--> ABSTRACT: Extended Tricyclic Terpanes: Molecular Markers in Lower Jurassic Synrift Lacustrine Black Mudstones of the Hartford Basin, Connecticut, by Michael A. Kruge, John F. Hubert, Jay Akes, and Paul Meriney; #91022 (1989)

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Extended Tricyclic Terpanes: Molecular Markers in Lower Jurassic Synrift Lacustrine Black Mudstones of the Hartford Basin, Connecticut

Michael A. Kruge, John F. Hubert, Jay Akes, Paul Meriney

The East Berlin Formation (lowest Jurassic, Hartford basin, Connecticut) is distinctive for its six cyclic black and gray lacustrine mudstone units. The black mudstones are each about a meter thick and were deposited in subtropical, thermally stratified, oligomictic lakes, the youngest of which (lakes 3 through 6) were large enough to flood most of the basin and attained water depths of several tens of meters.

From a fresh roadcut 8 km north of Middletown, organic-rich black mudstones of lakes 3, 4 and 6 were analyzed for biological markers. In each case, the saturate fraction predominates, making up 69-75% of the total solvent extract. The n-alkanes in turn dominate the saturate fraction, with a maximum at C20 and extending to at least C37. There are also extended homologous series of alkycyclohexanes, alkylbenzenes, and branched-chain alkanes. The methylphenanthrene indices of these samples range from 0.99 to 1.12, indicating middle to late oil window maturity levels. The absence of aromatic steroids, except for the C20 and C21 triaromatics, confirms the high maturity.

The most striking feature of these samples is the presence of a series of extended tricyclic terpanes from C20 to at least C46. Hopanes are not detectable in the mudstones of lakes 4 and 6. In lake 3, hopanes are present, but only in about the same concentrations as the high-molecular-weight tricyclics.

The samples are depleted in hopanes in part because of the elevated maturity level. There is no evidence for biodegradation. The tricyclics are present because they are more resistant to thermal stress and are also apparently neoformed by thermal alteration of kerogen and resins. In addition, the original organic matter may have been exceptionally rich in tricyclic terpane precursors, i.e., fossil lipids of the archaebacteria favored by these specific depositional environments. Tricyclic terpanes can thus provide an important correlation tool for mature lacustrine source rocks and oils.

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