Bulletin--KGS

Abstract

Devonian black shales constitute a major economic resource in Kentucky. These shales, known variously as the Ohio, Chattanooga, and New Albany Shales, are between 50 and 1, 700 feet thick and occur both in outcrop and in the subsurface, buried as deep as 4,200 feet below sea level.

Total gas in place for the Devonian shales in Kentucky is estimated to be between 63 and 112 trillion cubic feet; between 2 and 28 percent is recoverable. Known shale gas accumulations include the giant Big Sandy Field of eastern Kentucky and adjacent \/Vest Virginia, as well as a number of smaller fields in eastern and western Kentucky.

Geochemical evidence shows that Devonian shales haw acted as effective source rocks for the bulk of the oil in both the Illinois and Appalachian Basins. This finding is consistent with a vitrinite reflectance map of the Devonian shales of Kentucky, which shows that much of the New Albany Shale and the Ohio Shale has reached a level of thermal maturity adequate for abundant oil generation. Devonian shale gas is thermogenic, and associated with the widespread generation of oil. The predominance of gas production from fractured Devonian shales is attributed to evaporational fractionation during hydrocarbon migration through the organic-rich, low permeability reservoirs.

Natural fractures of different types are essential for effective gas production from Devonian shales. One common fracture type is closely associated with fault zones. Such fractured reservoirs may be poorly sealed, and productive only at the relatively low rates of migrating gas. Another common fracture type is closely associated with low-amplitude flexures. These fractured reservoirs should be well sealed, and productive at relatively high rates. Devonian shale reservoirs are commonly underpressured, but have a geological history of overpressuring that has been important in facilitating fracturing. Horizontal extension fractures in the Ohio Shale of the Big Sandy Field indicate a past episode of overpressuring that exceeded the lithostatic pressure gradient. Current underpressuring of Devonian shale reservoirs is probably caused by cooling resulting from uplift and erosion since the period of maximum burial in the Mesozoic. Calculation of paleoreservoir pressure using published values of eroded overburden and aquathermal pressure gradient suggests that reservoir pressure in the Devonian shales of the Big Sandy Field exceeded the lithostatic pressure gradient, locally resulting in horizontal extension fractures observed in core.

Regions of maximum potential for Devonian shale gas production can be identified interior to the Rome Trough in eastern Kentucky, and interior to the Moorman Syncline in western Kentucky. In these regions, the lack of surface faulting increases the potential for maintaining a good seal to the Devonian shale gas accumulations. Thermal maturity in these regions is adequate for the generation of oil and associated gas, with accompanying overpressuring. The degree of gas generation and overpressuring would have increased to the southeast in the Rome Trough, and to the west in the Moorman Syncline. Intensive drilling in the deep Rome Trough has resulted in the development of the Big Sandy Field. In contrast. the deep Moorman Syncline is virtually unexplored. In addition to avoiding surface faulting and seeking out maximum thermal maturity, effective exploration in the deep Moorman Syncline must locate those specific flexures that would have resulted in extensive reservoir fracturing in the overpressured Devonian shales.

Publication Date

1993

Series

Series XI

Report Number

Bulletin 4

Digital Object Identifier (DOI)

https://doi.org/10.13023/kgs.b04.11

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