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Thermal maturity of the Barnett Shale determined from well-log analysis

¹ 3906 Dunwich Drive, Richardson, Texas 75082; hankzhao{at}sbcglobal.net
² EnCana Oil & Gas (USA), Dallas, Texas 75240; natalie.givens{at}encana.com
³ Republic Energy Inc., Dallas, Texas 75206; bcurtis{at}republicenergy.com

Hanqing "Hank" Zhao is currently an independent geologist. In his more than 20-year career in oil and gas, he had been with Republic Energy, mainly working on Barnett Shale; Southwestern Energy, working on Fayetteville Shale; and Dagang Geophysical Exploration and Southwest Petroleum University in China. He received his Ph.D. in geology from the University of Wyoming, and his M.S. and B.S. degrees in petroleum geology from Southwest Petroleum University in China. His areas of interest are mainly on the geological and geophysical aspects of unconventional gas.

Natalie is a geologist concentrating on unconventional oil and gas plays. She received her M.S. degree in geology from the University of Kansas in 2006 and her B.S. degree in geology from the Southern Methodist University in 2000. Natalie spent 3 years with Republic Energy, Inc., prior to continuing her education and obtaining her M.S. degree.

Brad Curtis is vice president of Geoscience and has been with Republic Energy since 1990. He received his B.S. degree in petroleum geology from Midwestern State University in 1983 and then worked for Expando Oil Co. in Wichita Falls, generating prospects in the Fort Worth and East Texas basins.

Intensive development with large-scale fracturing treatments has made the Barnett Shale play (Newark East field) in the Fort Worth Basin the largest shale-gas field in the world. The Mississippian Barnett Shale is an organic-rich, self-sourced reservoir rock. Thermal maturity, thickness, and total organic carbon are the most important geological factors for commercial gas production from this shale formation. The log-derived thermal-maturity index (MI) has been developed in an effort to better understand and predict hydrocarbon phases across the basin. Maturity index was calculated using three types of open-hole logs: neutron porosity, deep resistivity, and density porosity (or bulk density). The derivation of MI is based on the hypotheses that shale gas is generated and stored locally without apparent migration from outside sources, and that the water saturation and the density of generated hydrocarbons decrease with an increase in thermal maturity. Maturity index correlates well with initial gas:oil ratios (GOR) from well production data. Based on this correlation, an empirical relationship has been demonstrated for the Fort Worth Basin. This method is useful in understanding the thermal-maturity levels of Barnett Shale source rock in the gas-generation window. Mapping MI, GOR, and gas heating value from hundreds of wells identifies the various maturity stages and areas of Barnett Shale that generate oil, condensate, wet gas, or dry gas in the Fort Worth Basin.