Abstract

Barnett Gas Isotope Geochemistry and Thermal Maturity

Harold Illich¹, Kevin Ferworn², Gary Nilson¹, Frank Hernandez¹, and David Williamson³
¹Pioneer Natural Resources USA, Inc., Irving, TX 75039;
²GeoMark Research, Ltd., Houston, TX 77095;
³Forge Energy, San Antonio, TX 78230

Geochemical analysis of Barnett gases collected from producing wells provides insight into basin evolution and thermal maturity at a resolution equal to or superior to other maturity tools. Gas geochemistry (bulk composition and stable isotopic gas composition) is a direct measurement of the product we wish to exploit and bring to sales. Gas geochemical data obtained from producing wells are largely independent of drilling and completion practices and other metrics that introduce complex, irresolvable variables.

Bulk compositional gas data clearly demarcate dry gas from liquids-rich windows in the Barnett resource play. Additionally, bulk composition of these gases reveals the influence of noncombustible components on the heating value of produced gases.

The isotopic chemistry of gases reflects source geochemistry as well as the level of thermal maturity. Methane gas isotopes, unlike heavier methane homologues, behave in a strongly linear fashion over a wide maturity range. Methane carbon and hydrogen isotopic compositions become more positive (isotopically “heavier”) with increasing maturation (the isotope effect). Thermal maturity derived from the study of hydrogen and carbon isotopes of methane provides a better regional characterization of the Barnett than other maturity proxies. Ethane-methane isotope data become useful maturity indicators at lower levels of maturation (for example, the Montague County area). Gas isotope geochemistry can be an effective forward-modeling tool for resource play development and high-grading existing leasehold.

AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013