Mississippian Barnett Shale: Characterization and concretions

Authors: R. J. Day-Stirrat, R. G. Loucks, K. L. Milliken, and B. A. van der Pluijm

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 (http://www.aapg.org)

Abstract: 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.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, “Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the United States.” The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological-, engineering-, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)