Geochemistry of Organic Matter-Rich Mesozoic Lacustrine Formations of Eastern United States

C. C. Walters, R. K. Kotra

During the initial extension of Laurasia, a series of rift basins formed along the eastern coast of North America. Cyclic lacustrine sedimentary sequences are found in the exposed basins, which extend from Nova Scotia to North Carolina. These lacustrine deposits, collectively known as the Newark Supergroup, consist of transgressive-regressive cycles. Muds (now shales) rich in organic matter were deposited during highstands of the lakes. We have conducted a detailed geochemical examination of lacustrine units from the Hartford, Newark, and Culpeper basins in order to determine their depositional environment, hydrocarbon source potential, and thermal history. Correlations between migrated bitumens and produced oils from the Richmond and Sanford basins were also made.

In the moderately mature Lower Jurassic units from the Newark basin, total organic carbon typically exceeds 1% and can reach 4% in the dark shales. Hydrogen indices exceed 400, and visual kerogen studies indicate an abundance of well-preserved type I kerogen in the dark shales. Many samples contain terrigenous spores that have been severely degraded by microbes. High abundance of carbonaceous residues were also noted in highly thermally altered (R_o > 2.0) Triassic rocks. We conclude that black shales with excellent potential for hydrocarbon generation were widely deposited throughout the East Coast rift system during the highstands of most of the ancient lakes. Although the shales are thin, their frequency and high potential yields suggest that the lacustrine sediments could have generated significant oil and gas accumulations.

Analyses of bitumens show that considerable variations in organic facies exist both regionally across a cycle and between sequential cycles. Sedimentation, though highly repetitive, was dynamically variable geographically and temporally in depositional environment and organic material input. Considerable variations are noted in the abundance and distribution of gammacerane ß-carotane, triterpanes, and other biological marker compounds.

Various thermal parameters derived from biomarker compounds indicate that maturation occurred under conditions of high heat flow characteristic of active rifting. However, superimposed on regional trends are anomalous variations in thermal maturation. We attribute them to increased heat flow associated with igneous events and migration of hot hydrothermal fluids. These fluids may also have played a significant role in the migration and accumulation of hydrocarbons and metals within the basins.

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