Pyrite Laminae In Black Shale - An Additional Source of Permeability In Unconventional Reservoirs

Most unconventional black shale reservoirs are thought of in terms of a dual porosity system comprised of matrix and fracture porosity. However, newly obtained data from the Middle Devonian Marcellus Shale suggest an additional and meaningful source of porosity that may exist in other organic-rich shales. The high-TOC (≈ 10%) basal interval of the Marcellus Shale defines a porosity of 3- 4%; however, these rocks are characterized by a two-fold increase in permeability over that of the overlying less carbonaceous shale. The source of the enhanced permeability appears to be thin laminae of incompressible pyrite framboids that preserved porosity throughout the burial history of these rocks. The presence of abundant pyrite in high-TOC basal intervals of the Marcellus Shale elsewhere in the Appalachian Basin is indicated by higher bulk density and lower resistivity than would be expected of such organic-rich rocks. The carbonaceous basal intervals of Devonian black shale units of the Appalachian Basin, including the Marcellus Shale, probably accumulated as transgressive systems tract /condensed sequence (TST/CS) deposits in response to rapid relative rises in seal level. Such conditions, in tandem with increased primary production, would have favored the establishment of anoxic bottom conditions leading to the preservation of abundant organic matter. Slow sedimentation rates and winnowing bottom currents known to accompany rapid transgression and accumulation of TST/CS deposits produced the described permeable framboidal lag deposits in the toe of a clinoform. Preliminary results of this ongoing work suggest that organic-rich TST/CS deposits, commonly thought of as excellent seals, are likely candidates for enhanced migration of gas from matrix porosity via permeable pyrite laminae deposits into either induced artificial or stimulated natural fractures.