ALBERT

Description: We thank Tom Ewing for his informative discussion to our recent article on the Barnett Shale. Ewing provides an excellent, useful review of geologic data concerning uplift and burial history in the Fort Worth basin. His discussion of this specific topic improves upon our own, adding significant detail that should aid future analyses of the Barnett. In particular, his inclusion of a major Mesozoic erosional event as a correction to our burial history curve for Eastland County (figure 7) is welcome. We also credit Ewing for noting the limits to existing data. He emphasizes that the amount of late Paleozoic subsidence and the precise timing of post-Permian uplift remain unknown, and that, as a consequence, reconstructions of burial history must therefore include both facts and inferences. At the same time, we find problematic other interpretations in Ewing's discussion. One of these involves the use of burial history information (sedimentary and tectonic loading) to explain maturation patterns in the Barnett Shale. The other issue of concern relates to the westward extent of Ouachita thrusting. As emphasized in our article, Barnett maturity levels in many parts of the Fort Worth basin are too high and variable to be explained by the present-day basin setting. No consistent relationship exists between depth …

Description: This article describes the primary geologic characteristics and criteria of the Barnett Shale and Barnett-Paleozoic total petroleum system (TPS) of the Fort Worth Basin used to define two geographic areas of the Barnett Shale for petroleum resource assessment. From these two areas, referred to as “assessment units,” the U.S. Geological Survey estimated a mean volume of about 26 tcf of undiscovered, technically recoverable hydrocarbon gas in the Barnett Shale. The Mississippian Barnett Shale is the primary source rock for oil and gas produced from Paleozoic reservoir rocks in the Bend arch–Fort Worth Basin area and is also one of the most significant gas-producing formations in Texas. Subsurface mapping from well logs and commercial databases and petroleum geochemistry demonstrate that the Barnett Shale is organic rich and thermally mature for hydrocarbon generation over most of the Bend arch–Fort Worth Basin area. In the northeastern and structurally deepest part of the Fort Worth Basin adjacent to the Muenster arch, the formation is more than 1000 ft (305 m) thick and interbedded with thick limestone units; westward, it thins rapidly over the Mississippian Chappel shelf to only a few tens of feet. The Barnett-Paleozoic TPS is identified where thermally mature Barnett Shale has generated large volumes of hydrocarbons and is (1) contained within the Barnett Shale unconventional continuous accumulation and (2) expelled and distributed among numerous conventional clastic- and carbonate-rock reservoirs of Paleozoic age. Vitrinite reflectance (Ro) measurements show little correlation with present-day burial depth. Contours of equal Ro values measured from Barnett Shale and typing of produced hydrocarbons indicate significant uplift and erosion. Furthermore, the thermal history of the formation was enhanced by hydrothermal events along the Ouachita thrust front and Mineral Wells–Newark East fault system. Stratigraphy and thermal maturity define two gas-producing assessment units for the Barnett Shale: (1) a greater Newark East fracture-barrier continuous Barnett Shale gas assessment unit, encompassing an area of optimal gas production where dense impermeable limestones enclose thick (≥300 ft; ≥91 m) Barnett Shale that is within the gas-generation window (Ro ≥ 1.1%); and (2) an extended continuous Barnett Shale gas assessment unit covering an area where the Barnett Shale is within the gas-generation window, but is less than 300 ft (91 m) thick, and either one or both of the overlying and underlying limestone barriers are absent. Rich Pollastro received an M.A. degree in geology from the State University of New York at Buffalo in 1977. Rich joined the U.S. Geological Survey in 1978 and serves as a province geologist on the national and world energy assessment projects. His recent accomplishments include petroleum system assessments of the Fort Worth, Permian, and South Florida basins and the Arabian Peninsula. Dan Jarvie is an organic geochemist and president of Humble Geochemical Services. Dan earned his B.S. degree from the University of Notre Dame and was mentored in geochemistry by Don Baker and Wallace Dow. He has studied unconventional oil and gas systems extensively since 1984. Dan's work on the Barnett Shale spans much of the last decade, which has resulted in several AAPG awards. Ronald Hill specializes in petroleum geochemistry and has more than 12 years of oil industry experience. Currently, he is a research geologist for the U.S. Geological Survey. His research interests include shale-gas resources and processes that control petroleum generation. Ron holds geology degrees from the Michigan State University (B.S. degree) and the University of California, Los Angeles (Ph.D.), and a geochemistry degree from the Colorado School of Mines (M.S. degree). Craig Adams worked as an exploration geologist with Amoco Production Company for 13 years before becoming an independent in 1996. As president and co-owner of Adexco Production Company, his primary focus is conventional and unconventional oil and gas in Texas and the mid-continent. Craig has worked the Barnett Shale for 6 years, where his company was one of the first to expand beyond the core area of the Newark East field.

Description: Shale-gas resource plays can be distinguished by gas type and system characteristics. The Newark East gas field, located in the Fort Worth Basin, Texas, is defined by thermogenic gas production from low-porosity and low-permeability Barnett Shale. The Barnett Shale gas system, a self-contained source-reservoir system, has generated large amounts of gas in the key productive areas because of various characteristics and processes, including (1) excellent original organic richness and generation potential; (2) primary and secondary cracking of kerogen and retained oil, respectively; (3) retention of oil for cracking to gas by adsorption; (4) porosity resulting from organic matter decomposition; and (5) brittle mineralogical composition. The calculated total gas in place (GIP) based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf/section (5.78 × 109 m3/1.73 × 104 m3). We estimate that the Barnett Shale has a total generation potential of about 609 bbl of oil equivalent/ac-ft or the equivalent of 3657 mcf/ac-ft (84.0 m3/m3). Assuming a thickness of 350 ft (107 m) and only sufficient hydrogen for partial cracking of retained oil to gas, a total generation potential of 820 bcf/section is estimated. Of this potential, approximately 60% was expelled, and the balance was retained for secondary cracking of oil to gas, if sufficient thermal maturity was reached. Gas storage capacity of the Barnett Shale at typical reservoir pressure, volume, and temperature conditions and 6% porosity shows a maximum storage capacity of 540 mcf/ac-ft or 159 scf/ton. Dan Jarvie is an analytical and interpretive organic geochemist. He has worked on conventional hydrocarbon systems and unconventional shale-oil and shale-gas hydrocarbon systems, including the Barnett Shale since 1989. He earned a B.S. degree from the University of Notre Dame and was mentored in geochemistry by Wallace Dow and Don Baker of Rice University. He is president of Humble Geochemical Services. Ronald Hill specializes in petroleum geochemistry and has more than 12 years of professional experience, including those with ExxonMobil and Chevron. Currently, he is a research geologist for the U.S. Geological Survey. His interests include investigation of shale-gas resources and the processes that control petroleum generation. He holds geology degrees from Michigan State University (B.S. degree) and the University of California, Los Angeles (Ph.D.), and a geochemistry degree from the Colorado School of Mines (M.S. degree). Tim Ruble is a petroleum geochemist with Humble Geochemical Services and is currently involved in studies focused on the assessment of shale-gas resources. He has had a diverse professional career that has included periods with the Commonwealth Scientific and Industrial Research Organization in Australia, Mobil Oil, and the U.S. Geological Survey. He has published on a variety of geochemical topics, including lacustrine petroleum systems, oil-bearing fluid inclusions, native bitumens, biomarker analyses, and hydrocarbon generation kinetics. Tim earned his B.S. degree in chemistry from Truman State University and his M.S. degree and his Ph.D. in geology from the University of Oklahoma. Richard Pollastro received an M.A. degree in geological science from the State University of New York at Buffalo in 1977. He joined the U.S. Geological Survey in 1978 and has served as a province geologist on the national and world energy assessment projects. His recent accomplishments include petroleum system analysis and resource assessment of the Bend arch–Fort Worth Basin, with particular focus on Barnett Shale, South Florida Basin, and the Arabian Peninsula.

Description: The Barnett Shale (Mississippian) in the Delaware Basin has the potential to be a prolific gas producer. The shale is organic rich and thermally mature over large parts of the basin. Depths to the Barnett range from 7000 ft (2133 m) along the western edge of the basin to more than 18,000 ft (5486 m) along the basin axis. The Barnett Shale began generating petroleum about 250 Ma and reached its maximum temperature about 260 Ma. Present-day thermal maturity is indicative of maximum burial and temperature. Wells in northern Reeves County are in the gas window based on measured vitrinite reflectance values and kerogen transformation ratios. The shale can be divided into an upper clastic unit and a lower limy unit by changes in resistivity. The lower unit can be subdivided into five subunits by distinctive well-log markers. Preliminary analyses suggest that intervals in the lower Barnett marked by high resistivity and high neutron porosity readings on well logs have high gas contents. Areas in which to focus the future exploration in the lower Barnett can be delineated by mapping a net resistivity greater than 50 ohm m. The Barnett Shale contains significant gas resources in the Delaware Basin. Realizing the potential of these resources depends on the current efforts to optimize drilling and completion techniques for this shale-gas play. Travis Kinley obtained his B.S. degree in geology from the University of Wyoming in 2004, and his M.S. degree in geology from Texas Christian University in 2006. He is currently an associate geologist at XTO Energy and has been working in the Permian Basin since he first worked as an intern at XTO while pursuing his M.S. degree. Lance Cook is currently the manager of geology/Rocky Mountains for XTO Energy. He received his B.S. degree in geology from Texas Christian University and his M.S. degree in geology from the University of New Mexico. He has been involved in unconventional resource play developments and exploration plays across North America during his career with Shell Oil, Union Pacific Resources, the State of Wyoming, and XTO Energy. John Breyer is a professor of geology at Texas Christian University. He received his B.A. degree in geology from the University of Cincinnati in 1970 and his Ph.D. in geology from the University of Nebraska in 1974. John is a broadly trained sedimentary geologist with particular interests in sandstone petrology, petroleum geology, and the history of science. Currently, his students are actively engaged in research on the Barnett Shale in the Fort Worth Basin and other shale-gas systems. Dan Jarvie is president of Worldwide Geochemistry and is an analytical and interpretive organic geochemist. He has worked conventional petroleum systems around the world and has extensive experience on unconventional shale-gas, shale-oil, and oil-shale systems. He earned his B.S. degree from the University of Notre Dame in 1976 and was mentored in geochemistry by Wallace Dow and Don Baker of Rice University. He is now affiliated with the Energy Institute at Texas Christian University. Art Busbey is an associate professor of geology at Texas Christian University. He has B.S. and M.S. degrees in geology from University of Texas-Austin and a Ph.D. in paleontology/anatomy from the University of Chicago. He was an associate editor of the AAPG journal, GeoByte , in the early 1990s. He teaches courses in historical geology, paleontology, digital cartography, and geological statistics.