Pore Structure Formation Mechanism of Lacustrine Fine-Grained Sedimentary System in Fengcheng Formation, Junggar Basin, China

Abstract

The Fengcheng Formation in the Mahu Sag of the Junggar Basin exhibits complex depositional systems containing heterogeneous hydrocarbon resources across distinct subregions. Shale oil accumulations predominantly occur in the northwestern basin sector, where significant variations in reservoir-oil-bearing potential arise from depositional heterogeneity and differential pore structure development within target intervals, presenting substantial challenges for commercial development. This study systematically investigates pore architecture and its controlling factors in Fengcheng Formation reservoirs, with a specific focus on sedimentary microfacies, mineralogical composition, and diagenetic alterations. A multidisciplinary characterization approach integrating high-pressure mercury intrusion (HPMI), low-temperature nitrogen adsorption, field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and conventional petrophysical analysis was employed to evaluate reservoir characteristics across sedimentary microfacies. Key findings reveal pronounced petrophysical contrasts among microfacies: (1) displays low porosity primarily controlled by intense mechanical compaction and cementation processes, resulting in substantial pore space reduction and low permeability. (2) Tempestite microfacies: demonstrates favorable pore characteristics attributed to high-energy depositional environments and carbonate dissolution, with secondary porosity generation enhancing pore connectivity through well-developed macropore networks. (3) Quiet Water microfacies: exhibits moderate porosity but restricted permeability due to pore-throat obstruction by felsic minerals and clay authigenesis, significantly limiting fluid mobility. (4) Slump microfacies: characterized by low porosity influenced by differential compaction and complex mineralogical assemblages, it leads to poorly connected pore systems with correspondingly low permeability. This study establishes a systematic analytical framework for pore structure evaluation in heterogeneous reservoirs of the Fengcheng Formation and elucidates the critical controls exerted by sedimentary microfacies differentiation, mineralogical constraints, and diagenetic overprinting on reservoir quality evolution.