Pore Types and Their Distribution Within Different Facies Types in the Devonian Ohio Shale of Martin County, Kentucky — SEM Observations on Ion-Milled Samples

Abstract

Middle and Late Devonian Strata of the Appalachian Basin in the Eastern Part of Kentucky have been an important energy resource since the discovery of natural gas in these shales in the 1890s. Most of these gas wells in that area extend over an area of 27,632 square kilometers and are known collectively as the Big Sandy gas field with an annual production between 50 to 70 billion cubic feet of gas. The source of this gas, the Devonian Ohio shale, consists of interbedded black, dark gray, gray and brownish gray organic rich mudstones that were deposited east of the Cincinnati Arch in the Appalachian Basin. It varies in thickness from about 300 meters in Martin County (easternmost Kentucky) to about 47 meters in Estill County (central Kentucky), near the outcrop belt of these Devonian strata. Based on lithology and sedimentary structures examined in core, the following facies have been recognized in the Ohio Shale: black silty mudstone, gray silty mudstone, dark gray silty mudstone, interlaminated dark and pale gray silty mudstone, dark gray silty mudstone with pyritic enrichment and compositional banding, and black silty mudstone with sub-mm to mm scale silty laminae. Thin intervals of coarse siltstone to very fine sandstone with abundant ripple cross-lamination are interspersed with finer grained lithologies. SEM examination of ion-milled samples shows three pore categories: Phyllosilicate framework (PF) porosity, organic matter (OM) porosity and carbonate dissolution (Cd) pores. PF pores (typically 50-500 nm), are triangular and best developed in pressure shadows of compaction resistant grains (quartz silt, pyrite framboids, marcasite grains). They are ubiquitous throughout the Ohio shale and appear to provide the bulk of visible porosity in the shale matrix (may be open or filled with kerogen/bitumen). OM porosity (pore sizes in 10-100 nm range) occurs in amorphous organic matter and pore filling (PF pores) bitumen, and is poorly developed owing to comparatively low maturity (Ro range 0.5-1.0%). OM porosity is most abundant in organic rich facies (black and dark gray mudstones). Due to the overall small amounts of diagenetic carbonates (dolomite, ankerite, siderite) in these rocks, the contribution to overall porosity by carbonate dissolution is negligible. PF porosity is strongly dominant in in all facies types, and without it it seems unlikely that the Ohio Shale as a whole could function as an unconventional reservoir.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016