Characteristics of ultra-deep buried strike-slip fault system in the Bozhong Uplift of the Bohai Bay Basin and its control on volcanic reservoir development

Mesozoic volcanic buried hills constitute the most extensively exposed basement-type paleo-buried hills in the Bohai Sea area. Recently, CNOOC Bohai Oilfield achieved a major breakthrough by discovering a super-thick hydrocarbon pay zone within Mesozoic volcanic rocks at depths exceeding 5000 m in the Bozhong Uplift, setting a new record for natural productivity from deep offshore exploratory wells in China. This discovery confirms the substantial exploration potential of deep to ultra-deep Mesozoic volcanic buried hills in the Bohai Sea area. Focusing on the Mesozoic volcanic edifice assemblages in the Bozhong Uplift of the Bohai Bay Basin, this study integrates three-dimensional seismic data, mud logging, thin-section petrography, well logging, porosity–permeability measurements, mercury intrusion capillary pressure analyses, and zircon isotopic geochronology to comprehensively characterize the geometry, genesis, and evolution of the conjugate strike-slip fault system in the Bozhong Uplift. On this basis, the controlling effects of the fault system on volcanic eruption and emplacement, fracture development, and the formation of high-quality reservoirs are systematically elucidated. The results indicate that: (1) Three major fault sets are developed in the Bozhong Uplift, striking N-S, NW-SE, and NE-SW. Among them, the N-S- and NW-trending faults constitute a conjugate left-lateral strike-slip fault system consistent with the Andersonian faulting model. In map view, these faults mutually intersect, while in cross-section they display a series of characteristic strike-slip structural styles, including positive flower structures, negative flower structures, vertical structures, and the “dolphin effect.” (2) During the Yanshanian, the inherited activity of the N-S- and NW-trending conjugate strike-slip faults provided preferential pathways for the ascent of deep-seated intermediate to acidic magmas, thereby controlling typical central-type volcanic eruptions. Under this structural control, medium- to high-relief volcanic edifices are distributed in a pronounced checkerboard pattern along the conjugate fault zones. Concurrently, widespread intrusion and diapirism of acidic magmas during this period ultimately shaped the transverse anticline structure of the Bozhong Uplift. (3) Segmental overlap of the major strike-slip faults generated broad overlap zones that served as loci for concentrated tectonic stress release. Intense stress release strongly modified the volcanic rocks and produced abundant secondary fractures. The coupling of faults and fractures ultimately formed a highly connected fracture network, providing efficient pathways for the dissolution by various acidic fluids and leading to significant enhancement of porosity and permeability. Consequently, even in medial to distal volcanic facies away from eruption centers, favorable conditions for the development of high-quality reservoirs persist. Based on these insights, a ternary coupled reservoir-forming model for volcanic rocks in the Bozhong Uplift is proposed, characterized by “dominance of intermediate-acidic lithologies, superimposed modification by strike-slip faults, and multi-fluid dissolution-induced porosity enhancement.” This model elucidates the formation mechanism of laterally extensive, high-porosity and high-permeability fractured-vuggy reservoirs and large-scale volcanic hydrocarbon accumulations.