Timing and characterization of multiple fluid flow events in the Beibuwan Basin, northern South China Sea: Implications for hydrocarbon maturation

Abstract

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS.