Print this pageTiming of Cleat Development in Coal Beds
SCOTT, ANDREW R.
Cleats are systematic open-mode fractures in coal that reflect maximum horizontal stresses that existed in the basin at the time of cleat genesis. Cleat geometry is important for designing maximum extraction efficiency in coals mines and for coalbed methane exploration where effective permeability is restricted to the cleat system. The initial development of cleats during coalification probably occurs between vitrinite reflectance values of 0.3 and 0.5% (peat to lignite) in response to shrinkage associated with the maximum rate of moisture loss. Moisture loss is more complex than simple "desiccation" because it involves rearrangement of the coal structure rather than simple loss of interstitial water. Cleat spacing in vitrinitic macerals is smaller than in hydrogen-rich macerals because of greater moisture loss and subsequent shrinkage of the coal structure. Similarly, cleat spacing decreases with increasing rank because of continued loss of molecular water and volatile components.
Regionally, face cleats probably do not develop simultaneously among formations of different ages or within one formation across an entire basin because the coals undergo different burial and coalification histories. The time required for initial cleat development after burial probably ranges from several million years to tens of million years, depending on burial history. Annealing of cleats during coalification probably occurs over the peat to anthracite ranks and may be more common than previously thought. The development of opening-mode fractures in higher rank coals occurs in direct response to regional tectonic stresses and is not associated with desiccation and coal shrinkage. These open-mode fractures may cross-cut existing desiccation cleats or develop after the annealing of cleats formed during earlier coalification. Microfractures that develop during active gas generation probably do not contribute significantly to effective permeability because of limited continuity and/or aperture size under in situ conditions. Understanding the timing of cleat genesis is important in evaluating changes in paleostress direction and predicting cleat geometry and permeability trends in the subsurface.