Anisotropic dispersion mechanism of inter-salt shale oil reservoir in terrestrial saline lake sediments using cross-band experiments

The rock mechanical properties and elastic anisotropy of terrestrial shale oil reservoirs are affected by various factors, such as lithology, structure, pores, fractures, and fluids. The experimental study of dynamic and static elastic properties can provide important mechanism analysis for the prediction of geological and engineering “sweet spots” in shale reservoirs. There are a large number of studies on the measurement of static mechanical properties of shale, but the experiments on dynamic cross-band elastic anisotropy of terrestrial shale have not yet been conducted thoroughly. Therefore, we report 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 for different confining pressures and fluid saturation conditions. The experiments were conducted by the cross-band rock physics measurement technology that comprised low-frequency stress-strain measurements and a high-frequency ultrasonic test. The experimental results indicated that: (1) The elastic property dispersion of the terrestrial shale was stronger than that of marine shale due to the high viscosity of the medium oil in the terrestrial shale. The lamellar structures and interbedded fractures were the main factors that determined the strong anisotropy of the terrestrial shale. (2) The dispersion of elastic properties from low to high frequencies in a partial oil saturation state ranged from strong to weak; the wave-induced fluid flow or intrinsic dissipation of viscoelastic inclusions may be the dominant mechanisms that caused the seismic dispersion. (3) The elastic parameters measured in the direction vertical to the bedding plane had stronger dispersion and pressure sensitivity than those measured in the direction parallel to the bedding plane, and the anisotropy and pressure sensitivity at seismic frequencies were higher than those at the ultrasonic frequencies. (4) Fluid filling reduced the pressure sensitivity of the elastic parameters along the direction vertical to the bedding plane, whereas the opposite trend was observed along the direction parallel to the bedding plane. (5) The anisotropic Gassmann theory could explain the P-wave velocity well at an extremely low frequency, but the prediction of S- and P-wave velocities at a relatively high frequency remained insufficient. Overall, our study can serve as a reliable mechanism reference for the study of frequency-dependent properties of azimuthal anisotropy, and provide important guidance for the seismic prediction of “sweet spots” in shale oil reservoirs.