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41882-B8
Sedimentology, Petrography and Sequence Stratigraphy of the Greasy Ridge Oilfield, Southeastern Ohio
Sequence stratigraphic analysis of the Lower-Middle Pennsylvanian Pottsville Group requires recognition of unconformities and their correlative surfaces. In general, incised-valley fill sandstones are known to be good hydrocarbon reservoirs, and the capacity to identify them within the Pottsville may facilitate future exploration. The base of the Pottsville Group in Ohio coincides with the Absaroka-Kaskaskia cratonic sequence boundary which has been interpreted by previous workers to have been caused by a global sea level lowstand combined with crustal uplift. Paleovalleys with as much a 122 m (400 ft) of relief are believed to have developed in portions of southern Ohio and northeastern Kentucky.
The Sharon Sandstone/Conglomerate is a quartz arenite/conglomerate that comprised the lower part of the Pottsville and fills paleotopographic lows along the unconformity. In SE Ohio, two contrasting hypotheses have been expressed regarding the fluvial drainage systems responsible for the Sharon: 1) north-flowing stream emptying into the Jackson-Gallipolis paleovalley, and 2) South-southwest flowing stream emptying into NE Kentucky. Previous workers had very few data points for Lawrence County.
Two undergraduates were employed to reevaluate available well data over a period of several weeks during 2007. As a ‘first look', drillers logs from 296 completion cards were reviewed through the Ohio Geologic Survey's interactive oil and gas map. The objective was to gather thickness data for the Mississippian Maxville Limestone and develop an isopach map for the study area. Previous workers had recognized valley incision during the early Pennsylvanian downcut into the Maxville and completely removed it in some areas. Our goal was to try to more accurately identify these valley systems in the study area. The results indicated an irregular SSW trending paleovalley which extended through Mason, Lawrence and Perry Townships. As a follow-up, a comparison was made between drillers log calls for the Maxville and gamma ray and density logs for those wells where these were available. It was found that the Maxville was misidentified often enough to make a portion of the preliminary database suspect. A second set of data was developed using only 83 wells, where GR and density confirmed the presence and thickness of the Maxville Limestone. A second isopach map of the Maxville was generated from these data which clearly identified a SSW-trending paleovalley where the Maxville was missing, flanked to the northwest by 18-21 m (60-70 ft) of Maxville in what we interpret as a drainage divide extending from Ironton to Gallia Ohio. Two subordinate linear minima intersecting the SSW trending trunk paleovalley suggest the presence of NW flowing tributaries in the SE corner of Lawrence County.
Two-incised valley-fills up to 30 m (100 ft) thick and filled with quartz arenites occur in the Lower Pottsville. An isopach of the lowest IVF indicates a SSW trend coincident with the trunk valley identified by isopaching the Maxville. The middle and upper portions of the Pottsville contain IVFs up to 18 m (60 ft) thick filled with sublithic arenites. High quality well data including GR and density logs through the Pottsville are restricted to Mason Township. The density logs are necessary to identify coals which are key beds in a high resolution stratigraphic framework. Without density logs or core data, it difficult to project regional trends to these valley systems. Local trends in Aid and Mason Township are commonly toward the SW.
