones near the source rocks of Member 3 of Xujiahe Formation in the eastern Yuanba area, usually occurring in the middle of thick sand bodies with the thickness of about 1-2 m, high feldspar or dolomite fragment contents but low matrix contents. The main diagenesises of tight reservoirs include compaction as well as the calcite cementation and replacement. The calcite cements and replacements were mainly formed in eodiagenesis stage B, when the formation temperature was between 60℃ and 80℃ and the source rock was at the peak of organic acid generation. Because the source rocks of Member 3 of Xujiahe Formation in the western Yuanba area contain high contents of carbonate minerals, the organic acid-rich fluids discharged from them can carry more Ca²⁺ and HCO₃^- ions to the sandstone layers. When the sandstone is rich in alkaline minerals such as feldspar and dolomite fragments, the Ca²⁺enriched rich organic acid fluid can be effectively buffered, forming a microenvironment favorable for the precipitation of calcite cements and replacements, thus resulting in a large amount of calcite precipitation.