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.

Pyrite framboid diameters were examined in 31 samples taken from 2 Marcellus Shale cores recovered from Greene County, Pennsylvania, and Upshur County, West Virginia (USA). Analysis of framboid diameters in those samples from the more proximally located Upshur County core suggests that anoxic to anoxic-euxinic conditions persisted during accumulation of the transgressive-regressive cycle (MSS1) that comprises the Union Springs Member of the Marcellus Shale, with intermittent episodes of dysoxia. An increased abundance of large framboids documented from the overlying transgressive-regressive cycle (MSS2), which comprises the bulk of the Oatka Creek Member of the Marcellus Shale, indicates improved bottom-water conditions. Redox conditions recorded by framboid diameters of the MSS1 cycle of the Greene County core are generally similar to those of the Upshur County core; however, conditions in that region of the basin from which the Greene County core was recovered appear to have remained dominantly anoxic to anoxic-euxinic. Furthermore, the presence of small syngenetic framboids and large diagenetic framboids in the same thin section samples suggests that redox conditions fluctuated on a temporal scale beyond that observed at the scale of a centimeter-scale thin section. Framboid diameter trends established for both cores enhance our understanding of how much redox conditions varied both spatially and stratigraphically during accumulation of the Marcellus Shale.