THE INFLUENCE OF BASEMENT STRUCTURES ON ORDOVICIAN AND DEVONIAN BLACK SHALE DEPOSITION AND POST-DEPOSITION IN THE NORTHERN APPALACHIAN BASIN

The stratigraphy of the northern Appalachian basin includes six thick, potentially economic black shales: the Ordovician Utica Fm. and the middle to late Devonian Marcellus, Geneseo, Middlesex, Rhinestreet and Dunkirk formations. Traditional models show the regional maximum black shale thickness successively steps farther west with the development of a gentle, structurally inactive clinoform. However, in the northern region of the Appalachian Foreland Basin, our field and well studies in New York and Pennsylvania in the 1990s indicated Clarendon-Linden Fault System (CLF) depositional control on the Devonian black shales and in the 2000s we have continued detailed analysis of well logs to expand the data base to over 6000 wells in NYS and PA. These data demonstrate that many of the areas of thickest black shale deposition and sharp variations in shale thickness coincide with faulting. These faults commonly extend upsection as reactivated basement-affected structures, and include northerly-trending fault systems (e.g., CLF) and the arcuate (in map view) Iapetan-opening structures.

Thicker deposits of both the Marcellus (~61 m/ 200 ft) and Utica formations (~183 m/600 ft) correspond closely with Iapetan-opening structures, whereas other potential black shale reservoirs display a combination of Iapetan-opening structures and obliquely trending basement structures. Thick accumulations of the Geneseo Fm (~55 m/180 ft) follow Iapetan-opening structures in Pennsylvania, but trend north-south in New York along the CLF. In addition to structural influences on regional depositional trends, we have observed localized accumulation of black shales in areas of syndepositional fault, readily apparent in the minor, thin black shale beds of the Pipe Creek and Hume formations, but also present in the major Devonian black shales.

Post-depositionally, basement structures can also be shown to influence thermal maturities in the Northern Appalachian Basin. For example, a northwestern deflection displayed in Devonian vitrinite reflectance contouring (Repetski et al., 2002) coincides with the Tyrone-Mt Union Lineament-a series of NW-trending faults. The general pattern of Upper Devonian oil and gas sandstone reservoirs is consistent with such patterns of underlying source rock thermal maturity.