The identification of “organoporosity” (microscale and nanoscale pores within organic matter in shales), its importance to storage and perhaps transfer of gas molecules through shales, and methods for gathering three-dimensional images of the pores, such as by argon-ion milling and/or field emission scanning electron microscopy, have all been well documented and discussed for unconventional gas shales. However, other types of pores exist within shales that can be important to storage and migration of gas (and oil), and other technologies are available for their identification and imaging. The different pore types found in the Barnett and Woodford shales are described and classified in this article. Scanning electron microscopy revealed the presence of porous floccules in both the Barnett and Woodford shales, which appear similar to laboratory-produced floccules and to those in other ancient shales. Published experimental and observational studies indicate that floccules are hydraulically equivalent to coarser grains and are transported by traction processes. Current-induced microsedimentary structures and textures within the Barnett and Woodford shales, as well as the preserved floccules, suggest such processes were active during transport and deposition. Pore spaces between the floccules are open and can provide storage space as well as permeability pathways for gas molecules through the shales. Pores are also found within organic matter that occurs both as discrete particles and as adsorbed coatings around clay grains in the shale. They are referred to here as “organopores.” Porous fecal pellets are also common in the Barnett Shale. Preserved fossil fragments such as organic-walled spores and inorganic sponge spicules have hollow central chambers, which may remain partially or completely open even after burial. Intraparticle pores occur between grains of various minerals (e.g., pyrite framboids). Microchannels within shale matrix, which may be the bounding surfaces of scours or microsedimentary structures, may also provide permeability pathways for hydrocarbon migration. Mircrofractures are also common, and their initiation might be related to mineral crystal structure. When present in sufficient quantity, these pore types offer potential gas (and oil) molecule storage spaces and permeability pathways through the shales.

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 mudstone 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 gamma-ray 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.

The recent revelation of vast global quantities of potentially gas- (and oil-) productive shales has paved the way for rapid advancements in understanding their geological properties, many of which affect their geomechanical properties, and ultimately reservoir operations and performance. Because of shale's unique and very intricate geological, physical and mechanical properties at all scales (from nano- to macro-scale (Abousleiman et al., 2010) compared with sandstones and carbonates, the ad-jective “unconventional” has often been applied to their characterization. Although many shale characteristics are different than those of sandstones and carbonates, and require new technologies to measure them, other characteristics, mentioned below, are similar. As such, customized conventional methodologies and concepts can be applied to exploration and production of shale gas (and oil).