--> ABSTRACT: Pore Geometry and Microfabrics of Gas-Bearing Devonian Shales, by David K. Davies, William R. Bryant, Richard K. Vessell, and Patti J. Burkett; #91022 (1989)

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Pore Geometry and Microfabrics of Gas-Bearing Devonian Shales

David K. Davies, William R. Bryant, Richard K. Vessell, Patti J. Burkett

Devonian shales of the Appalachian basin comprise a significant domestic gas resource. Economic production from nonfractured shale lithologies has been well documented in several areas of the basin. However, the pore geometry and microfabric of gas-productive shales are poorly understood. The present investigation is based on analysis of more than 100 Devonian shale samples.

Combined SEM/TEM observations reveal that pores in the Devonian shales are of primary interparticle origin and are commonly lined with minute (0.1 µm), euhedral, authigenic crystals of pyrite and possibly apatite and zeolite. Imaging tomography, using micrographs of ultra-thin sections (800-1200 A) has allowed for the reconstruction of shale pores in three dimensions. Pores are of variable size and shape. The average shale pore has elongate, platelike morphology and is 1.2 µm in diameter. There is a direct relationship between pore size and particle size: pore size increases with an increase in particle size. Selected area diffractograms and x-ray diffraction analyses reveal that the detrital shale particles are highly crystalline illites. They are characterized by partial i lite overgrowths which extend into adjacent pores as euhedral crystal faces.

Porosity values range from 1% to 8% (mean = 4%). Porosity is not affected by significant changes in shale microfabric. Permeability values range from 10 to 10-2 µd (mean = 2.23 µd). Permeability values are controlled strongly by shale microfabric. Shales with parallel microfabric have lower permeabilities and significantly lower rates of gas production than shales with chaotic microfabric.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.