Gas Capillary Sealing as a Mechanism of Seal Development in the Upper Devonian Dunkirk Shale, Western New York State

Lash, Gary G.

Dept. of Geosciences, SUNY-Fredonia, Fredonia, NY 14063   

The Upper Devonian Dunkirk black shale, western New York State, was top seal to fluids migrating upward from deeper in the sediment pile.  The high sealing capacity of the organic-rich shale reflects a number of factors, including a strongly oriented platy grain microfabric, abundant organic matter, and an anoxic depositional environment.  However, the Dunkirk shale is not an especially thick seal by modern standards.  The duration of a seal before capillary failure is proportional to the square of the seal thickness and inversely proportional to the permeability of the seal.  Assuming that the Dunkirk shale had been compacted to its present 17-m thickness by the time it halted vertically migrating fluids and using a mean permeability of 2.6 x 10^-21 m² for the Dunkirk shale based on porosimetry measurements indicates that the black shale seal would have been compromised after only 270 years.  Indeed, confinement of overpressured fluids by the Dunkirk shale for 1 MY would have required a permeability less two orders of magnitude lower than the lowest measured permeabilities.  The extraordinarily high sealing capacity of the Dunkirk shale may reflect the formation of a gas capillary seal, an especially durable barrier that can form in layered sequences of variable grain size and in the presence of free methane.  Moderately depleted carbon isotope values of carbonate samples collected from Dunkirk shale concretions probably reflects the generation of biogenic methane in these deposits.   Interlaminated siltstone and claystone in the lower Dunkirk shale provided the framework within which to segregate biogenic methane and water.