Comparison of pore fabric based on high-resolution X-ray computed tomography and magnetic pore fabric in sedimentary rocks

Pore fabrics characterize pore geometry and network in rocks. The pore size, connectivity and elongation direction determine the permeation ability and preferred permeation direction. X-ray micro-tomography (XRCT) is a widely used technique to visualize the inner structure of rock samples. Based on XRCT data, digital rock models can be generated and analyzed to visualize and quantify pore shape distribution, pore sizes and the connectivity of pores. To measure the magnetic pore fabric (MPF), samples are impregnated with ferrofluid prior to measuring anisotropy of magnetic susceptibility. This technique could be complementary to existing techniques to capture smaller pores. Empirical relationships exist between pore fabric or permeability anisotropy and MPF, and the aim of this study is to quantitatively test these relationships. In this study, Upper Marine Molasse sandstone (OMM, Belpberg, Switzerland) with 10-20% porosity and relatively homogeneous pore structure, and Plio-Pleistocene calcarenite (Apulia, Italy) with ~50% porosity and complex pore structure, are tested. To understand the pore networks of these rock types, an integrated approach has been applied including standard pycnometer porosity measurements, MPFs, XRCT, and porosity and permeability simulations based on XRCT analyses. The average equivalent diameter of pores based on micro-CT is ~150 μm for the Molasse sandstone, and ~300 μm for calcarenite. XRCT data indicate preferential alignment of the long axes of the pores, and both MPFs and simulated permeabilities are anisotropic in these samples. For calcarenite with large pores, the direction of the maximum magnetic susceptibility coincides with the direction of the maximum grouping of long pore axes. Simulated permeability is affected by other factors in addition to the grouping of long pore axes, including porosity, pore size, connectivity and tortuosity of pores. Therefore, the next step of this study will compare laboratory-measured directional permeabilities with permeability simulations and with MPFs, to investigate their potential for predicting the preferred fluid flow direction in these samples. For the full understanding of MPFs, more types of sedimentary rocks will be analyzed. If MPFs prove a good and quantitative proxy for pore fabric characterization in hydrocarbon and geothermal studies, more measurements can be made in the future, making it possible to investigate regional-scale variations.