PESA ACT/NSW April Technical Talk

Source rock deformation and hydrocarbon accumulation controlled by a metamorphic core complex in the Kaiping Sag, northern South China Sea rifted margin

The recent discovery of hundred-million-ton-class oil fields in the Kaiping Sag, on the northern South China Sea rifted margin, is considered the first global breakthrough in hydrocarbon exploration within supra- detachment basins controlled by mature metamorphic core complexes (MCCs) at passive margins. We utilize 3D seismic, petrological and geochemical data to reconstruct source rock deformation and hydrocarbon accumulation in the context of detachment tectonism associated with uplift of the Kaiping MCC. In the pre-MCC stage, high-quality deep-water lacustrine source rocks of the Lower Wenchang Formation were deposited during the intense rifting. During the late Wenchang stage, the Kaiping MCC was uplifted via a rolling-hinge process, which caused the transformation of the original attitude and position of the source rock layers in the Lower Wenchang Formation. The distribution of the source rocks can be characterised by three distinct patterns: disconnected with antithetic dip direction (Model I), moderate continuous and V-shaped (Model II), and continuous with primitive original dip direction (Model III). These models control the dominant migration direction of hydrocarbons. Furthermore, the extensive development of fault systems and block rotations associated with the Kaiping MCC linking source rocks to traps and influence local oil migration pathways. The breakthrough of hydrocarbon exploration in the Kaiping Sag verifies that supra- detachment basins associated with MCCs also have the potential to develop high-quality source rocks, particularly during the rifting phase preceding the uplift of the MCC. The source rocks distribution models, antithetic faults, and rotating fault blocks resulting from the Kaiping MCC can serve as a reference model for evaluating source rocks and analysing hydrocarbon accumulation patterns in similar types of basins.