Energy Policy Act of 2005 (Ultra-deepwater and Unconventional Resources Program)
Geological Foundation for Production of Natural Gas from Diverse Shale Formations
The primary objective of this research is to develop a conceptual and procedural foundation for the exploration and development of Alabama gas shale resources.
Geological Survey of Alabama, Tuscaloosa, AL 35486-6999
Unconventional gas plays require an integrated, multidisciplinary approach to exploration and development. However, broadly applicable geologic models of resource distribution and producibility have yet to be developed for Alabama gas shale formations. This study will employ field and laboratory techniques to characterize the stratigraphy, sedimentology, geologic structure, hydrodynamics, geothermics, petrology, geochemistry, and resource/reserve base of the gas shale reservoirs in the Black Warrior Basin and the Appalachian Thrust Belt of Alabama. The area contains a diversity of emerging gas shale plays in Cambrian (Conasauga Formation), Devonian (Chattanooga Shale and unnamed shale units), and Mississippian (Floyd Shale) strata. Development of these reservoirs has been slowed by a series of technical challenges, including uncertainty about the best practices for exploration, drilling, and well completion. This uncertainty stems largely from inadequate characterization of the geologic framework of the targeted shale formations and is compounded by major differences in composition, thickness, geometry, and fracture architecture that exist between these formations and gas shale reservoirs in other regions.
This study is designed to reduce exploration risks for these gas shale plays and to provide an accurate assessment of their gas resources and reserves. The study will also assist industry in the formulation of exploration and development strategies that are optimized for each gas shale play. The results will include general concepts that can be applied to other emerging and frontier unconventional gas plays.
Project deliverables will include monthly status reports, a topical report, and a final report. The Geologic Survey of Alabama will build and maintain a web site with information about the project and update it as needed.
To date, no formal, publically available assessment has been made of resources and reserves in the gas shale plays of the Black Warrior Basin and Appalachian Thrust Belt. Detailed assessments are required to characterize the gas potential of each play. Publication of a comprehensive report characterizing the resource will accelerate development by reducing the risk of E&P for smaller operators with less capability for carrying out their own resource assessments. By increasing the efficiency and reducing errors in exploration and development, environmental impacts will be minimized as regional development accelerates.
Although specifically directed at the Black Warrior Basin and Appalachian Thrust Belt, the concepts and methods developed during this study will be broadly applicable to gas shale prospects throughout North America. Key contributions will be increased understanding of the development potential of gas shale in areas with complex geologic structure and the formulation of development strategies for shale formations where reservoir thickness is limited. The project will also result in an improved understanding of gas storage and mobility in shale formations, across a broad range of shale compositions.
The research team completed a Project Management Plan (Task 1.0) and a Technology Status Assessment (Task 2.0), and from there made a presentation on shale gas potential of thrust belts at Geological Society of America Annual Meeting (Task 3.0). The team also prepared a paper on shale gas plays of the Southern Appalachian Thrust Belt under Task 5.0 and presented and published it in the International Coalbed and Shale Gas Symposium Proceedings. A presentation on Devonian shale prospects is scheduled for the 2010 AAPG Annual Meeting, and a field trip called “Geology of Unconventional Gas Plays in the Southern Applachians” will be run in conjunction with that meeting.
The design of subsurface database for Tasks 4.0, 5.0, and 6.0 have been completed. The team developed structural and hydrodynamic models of shale gas plays in thrust belts, analyzed sedimentology of Conasauga Shale, defined facies trends and prospect areas for pre-Chattanooga Devonian shale and Mississippian Floyd Shale, and completed a compilation of basic geochemical data from Cambrian and Mississippian shale. They also received isotherm and tight-rock analysis data from Terra-Tek, thus completing Year 1 analyses of reservoir petrology.
Results show the primary rock fabric dominated by platy clay, quartz grains, and pyrite framboids. Secondary Illite laths provide evidence for significant open pore space in the shale matrix. An analysis of isotherm and Rock-Eval pyrolysis data indicate near-perfect correlation between adsorbed gas capacity and total organic carbon.
The key tasks to be undertaken are outlined below. The project team has made considerable progress on several of these tasks.
Analyze stratigraphy and sedimentation GSA personnel are describing all available cores, cuttings, and geophysical well logs of the gas shale units using standard stratigraphic and sedimentologic techniques. Basic subsurface data are being compiled from well logs and consolidated in a database using Petra software, which is commonly used by independent producers in the region. Stratigraphic cross sections have been made to establish stratal geometry and facies relationships and help characterize reservoir quality in the gas shale units and adjacent strata. Isopach, isolith, and other subsurface maps that may be useful are being constructed using Petra software. The final step in this task is to develop comprehensive and predictive sequence-stratigraphic, depositional, and paleogeographic models of each gas shale formation.
Determine structural geology The structural geology of the gas shale formations in the study area is being characterized. A variety of field and subsurface techniques are being used to characterize the fold and fault systems. Structural contour maps showing the elevation of key stratigraphic markers and the traces of faults and fold axes are being constructed using Petra software. Balanced structural cross sections have been constructed using the available well, seismic, and outcrop control. Work will also focus on the characterization of natural fracture systems in the gas shale formations and will employ a combination of field and subsurface techniques. Data on fracture networks will be collected from cores and outcrops. These data will be used to quantify the orientation, spacing, height, kinematic aperture, and cross-cutting relationships of the fracture networks.
Assess basin hydrodynamics and geothermics The basin hydrodynamics subtask is focusing on characterization of subsurface fluid chemistry and reservoir pressure. Chemical and pressure data are being synthesized into hydrodynamic models for each shale formation studied. Under the geothermics subtask, temperature data will be compiled, and bottom-hole temperatures will be corrected to estimate true formation temperatures. Geothermal gradients will be calculated from the temperature data, and the temperature of proven and prospective reservoir intervals will be determined. After the basic data are compiled and analyzed, maps of geothermal gradient and reservoir temperature will be constructed for each shale formation using Petra software.
Assess petrology and geochemistry Petrographic and geochemical information are being collected and analyzed to help refine the depositional models and provide critical information on mineralogy and pore systems in shale. The work under this task will be divided into organic and inorganic studies and will include microscopic and geochemical analyses performed in the laboratory.
Determine gas storage and mobility The principal objective of this task is to quantify and understand the basic controls on adsorption and free gas storage in the Alabama shale formations. Adsorption isotherms are being determined for methane, and adsorption capacity is being correlated with petrologic variables. Porosity data will be used to determine the volume of reservoir available for free gas storage, and pressure-volume-temperature properties for methane and other produced gases will be used to determine free gas capacity under a range of reservoir conditions. To aid in the characterization of free gas storage and gas mobility, gas porosimetry data will be measured. These data will provide critical information on total porosity and the size distribution of effective pore throats and will be used to quantify reservoir permeability.
Assess reserves and identify strategies for gas shale development Resources and reserves will be calculated for each shale formation using a volumetric approach. Resource maps for each formation will be constructed using Petra software and will include maps of gas-in-place per unit volume of reservoir, as well as maps of gas-in-place per unit area. Estimation of gas reserves will require evaluation of ultimate gas recoverability. Recoverability will be estimated partly by determination of the quantity of free and adsorbed gas that can be recovered at mature reservoir pressures and partly by employing recovery factors and other appropriate variables based on the performance of wells in the target formations and in analogous reservoirs. Reserve maps for each formation will be constructed in Petra software and will include maps of estimated gas reserves per unit volume of reservoir, as well as maps of estimated reserves per unit area. Total resources and reserves will be estimated for each formation, and ranges of uncertainty will be calculated for these estimates.
The final step in this study will be to suggest development strategies that are tailored to the diverse geologic conditions in each shale formation. The Alabama shale plays will be screened based on depositional, structural, hydrodynamic, and petrologic variables to identify the best opportunities for economic production. Drilling, completion, and production strategies for each target formation will be developed and published.
Project Start: August 19, 2008
Project End: July 30, 2011
DOE Contribution: $ 497,459
Performer Contribution: $ 346,593
RPSEA – Kent Perry (email@example.com or 847-768-0961)
NETL – Virginia Weyland (Virginia.Weyland@netl.doe.gov or 281-494-2517)
Performer Company – Jack C. Pashin (firstname.lastname@example.org or 205-247-3558)
Final Project Report [PDF-29.4MB] July, 2011
Aticle in Spring 2009 E&P Focus [PDF]