ABSTRACT The Devonian Woodford Shale and Cretaceous Mowry Shale consist of relatively deep (below storm wave base) intracratonic basin deposits commonly referred to as “shales” because of their dark gray to nearly black color, very fine-grained nature, pelagic fossils such as radiolarians, and common amorphous marine kerogen. These shales typically contain less than 30% detrital clay by weight and more than 50% quartz (locally up to 80%). The quartz is a mix of biogenic grains, mainly radiolarians, and authigenic silica along with some detrital quartz silt of extrabasinal origin. The authigenic silica is dominantly microcrystalline (< 1 micron) and forms a major component of the matrix in these formations, but the rocks also contain authigenic pyrite, commonly as framboids, minor carbonates including magnesite, and quartz overgrowths, but together these authigenic minerals form less than 10% of the rock. Authigenic quartz in the Woodford and Mowry samples commonly takes the form of silica nanospheres, a type of microquartz less than a half micron in diameter. Textures of this microquartz are best observed directly with a high-resolution electron microscope. In many Woodford and Mowry samples, the silica nanospheres, which tend to be associated with organic matter, form more than 50% of the rock. The large volume of the authigenic quartz, together with “floating” detrital components and the close association with pyrite framboids, indicates that the silica nanospheres formed very early, perhaps in association with microbial activity on or in the seafloor sediments. These early silica nanospheres, which are only weakly luminescent, helped create a lithified sediment during or soon after deposition. Where the silicification process ceased prior to complete silica cementation, the early silica nanospheres are associated with up to 15% interparticle microporosity. This gives the Woodford and Mowry good potential reservoir quality, at least locally. The authigenic silica nanospheres also enhance the mechanical properties and brittleness of these siliceous mudrocks to a degree much greater than the presence of the detrital quartz particles alone.

ABSTRACT A new model is proposed to predict porosity in organic matter for unconventional shale reservoirs. This model is based on scanning electron microscopic (SEM) observations that reveal porosity in organic matter is associated with secondary porosity developed within organic matter cement that fills void space preserved prior to oil generation. The organic matter cement is interpreted as solid bitumen resulting from the thermal alteration of residual oil retained in the source rock following oil expulsion. Pores are interpreted to develop within the solid bitumen as a result of thermal cracking and gas generation at increased levels of thermal maturity, transforming the solid bitumen to pyrobitumen. The pyrobitumen porosity model is an improvement over existing kerogen porosity models that lack petrographic validation. Organic matter porosity is predicted by first estimating the potential volume of organic matter cement by deriving the matrix porosity available at the onset of oil generation from extrapolations of lithologic specific compaction profiles. The fraction of organic matter cement converted to porosity in the gas window is then calculated by applying porosity conversion ratios derived from SEM digital image analysis of analogous shale reservoirs. Further research is required to refine and test the porosity prediction model.

In 1907, Barnum Brown named the Hell Creek beds (Formation) for the strata exposed in the Hell Creek Valley and other downstream tributaries of the Missouri River. In the absence of a stratotype section, a lectostratotype is herein proposed for the Hell Creek Formation based on 84.2-m-thick exposures at Flag Butte (local name) in Ried Coulee (archaic use; East Fork of Hell Creek) and East Ried Coulee, tributaries of Hell Creek, Garfield County, Montana (sec. 29, T. 21 N., R. 38 W., NAD27 CONUS; base 47.55931°N, 106.88111°W; top 47.55533°N, 106.86810°W). The formation is underlain with general conformity by sandstone beds of the Fox Hills Formation (as characteristically known, the Colgate Member is absent) and is for the most part conformably overlain locally by the Tullock Member of the Fort Union Formation. The upper contact at Flag Butte is demarcated at the base of the IrZ lignite bed (above an iridium anomaly). The boundary has been demonstrated to be somewhat unconformable in areas to the west. The IrZ bed is also missing at Bug Creek in McCone County. In its type section, the Hell Creek Formation is subdivided (simply and informally) into Ried Coulee (lower Hell Creek), East Ried Coulee (middle Hell Creek), and Flag Butte (upper Hell Creek) units, each containing a sandstone and a mudstone lithofacies. Formational thickness varies with local depositional and erosional history of various coastal-deltaic environments across the Williston Basin and a trend of overall thinning to the east and northeast.