Petrophysical Acoustic Characterization of Weathered Granite: A Case Study of Mesozoic Granites in the Coastal Area of Fujian Province, China

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 Ip(2) - 2.03 Is(2), lambda - 0.03 mu, lambda, lambda/mu, E - 2.03 mu, sigma, K/mu, K, Ip, Vp/Vs, Vp, E, mu, 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, lambda) and combination parameters (Ip(2) - 2.03 Is(2), K/mu, lambda- 0.03 mu, E - 2.03 mu, lambda/mu) 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.