Mechanisms of Overpressure Formation and Distribution Prediction in Ultra‐Deep Carbonate Reservoirs in the YB Area, Sichuan Basin

Understanding the formation mechanisms and predicting the distribution of abnormally high pressure in marine carbonate reservoirs are fundamental to ultra‐deep hydrocarbon exploration. This study investigates the ultra‐deep carbonate grain‐beach reservoirs of the second member of the Feixianguan Formation in the YB area, northeastern Sichuan Basin. Through an integrated analysis of geological data, well logs, and seismic attributes, we systematically unravel the multi‐factor coupled genesis, reservoir‐specific pressure responses, and predictive workflows for abnormally high pressure within this interval. The key findings are as follows: (1) Formation pressures in the second member of the Feixianguan Formation exhibit strong heterogeneity, with pressure coefficients spanning 1.00–2.13, allowing classification into normal‐pressure, high‐pressure, and ultra‐high‐pressure regimes. (2) Overpressure development is controlled by the synergistic effects of three primary factors—reservoir lithofacies, hydrocarbon charge and retention, and tectonic deformation. High‐energy shoal‐facies oolitic limestones and dissolution‐vug networks provide superior storage capacity, whereas tight micritic limestones and evaporites act as competent pressure‐sealing barriers. Thermogenic pressurization from advanced organic maturation serves as the dominant pressure source, whereas multiphase tectonic compression simultaneously promotes fracturing—enhancing local permeability—and forms structurally confined sealing units that preserve overpressure. (3) For pressure prediction, we propose a ternary criterion integrating fracture intensity, acoustic impedance, and porosity, which enables the differentiation of ultra‐high‐pressure, high‐pressure, and normal‐pressure zones. (4) The predicted pressure distribution closely matches measured coefficients, clearly outlining the ultra‐high‐pressure fairways. These areas correspond to “pore–fracture–overpressure” composite sweet spots; nevertheless, the overpressure regime imposes substantial risks to wellbore integrity, demanding rigorous well‐control protocols and corrosion‐resistant material selection. Collectively, this study furnishes both a theoretical underpinning and a practical framework for identifying and mapping abnormally high pressure in ultra‐deep carbonate grain‐beach reservoirs, with direct applicability to drilling hazard assessment, sweet‐spot targeting, and the safe, efficient development of analogous deep to ultra‐deep carbonate gas plays.