Geologic Controls on Reservoir Properties in Gas-Bearing Middle and Upper Devonian Rocks, Southern Appalachian Basin

Richard K. Vessell, David K. Davies

Porosities and permeabilities have been measured for a wide range of nonfractured Devonian lithologies in 23 wells from southeastern Ohio, eastern Kentucky, West Virginia, and Virginia. These reservoir properties can be related directly to the geometry of the pore system. Pore geometry, in turn, is a function of rock lithology and mineralogy. Despite the lithologic complexity of the Devonian sequence, reservoir quality can be related to a small number of differing pore geometries.

Pore type 1 (intergranular macroporosity):
This geometry is restricted to sandstones and clean siltstones that are interlaminated with shale. These rocks represent the more conventional reservoirs in the Devonian sequence. Mean pore diameters are larger than 5 µ, and reservoir quality is controlled strongly by grain size. Average core measured values are: for sandstones, porosity = 7.3%, permeability = 260 md; for siltstones, porosity = 5.1%, permeability = 20 md. These lithologies are generally accepted as being potentially productive in the study area.

Pore type 2 (chaotic microporosity):
This pore type occurs in nonlaminated, very shaly siltstones and sandstones (shale content > 30%), and in silty and gray shales with a chaotic fabric. A dual porosity system exists which consists of interparticle pores (1-5 µ) and intraparticle pores (0.5 µ). Pores are arranged chaotically. Average core measured values are: for shaly sandstones and siltstones, porosity = 4.1%, permeability = 9.5 microdarcys; for silty and gray shales, porosity = 3.6%, permeability = 3.5 microdarcys. These rocks have the highest measured permeabilities of all shale lithologies in the Devonian sequence. The pore system has a geometry that appears to be capable of flow, particularly if hydraulically or naturally fractured.

Pore type 3 (laminated microporosity):
This pore geometry is characteristic of kerogen-rich black shales. Interparticle pore diameter is 0.1-5µ, and pores have a strong preferred orientation parallel with bedding. Average core measured values are: porosity = 4.2%, permeability = 0.8 microdarcys. The laminated and strongly overlapping arrangement of the constituent particles in the black shales results in a pore system that appears incapable of sustaining gas production, even when fractured. The black shales are interpreted as the source rocks for the abundant gas in the Devonian shale sequence.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.