New insights into Cretaceous hydrocarbon accumulation and its significance for hydrocarbon exploration in Hashan area, northwestern margin of Junggar Basin

To address the challenges in understanding the ambiguous hydrocarbon accumulation patterns and improving the exploration effectiveness in the Cretaceous system of the Hashan area along the northwestern margin of the Junggar Basin, this study aims to delineate the differences in hydrocarbon accumulation models between the Cretaceous and the Jurassic, identify the key controlling factors for Cretaceous hydrocarbon enrichment, and expand exploration in basin-margin overlapping zones. By integrating data from rock cores, thin sections, biomarker compounds, and fluid inclusions, a multidisciplinary geological and geochemical method was employed to conduct systematic studies on oil and source correlation, hydrocarbon migration pathways, and transport system configurations. Key investigations focused on source rock characteristics, crude oil properties, accumulation stages, and the coupling relationship between strike-slip faults and sand bodies. A Cretaceous accumulation model was established and validated through 3D seismic interpretation and drilling data. The study found that: (1) Oil source differentiation: Cretaceous crude oils originated from alkaline-saline lithofacies of the Permian Fengcheng Formation in the Hashan Sag, characterized by C₂₈/C₂₉ sterane ratios of 0.6 to 1.1 and gammacerane/C₃₀ hopane ratios of 1.58 to 2.02. Jurassic oils were mainly derived from brackish dolomitic lithofacies. (2) Transport systems: Cretaceous accumulation was controlled by a strike-slip fault-lobate sand body dual transport mechanism. Strike-slip faults (e.g., Haqian 23 to Haqian 34 faults) vertically connected deep hydrocarbon source rocks with shallow sand bodies (porosity greater than 20% and permeability greater than 200×10^-3 μm²), forming seven oil-bearing layers. In contrast, Jurassic reservoirs were predominantly controlled by lateral "fault-blanket" type transport mechanism. (3) Accumulation stages: Homogenization temperatures of inclusions (110 to 140 ℃) and aromatic maturity parameters (R_c=1.21%-1.56%) indicated that the Cretaceous experienced a single-phase, high-maturity hydrocarbon charging, whereas the Jurassic underwent a mixed dual-phase charging. Based on these, several oil-bearing blocks, such as Haqian 23 to Haqian 10, have been identified, and the northern region of Haqian 24 is predicted to hold 45 million tons of reserves. The Cretaceous system is projected to develop a 50-million-ton exploration target. Breaking from the traditional "fault-blanket" theory, this study proposes a new transport mechanism of near-source vertical supply coupled with strike-slip faults and lobate sand bodies for the Cretaceous hydrocarbon accumulation. The inherent spatial configuration of efficiently conductive strike-slip faults and sand bodies distributed along grooves is critical for reservoir formation. The new theory redirects exploration focus from isolated targets toward regional-scale accumulation zones, significantly expanding the exploration potential along the northwestern margin of the Junggar Basin.