Since fractured reservoirs are essential to production, exploration rationales specific to the production, exploration rationales specific to the Devonian shale unconventional gas resource are characterized by a geologic fracture creating mechanism. One such predicts intense fracturing in the shale wherever proximally associated with the major thrust faults of the Appalachian overthrust belt. Gruy Federal No. 1 Grainger Co. (DOE EGSP-TN9), a U.S. Department of Energy (DOE)-funded rank wildcat located in Grainger County, TN, successfully tested that rationale. The well penetrated the Saltville Thrust Fault (stratigraphic throw: over 10,000 feet) and encountered the Devonian Mississippian Chattanooga Shale in the lower plate. The shale proved to be 720 feet thick, of which 480 feet or 67% was anomalously radioactive (25 API above shale base line). Two hundred twenty (220) feet of core was recovered from the most highly radioactive (organic rich) intervals and a full suite of wireline logs was run. Widespread and locally intense fracturing in the shale, especially in the organic sections, observed in the core and evidenced by the logs vindicates the exploration rationale. The well will be hydraulically fractured and tested to evaluate production potential.
The classical trinity of essential factors in natural gas exploration, source, reservoir, and trap, is as relevant to unconventional resources as it is to conventional. In the unconventional resource the three are merely manifested in an unusual way and/or bear an unusual relationship to each other. The nature of the resource must be taken into account when charting exploration strategy, i.e., both exploration rationales and exploration techniques must be tailored to the specific resource. A conventional rationale is typically based on and characterized by some trapping mechanism. Exploration rationales for the unconventional Devonian shale resource, however, are distinguished by their fracture creating mechanism, which for the ultra-tight shale is equivalent to the reservoir creating mechanism. In as much as fracture systems are necessary, but not alone sufficient, ancillary factors, especially those relating to source potential, become important in evaluating Devonian shale rationales. A number of exploration rationales and techniques have been identified as holding promise for the Devonian shale. The Eastern Gas Shales Project's (EGSP) development of prospects in eastern Tennessee through the application of some of the techniques to one of the rationales and subsequent testing of one of them serves as a case study.
From January 3 to 21, 1980, U.S. Department of Energy (DOE), through its contractor Gruy Federal, Inc., drilled a Devonian shale wildcat well in Grainger County, TN. The geographic coordinates of Gruy Federal No. 1 Grainger Co. (DOE EGSP-TN9) are 30 deg. 18' 56" N latitude by 83 deg. 24' 33" W longitude (Tennessee coordinates 710,300 N by 2,762,000 E). The ground elevation of the well is 1140 feet above mean sea level. All depths were measured from the kelly bushing, which stood 10 feet higher at 1150 feet. The site is on lot A-8 of the Grainger County Industrial Park, public land belonging to the county situated Park, public land belonging to the county situated along U.S. Highway 11-W midway between Rutledge and Bean Station (Figure 1). Gruy Federal, Inc., operated the well under an agreement with Grainger County whereby the former received permission to drill, core, log, and test the well, while the latter retained the mineral rights and hence ownership of any producible hydrocarbons. The primary targets were the dark, organic rich primary targets were the dark, organic rich intervals within the Devonian-Mississippian Chattanooga Shale.
The drilling and testing of this wildcat well in Grainger County, TN, is a field demonstration project of the DOE's Eastern Gas Shales Project under the Unconventional Gas Recovery (UGR) Program.