Mississippian Barnett Shale: Characterization and Concretions

Ruarri J. Day-Stirrat¹, Robert G. Loucks¹, Kitty L. Milliken², and Ben A. van der Pluijm^3
1Bureau of Economic Geology, University of Texas, Austin, TX
²Department of Geological Sciences, University of Texas, Austin, TX
³Department of Geological Sciences, University of Michigan, Ann Arbor, MI

The Barnett Shale is a large gas field located in the Fort Worth Basin, Texas. It contains ~26tcf of natural gas held in a tight gas reservoir. The unit acts as reservoir, source and seal for its high kerogen content. More than 150 operators are working the play applying fracture techniques to extract the natural gas reserves. Despite its economic importance, the complex petrologic character of the Barnett has received remarkably little attention. Core material from 4 wells has been subjected to analysis via optical petrography, backscattered-electron imaging (BSE-I), X-ray mapping, X-ray powder diffraction (XRPD), and high-resolution X-ray diffraction (HRXTG). These techniques reveal a variety of depositional fabrics as well as mineralogical assemblages. Clay minerals only comprise an average of around 30% of the rock and are predominantly high illite IS, this figure drops to ~5% in the concretions which are predominantly calcite. HRXTG reveals random orientations of phyllosilicates in carbonate-rich concretions, pointing to early formation. Carbonate-poor lithologies adjacent to the concretions reveal more strongly aligned clay fabrics indicative of burial and effective stress increases, with amount of clay material and the presence of non-platy phases dictating the degree of alignment. BSE-I and X-ray Mapping help to clarify the distinct textures and compositional variations that characterize different lithologies. Potassium X-ray maps provide visual confirmation of the HRXTG data as R1 ordered IS has potassium as the prevailing interlayer cation. In addition, distinct depositional segregation of IS material within horizons of high clay content are visible within some of the carbonate-rich lithologies. In these latter zones, the thin clay drapes revealed by this technique may prove to be important zones of mechanical weakness for fracture propagation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas