Experimental Studies on Anisotropic Dispersion of inter-salt Shale Oil

The mechanical properties and elastic anisotropy of shale oil reservoirs are influenced by factors such as lithology, pore structures, and fluids. Studying the dynamic and static elastic properties is crucial for predicting geological and engineering “sweet spots” in these reservoirs. While there has been extensive research on the static properties of shale, dynamic multi-frequency elastic anisotropy remains under-explored. This study investigates the anisotropic dispersion mechanism of favorable lithofacies (lamellar dolomitic shale with vertical and horizontal bedding) in the inter-salt shale oil reservoir of the Qianjiang Formation, by using multi-frequency measurement techniques including low-frequency stress-strain and high-frequency ultrasonic tests. Key findings include: 1) Terrestrial shale exhibited stronger elastic property dispersion than marine shale, attributed to high viscosity medium oil. Lamellar structures and interbedded fractures significantly contributed to this anisotropy; 2) Elastic property dispersion in partially oil-saturated states decreased from strong to weak with increasing frequency, likely driven by wave-induced fluid flow or intrinsic dissipation; 3) Elastic parameters measured perpendicular to bedding showed greater dispersion and pressure sensitivity than those measured parallel, with higher anisotropy and sensitivity at seismic frequencies; 4) Fluid saturation reduced pressure sensitivity of elastic parameters in the vertical direction while increasing it in the parallel direction; 5) Anisotropic Gassmann theory effectively explained P-wave velocity at low frequencies, though predictions for S- and P-wave velocities at higher frequencies were less accurate. This study provides a solid reference for understanding frequency-dependent properties of azimuthal anisotropy and offers valuable insights for seismic prediction of “sweet spots” in shale oil reservoirs.