Paper No. 4
Presentation Time: 8:45 AM
Subsurface Interpretation of Deltas In the Bossier Shale of the East Texas Basin by Comparison with Outcrop Expression of the Panther Tongue In Northern Utah
The Upper Jurassic Cotton Valley Group of the East Texas Basin was deposited as a prograding clastic system that contains updip alluvial units assigned to the Schuler Formation, intermediate shoreline/deltaic units assigned to the Taylor Sand and basinal muds and sands assigned to the Bossier Shale. The economically important lower Bossier Shale is difficult to interpret and correlate because of its deep burial to over 3,900 m (13,000'), combined with faulting, salt movement and frequent poor seismic and biostratigraphic resolution. Wireline log data combined with a small number of cores suggest that the lower Bossier Shale was deposited as a series of small, elongate to lobate deltas with laterally compensating thicknesses of less than 30 m, widths of 3-5 km and extension of only 1-2 km basinward. Delta geometry indicates primarily river and wave dominated morphologies over four parasequences. Log and core data suggest upward cleaning delta front shales and sands overlain by blocky distributary mouth bars. Seaward of mouth bars, gravity flows deposited sands into the prodelta environment. Interpretation and visualization of the lower Bossier Shale deltas has been aided by comparison with well-exposed outcrop examples of the Upper Cretaceous Panther Tongue Member of northern Utah. Here, relatively steep, river dominated deltas prograde 15-20 m thick delta parasequences consisting of turbidite sands underlying coarsening-upward delta front and distributary mouth bar deposits. Three dimensional exposures of the Panther Tongue enable a clear delineation of deltaic geometry and facies, providing a useful analog for assisting interpretation of the lower Bossier Shale.