Mudstone resource plays have become a focus of industry activity over the last decade. Understanding the depositional limits of the play fairway can increase the probability of locating the sweetspot of the play. This paper outlines the importance of distinguishing a suite of mudstone sedimentary facies within high-resolution stratigraphy to define the vertical and lateral extents of source rock related to depositional environment. We examined cores containing mudstone and carbonate successions of the Trenton, Lexington, Pt. Pleasant, and Utica Formations deposited on an Ordovician platform sub-basin of eastern Ohio. The wells are located updip of the Rome Trough (i.e., Appalachian Basin to the east) in the Pt. Pleasant sub-basin associated with the Trenton carbonate platform.
Combining classical carbonate and siliciclastic core description techniques permitted the identification of five subtidal and seven deep-water carbonate facies associated with the unconventional mudstone play fairway. Petrophysical work from multiple cores, including SEM and thin-section analysis, identified a dramatic decrease in carbonate content and increase in argillaceous clay content upward across the Lexington - Pt. Pleasant contact. Stable isotope analyses (δ13C) of these cores also helped to deconvolve key stratigraphic intervals in the overall succession.
Detailed regional stratigraphic correlation and mapping of the Lexington and Pt. Pleasant Formations document apparently compensating southward-shifting intraplatform "sub-basins." However, core analyses indicate that the underlying Lexington contains rippled mudstone that passes southward into suspension-dominated mudstone and bioclast gravity flow deposits. This is associated with a thicker slope environment thinning southward onto a platform, and not the fill of a sub-basin.
The Pt. Pleasant thickens southward and facies transition from an overall lower proportion of bioclast gravity flow deposits and rippled mudstone into high TOC suspension mudstones comprising the fill of an anoxic sub-basin and the "sweet spot." Furthermore, a conspicuous erosional surface identified in the Miller-1 core of NE Trumbull County marks the pinchout of the Pt. Pleasant, which correlates with a significant δ13C isotope excursion (~2 per mil) seen in regional outcrop examples suggesting possible meteoric diagenesis and subaerial exposure. These relationships help to further distinguish the genetic relationship with the underlying Lexington.
This work demonstrates the importance of integrating log, core, and geochemical data to build a sequence stratigraphic view of the depositional history. Incorporating these data into the construction of a depositional model for the basin along with isopach maps of high-resolution sequences is critical in identifying the "play fairway" for access. Validation of the work is seen by plotting the IP rates and EUR bubbles on top of the play fairway. The actual Pt. Pleasant sweetspot, defined here as the area within the play fairway with the best rate of return, is further south in the play fairway than the overall depositional thick seen by mapping the Lexington and Pt. Pleasant interval.