SEDIMENTOLOGY AND RESERVOIR CHARACTERISTICS OF PENNSYLVANIA’S LOWER SILURIAN MEDINA GROUP

The Lower Silurian Medina Group is regionally extensive across several states in the Appalachian basin, has good geophysical well control, and years of hydrocarbon production histories from depleted oil and gas fields. This allows for a detailed examination of the sedimentology and reservoir characteristics both regionally and locally. We performed a case study of sixteen Medina Group wells drilled on 1,300 to 2,000 foot spacing in Warren County, Pennsylvania, which indicates the greatest porosity (as measured by bulk density log), and permeability (as inferred by gas production), are most associated with fluvial and upper shoreface facies of incised-valley fills (from gamma ray log signature). This facies is present in the basal Whirlpool Sandstone, which formed either during late highstand or initial transgression. The transgressive Cabot Head Shale shows low porosity, as do the regressive, progradational tidal flat facies in the Grimsby Sandstone. Intermediate porosity and lower gas production values are exhibited in the Grimsby’s tide-dominated shoreline facies. When compared to regional cross-sections in northwestern Pennsylvania and eastern Ohio, geophysical well logs in this pool exhibit similar character and variation. The fluvial-deltaic facies of the Medina Group form discontinuous sand bodies that vary both vertically and laterally. However, they possess a common depositional energy, water depth, and sediment supply that creates recognizable patterns of deposition. These patterns can be mapped using closely-spaced geophysical well log control in Medina gas development fields, to identify facies and characteristic porosities associated with fluvial-deltaic environments.