Types, origin, and geological implications of calcites in shales

Calcites, brittle minerals that are extensively distributed in shales, hold great significance for indicating shale oil and gas occurrence, reservoir stimulation, and sedimentary environments. In recent years, increasing studies have been conducted on the types, genetic mechanisms, and geological significance of calcites in shales. However, systematic reviews on these topics remain limited. In this study, calcites in shales are categorized into two types initially: micritic calcites (including dispersed granular, nodular, and lamellar calcites, calcite cements, and biogenic calcites) and sparry calcites (including fibrous, bladed, and equiaxed calcites). The subsequent results indicate that the formation of calcite veins is jointly driven by fluid overpressure, tectonic stress, and crystallization forces. The formation process is affected by microbial reduction in the early stage, relates to the decarboxylation of organic matter in the middle stage, and is associated with the methane thermochemical oxidation-related fluid activity in the late stage. During the thermal evolution of calcite veins, dissolution windows and retrograde dissolution also occur. Calcites in lacustrine and marine shales generally share similar formation mechanisms, with differences attributed primarily to sedimentary environments. The high degree of enrichment of micritic calcites, the combination of shales and bioclastic layers, and calcite inclusions all provide valuable indications for the reconstruction of paleosedimentary and diagenetic environments. Regarding shale brittleness, calcites both enhance the shales' brittleness index and promote the propagation of hydraulically stimulated fracture networks as brittle minerals. However, their cementation reduces reservoir porosity, creating dual effects on reservoir stimulation. Additionally, calcite veins preserve records of the hydrocarbon migration process, while their dissolution pores and bedding-parallel fractures offer important reservoir spaces for shale oil and gas. Nevertheless, the genetic mechanisms of calcites in ancient, highly evolved shales and their coupling relationship with the hydrocarbon generation process of organic matter remain poorly understood. Furthermore, the accuracy of U-Pb dating techniques for calcites in shales is yet to be substantially improved, and further improvements in the precision of relevant detection techniques are required to further reveal the complex formation process of the calcites. Advancements in these areas will provide more improved theories for the sweet spot prediction and efficient exploitation of oil and gas from calcite-rich shales.