Characteristics and Genesis of Buried-Hill Reservoirs in a Compressional–Extensional Setting: Insights from the Baiyun Sag, Pearl River Mouth Basin, South China Sea

Abstract

Buried-hill reservoirs represent crucial deep hydrocarbon targets. However, their characteristics and genesis in a compressional–extensional tectonic setting remain poorly constrained because of complex interplay among tectonic compression, weathering and extensional faulting. The Baiyun Sag in the northern South China Sea (SCS) underwent a Meso–Cenozoic transition from compression to extension. Integrating petrological observations, wireline logging and 3D seismic datasets, this study confirms that its basement is predominantly composed of Yanshanian intermediate–acid intrusive rocks. Three buried-hill reservoir types were identified: Type I (gradually changing reservoir), formed by prolonged weathering following Late Mesozoic compression is widely distributed at the tops of buried hills and consists of a weathering crust and an inner fractured zone. Type II (interbedded reservoir), featuring alternating weathered and tight zones (TZs), occurs on the flanks of buried hills. It formed through weathering along reactivated faults during late Mesozoic and Cenozoic extension. Type III (transitional reservoir), located on the tops of buried hills proximal to boundary faults, comprises a weathering crust and multiple alternating weathered and TZs. These reservoirs formed through differential tectonism and weathering responses to late Mesozoic compression from Paleo-Pacific Plate subduction and Cenozoic extension from SCS opening. Differential tectonism primarily controls buried-hill reservoir types in such settings.

Keywords

Baiyun Sag / buried-hill reservoir / genesis / granite / Pearl River Mouth Basin / transitional tectonic setting

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2026 Geological Society of China.