A study on the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles

Marlstone reservoirs are closely related to the degree of fracture development and their hydrocarbon accumulation are also controlled by the distribution of fractures. Hence, the prediction of fracture development zones is very important to the discovery of fractured oil and gas reservoirs. Tectonic movements and their intensities are one of the key factors affecting the formation of fracture systems and the distribution of tectonic stress field has great impact on the generation of fractures. Therefore, taking marginal subsags in the Jiyang Depression, Eastern China, as an example, this paper studied the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles. This study proposed the equivalent geological model and elastic characterization parameters for the fractured marlstone reservoirs in study area, analyzed the seismic response patterns, anisotropy characteristics, and seismic wave-field distribution of the marginal subsags in the Jiyang Depression, discussed the influences of fracture filling fluid, density, inclination, orientation, and layer thickness on the characteristics of seismic wave propagation in fractured marlstone reservoirs, and finally explored the shear wave splitting phenomenon and the exploration and development significance of the seismic wave propagation in fractured marlstone reservoirs. The study results show that the fracture filling fluid can lead to the dispersion and attenuation of seismic wave propagating in the medium, and rock saturated with fluid can exhibit frequency-dependent attenuation and dispersion due to wave-induced fluid flow between pores and fractures. The change of fracture density can make seismic wave appear anisotropic and biphasic and the difference between the high and low frequency of the longitudinal wave phase velocity becomes more and more obvious. The change of fracture inclination and orientation can cause the shear wave splitting phenomenon, in which the fast shear wave polarized parallel to the fracture with a fast velocity and a slow attenuation and the slow shear wave is polarized in the direction perpendicular to the crack with slow velocity and fast attenuation. The change of fracture layer thickness can make because the change of phase velocity and attenuation and the larger the thickness of the fracture layer is, the greater the difference between the reflection amplitude of the bottom interface and that of the non-fracture layer is. The study of characteristics of seismic wave propagation in fractured marlstone reservoirs can effectively obtain the multi-directional shear wave impedance and then fine characterize the fracture strike and density of tight oil fractured reservoir, which has important theoretical research and practical exploration significance. The study results provide a reference for further researches on the characteristics of seismic wave propagation in fractured marlstone reservoirs based on mechanical principles.