The organic-rich upper shale member of the upper Devonian–lower Mississippian Bakken Formation (Williston Basin, North Dakota, USA) has undergone significant diagenetic alteration, irrespective of catagenesis related to hydrocarbon generation. Alteration includes precipitation of numerous cements, replacement of both detrital and authigenic minerals, multiple episodes of fracturing, and compaction. Quartz authigenesis occurred throughout much of the member, and is represented by multiple generations of microcrystalline quartz. Chalcedonic quartz fills radiolarian microfossils and is present in the matrix. Sulfide minerals include pyrite and sphalerite. Carbonate diagenesis is volumetrically minor and includes thin dolomite overgrowths and calcite cement. At least two generations of fractures are observed. Based on the authigenic minerals and their relative timing of formation, the evolution of pore waters can be postulated. Dolomite and calcite resulted from early postdepositional aerobic oxidation of some of the abundant organic material in the formation. Following aerobic oxidation, conditions became anoxic and sulfide minerals precipitated. Transformation of the originally opaline tests of radiolaria resulted in precipitation of quartz, and quartz authigenesis is most common in more distal parts of the depositional basin where radiolaria were abundant. Because quartz authigenesis is related to the distribution of radiolaria, there is a link between diagenesis and depositional environment. Furthermore, much of the diagenesis in the upper shale member preceded hydrocarbon generation, so early postdepositional processes were responsible for occlusion of significant original porosity in the member. Thus, diagenetic mineral precipitation was at least partly responsible for the limited ability of these mudstones to provide porosity for storage of hydrocarbons.

Abstract A study of the Upper Devonian-Lower Mississippian Woodford Shale was undertaken on samples at low thermal maturity from the Arbuckle Mountains, southern Oklahoma, to dientify possible mechanisms by which natural gas might be stored in Woodford reservoirs in the adjacent Anadarko Basin. The two main lighologies in the Woodford, chert and mudstone, display different inorganic and organic characteristics. Cherts have (1) variable porosity from 0.59% to 4.90%, (2) low calculated permeabilities, and (3) small mean pore apertures. Intercrystalline pores dominate in cherts. In contrast, mudstones generally have (1) porosities ranging from 1.97% to 6.31%, (2) low calculated permeabilities, and (3) small mean pore apertures. Interparticle, intraparticle, and moldic pores all are present in mudstones. Because of their high quartz content, cherts are brittle and commonly demonstrate microfracturing that is lithologically controlled and bedding perpendicular, whereas much less microfracturing exists in mudstones. The early diagenetic intercrystalline porosity in cherts has likely been preserved since it formed because of the rigid, internal framework provided by the abundant authigenic quartz. Coupled with their relatively high TOC contents, cherts then may be important intervals of gas generation and storage in the Woodford. Where abundant, cherts may then play a significant role as source and reservoir intervals within the formation in the Anadarko Basin.