Origins of non-tectonic fractures in shale

Natural fractures are widespread in shales in sedimentary basins and can significantly increase the bulk permeability and enhance fluid flow, which are thereby considered to be a key factor in shale gas development, seal integrity analysis, site selection for CO2 sequestration and nuclear waste disposal. Although many fractures in shale owe their origin to tectonism, a substantial proportion of fractures are induced by shale diagenesis, hydrocarbon generation and expulsion during different stages of burial. The conditions under which non-tectonic fractures are generated during shale diagenesis generally include (1) fluid overpressure due to clay mineral dehydration (e.g., smectite to illite transition); (2) differential compaction caused by abrupt porosity reduction during silica diagenesis (e.g., opal-A dissolution); and (3) pressure solution in the rock matrix and neomorphic calcite. With an increasing thermal maturity of organic matter, natural hydraulic fractures can be produced in organic-rich shale during (1) early generation of biogenic gas; (2) oil generation and primary migration; and (3) cracking of oil to gas. At the early stage of the oil window, contraction fractures may also be formed in organic matter due to thermal shrinkage. A better understanding of the origins of non-tectonic fractures induced during shale burial may yield important implications for the interplay between fracture mechanics, inorganic/organic matter evolution and fluid flow in shale.