Concepts and Methods for the Recognition of Cyclicity in the Middle Devonian Marcellus Shale

Emmanuel, Olusanmi O.; Sonnenberg, Stephen

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 Black sea depositional model for black shale sedimentation.

Sedimentological and stratigraphic work involving eight core data, over 100 thin sections, SEM 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 such as starved ripples, grading, burrows, bioturbation, reworking of authigenic minerals, aligned fossils and basal lag deposits were interpreted to be indicative of shallow marine current activity. These observations suggest that Marcellus Shale facies were deposited in an aerated, but shallow muddy epeiric sea. In such settings, the key control on sedimentation was a combination of local geologically rapid subsidence/uplift events, seasonal variations in nutrient sourcing of algal blooms, changes in salinity and clastic influx rates, rather than water depth. Given a depositional model that is not driven primarily by sea-level fluctuations, some concepts and methods for recognizing and describing cyclicity are discussed in this study.

TOC content, mineralogical composition, sedimentary structures and log data were integrated and used as proxies for defining cyclicity. New terms were introduced to describe important zones and surfaces. These terms include Preservation Shutdown Surface, Preservation Initiation Surface, Maximum Preservation Surface, Shutdown Systems Tract, Initiation Systems Tract and Preservation Systems Tract. The first three terms describe depositional surfaces while the last three terms describe the systems tracts that are contained by them. The results suggest that up to four cycles can be recognized from the base of the Onondaga Limestone, below the Marcellus to the base of the Stafford Limestone (or base of the Mahantango 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.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013