--> ABSTRACT: Deposition, Diagenesis, and Maturation of Organic Matter in Rift-Basin Lacustrine Shales of Triassic-Jurassic Newark Supergroup, by L. M. Pratt; #91022 (1989)
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Deposition, Diagenesis, and Previous HitMaturationTop of Organic Matter in Rift-Basin Lacustrine Shales of Triassic-Jurassic Newark Supergroup

L. M. Pratt

Evaluating the petroleum source potential of rift basins in frontier areas is difficult because rift lakes vary greatly in areal extent, water depth, water chemistry, and longevity. In addition, climatic cycles are often strongly expressed in lacustrine settings. Triassic-Jurassic deposits in the continental rift basins on the Atlantic margin of North America are one of the few cases where lateral and vertical variations in the content (Corg, hydrogen index) and thermal maturity (Tmax, Ro) of bulk organic matter are well documented. Relatively little is known, however, about sulfur content (Sorg, Stotal) and the composition of biomarkers in these strata. New data on low-maturity shales (Ro < 0.4%) in the Jurassic portion of the Hartford basin show wide variation in Stotal/Corg ratios, suggesting that paleolimnological conditions ranged from fresh to alkaline or saline. Stratigraphic profiles of Corg and Stotal through individual shale beds indicate rapid and repeated changes in water chemistry during each lacustrine cycle. Bitumens extracted from samples with less than about 0.05% Stotal are characterized by dominance or high abundance of carotanes and extended regular isoprenoids in the saturated hydrocarbons. These biomarkers are probably derived from carotenoid pigments of algae or bacteria. Early diagenesis in the absence of hydrogen sulfide may account for the unusual preservation of extended isoprenoids and carotanes in the bit men.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.