CHERT BEDS IN THE UPPER DEVONIAN-LOWER MISSISSIPPIAN WOODFORD SHALE IN OKLAHOMA—POSSIBLE RESERVOIR ROCKS?

How gas is stored in shale-gas systems is a critical element in characterizing these potentially prolific, low porosity/permeability reservoirs. We have undertaken an integrated mineralogic, geochemical, and porosity/permeability study of the Upper Devonian-Lower Mississippian Woodford Shale, Arbuckle Mountains, OK, at locations previously described through detailed stratigraphic and spectral gamma surveys, to further our understanding of possible mechanisms by which natural gas is stored in Woodford reservoirs in the adjacent Anadarko Basin.

Rock types of interest in the Woodford are broadly divided into chert and mudstone lithofacies that display different characteristics. Woodford cherts (>85 wt% quartz, <5 wt% clays) have an average TOC content of 4.5%. Quartz, which occurs in cherts as mosaic or granular chalcedony, infilling radiolarian tests and composes much of the rock volume, formed diagenetically early from recrystallization of radiolarian skeletal parts. Organic matter in cherts is present as amorphous organic material (AOM) and fills micropores 1) between minute quartz crystals within chalcedonic masses; or 2) between the colloform, bulbous masses of chalcedony. In contrast, Woodford mudstones (26-77 wt% quartz, 10-40 wt% clays), are more organic rich (avg. TOC 13.1%), with organic matter largely present as AOM and Tasmanites microfossils. Quartz in mudstones is largely detrital but some is also authigenic monocrystalline “grains” infilling Tasmanites. Mercury injection capillary pressure analyses (at 50% Hg saturation) reveal that cherts have 1) variable porosity (0.59-4.90%), 2) low permeability (0.003-0.274 mD), and 3) small pore mean apertures (5.8-18.6 nm). In comparison, mudstone porosities are variable (1.97-11.9%), as are mudstone permeabilities (0.011-0.089 mD). Mean pore apertures of mudstones (6.2-17.8 nm) are similar to those of cherts. Microfractures also contribute to rock porosity, but appear to be lithologically controlled and are best developed in cherts (brittle), but poorly developed or absent in adjacent clay-rich mudstones (ductile).

Owing to their porosity, Woodford cherts may provide important, overlooked sites of gas storage in the formation, and upon artificial stimulation may contribute a significant portion of the gas that is produced.