Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A.

Michael Kruge, John F. Hubert, David F. Bensley, John C. Crelling, R. Jay Akes, Paul E. Meriney

Research output: Contribution to journalArticle

20 Citations (Scopus)

Abstract

Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hanford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray silstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C20 to at least C41, and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly lessextractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the manual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks.

Original languageEnglish
Pages (from-to)689-701
Number of pages13
JournalOrganic Geochemistry
Volume16
Issue number4-6
DOIs
StatePublished - 1 Jan 1990

Fingerprint

organic geochemistry
Geochemistry
black shale
Shale
siltstone
Jurassic
Lakes
basin
asphalt
Water
lake
corrensite
Calcium Sulfate
analcime
vitrinite reflectance
Petroleum
Biomarkers
bitumen
Fractionation
Alkalinity

Keywords

  • Connecticut
  • Jurassic
  • alginite
  • biological markers
  • lacustrine depositional environment
  • migration of petroleum
  • rift basins
  • tricyclic terpanes

Cite this

Kruge, Michael ; Hubert, John F. ; Bensley, David F. ; Crelling, John C. ; Akes, R. Jay ; Meriney, Paul E. / Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A. In: Organic Geochemistry. 1990 ; Vol. 16, No. 4-6. pp. 689-701.
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Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A. / Kruge, Michael; Hubert, John F.; Bensley, David F.; Crelling, John C.; Akes, R. Jay; Meriney, Paul E.

In: Organic Geochemistry, Vol. 16, No. 4-6, 01.01.1990, p. 689-701.

Research output: Contribution to journalArticle

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T1 - Organic geochemistry of a lower jurassic synrift lacustrine sequence, Hartford Basin, Connecticut, U.S.A.

AU - Kruge, Michael

AU - Hubert, John F.

AU - Bensley, David F.

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AU - Akes, R. Jay

AU - Meriney, Paul E.

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N2 - Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hanford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray silstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C20 to at least C41, and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly lessextractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the manual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks.

AB - Synrift terrestrial strata of the Lower Jurassic East Berlin Formation (Hanford basin, Connecticut, U.S.A.) record cyclical expansion and contraction of major lakes, six of which were deep enough to develop anoxic bottom waters. We have studied one representative lacustrine sequence in detail, sampling a new roadcut near the village of East Berlin. The section examined is 4 m thick, with a gray silstone at the base, deposited in shallow water, overlain by an organic-rich black shale (deep water), succeeded in turn by another gray siltstone, deposited as the lake waters gradually receded. The upper gray siltstone is chemically distinct from the lower siltstone, as it contains small amounts of corrensite, analcime and gypsum, reflecting the increasing salinity and alkalinity of the contracting lake. The samples in the center of the black shale unit contain laminae of thermally-altered, yellowish orange-fluorescing, mottled telalginite. The fluorescence properties indicate a peak oil generation maturity level, confirmed by a vitrinite reflectance of 1.13% and a Methylphenanthrene Index of 1.08. The other samples have less organic matter, becoming increasingly lean towards the top and bottom of the sequence. Samples in the middle of the black shale unit are distinguished by the presence of an homologous series of tricyclic terpanes extending from C20 to at least C41, and by the near absence of hopanes and steranes. Moving upsection into the gray siltstone, the samples contain markedly lessextractable organic material (EOM) and the concentration of tricyclic terpanes relative to hopanes steadily decreases. In the uppermost sample, hopanes are the predominant terpanes. Moving downsection from the black shale into the lower gray siltstone, EOM and the ratio of tricyclic terpanes also decrease, except in the lowermost samples, which contain terpane distributions like those of the middle part of the black shale. This likely is migrated material, because bitumen fills microfractures and extensive megafractures. The lack of hopanes and steranes in the black shales cannot simply be a maturation effect, as these biomarkers appear in the adjacent beds. Instead, the manual terpane distributions may indicate a changing depositional environment, documenting the geochemical evolution of the lake. Or, more likely, they may result from fractionation during expulsion of petroleum from these mature source rocks.

KW - Connecticut

KW - Jurassic

KW - alginite

KW - biological markers

KW - lacustrine depositional environment

KW - migration of petroleum

KW - rift basins

KW - tricyclic terpanes

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