Lacustrine Source Rock Potential, Culpeper Basin, Virginia

Michael A. Smith, Coleman R. Robison

Sediments of the Waterfall formation were deposited during a 5-m.y. period of the Early Jurassic in an open, oligomictic marl lake in the western part of the Culpeper basin. These black shales exhibit good organic richness with an average organic-carbon content of nearly 2% and average Rock-Eval pyrolysis yield (S₁ + S₂) of more than 7 mg HC/g rock. The kerogen is typical of nonmarine source rocks, consisting of equal amounts of fluorescent amorphous sapropel and palynomorphs with less abundant vitrinitic and inertinitic material, and it appears to be predominantly Type I and II. Vitrinite reflectance and other maturity indicators suggest that the Waterfall formation is slightly undermature in outcrop. Lacustrine facies of the underlying Late Triassi Bull Run Shale and Early Jurassic Buckland formation are also marginally immature and are more organically lean and gas prone than the Waterfall shale.

Lake deposits in the Culpeper basin are much like those in other eastern U.S. early Mesozoic basins. The Culpeper lake, located at paleolatitude 12°N, covered approximately the same area as present-day Lake Turkana in the Eastern Rift Valley of Africa and had a similar water depth of about 35 m. The varied and abundant population of spores and pollen indicate a humid, tropical climate for the Culpeper basin during the Early Jurassic. Good preservation of organic material was promoted by the relatively high sedimentation rate and anoxic lake bottom water; with an adequate thermal history, the graben would produce a high-wax crude oil from these sediments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.