Geologic Controls on Gas Production from Appalachian Basin Devonian Shales
Daniel J. Soeder
Recent investigations, including precision core analysis, indicated several factors that influence or control natural gas production from organic-rich black shales in the Appalachian basin. Data from core analysis on samples of the Upper Devonian Huron Member of the Ohio Shale, obtained from six wells in the western part of the basin, consistently showed gas permeabilities of less than 0.1 nanodarcys and gas porosities below 1%. These values are extremely low (even for shale) and appear to result from petroleum in the pores. Because the characteristic pore size in these shales is only about 0.05 µm, fairly low partial oil saturations form a mobile liquid phase that blocks gas flow. Consequently, oil-bearing black shales normally will not be gas productive, although e idence suggests that these impermeable black shales may form stratigraphic traps for gas contained in intertongued, organic-lean gray shales.
Core analysis was also conducted on a sample of Middle Devonian Marcellus Shale from a well in the more central part of the Appalachian basin. Gas permeability in the Marcellus was mostly above 5 microdarcys, and porosity to gas was about 10%. The Marcellus Shale has potential as a commercial gas producer, although the strong stress-dependence of permeability observed from the core analysis may cause difficulties in gas recovery during later stages of reservoir drawdown. The degree of thermal maturity of the Marcellus Shale in the central Appalachian basin is significantly higher than that of the basin-margin Huron Shale, and the Marcellus core did not appear to contain any of the mobile liquid hydrocarbons that blocked gas flow in the Huron samples.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.