Tectonic deformation and its petroleum geological significance of continental margin necking zone in deepwater area of Pearl River Mouth Basin, South China Sea

Abstract

Based on a set of high-resolution 3D seismic data from the northern continental margin of the South China Sea, the lithospheric structure, thinning mechanisms and related syn-rift tectonic deformation response processes in the crustal necking zone in the deepwater area of the Pearl River Mouth Basin were systematically analyzed, and the petroleum geological significance was discussed. The necking zone investigated in the study is located in the Baiyun Sag and Kaiping Sag in the deepwater area of the Pearl River Mouth Basin. These areas show extreme crustal thinned geometries of central thinning and flank thickening, characterized by multi-level and multi-dipping detachment fault systems. The necking zone exhibits pronounced lateral heterogeneity in structural architectures, which can be classified into four types of thinned crustal architectures, i.e. the wedge-shaped extremely thinned crustal architecture in the Baiyun Main Sub-sag, dumbbell-shaped moderately thinned crustal architecture in the Baiyun West Sub-sag, box-shaped weakly thinned crustal architecture in eastern Baiyun Sag, and metamorphic core complex weakly thinned crustal architecture in the Kaiping Sag. This shows great variations in the degree and style of crustal thinning, types of detachment faults, distribution of syn-rift sedimentary sequences, and intensity of magmatism. The thinning of the necking zone is controlled by the heterogeneous rheological stratification of lithosphere, intensity of mantle-derived magmatism, and deformation modes of detachment faults. The syn-rift tectonic deformation of the necking zone evolved through three phases, i.e. uniform stretching during the early Wenchang Formation deposition period, necking during the late Wenchang Formation deposition period, and hyperextension during the Enping Formation deposition period. The crustal thinning extent and architectural differentiation in these phases were primarily controlled by three distinct mechanisms, i.e. the pure shear deformation activation of pre-existing thrust faults, the simple shear deformation of crust-mantle and inter-crust detachment faults, and differential coupling of lower crustal flow and ductile domes with main detachment faults. The hydrocarbon accumulation and enrichment in the necking zone exhibit marked spatial heterogeneity. Four distinct crustal thinned architecture-hydrocarbon accumulation models were identified in this study. The hydrocarbon accumulations in the shallow part exhibit significant correlations with their deep crustal thinned architectures. The unique lithospheric structure and deformation process predominantly control the favorable hydrocarbon accumulation zones with excellent source-fault-ridge-sand configurations, which is critical to reservoir-forming. The most promising exploration targets are mainly identified on the uplift zones and their seaward-dipping flanks associated with the middle and lower crustal domes. This research provides additional insights into lithospheric thinning-breakup process at intermediate continental margins of marine sedimentary basins, being significant for guiding the deepwater petroleum exploration in the Pearl River Mouth Basin.