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The AAPG/Datapages Combined Publications Database
Several hundred shale samples have been characterized by us with respect to mineralogic composition, chemical composition, and petrophysical properties. However, no study based on composition reveals the fabric or manner in which the components are put together to form the resultant rock. Classification by fabric elements, based on X-radiography and direct prints from thin sections, is practical and appears useful for interpretation of both depositional environments and reservoir properties of shale.
Six lithotypes based on fabric elements are defined in Devonian shales of West Virginia: (1) sharply banded shale, (2) thinly laminated shale, (3) lenticularly laminated shale, (4) nonbanded shale, (5) siltstones, and (6) concretions. A gradational series exists from thinly laminated through lenticularly laminated to nonbedded. Thinly laminated and organic-rich lenticularly laminated shales appear the most favorable types for gas productivity because the laminated fabric permits the best lateral continuity of the low porosity (about 2%) present. The higher organic content of these lithotypes also probably acts as sites for significant sorption of gas, which is slowly released during production.
No microfracture system appears to exist, and macroscopic fractures do not necessarily coincide with productive zones. Wells without natural fractures prove productive after stimulation, which suggests that outgassing of a favorable rock type is responsible for the bulk of productive gas in Devonian shale wells. Natural fracture porosity, while important in extending effective volume tapped by a given well, represents only a minor portion of reservoir storage volume.
Thinly laminated shales are interpreted as being generally representative of shallow-marine deposits (dominantly shelf deposits) accumulating under anoxic conditions. Nonbanded and lenticularly laminated shales are deeper water accumulations deposited farther from shore. Sharply banded shales and some siltstones generally represent pulses of more rapid sedimentation in the prodelta environment. A single sample in itself is not diagnostic of depositional environment, but relative abundance of specific lithotypes in a given vertical section is useful for general environmental reconstruction. Better definition of the diagnostic worth of shale fabric-element lithotypes awaits results of more detailed fabric-element studies of modern fine-grained clastics and marine muds.
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