Identification of Gas-Productive Shale Intervals in Upper Devonian of Southern Appalachian Basin

Richard K. Vessell, Raymond L. Campbell, Jr., David K. Davies, Robert B. Truman, Richard J. Scheper

Devonian shale gas production is influenced strongly by lithology as well as gas saturation. Petrographic analysis reveals five rock types in the shale interval: (1) clean sandstones and siltstones, (2) shaly sandstones and siltstones, (3) silty shales, (4) gray shales, and (5) black shales. The permeability of each is a function of pore geometry. Rocks with the highest permeabilities are characterized by intergranular and chaotic pore systems. The lowest permeabilities occur in strongly laminated pore systems. Pore geometry, lithology, and mineralogy are controlled by depositional environment. Thus the distribution of rock types, their permeability, and mineralogy are ordered, i.e., nonrandom.

Only major constituents are important for subsurface identification of rock types: the bulk volumes of quartz, heavy minerals (siderite, pyrite), and clays (including muscovite) and weight percent total organic carbon (TOC). Two ratios are used simultaneously to identify the rock types: (1) quartz to clay plus heavy minerals, and (2) TOC to heavy minerals. Gas production is a function not only of rock type but also of effective porosity and bulk volume gas. These latter elements can be determined reliably using currently available log interpretation methods.

All rock types in the Devonian sequence can yield gas to the well bore. Analysis of production logs demonstrates that the highest production rates are achieved consistently from rock types 1, 2, and 3 (sandstones, siltstones, and silty shales). The gray and black shales of rock types 4 and 5 have significantly lower rates of production.

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