Print this pageVitrinite Suppression in Devonian Black Shale, Western New York State: Preliminary Results
Gary G. Lash
Dept. of Geosciences, SUNY Fredonia, Fredonia, NY 14063, [email protected]
Vitrinite reflectance (%Ro) is perhaps the most widely cited 
index
of thermal maturity in a source rock. However, there is increasing evidence that
the %Ro of some hydrogen-rich source rocks is suppressed. Suppression
of vitrinite is most readily recognized by anomalously low %Ro values
on maturity profiles. It has been described from some Devonian-Mississippian
black shales of the Appalachian Basin by comparison of measured %Ro
with that of overlying Pennsylvanian coals as well as by burial/thermal models
constrained by maturity indices other than %Ro. 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 %Ro 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
%Ro. 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