ABSTRACT Organic-rich mudstones of the Appalachian Basin hold a sizable portion of the natural gas produced in the United States. Indeed, in 2015, Pennsylvania and West Virginia accounted for 21% of produced natural gas, driven in part by production from the Point Pleasant Limestone. The critical role that unconventional reservoirs will play in future global energy use necessitates the need for an enhanced understanding of those geological aspects that shape and influence their reservoir architecture. Foremost among these is a clearer understanding of the preservation and accumulation of organic carbon, as it is the source of hydrocarbons, and often provides the dominant host of interconnected porosity and hydrocarbon storage. To this end, pyrite morphology can offer insight into the redox conditions of the bottom and pore water environment at the time of sediment deposition and early diagenesis and can be especially useful in the analysis of deposits devoid of redox sensitive trace metals. Pyrite contained in cuttings and core chips retrieved from vertical and horizontal Point Pleasant Limestone wells were analyzed by scanning electron microscope. Results demonstrate a dearth of pyrite in the Point Pleasant (0.02–1.7% of the surface area analyzed). Pyrite morphology is dominated by euhedral grains and masses (~80% of pyrite encountered) co-occurring with infrequent framboids. Framboids are uniformly small (average = 4.7 μm) with just a few examples >10 μm. The presence of small amounts of euhedral pyrite grains and masses is consistent with accumulation under a dysoxic water column. Conversely, the size of the framboids suggests that they formed in a water column containing free hydrogen sulfide. A model invoking a lack of reactants necessary to sustain diagenetic pyrite growth in anoxic pore waters may explain this apparent paradox. In such a case, the framboid size distribution may reflect newly forming diagenetic framboids competing for a finite amount of reactants resulting in a population of small framboids and few large examples. Indeed, the low total iron/aluminum (Fe/Al) content of the Point Pleasant (average Fe/Al = 0.45) would indicate a low delivery of reactive iron to the seafloor during Point Pleasant deposition. The data suggests a model in which organic carbon preservation occurred by rapid burial and removal from oxygen-bearing water. In turn, more organic-rich and potentially higher quality reservoir facies of the Point Pleasant Limestone occur in areas of higher clastic delivery to basin.

ABSTRACT Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections ( n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite. Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval. The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

ABSTRACT The lower Permian Wolfcamp Shale in the Permian Basin is a major unconventional resource play composed of organic-rich, siliceous and calcareous mudstones interbedded with carbonate turbidites and debrites. Using two cores that comprise the Wolfcamp Shale near the eastern margin of the Midland Basin, this study reconstructs the complex diagenetic history of both the mudstone and carbonate facies. These cores were analyzed using petrographic and SEM techniques to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness. Early, middle, and late phases of chemical diagenesis are defined in this study. Mineral precipitation and dissolution events occur from the passage of fluids through both interstitial and fracture pore space. Early authigenic mineral precipitation (calcareous and phosphate concretions, sphalerite, barite, framboidal pyrite, quartz, dolomite, and ferroan dolomite) resulted in destruction of primary porosity within the mudstone facies, before and during the mechanical compaction event. Destruction of porosity in the carbonate turbidites facies occurred through carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) during early to middle diagenesis. An episode of dissolution and dolomitization in the carbonate facies resulted in the creation of moldic and intercrystalline porosity respectively. Within mudstones intercrystalline porosity is observed between pyrite framboids and clay sheets of chlorite. Diverse fracture types occur in all facies within the Wolfcamp Shale and play a critical role in the migration of diagenetic fluids and hydrocarbons. Horizontal fractures are filled by “beef”-type calcite, and vertical fractures are filled with equant calcite and/or celestine-barite. Mineralized fractures contain porosity, some of which contain ferroan dolomite rhombs within pores, which supports diagenetic fluid movement through fractures after an initial stage of mineralization. Fluid inclusion data suggest that some mineralized fractures acted as fluid conduits for externally derived, warm, high-salinity brines, suggesting the Wolfcamp Shale was an open system during it burial history.