Jurassic–Cretaceous Unconformity Architecture as a Reservoir–Seal Factory: Controls on Multi‐Phase Hydrocarbon Migration in the Deep Central Junggar Basin

ABSTRACT

Unconformities serve as key interfaces in hydrocarbon systems, yet the three‐dimensional (3D) architecture of continental unconformities and their controls on deep‐buried reservoir evolution remain poorly constrained. We characterise the Jurassic–Cretaceous unconformity zone in the Yongjin area of the central Junggar Basin using an integrated dataset of well logs, high‐resolution 3D seismic data, seismic facies analysis, and quantitative core observations. Four seismic reflection patterns (parallel–parallel, parallel–truncated, onlap–parallel, onlap–truncated) are identified, and a tripartite vertical subdivision of the unconformity zone is proposed. This subdivision includes three units: post‐unconformity Cretaceous sandstones (20–36 m thick; porosity 6%–16%), a laterally discontinuous weathered clay seal (0–18 m thick), and semi‐weathered Jurassic sandstones (18–52 m thick; porosity 3%–12%). Provenance‐controlled thickness variations indicate a NE‐sourced, NW–SE‐prograding carrier‐reservoir system that transitions downdip into distal mudstone baffles. Fluid inclusion microthermometry (±2°C accuracy) documents three distinct hydrocarbon migration phases, with a dominant south‐to‐north trend that opposes the present‐day structural dip: Phase I (Late Jurassic) is associated with vertical migration along reactivated faults during early uplift; Phase II (Late Cretaceous) is characterised by lateral migration along carrier–reservoir layers during peak subsidence; and Phase III (Neogene–Quaternary) is attributed to Himalayan orogenesis, which introduced late gas‐condensate charges. This multi‐phase charging history records a progressive shift from fault‐dominated to stratigraphy‐controlled and finally tectonically reactivated fluid flow. The unconformity architecture governs two main trap types: dissolution‐enhanced onlap unconformity traps and overpressure‐stabilised erosional unconformity traps (pressure coefficients up to ~1.8). On the basis of these observations, we propose a “reservoir–seal factory” conceptual framework, in which syndepositional arid weathering, deep burial diagenesis, and fault–fluid interactions collectively enhance both reservoir storage and seal capacity. This mechanism differs from that of typical marine analogues. The proposed integrated workflow provides a systematic predictive framework for evaluating deep (> 5000 m) targets in tectonically complex continental basins worldwide.