Petrophysical properties of the major marine shales in the Upper Yangtze Block, south China: A function of structural deformation

A series of organic-rich shale samples from various shale gas reservoirs that experienced complex structural change around the Sichuan Basin of the Upper Yangtze Block, South China, have been studied with respect to their microstructural and petrophysical variations, using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Deformed samples were chosen as being representative of various degrees of deformation intensity, and undeformed samples (reference primary structure shales) were sampled outside the main deformed zones. In most of the examples explored, porosity, permeability, and fracture varied with the degree of structural deformation at the micrometer or even nanometer scale, changing and in most cases increasing the deformed shales's ability to store and transport hydrocarbon molecules. Quartz and carbonates are primarily responsible for petrophysical variations, but tectonism can strongly influence these existing relations. Porosity varies by up to one to two times in extreme cases, but the average porosity increase is about one time. Permeability increases by up to three to four orders of magnitude in extreme cases, but the average permeability increase is about two to three orders of magnitude. These ranges depend heavily on the development of abundant natural micro-fractures and micro-channels. Six major fracture types, including matrix micro-fracture, mineral-related micro-fracture, OM-related micro-fracture, fracture-edge micro-channel, pore-edge microchannel, and intragranular micro-channel, have been identified and classified by size, location, and geometry. Furthermore, possible formation mechanisms for these fractures, such as grain crushing, grain cracking, grain folding, and grain spacing, have been proposed and discussed. This study highlights the tectonic processes for natural fracture formation and their changes in porosity and permeability. These results have significant implications for understanding methane storage and fluid migration in naturally deformed shale reservoirs, and they could prove important for evaluating and predicting producible resources.