Abstract:

The Archean basement of the Jiyang Depression has emerged as a key target for oil and gas exploration; however, an unclear understanding of the reservoir formation mechanisms has severely limited exploration and discovery efforts. This paper systematically studies the diagenesis of Archean basement rock in the Jiyang Depression and its influence on reservoir development through field geological investigation, core observation, thin section identification, cathodoluminescence, X-ray diffraction (XRD), scanning electron microscopy, and fluid inclusion analysis. Diagenetic characteristics are identified based on core analysis, flake and elemental analysis results, and fluid inclusion homogenization temperatures. The impacts of these characteristics on reservoir development are characterized through flake face rates and physical property logging data. The results indicate that the lithology of the basement rock in the study area is primarily divided into two categories: magmatic and metamorphic rocks. Magmatic rock has undergone four successive diagenetic stages: condensation consolidation, post-magmatic hydrothermal alterations, weathering denudation leaching, and buried rock formation. During these stages, magmatic rock has experienced processes such as condensation consolidation, compaction, dissolution, filling and cementation, as well as alteration and metasomatism. Eventually, metamorphic rock is formed through metamorphic deformation. Condensation consolidation can create a small number of primary matrix pores, while corrosion processes generate corrosion pores and fractures, contributing to up to 80% of the porosity, which is crucial for enhancing reservoir quality. Conversely, compaction, alteration, and metasomatism tend to destroy these pores, as calcite, clay minerals, and other secondary minerals fill and cement fractures, with a filling degree ranging from 30% to 70%, adversely affecting reservoir space. A comprehensive analysis reveals that the physical properties of gneiss reservoirs are superior to those of granite. Favorable reservoirs are mainly located in the weathering crust and the inner fracture zone. The weathering crust, which is situated at the top of the buried hill of the basement rock, displays longitudinal zoning and is primarily influenced by weathering, leading to the development of I and II reservoirs. The inner fault zone, located in the middle section of the buried hill and aligned along fault lines, predominantly develops II and III reservoirs. The findings of this study provide valuable insights into the genetic research of basement rock reservoirs.

Key words: Jiyang Depression, Archean, basement rock reservoir, diagenesis stage, buried hill