Non-Tectonic Aspects of Shale Fracture Formation, Permeability, and Porosity of Late Devonian Black Shales in the Eastern US

The Late Devonian Chattanooga Shale and laterally equivalent strata in the eastern US record a complex depositional history. These shales can be subdivided into three packages that are separated by significant subaerial erosion surfaces. Tracing of shallow marine erosion surfaces allows further subdivision of these packages into as many as 14 erosion bounded sequences. All shale intervals exhibit distinct compositional, sedimentary, and textural features that influence their capacity to facilitate or impede fluid movement. Abundant organic matter is associated with low fracture density, whereas abundant silt and sand beds are associated with increased fracturing. Interbedded gray mudstones show abundant fractures, but continuity is limited due to the more plastic behaving black shale interbeds. Although fracture patterns generally reflect regional stress fields, fracture density may abruptly change between sequences because of compositional changes. Intermittent subaerial exposure of some shale units during the Devonian has also resulted in fracturing due to unloading and to leaching of pyrite and dolomite along fractures and laminae, both resulting in increased porosity and permeability. Permeability, porosity, and sealing capacity of the various black shale intervals in the Devonian black shale succession is determined by a combination of oceanographic (organic matter, cement minerals), sedimentologic (winnowing), and biological (bioturbation) factors, and by their history along the continuum of deposition, exposure, erosion, weathering, and final burial.