Tropical lakes, copropel, and oil shale

GSA Bulletin



During a long-continued study of the lacustrine beds of the Eocene Green River Formation, I have tried to interpret past events from observation of present-day processes. After a search of some 40 years, four lakes have been found that are producing a kind of organic ooze judged to be a modern analogue of the precursors of rich oil shale. Two of the lakes are in central Africa and two are in Florida. All four are shallow. The ooze in all four is predominantly algal, entirely in the form of minute fecal pellets, and does not decay in warm, wet, oxidizing environments.

Several of the most unusual, mummified microorganisms found in the oil shale of the Green River are illustrated as testament to the inference that the Eocene organic oozes also were resistant to decay.

Studies to determine why these algal oozes do not decay are in progress, but as yet no satisfactory explanation is available. The ooze from Mud Lake, Florida, contains very few living bacteria but a great many bacterial spores, suggesting some active inhibitor. Gentle and slow anaerobic decay takes place in the ooze 1 foot or more below the mud-water interface.

The algal ooze accumulates slowly. That at a depth of 3 feet below the mud-water interface has a C14 age of 2280 ± 200 years. If compacted, this 3-foot layer would amount to a layer only about 0.5 inch thick.

The air-dried algal mud (from Mud Lake) looks much like oil shale and has a C-H ratio essentially like that of the organic matter in oil shale. The oxygen of the mud, however, is roughly 5 times as high as in oil shale. The calorific value of dried Mud Lake algal ooze is about 6600 cal/g, whereas the organic matter from Green River oil shale averages about 9500 cal/g.

Analyses show that the dried algal ooze from Mud Lake contains small quantities of higher fatty acids (C12–C34), with C16 being dominant. It also contains some unsaturated fatty acids and about 0.3 per cent of n-alkanes, predominantly odd carbons, with C29 being dominant. A few qualitative analyses show that carotenoid pigments and terpenes are also present.

A possible source of hydrocarbons is the long branched side chain of the chlorophyll molecule. Small crustaceans liberate this in the form of phytol, which, by dehydration, can go over into a series of saturated and unsaturated hydrocarbons, including phytanc and pristane, both of which are common in Green River oil shale. Search for other precursors of hydrocarbons continues.

Publication type Article
Publication Subtype Journal Article
Title Tropical lakes, copropel, and oil shale
Series title GSA Bulletin
DOI 10.1130/0016-7606(1966)77[1333:TLCAOS]2.0.CO;2
Volume 77
Issue 12
Year Published 1966
Language English
Publisher Geological Society of America
Description 5 p.
First page 1333
Last page 1337
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