Characteristics and Controlling Factors of Tight Sandstone Reservoirs in the Deep Sags of Rifted Lacustrine Basins: A Case Study of the Ek 2 Member, Cangdong Sag, Bohai Bay Basin

Abstract

Deeply buried tight sandstones have emerged as important exploration targets in rifted lacustrine basins. However, reservoir quality generally declines significantly at burial depths exceeding 5000 m. The mechanisms responsible for the preservation of effective pore systems remain poorly understood. This study investigated the deeply buried Ek 2 Member of the Cangdong Sag, Bohai Bay Basin, where locally preserved high‐quality reservoirs occur, despite extreme burial depths. A total of 26 core samples from Well Xinhua 1 were analysed using petrophysical analysis, thin‐section petrography, scanning electron microscopy (SEM), high‐pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), fluid‐inclusion laser Raman spectroscopy (LRS) and well‐log interpretation. The sandstones are composed predominantly of lithic arkose and arkosic litharenite (porosity: 0.85%–7.64%, permeability: 0.004–0.058 mD). Residual intergranular pores, dissolution pores and intercrystalline pores constitute the dominant reservoir space, whereas microfractures contribute only marginally. Integrated HPMI‐NMR analyses identified three pore‐structure assemblages: Type I, comprising small pores and mesopores; Type II, dominated by small pores; and Type III, dominated by micropores. Type I occurs predominantly in subaqueous distributary‐channel deposits, whereas Type II and Type III are mainly associated with mouth‐bar and sheet‐sand microfacies, respectively. These results indicate that sedimentary microfacies establish the primary reservoir framework, while pore preservation and connectivity are governed by the combined effects of compaction, cementation, dissolution and overpressure‐related fluid activity. By demonstrating that the preservation of deeply buried tight sandstone reservoirs is influenced by the coupled effects of depositional architecture and diagenetic evolution, this study provides new insights into the reservoir‐quality evolution of the Ek 2 Member and offers a geological analogue for the exploration of deeply buried tight sandstone reservoirs in rifted lacustrine basins.