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 Beautifully fossiliferous strata in the Hamilton Group (Middle Devonian, central New York) constitute a rich “ecological archive” sufficient to probe and test foundational concepts in paleontology. The evident community stability of Hamilton faunas over 4–6 m.y.—including two proposed mechanisms for coordinated stasis—has ignited controversy. Resolving community structure and both taxonomic and ecological temporal persistence within the Hamilton Group thus becomes critical to testing whether these Hamilton communities are stable and whether they are ecologically “locked.” Toward this end, we conducted multivariate analyses (cluster and correspondence analysis) of marine faunas in 81 large samples (~300 specimens each) in shallowing-upward sequences of the Cardiff and Pecksport Members (Marcellus Subgroup, Oatka Creek Formation) of the Hamilton Group. Eight statistically and ecologically distinctive benthic communities characterize the vertical gradient, from depauperate, deeper-water dark shales below to species-rich shelf siltstones above. These communities correlate strongly with grain size, bioturbation intensity, bedding thickness, density of fossils, and faunal and ecological diversity. Species richness varies inversely with weight percent organic matter. We characterized taxonomic distributions using multivariate statistics; these statistical analyses were based on percentages of 50 taxa. In order of decreasing depth, the communities are: Cephalopod- Pterochaenia , Pterochaenia-Eumetabolotoechia , Eumetabolotoechia , Emanuella , Eumetabolotoechia-Ambocoelia , Arcuaminetes-Eumetabolotoechia , Arcuaminetes-Ambocoelia , and Mucrospirifer- Ambocoelia . The Cephalopod- Pterochaenia community represents a mixed benthic-pelagic fauna associated with the deepest and finest-grained facies. The Pterochaenia-Eumetabolotoechia , Eumetabolotoechia , and Emanuella communities have low to moderate species richness and are dominated by epifaunal, active suspension feeders, especially the small epibyssate bivalve Pterochaenia fragilis , and the pedunculate brachiopods Eumetabolotoechia multicostata and Emanuella subumbona . The Pterochaenia-Eumetabolotoechia community is an opportunistic fauna that developed when the substrate first became favorable for colonization by benthic organisms. To a lesser extent, this probably also holds true for the Eumetabolotoechia assemblage. Communities near the top of the shallowing-upward cycle— Eumetabolotoechia- Ambocoelia , Arcuaminetes-Eumetabolotoechia , Arcuaminetes-Ambocoelia , and Mucrospirifer-Ambocoelia —have higher taxonomic and ecological heterogeneity, with a more diverse array of trophic and locomotory groups than their counterparts in the finer-grained, and inferred deeper, facies. Cluster significance tests applied to all pairs of communities known from adequate numbers of samples demonstrated that the communities are statistically valid and distinctive. Multivariate means of all communities were significantly different; furthermore, most pairs of communities were drawn from populations that showed no overlap in terms of rectangular distributions. The community sequence and an ordination derived from the first two axes of the correspondence analysis provided relative depth curves. Our communities, with two exceptions, do not have clear counterparts among upper Hamilton Group faunas. The ecological locking model proposed to explain the stability of Hamilton faunas is not supported by our quantitative tests to date.

ABSTRACT The Middle Devonian Marcellus shale play has emerged as a major world-class hydrocarbon accumulation. It has rapidly evolved into a major shale gas target in North America and represents one of the largest and most prolific shale plays in the world with a prospective area of approximately 114,000 km 2 (44,000 mi 2 ). Two major core areas have emerged, each with a unique combination of controlling geologic factors. Production from the Marcellus play reached 16 billion cubic feet of gas equivalent per day (BCFepd) in 2015, and it has been recognized as the largest producing gas field in the United States since 2012. The organic-rich black shales comprising the Marcellus shale were deposited in a foreland basin that roughly parallels the present-day Allegheny structural front. The Marcellus shale accumulated within an environment favorable to the production, deposition, and preservation of organic-rich sediments. The key geologic and technical factors that regionally define the Marcellus play core areas include organic richness, thermal maturity, degree of overpressure, pay thickness, porosity, permeability, gas in place, degree of natural fracturing, mineralogy, depth, structural style, lateral target selection, completion design, and important rock mechanics issues such as the ability to be fractured, rock brittleness versus ductility, and the ability to generate complex fractures. Structural setting and deformation styles are critical to address natural fracture trends, potential geologic hazards such as faulting and fracturing in structurally complex areas, and fracture stimulation containment issues. Since the Marcellus shale unconventional shale gas reservoir discovery in 2004 until May 2015, more than 8600 horizontal Marcellus shale wells had been drilled in Pennsylvania, West Virginia, and limited portions of eastern Ohio. Many decades of future drilling potential remain due to the enormous extent of the Marcellus shale play. Horizontal Marcellus wells report initial production rates ranging from less than 1 MMCFe/day to over 47.6 MMCFe/day. Despite the large number of wells drilled and completed to date and production of 16 BCFepd in 2015, the play is still in its infancy due to its vast geographic extent and production potential. The Marcellus shale represents a continuous-type gas accumulation and when fully developed will comprise a large continuous field or series of fields. Over its productive trend, the Marcellus shale play has significant additional reserve potential in the overlying organic shales in the Devonian Age Rhinestreet, Geneseo, and Burket units as well as deeper potential in the Ordovician Age Utica/Point Pleasant units. Estimates of recoverable reserves from the world’s largest gas fields combine their reserve estimates for all key productive units in the field/play trend. Likewise, estimates of in-place gas resources for the Marcellus play range from 2322 tcf for the Marcellus (Hamilton Group) to over 3698 tcf for the combined Devonian Age Marcellus-Geneseo-Rhinestreet system. This represents the largest technically accessible in-place gas resources in the world.

Abstract A Marcellus-Burket/Geneseo field trip in the Appalachian Valley and Ridge features both brittle and ductile structures. The degree to which these structures have developed depends on both lithology, which is a function of the stratigraphic architecture of the Devonian Appalachian Basin and position relative to the foreland during the Alleghanian Orogeny. Joints are best developed in the black shales and the units immediately above with the J 2 joint set most prominent in the Brallier Formation just above the Burket/Geneseo Formation. Faults are seen in the form of cleavage duplexes and bedding-parallel slip accompanying flexural-slip folding. Cleavage duplexes are found in the Marcellus whereas bedding-parallel slip is more common in the overlying Mahantango Formation and further up the section in the Brallier Formation. Layer-parallel shortening decreases from greater than 50% to approximately 10% when crossing the Jacks Mountain–Berwick Anticline structural front from the hinterland portion to the foreland portion of the Valley and Ridge. Disjunctive cleavage, the primary mechanism for layer-parallel shortening, is best developed in carbonates whereas pencil cleavage is best developed in shales.