Screening Criteria for Shale-Gas Systems

Fred P. Wang and Julia F. W. Gale

Bureau of Economic Geology, Jackson School of Geosciences,

The University of Texas at Austin, University Station, Box X, Austin, Texas  78713

ABSTRACT

In North America alone, more than 70 shale-gas plays have been identified.  With this rapid increase in shale-gas production, shale-gas systems have presented many new challenges to production technology and the basic understanding of shale-gas reservoirs.  Because gas shales are complicated and highly variable, the objective of this study was to indentify key controls on shale-gas productivity and to develop criteria for screening shale-gas systems.

Gas shales, including biogenic and thermogenic systems whose depths range from several hundred feet to >18,000 ft, are a complex function of total organic content (TOC), thermal maturation, gas content, thickness, and brittleness.  Because fixed-value criteria cannot adequately be applied to all shale-gas systems for screening, a set of depth-dependent screening criteria for shale-gas systems was developed that can be applied to biogenic and thermogenic systems, as well as regional plays and local lease areas.  Gas content, adsorption, and shale thickness are the depth-dependent parameters for this screening of shale-gas systems.

Brittleness, a measure of the ability of rock to fracture, is a complex function of lithology, mineral composition, TOC, effective stress, reservoir temperature, diagenesis, thermal maturity, porosity, and type of fluid.  The enormously successful performance of Barnett Shale in the Fort Worth Basin, Texas, stems from its favorable tectonic setting and burial history, which resulted in a double enhancement of brittleness.  Its early deep burial made the Barnett thermally mature and brittle.  Subsequent exhumation and uplift made it cheap to drill and easy to frac through enhancement of brittleness by reduction of effective stress.

Wang, F. P., and J. F. W. Gale, 2009, Screening criteria for shale-gas systems:  Gulf Coast Association of Geological Societies Transactions, v. 59, p. 779-793.

AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana