Estimation of Permeability Anisotropy and Depositional Cycles in Organic-Rich Chalk by NMR Restricted Diffusion
Authors:   Rice University, Wang Xinglin , Vinegar Eva G. ,   The University of Texas at Austin and ; Eva G. Vinegar, The University of Texas at Austin and Vinegar Technologies, LLC, Liu Yunke ,   Rice University, Singer Philip M. ,   Rice University, Hirasaki George J. ,   Rice University, Vinegar Harold J. ,   Vinegar Technologies, LLC and Ben-Gurion University of the Negev
Published: November 2024

We present a new method for studying permeability anisotropy and paleo-depositional cycles by combining NMR anisotropic restricted diffusion measurements and scanning electron microscope (SEM) images on the core. In particular, the method is applied to measuring a depositional cycle from the Tethys Sea in the late Cretaceous period in the Ghareb Formation, which appears to be equivalent to the present-day El Nino-Southern Oscillation cycle. The NMR anisotropic restricted diffusion measurements were made with a 2.3-MHz NMR core analyzer on adjacent 1-in. core plugs drilled parallel (horizontal) and perpendicular (vertical) to the bedding plane. The cores at connate water saturation were then saturated with methane at 1,200 psi and then saturated with decane for NMR measurements using unipolar stimulated-echo pulse sequences. Different values of diffusion time were used to probe both the short L_D (diffusion length) regime with decane to determine surface-to-volume ratio S/V and the long L_D regime with methane to determine 1/tau, where tau is the diffusive tortuosity. Pore size and tortuosity were estimated based on the NMR restricted diffusion vs. diffusion length data and then used in a modified Carman-Kozeny model to predict the permeability anisotropy. The figure shows NMR anisotropic restricted diffusion measurements (restricted diffusivity (D/D_0) vs. diffusion length (L_D)) on decane-saturated cores with connate water (C10(H2O)) and methane-saturated cores with connate water in horizontal and vertical directions. The S/V is the same for horizontal and vertical directions, indicating the pore size is the same in the two samples. The permeabilities, computed from a modified Carman-Kozeny model, show that tortuosity is the main factor in the anisotropy of the measured core permeabilities. The diffusive tortuosity is much greater in the vertical direction than in the horizontal direction due to the additional diffusional restriction from the depositional laminations. We find that the L_D at which the vertical core reaches its tortuosity limit is significantly shorter than in the horizontal direction. We interpret the value of L_D ~100 µm, where the vertical diffusion reaches the asymptotic limit, as the half-spacing between laminations due to the depositional cycle. SEM images of the organic-rich chalk in this zone show several layers of shell fragments with the half-spacing between laminations of about 100 µm, which is consistent with NMR restricted diffusion results. We propose a new method to measure the permeability anisotropy using NMR restricted diffusion and the Carman-Kozeny model. This method can reduce the diffusive coupling using hydrocarbon saturation on cores with connate water and make a more accurate permeability estimation. The Ghareb Formation has been carefully dated in this region, and the rate of deposition is known. Thus, the laminations shown in the NMR restricted diffusion enable us to estimate the duration of the depositional cycle in this late Cretaceous period (~69 Mya). Surprisingly, we find that the timing of this paleo-depositional cycle was very close to the present-day El Nino-Southern Oscillation cycle.