Geologic characterization and the recognition of cyclicity in the Middle Devonian Marcellus Shale, Appalachian Basin, NE USA

Abstract

The Middle Devonian Marcellus Shale in the Appalachian basin is one of the leading gas producing formations in the United States. It is clear that we currently do not have good methods for establishing sequence stratigraphic frameworks in these mudrock successions as recent attempts show a bias towards the deep basin depositional model that is patterned after the Black Sea. Geologic characterization involving eight core data, over 100 petrographic thin sections, SEM, QEMSCAN and XRD data, field studies of Marcellus Shale exposures in New York and Pennsylvania as well as over 800 electrical wireline logs was carried out to better characterize the Marcellus Shale and define cyclicity within the succession. Sedimentary structures and features such as laminations, starved ripples, grading, burrows, bioturbation, reworking of authigenic minerals, abundant and aligned benthic fossils and basal lag deposits were interpreted to be indicative of significant oxygenation and current activity. These observations suggest that Marcellus Shale facies were deposited in a non-permanently anoxic, shallow muddy epeiric sea. The key control on sedimentation was a combination of local geologically rapid subsidence/uplift events, seasonal variations in nutrient sourcing of algal blooms, climate and sediment supply, rather than water depth. Essentially, during Marcellus Shale deposition, subsidence/uplift events controlled both the creation accommodation space and the character of the sediment supply into the Appalachian Basin. Climate and nutrient sourcing of algal bloom controlled the creation of seasonal anoxia in the bottom waters and together with variations in the bottom water chemistry, these factors dictate whether siliciclastics sediments and organics are deposited or whether carbonates are deposited during a particular depositional cycle. Given a depositional model that is not driven primarily by sea-level fluctuations, new concepts and methods for recognizing and describing cyclicity have been developed. The concepts and methods involved the use of proxies to define important depositional surfaces and packages. The proxies were developed by integrating total organic carbon content, mineralogical composition, facies description and log signatures. New terms were introduced to describe important surfaces and packages. These terms include Preservation Shutdown Surface, Preservation Initiation Surface, Maximum Preservation Surface, Preservation Shutdown Tract, Preservation Initiation Tract and Preservation Decline Tract. The first three terms describe depositional surfaces while the last three terms describe the shale packages (deposits) that are contained by them. The results indicate that the preservation initiation tract (PIT) deposits together with the deposits of the Maximum Preservation Surface (Zone) generally have the highest total organic carbon contents, the best reservoir properties and also contain the most brittle facies. The most organic-rich interval occurs at the base of the Marcellus Shale within the preservation initiation tract deposits of the Lower Union Springs Member. TOC usually range between 4% and 12% in these deposits, but may be up to 20% in very organic-rich intervals. The most brittle facies are the pyritic sandstone, the noncalcareous siliceous-argillaceous mudstone and the siliceous mudstone facies. The results also suggest that up to four cycles can be recognized from the base of the Onondaga Limestone, below the Marcellus Shale to the base of the Stafford Limestone above the Marcellus Shale. With limited age-datable fossils, it is difficult to establish a high resolution chronostratigraphic framework for the cycles. This work shows that it is possible to define cyclicity in organic matter-rich mudrocks-dominated successions without bias towards the deep-basin depositional model.