CURRENT-INDUCED TRANSPORT AND DEPOSITIONAL PROCESSES OF THE BARNETT SHALE IN FORT WORTH BASIN, TEXAS USA

The Mississippan-age Barnett Shale has been the subject of numerous studies over the past few years because it is the longest-producing unconventional gas shale in the U.S., Stratigraphic studies (Loucks and Ruppel, 2007; Singh, 2008; Slatt et al., in press; Abouelresh and Slatt,in press) have suggested that the lower Barnett was deposited mainly in a relatively low energy, deep water environment, relatively far from a terrigenous source area which probably lay to the south and the west. Implications of thick sequences of the Barnett and other thick, aerially extensive shales is that deposition was mainly by ‘hemipelagic rain’ of clay (and silt) sized, detrital and/or biogenic particles.

In this work, six main sedimentary facies were identified in terms of processes and revealed a wide spectrum of vertical facies transitions and bed types, these facies are: Massive mud facies, Rhythmic silty clay facies, Ripple, low-angle and hummocky laminated facies, Graded Mud Facies, Clay cap facies and Spicule-rich facies.

Core, thin section and SEM analyses of Barnett lithofacies has revealed a variety of sedimentary textures and structures which indicate other transport, depositional, and post-depositional processes were active during Barnett Shale deposition. These processes may have included hyperpycnal flows, turbidity current flows, tempestites (storm deposits) and/or bottom-hugging slope contour currents. Current-induced features of mudstone facies include mm- to cm-scale cross- and parallel-laminations, scour surfaces, clastic/biogenic particle alignment, and normal- and inverse-size grading. Current-induced sedimentation may have been prompted by flocculation of clay particles into silt- or sand-size grains; such preserved Barnett floccules can be observed under the SEM. Micro-vertical successions suggestive of waxing-waning flows, lofting facies indicative of reversal in buoyancy by transformation from concentrated to dilute flows, and the occurrence of terrestrial materials and organic remains support a hyperpycnal origin for at least a portion of the strata.