Characteristics and genesis of the high-quality Carboniferous volcanic reservoirs in the periphery of the Western Well Pen-1 Sag, Junggar Basin

In recent years, continuous breakthroughs have been achieved in exploring the Carboniferous volcanic hydrocarbon reservoirs in the periphery of the Western Well Pen-1 Sag within the central Junggar Basin. However, challenges such as the strong heterogeneity of volcanic reservoirs, complex genesis of high-quality reservoirs, and the unclear reservoir distribution patterns have significantly increased the costs of hydrocarbon exploration and development. To address these issues, we systematically characterize the lithology and lithofacies of volcanic rocks, as well as reservoir space types and their distribution patterns, using multiple techniques including petrology, geochemistry, micro-computed tomography (micro-CT), well logging, and seismic surveys. Through paleogeomorphic reconstruction and tectonic evolution analysis, we determine the durations of weathering and denudation and explore how tectonic movements, together with the intensities of weathering and leaching, control reservoir occurrence. In combination with differences in lithology, structure, and the intensity of weathering and leaching across various areas, we establish a developmental model for large-scale, high-quality volcanic reservoirs. The results indicate that the volcanic rocks in the study area are dominated by effusive phase (that is, basaltic, andesitic, dacitic, and rhyolitic types) and the pyroclastic flow sub-phase of explosive phase (i. e., resulting in volcanic breccia lavas and welded tuffs). The spatiotemporal distribution of the lithology and lithofacies is governed by multi-stage eruption events. The reservoir spaces are dominated by secondary pores and fracture systems, with tectonic fractures and intergranular dissolution pores representing high proportions. The reservoirs exhibit significant differences in vertical zoning. For instance, the Mobei Uplift, located on a structural slope, preserves a complete four-layered structure consisting of clay zone, leaching zone, disintegration zone, and parent rock zone. In contrast, the Shixi Uplift, located at a structural high during the uplifting and denudation stage, experienced the denudation and transport of the clay zone. Weathering and leaching are key geological processes in the formation of weathered crustal reservoirs. A duration of sustained weathering and denudation of approximately 40 Ma is identified as the critical threshold for significant improvement in reservoir physical properties. Pronounced improvements in physical properties are observed in volcanic breccias, basalts, and andesites as these lithologies are more sensitive to weathering and dissolution. The Hercynian to Indosinian tectonic movements significantly improve the storage capacity of interior reservoirs through fracture reactivation, leading to the formation of a dual-layered reservoir structure comprising weathered crustal and interior reservoirs. Preferential lithologies for this reservoir type include dacites, rhyolites, and volcanic breccias. Accordingly, a volcanic reservoir developmental model proposed is characterized by a material basis comprising favorable lithologies and lithofacies, along with the dual control of supergene weathering and tectonic evolution. The superimposed zones of the Hercynian-Indosinian tectonically active areas and areas subjected to long-term weathering (> 40 Ma) are identified as key targets for future exploration of high-quality volcanic reservoirs.