Jia Fanjian, Zeng Zhiping, Guo Ruichao, Zhang Yang, Wang Zhukun, Zhao Leqiang, Wang Jianwei, Wu Zhiping
School of Geosciences, China University of Petroleum (East China), Qingdao 266580, China.
Exploration and Development Research Institute of Shengli Oilfield, Sinopec, Dongying 257000, China.
ACS Omega. 2024 May 8;9(20):22371-22385. doi: 10.1021/acsomega.4c00333. eCollection 2024 May 21.
The occurrence and genesis of grain-coating chlorite were investigated in order to evaluate the impact of grain-coating chlorite on preserving porosity in the deep-buried Triassic Karamay volcaniclastic sandstones based on thin sections, scanning electron microscopy, and an electron probe. Grain-coating chlorite was formed during the eogenesis, originating from the precursor of smectite through the solid-state transformation (SST) mechanism. The hydration and dissolution of unstable, intermediately basic volcanic rock fragments provided essential Fe and Mg ions for the formation of grain-coating chlorite. Due to relatively high stability and low susceptibility to dissolution, acidic volcanic rock fragments could not promote chlorite formation but resulted in authigenic quartz and clays as pore-filling cements. This process would destroy reservoir properties. Under high hydraulic conditions, medium- to coarse-grained sandstone experienced saltation transport, creating significant velocity differentials and pressure differentials on grain surfaces. Subsequently, clay grains adhere to the surfaces, forming grain-coating chlorite during diagenesis with good continuity. In contrast, pebbly sandstone undergoes rolling transport, resulting in smaller velocity differentials on grain surfaces. This makes relatively ineffective clay adsorption and leads to discontinuous grain-coating chlorite in subsequent stages. Under weak hydraulic conditions, grains and clay particles in fine-grained sandstone undergo suspended transport, lacking mutual movement and velocity differentials. Clay particles cannot effectively cover particles but instead fill the pores between them. Therefore, continuous grain-coating chlorite is more commonly developed in the medium- to coarse-grained sandstones and is crucial for inhibiting quartz cementation with a coverage rate exceeding 80%. Inadequate coatings fail to inhibit quartz cementation effectively, while excessive coatings may block pore throats. Optimal protection of primary porosity could occur only when grain-coating chlorite is moderately developed.
为了基于薄片、扫描电子显微镜和电子探针评估粒包膜绿泥石对深埋三叠系克拉玛依火山碎屑砂岩孔隙度保存的影响，对粒包膜绿泥石的赋存和成因进行了研究。粒包膜绿泥石形成于早成岩阶段，通过固态转变（SST）机制由蒙脱石前驱体形成。不稳定的中基性火山岩碎屑的水化和溶解为粒包膜绿泥石的形成提供了必需的铁和镁离子。由于酸性火山岩碎屑相对较高的稳定性和较低的溶解敏感性，它们不能促进绿泥石的形成，而是导致自生石英和粘土作为孔隙充填胶结物。这个过程会破坏储层性质。在高水力条件下，中粗粒砂岩经历跳跃搬运，在颗粒表面产生显著的速度差和压力差。随后，粘土颗粒附着在表面，在成岩过程中形成连续性良好的粒包膜绿泥石。相比之下，含砾砂岩经历滚动搬运，颗粒表面的速度差较小。这使得粘土吸附相对无效，并导致后续阶段粒包膜绿泥石不连续。在弱水力条件下，细粒砂岩中的颗粒和粘土颗粒进行悬浮搬运，缺乏相互运动和速度差。粘土颗粒不能有效地覆盖颗粒，而是填充它们之间的孔隙。因此，连续的粒包膜绿泥石更常见于中粗粒砂岩中，对于抑制石英胶结至关重要，其覆盖率超过80%。包膜不足不能有效地抑制石英胶结，而包膜过多可能会堵塞喉道。只有当粒包膜绿泥石适度发育时，才能对原生孔隙度进行最佳保护。
