FABRIC OF THE BARNETT SHALE AND ITS CONCRETIONS; A MULTI-TECHNIQUE ANALYSIS

The Barnett Shale is a large onshore natural gas field located in the Fort Worth Basin, Texas. It is proven to contain 26tcf of natural gas held in what is described as a tight gas reservoir. The unit acts as both source and seal for its high kerogen content. Currently, there are more than 100 operators working in 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 has been subjected to analysis via optical petrography, backscattered-electron imaging (BSE-I), X-ray mapping, X-ray diffraction (XRD), 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 I/S. Minor quantities of chlorite have also been recorded, with quartz being the dominant grain type. Fossil lags and zones of reworked phosphate clasts are common and break up the typical mudstone fabric. Carbonate-rich concretions prevalent in outcrop, in San Saba County Texas, are also present in core from the Fort Worth Basin. HRXTG reveals random orientations of phyllosilicates in carbonate-rich concretions. Carbonate-poor lithologies adjacent to the concretions reveal more strongly aligned clay fabrics indicative of later burial diagenesis 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 in the Barnett. Potassium X-ray maps provide visual confirmation of the HRXTG data. In addition, distinct depositional segregation of micaeous 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.