How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: Observations and experiments
How clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones: Observations and experiments
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DOI：
10.1306/02211211075
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发表时间：
2012-11
期刊：
影响因子：
3.5
通讯作者：
J. Ajdukiewicz;R. E. Larese
中科院分区：
文献类型：
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作者：
J. Ajdukiewicz;R. E. Larese
通过观察和热液实验，获得了粘土颗粒涂层如何抑制石英胶结并保持深埋砂岩孔隙度的新信息。深埋的多孔砂岩样品具有不同类型的粘土层，被分成两部分，每对裂缝中取出一层，用扫描电子显微镜检查颗粒表面。埋藏至115℃的河流三角洲砂岩中的石英颗粒，在被成岩绿泥石外壳清洁的颗粒表面上没有明显的自生石英，尽管在附近的自然未被包裹的颗粒上出现了发育良好的过度生长。然而，在埋藏至164C的类似砂岩中，绿泥石涂层去除暴露的石英颗粒表面覆盖着小（5 m）的、主要为倒面体的合成石英生长体。在埋藏温度高达215℃的多孔风成砂岩中，在各种碎屑和成岩粘土层中也观察到类似的过度生长。我们得出结论，粘土涂层可能会在中等温度下阻碍石英成核，但在高温下，许多涂层允许石英成核，并通过限制水泥生长来保持孔隙度。为了研究水泥生长限制机制，去除涂层的样品在水热反应器中经受石英胶结条件。在实验过程中，经粘土清洗后的颗粒上自然形成的小的过生长体聚结并生长，表明涂层中的粘土颗粒通过形成阻碍早期过生长体聚结的屏障来抑制水泥的生长。尽管颗粒表面覆盖的比例是涂层对水泥抑制作用的主要控制因素，但水泥生长干扰结构因涂层类型而异，这提供了一种机制，即涂层成分可能是抑制效果的次要控制因素。在深埋砂岩中，石英胶结剂可以填充成岩绿泥石外壳内的显著微孔隙，潜在地影响岩石的力学和岩石物理性质。
Observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones. Samples of deeply buried, porous sandstones with different types of clay coats were split in two, coats removed from one of each pair of splits, and grain surfaces inspected with scanning electron microscopy. Quartz grains in a fluvial-deltaic sandstone buried to 115C had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats, though well-developed overgrowths occurred on nearby, naturally uncoated grains. However, in similar sandstones buried to 164C, quartz-grain surfaces exposed by chlorite-coat removal were covered with small (5 m), mainly anhedral, syntaxial quartz overgrowths. Similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215C. We conclude that clay coats may retard quartz nucleation at moderate temperatures, but at high temperatures, many coats permit quartz nucleation and preserve porosity by limiting cement growth. To investigate cement growth-limitation mechanisms, samples with coats removed were subjected to quartz-cementing conditions in a hydrothermal reactor. During experiments, the naturally occurring small overgrowths on clay-cleaned grains coalesced and grew, suggesting that clay particles in coats inhibit cement growth by forming barriers to early-overgrowth coalescence. Although the fraction of grain-surface coverage is the primary control on cement inhibition by coats, cement growth–interference textures vary with coat type, providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness. In deeply buried sandstones, quartz cement can fill significant microporosity within diagenetic chlorite coats, potentially affecting mechanical and petrophysical rock properties.