Subsurface variation in facies is a main control on reservoir quality and can potentially be related to production. Previous studies of the Marcellus Shale have focused on the regional sequence stratigraphy and depositional models based on outcrops, wireline logs, and subsurface cores. A detailed sedimentological analysis was performed on one complete section of Marcellus core (310 ft) from northeastern Pennsylvania, consisting of core description, petrography (69 thin sections) and high-resolution geochemical analyses (every 2-inch), to better understand depositional processes and conditions and to characterize the vertical heterogeneity of the lithofacies that affect petrophysical and geomechanical rock properties. In addition, pore systems were systematically investigated in these core samples which are at near 3.0% R_o thermal maturity. Variations in biota, sedimentary structures, bioturbation, organic matter type and content, as well as sizes of pyrite framboids were integrated with geochemical data to define conditions at the sediment-water interface and within the water column. Fourteen lithofacies were defined on the basis of mineralogy, fabric, texture, and biota. The Marcellus Shale in northeastern Pennsylvania is interpreted to have been deposited in distal, relatively deep areas of the basin. Evidence of weak turbidity currents and bottom-water currents has been observed in the form of graded beds, multiple erosional surfaces, and thin-grain-supported silt laminae. Elevated concentrations of redox-sensitive and productivity-sensitive trace elements (U, Mo, V, Cr, Ni) in the basal shale member suggest deposition under reducing, sulfur-rich (anoxic to euxinic) conditions during times of elevated productivity. An upward decline in organic richness and trace-element abundance was observed, likely due to an increase in dilution and more oxygenated water column through time. The abundance of small conical calcitic shells—including tentaculitids and styliolinids—is significantly higher in the Cherry Valley member. The high abundance of algal cysts (Tasmanites)

in the basal Marcellus shale is correlative with the highest sulfur and TOC concentrations, representing an anomalous period of intense phytoplankton growth in the Appalachian Basin caused by additional nutrients, possibly from wind or rivers. The algal cysts bloom increased organic input and enhanced the production of organic matter (OM ) (Figure 1). The basal Marcellus Shale (BMS) that is dominated by laminated radiolarian and Tasmanites-rich argillaceous siliceous mudstone facies has the best reservoir quality (highest TOC, OM porosity, and lowest clay mineral content).

Both SEM and HIM imaging reveal that OM spongy pores whose sizes are mostly < 200 nm are predominant. Mineral pores are present, many associated with carbonate dissolution. The majority of OM is pyrobitumen which hosts spongy pores. Correlative relationships between total organic carbon (TOC) and porosity (R²=0.5) and TOC and permeability (R²=0.76) suggest that pyrobitumen and OM spongy pores form a connected network. The total porosity does not vary much throughout the Marcellus; however, the measured matrix permeability of the BMS is two orders of magnitude higher than that of the lower Marcellus Shale (LMS) and upper Marcellus Shale (UMS). The significant increase in permeability might be related to higher TOC, more Type II kerogen, and a less-compacted siliceous framework that helps preserve interparticle pores in the BMS than in the argillaceous framework in the LMS/UMS (Figure 2).