Microfracturing and microporosity in shales

Shales are ubiquitous rocks in sedimentary basins, where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories (waste waters, CO2, nuclear fuels). Moreover, when they contain organic matter, they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial, a process referred to as primary migration. And when the hydrocarbons cannot escape, these rocks can be exploited as oil or shale gas reservoirs. While the presence of fractures at the outcrop scale has been described, the existence of fractures at smaller scales, their link with microporosity, the mechanisms that created them, their persistence over geological times, and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data. Here, we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability. One question is whether such microfractures, when observed in outcrops or in drilled core samples extracted from boreholes, are related to decompaction and do not exist at depth. Another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation. The mechanisms of microfracture development are described. Some have an internal origin (fracturing by maturation of organic matter, dehydration of clays) while others are caused by external factors (tectonic loading). Importantly, the amount of microfracturing in shales is shown to depend strongly on the content in 1) organic matter, and 2) strong minerals. The nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale. Their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso- to macro-scales. Such percolating microfracture networks could control both the long-term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks. Finally, possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed