Prediction of lithologic distribution of basement in the Lishui Sag：a case study of the Lishui Sub-sag A

Basement is well developed in the Lishui sag, East China Sea Shelf Basin. Previous wells showed good oil and gas properties and great potential for further exploration. Using well cores, thin sections, zircon U-Pb dating, well log data, and 3D seismic data, we combined the facies-seismic model and petrophysical analysis to predict the basement facies qualitatively and semi-quantitatively and their lateral distributions. Results show that the basement of the Lishui Sag is composed of mainly gneiss, granitic gneiss, and granite. The gneiss is the Proterozoic metamorphic continental core, and the granite was the intrusive of the Mesozoic magma via deep and large faults. Seismic characteristics of gneiss show continuous bedding planes with medium-high frequency and medium-high amplitude, while granite shows chaotic reflectors with low-medium frequency and low-medium amplitude. Granitic gneiss displays seismic characteristics between these two. The petrophysical analysis indicate that the density is most effective parameter in differentiating lithologies, and pre-stack density inversion could predict better the distribution of basement rocks semi-quantitatively. It is suggested that the granite in the Lishui Sub-sag A is overlain by metamorphic rocks, and the facies in the basement top varies laterally in different tectonic units. The granite in the uplift regions has been severely eroded, and left with only gneiss and granitic gneiss basement. The places lacking deep and large faults in the sag are made up of metamorphic basement, while other areas are characterized by granitic basement.