Characteristics and quality determinants of Carboniferous volcanic reservoirs in the Hongche Fault Zone, Junggar Basin

The Carboniferous volcanic rocks within the Hongche Fault Zone in the Junggar Basin serve as pivotal exploration and exploitation targets. However, ambiguities regarding the characteristics and quality determinants of the volcanic reservoirs pose challenges for the forthcoming emplacement of exploratory and exploitation wells. Leveraging the data from core, ordinary thin section, and casting thin section observation, as well as image logs, we conduct a fine-scale lithological classification for these reservoirs. Furthermore, we identify the types of reservoir spaces and reveal the factors influencing the reservoir quality. This study seeks to provide a scientific basis for selecting the optimal exploration and exploitation targets. The results are as follows: (1) The volcanic reservoirs predominantly consist of volcanic breccia, tuffaceous volcanic breccia, tuff, basalt, and andesite. Their storage spaces are dominated by secondary pores and micro-fractures. These reservoirs are characterized by ultra-low porosity (average: 9. 69 %) and ultra-low permeability (average: 0. 24 × 10^-3 μm²). Notably, the Carboniferous volcanic breccia exhibits the most favorable physical properties, followed by andesite and basalt; (2) The physical properties of these reservoirs are collectively molded by volcanic rocks’ lithologies and lithofacies, diagenism, and tectonism. Specifically, the lithologies determine the types of primary pores and their development degree. In comparison, the lithofacies are instrumental in governing the types and distributions of lithologies, determining the material composition of volcanic rocks and fundamentally influencing the compaction and subsequent dissolution of volcanic rocks. Therefore, the lithofacies serve as the predominant factor influencing the reservoir quality. Several processes, including weathering, leaching, dissolution, and fault activity, contribute to the formation of a fracture-dissolution pore system, significantly enhancing the reservoir quality. In contrast, compaction and the filling process greatly reduce the number of primary pores, leading to poor physical properties of reservoirs.