Abstract

Ten Barnett Shale lithofacies have been recognized in a 223 ft. (67.9 m) thick core from Johnson County, Texas. Many of these lithofacies are similar to those described by Singh (2008) for the ‘core area’ of the Newark East Field to the north, but two new lithofacies have been identified in this core: re-sedimented spiculitic mudstone lithofacies and lag deposits, both of which are indicative of a relatively higher energy environment and downslope resedimentation of shallower water deposits. Based upon the internal characteristics of the lithofacies and their stacking patterns, a sequence stratigraphic framework has been established for the cored interval which consists of 7 depositional sequences in the lower Barnett Shale and 5 sequences in the upper Barnett Shale. The recognition of cyclical stacking patterns of the lithofacies, condensed sections and transgressive surfaces of erosion were the keys to establishing the sequence stratigraphic framework in these fine–grained rocks. Spectral gamma ray U and Th logs were particularly useful for this purpose, and are recommended for sequence stratigraphic studies of these and other shales. The sequence stratigraphic framework reveals that the lower Barnett Shale in this area was deposited mainly in a low energy, relatively deep water environment, relatively far from a terrigenous source area which probably lay to the northwest. By contrast, the upper Barnett Shale was deposited in a generally shallow water environment more conducive to related fluctuations in relative sea level possibly due to tectonic activity. The source area may have been more proximal, to the west and/or southwest.

High gamma-ray log responses are often associated with condensed sections in the Barnett (and other) shales due to their high TOC content, thus making them likely candidates for regional correlation. However, downslope transport of reworked phosphatic deposits can also provide high gamma-ray log responses.

Relative Hydrocarbon Potential (RHP) is an organic geochemical parameter obtained from Rock-Eval Pyrolysis and TOC data. It was applied to this core, and found to provide an indicator of marine transgressions and regressions. A similar conclusion was reached for core in the ‘core area’ of the Newark East Field (Singh, 2008; Slatt et al., in press). We recommend continued testing and use of the RHP parameter for high frequency sequence stratigraphic analysis of unconventional gas shales.