  To better understand the mechanisms of fracture pattern development and fluid
escape in low permeability rocks, we performed time-resolved in situ X-ray
tomography imaging to investigate the processes that occur during the slow
heating (from 60\degree; to 400\degree;C) of organic-rich Green River shale. At
about 350\degree;C cracks nucleated in the sample, and as the temperature
continued to increase, these cracks propagated parallel to shale bedding and
coalesced, thus cutting across the sample. Thermogravimetry and gas
chromatography revealed that the fracturing occurring at ~350\degree;C was
associated with significant mass loss and release of light hydrocarbons
generated by the decomposition of immature organic matter. Kerogen
decomposition is thought to cause an internal pressure build up sufficient to
form cracks in the shale, thus providing pathways for the outgoing
hydrocarbons. We show that a 2D numerical model based on this idea
qualitatively reproduces the experimentally observed dynamics of crack
nucleation, growth and coalescence, as well as the irregular outlines of the
cracks. Our results provide a new description of fracture pattern formation in
low permeability shales.
