Microtomographic analysis of controlling parameters on permeability and elastic velocities of black shales

Shales are sedimentary rocks characterized by their very fine grain size, and are basically composed of clays and silts. Important features of shales are their fissility, presenting thin layers almost always parallel to their bedding, low permeability and elastic velocities which can vary significantly, but whose controlling factors are not yet well understood. X-ray microtomography (microCT) is a widely-used technique to reconstruct the internal pore space of rocks and other materials. In this work parameters controlling permeability and elastic velocities of black shales are investigated through x-ray microCT digital image analysis. Results show that black shale permeability is controlled mainly by pore size distribution and pore connectivity, while elastic velocities of such rock type are controlled by porosity, pore size and the quality of grain-grain contacts. Mineral composition of the analyzed rock samples does not affect this analysis since all the samples have similar mineral content. MicroCT images show clearly the predominant pore locations along the fissility planes, which is a determinant factor in the difference between elastic velocities of horizontal and vertical rock samples. The macropores of the analyzed rock samples are not well interconnected, in contrast with the micropores, which present high pore connectivity and capillary pressure. A quality index for grain-grain contacts is proposed here which is the dominant factor controlling the elastic velocities of the analyzed black shale samples, followed by macro and micro porosities. Vertical samples, whose elastic velocities are lower than those of horizontal ones, present low values for the quality index of grain-grain contacts.