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Barnett & Woodford Shale Gas: from the Laboratory to the Field

Saison, Anne 1; Allen, Jon D.1; Philp, Paul R.1
1 School of Geology and Geochemistry, University of Oklahoma, Norman, OK.

Understanding the origin of shale gas requires coupling lab data with field data, that is to say finding the link between kerogen quality and natural gas isotopic composition. Kerogen quality is the key to determining both the quantity and type of petroleum generated, and the extent of this generation within the basin. The isotopic composition of the individual components in the gas provides invaluable information on the origin, alteration, maturity and genetic relationship between gas families.

Our method consists in (1) the determination of the stable carbon and hydrogen isotope compositions of the components of the natural gas sampled on field using a GC-IRMS device, and (2) the artificial maturation of the kerogen & the determination of the isotopic composition of the generated products by MSSV-Py-GC-IRMS. Facies within the source rock have been identified using a multi parameter approach, which involves the assessment of the genetic potential, petroleum potential, petrographic facies, and thermal stability of the sediments. The comparison of the isotopic composition of natural gas and artificially generated gas allows us to propose different scenari of gas generation and fate. This method has been applied to the Barnett and Woodford shales.

The study of the Barnett Shale is based on 150 gas samples and 55 core samples originating from 4 wells. 10 facies are presented with their specific geochemistry and generation kinetics. One of our major findings is that Barnett gas composition results from a complex combination between initial kerogen composition, and maturity-induced geochemical & structural changes, and migration pathways. The secondary cracking of the oil retained within the organic matrix into gas and the late release of adsorbed gas during thermal-induced decomposition of the organic matter play both a major role in the isotopic composition of the generated gases, showing that the evolution of the isotopic composition of the gas components along kerogen maturation is not straightforward.

The study of the Woodford Shale is based on 97 gas samples and 30 core samples and includes as well samples from the Caney Shale. Four organic facies were identified, each presenting specific petroleum potential and kinetic parameters and gas isotopic composition. Our preliminary results highlight the importance of a robust definition of organic facies within the Woodford shale and the role of the Caney Shale in the Arkoma petroleum system play.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009