There are numerous controversies surrounding the tectonic properties and evolution of the Proto-South China Sea (PSCS). By combining data from previously published works with our geological and paleontological observations of the South China Sea (SCS), we propose that the PSCS should be analyzed within two separate contexts: its paleogeographic location and the history of its oceanic crust. With respect to its paleogeographic location, the tectonic properties of the PSCS vary widely from the Triassic to the mid-Late Cretaceous. In the Triassic, the Paleo-Tethys and the Paleo-Pacific Oceans were the major causes of tectonic changes in the SCS, while the PCSC may have been a remnant sea residing upon Tethys or Paleo-Pacific oceanic crust. In the Jurassic, the Meso-Tethys and the Paleo-Pacific oceans joined, creating a PSCS back-arc basin consisting of Meso-Tethys and/or Paleo-Pacific oceanic crust. From the Early Cretaceous to the mid-Late Cretaceous, the Paleo-Pacific Ocean was the main tectonic body affecting the SCS; the PSCS may have been a marginal sea or a back-arc basin with Paleo-Pacific oceanic crust. With respect to its oceanic crust, due to the subduction and retreat of the Paleo-Pacific plate in Southeast Asia at the end of the Late Cretaceous, the SCS probably produced new oceanic crust, which allowed the PSCS to formally emerge. At this time, the PSCS was most likely a combination of a new marginal sea and a remnant sea; its oceanic crust, which eventually subducted and became extinct, consisted of both new oceanic crust and remnant oceanic crust from the Paleo-Pacific Ocean. In the present day, the remnant PSCS oceanic crust is located in the southwestern Nansha Trough.
Gravity and magnetic data play an irreplaceable role in regional research, especially in sea areas that lack seismic and drilling data. Gravity and magnetic data are widely used in lithologic identification. In order to ensure the rationality of lithologic identification based on gravity and magnetic data, we analyzed the relationships between the lithology and the density and magnetic susceptibility. Through this analysis, the sensitivities of these two parameters to the lithology were determined, thereby determining the identifiable rock types and the identification possibilities (Yang, 1998; Frank and Nowaczyk, 2008; Lang et al, 2011). In recent years, the deep-water area of the southern South China Sea has gradually become a hot spot for oil and gas exploration. Drilling in adjacent areas has revealed that there are abundant oil and gas resources in the Mesozoic-Cenozoic strata (Yao and Liu, 2006; Lu et al., 2014a; Zhao, 2018). Despite the scarcity of seismic and drilling data for this area, the high resolution seismic imaging of the Cenozoic strata can basically satisfy the needs of lithologic analysis. In comparison, the seismic imaging resolution of the Mesozoic strata is low, and it is
In recent years, with the exploration and development of granite buried-hill oil and gas reservoirs, petrophysics research has played an important role in the study of reservoir characteristics and fluid identification. Through analysis of the relationship between the fluid-bearing petrophysical parameters and the reservoir, the seismic response changes caused by reservoir fluid changes can be determined. Mesozoic granites in the coastal area of Fujian Province in eastern China were investigated as the research object of this project. The mineral composition, density, porosity, P-wave velocity, and S-wave velocity of the granite were measured and analyzed by X-ray diffraction, rock density, rock porosity, and rock acoustics methods. Therefore, the granite’s petrophysical properties, fluid response characteristics, and gas sensitivity parameters were analyzed. The result of the study shows that the granite is predominantly monzogranite. According to the type of reservoir space assemblage, the samples can be divided into two types: those containing fracture-dissolution pores and those containing only dissolution pores. All the samples were characterized by medium to high densities and low to extra-low porosity. There was a linear correlation between the P-wave velocity and S-wave velocity under gas and water-saturated conditions. Factors such as P-wave to S-wave velocity ratio, Poisson’s ratio, Lame coefficient, and other parameters of the samples were analyzed, and the threshold values that distinguished the water and gas-saturated states of the samples were measured and determined. In addition, there were negative correlations between the P- and S-wave velocities and porosity. The sensitivities of the petrophysical parameters to the gas capacity from high to low are Ip2 − 2.03 Is2, λ − 0.03 μ, λ, λ/μ, E − 2.03 μ, σ, K/μ, K, Ip, Vp/Vs, Vp, E, μ, Vs, and Is. For granite-buried hill reservoirs, the variation ranges of the parameters, such as the density, porosity, and P-wave velocity, of the fracture-dissolution pore granite samples were larger than those of the dissolution pore samples. The bulk parameters (Ip, Vp, K, λ) and combination parameters (Ip2 − 2.03 Is2, K/μ, λ− 0.03 μ, E − 2.03 μ, λ/μ) of the dissolution pore samples were more sensitive to the gas capacity. The results of this study provide a basis for the geophysical identification of granite-buried hill reservoirs.
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