Gary G. Lash

Dept. of Geosciences, SUNY Fredonia, Fredonia, NY 14063, [email protected]

Vitrinite reflectance (%R_o) is perhaps the most widely cited index of thermal maturity in a source rock. However, there is increasing evidence that the %R_o of some hydrogen-rich source rocks is suppressed. Suppression of vitrinite is most readily recognized by anomalously low %R_o values on maturity profiles. It has been described from some Devonian-Mississippian black shales of the Appalachian Basin by comparison of measured %R_o with that of overlying Pennsylvanian coals as well as by burial/thermal models constrained by maturity indices other than %R_o. Preliminary analysis of Devonian black shales of western New York state and northwest Pennsylvania suggests that these rocks, too, have been affected by vitrinite suppression. Measured %Ro of relatively close-spaced samples collected from the base of the Upper Devonian Rhinestreet and Dunkirk shales in western New York varies irregularly from 0.51 to 0.77. Moreover, comparison of the Rock-Eval hydrogen index (HI) and measured %R_o for > 50 Devonian black shale (principally Marcellus shale) samples reveals a trend similar to that of the New Albany shale, which has been affected by vitrinite suppression. Specifically, a reduction of HI from > 400 mg/g to ~ 125 mg/g is accompanied by little change in %R_o. At HI ~ 125 mg/g, however, %Ro increases sharply from ~ 0.75 to >2.5 as HI drops to near zero. These results suggest that vitrinite was suppressed in the lower or mid oil window maturities in the Marcellus and younger black shale units in this region of the Appalachian Basin, a factor that should be taken into account in exploration strategies.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York