Genetic mechanisms and classified evaluation of conventional and unconventional effective reservoirs in the whole petroleum system of continental sequences, western-central Sichuan Basin

Based on the theory of the whole petroleum system (WPS), we systematically reveal the differential characteristics of conventional and unconventional reservoirs in continental sequences within the western-central Sichuan Basin. Accordingly, the mechanisms underlying the control of these differential characteristics on hydrocarbon accumulation are explained. Through analyses and experiments, including core observations, thin section observations, high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM), we comprehensively analyze the reservoir characteristics and their variation patterns, as well as the genetic mechanisms of relatively high-quality reservoirs. Additionally, we determine parameters for the grading and classification evaluation of various types of reservoirs and establish the evaluation criteria. The results show that the conventional and unconventional reservoirs in the WPS of continental sequences in the western-central Sichuan Basin exhibit a spatially superimposed pattern in distribution. Specifically, the intermediate to shallow strata overlying the buoyancy-driven hydrocarbon accumulation depth (BHAD) exhibit strong free fluid activities, resulting in conventional high-quality reservoirs with low permeability under the joint control of sedimentary facies and fluids. The intermediate to deep tight sandstones exhibit multi-stage superimposed channel deposits and enhanced capillary resistance. Furthermore, fractures are developed in these sandstones under intense tectonic compression, resulting in multiple types of reservoirs, such as pore, fractured, and pore-fractured reservoirs, jointly governed by lithofacies, fluids, and faults. In contrast, deep lamellar shale reservoirs are generally dominated by nanoscale pores. Within hydrodynamic fields predominantly characterized by strong overpressure and diffusive forces, the occurrence of the shale reservoirs is significantly governed by the joint effects of lithofacies assemblages, organic matter abundance, and the overpressure system. For the intermediate to shallow conventional, low-permeability reservoirs, the grading and classification evaluation focuses on sedimentary and diagenetic facies, along with pore types. The evaluation results indicate that these reservoirs can be classified into four types, with types Ⅰ and Ⅱ serving as the high-quality ones. Both reservoir types primarily occur in the deltaic plain and front, with moderate to strong dissolution identified as their key genetic mechanism. The grading and classification evaluation of the intermediate to deep tight sandstone reservoirs principally considers pores, fractures, lithofacies, and pore structures. Types Ⅰ and Ⅱ of these reservoirs represent high-quality pore reservoirs, primarily occurring in medium-to coarse-grained sandstone in massive and parallel beddings. Integrating reservoir thickness, lithofacies assemblages, reservoir physical properties, degree of organic matter enrichment, brittle mineral content, and the volumetric proportion of movable oil, we classify the intermediate to deep shale reservoirs in the Da'anzhai Member of the Jurassic Ziliujing Formation into types Ⅰ, Ⅱ, Ⅲ, and Ⅳ. Types Ⅰ and Ⅱ, among others, are relatively high-quality reservoirs, consisting predominantly of pure shales and shales interbedded with shell layers.