ABSTRACT This study examines the usefulness of accommodation plots (Fischer plots) as a means of correlating mixed carbonate-siliciclastic strata in the subsurface. Fischer plots have been widely used to extract accommodation changes from carbonate platforms, but there are few published studies of siliciclastic or mixed carbonate-siliciclastic environments. The Middle Devonian of the Appalachian Basin is penetrated by thousands of wells, is exposed in numerous exceptional outcrops, and is an excellent place to test the usefulness of accommodation history plots as correlation tools. In the past, researchers have used cores, well cuttings, well logs, and outcrop gamma-ray profiles to correlate between outcrop and subsurface data, but all these methods have their limitations. Gamma-ray logs for wells penetrating the Middle Devonian from eight locations, from Preston County in the east to Wetzel County, West Virginia, in the west, were used in this study. Accommodation cycle thicknesses were measured from gamma-ray logs, printed at a vertical scale of one inch per ten feet (2.5 cm/3 m). The accommodation cycle thickness data were entered into Antun Husinec’s FISCHERPLOTS program to produce accommodation plots. Next, well-documented, outcrop-based sequence stratigraphy was used to help interpret the results of the accommodation plots. This study demonstrates that using accommodation plots is a novel way of overcoming the uncertainties and biases of other methods. The use of this approach in other mixed carbonate-siliciclastic successions with abundant subsurface data would help to demonstrate that Fischer plots are a novel and useful approach that can help remove many of the uncertainties and biases encountered in stratigraphic correlation.

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 An evaluation of an integrated data set collected over the past 12 years designed to identify the parameters controlling reservoir quality and production properties in organic, siliceous mudrocks reveals the key diagenetic processes affecting the development of brittleness in siliceous mudrocks. This work was motivated by the failure of early efforts to correlate brittleness to x-ray diffraction (XRD) mineralogy. The outcome of this analysis has been the recognition of two, often overlapping, pathways to brittleness that are determined at the time of deposition by the relative proportions of clay, detrital quartz, and biogenic silica present in the original sediment and are later affected by burial history. One pathway begins with a phyllosilicate–mud-dominated sediment, and the other begins with a sediment containing common or abundant biogenic silica (opal-A). Both pathways are characterized by compactional porosity loss and both eventually include the generation of authigenic quartz cement; however, the source of that authigenic quartz is different between the two pathways. The authigenic quartz that characterizes the first pathway is developed from the illitization of smectite and is precipitated as a cement within the argillaceous matrix. This authigenic quartz is detectable along with the detrital quartz by XRD analysis. All other factors being equal, the volume of brittle, authigenic quartz cement derived from the alteration of smectite is proportional to the volume of original clay. As a result, the effectiveness of this cement to increase the brittleness of the rock may be impacted by the presence of the ductile clays. In the alternate pathway, authigenic quartz is derived from the transformation of biogenic opal-A and is independent of the amount of clay. Much of the XRD quartz volume in rocks derived from biogenic–silica-rich sediment that contained little or no detrital quartz will comprise a brittle, authigenic cement.