In Search of a Silurian Petroleum System in the Central Appalachian Basin

Ryder, R.T.¹, Lentz, E.E.¹, Trippi, M.H.¹, Avary, K.L.², Harper, J.A.³, Kappel, W.M.⁴, and Rea, R.G.⁵

 ¹U.S. Geological Survey, Reston, VA

²West Virginia Geological Survey, Morgantown, WV

³Pennsylvania Geological Survey, Pittsburgh, PA

⁴U.S.Geological Survey, Ithaca, NY

⁵Ohio Geological Survey, Columbus, OH

      Oil and gas accumulations in Silurian reservoirs in the central Appalachian basin have yet to be conclusively correlated with a source rock.  The Ordovician Utica Shale, which underlies these accumulations and has high total organic carbon values (TOC~1.5-4.0 wt. %), is most commonly cited as the source rock.  However, other source rocks may exist, such as overlying Devonian black shale and Silurian shale and carbonate.

To evaluate the source rock potential of Silurian strata, well cuttings (n=302) and core (n=6) were sampled in NY, OH, PA, and WV and analyzed for their TOC content and RockEval parameters.  The following strata were sampled: the Lower Silurian Cabot Head Shale, Rochester Shale, and Rose Hill Formation; and the Upper Silurian McKenzie Formation, Lockport Dolomite, Wills Creek Formation, Tonoloway Limestone, and Salina Group.

Most samples (n=280) have TOC values that are too low (~0.20-0.49 wt. %) to qualify as a source rock.  However, 23 samples are fair source rocks (TOC =0.50-0.99 wt. %) and 5 samples are good to very good source rocks (TOC=1.00-3.35 wt. %).  The highest TOC values were measured in 150- to 500-ft-thick intervals of anhydritic dolomite and(or) gray limestone with thin black shale interbeds in the Salina Group in northwest Pennsylvania (Butler County) and in the McKenzie and Wills Creek Formations and Salina Group in northern West Virginia (Hancock, Monongalia, and Preston Counties).  We conclude that although local source rocks are present in Silurian strata, they are too lean and too widely dispersed to constitute a major petroleum system.