Ten Barnett Shale lithofacies have been recognized in a
223-ft (68-m)-long core from Johnson County, Texas. Eight of these lithofacies match those
previously identified in the main producing area of the Newark East (Barnett Shale) field in the
northern part of the Fort Worth Basin, but two new lithofacies have been identified in this core,
resedimented spiculitic mud-stone lithofacies and lag deposits, both of which are indicative of a
relatively higher energy environment and downslope resedimentation of shallower water deposits.
The recognition of cyclical stacking patterns of the lithofacies, condensed sections (CSs), and
transgressive surfaces of erosion were the keys to establishing the sequence-stratigraphic
framework in these fine-grained rocks, which consists of seven stratigraphic intervals in the
lower Barnett Shale and nine stratigraphic intervals in the upper Barnett Shale. Spectral
gammaray uranium and thorium logs aided in this objective and are recommended for future
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, somewhat far from a terrigenous source area, which probably
lies to the northwest. By contrast, the upper Barnett Shale was deposited in an oxygenated
shallower water environment, which had a source area from the west and southwest sides of the
basin. The higher frequency of sea level fluctuation during development of the upper Barnett
Shale most probably indicates periodic tectonic activity, perhaps associated with a structural
high that was susceptible to sea level fluctuations. Alternatively, it could have resulted from
the onset of glaciations in Gondwanaland during this time. This higher frequency may indicate
that the upper Barnett is Chesterian in age, because cyclicity was higher than during the Osagean
and Meramecian stages. If so, there may be more high-frequency cycles than recognized in this
core. Siliceous sponge spicules are more common in this core than in more northerly cores, so
more brittle facies might prevail in the southern part of the Fort Worth Basin. High gamma-ray
log responses, which are caused by a high total organic carbon, and/or in-situ phosphate minerals
are commonly found in CSs and can be used for regional correlations. However, high gamma-ray
phosphatic deposits that have been resedimented to downslope positions by sediment gravity flows
are an exception to the previous statement. Correlation of the Barnett stratigraphic intervals
now provides a north-to-southeast stratigraphic framework along the Fort Worth Basin. Relative
hydrocarbon potential (RHP) is an organic geochemical parameter applied to this core and found to
provide an indicator of marine transgressions and regressions. We recommend continued testing and
use of the RHP parameter for high-frequency sequence-stratigraphic analysis of unconventional gas
shales.
