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Geological Framework for Appraising Continuous Petroleum Resources in the Chattanooga Shale of the Southern Appalachian Basin


The U.S. Geological Survey (USGS) is reassessing technically recoverable, continuous petroleum resources in Upper Devonian–Lower Mississippian black shale strata in the Appalachian basin based on the total petroleum system concept. The Chattanooga Shale Gas Assessment Unit (AU) is assigned to the distal fringe of these black shale strata in the southern part of the Appalachian Basin. The AU extends south from south-central Kentucky through eastern Tennessee, northwest Georgia, northern Alabama and into eastern Mississippi.

Following a technique developed by the USGS for use in shale-gas assessments based on the well documented relationship between total organic carbon (TOC) and total gamma-ray response, gamma-ray logs were used to define two mapping parameters involving “high gamma-ray” (HGR; total gamma-ray response >150 API units) response: (1) Gross HGR is the total thickness of strata between the top of the shallowest HGR response and the base of the deepest HGR response within the study interval. (2) Net HGR is the sum of thickness of beds displaying HGR response within the gross HGR interval. These parameters have been used in other basins as reliable proxies for shale-reservoir thickness and shale-source-rock thickness, respectively.

HGR response in the Chattanooga Shale is generally >150 API over much of the AU area, and both gross and net HGR mimic Chattanooga Shale thickness trends. Maps of both gross and net HGR reveal a gradual thinning to the southwest from >100 ft at the northeastern AU boundary to <30 ft southwest of east-central Tennessee. Both parameters are generally < 30 ft throughout the remainder of the AU. The most significant anomaly in the AU occurs in Greene and Hale counties of central Alabama, where over 1400 ft of Devonian shale (>300 ft net HGR) is preserved in a large (>250 mi2) synclinorium. Relatively high gas potential within the Chattanooga Shale AU may be suggested by net HGR values >30 ft in three locations: 1) an area spanning from south-central Kentucky to northeastern Tennessee, 2) an area in northeast Alabama and northwest Georgia that includes Etowah County, Alabama and Chattanooga County, Georgia, and 3) the synclinorium in Greene and Hale Counties, Alabama.