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Preludes to the Entrapment of Organic Matter in the Sediments of Lake Maracaibo1

By
Alfred C. Redfield
Alfred C. Redfield
Woods Hole, Massachusetts
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Published:
January 01, 1958

Abstract

Sedimentation has continued in the Maracaibo Basin since the formation of its earliest oil-bearing strata. The present lake is reputed to be one of the most productive waters in the world. Perhaps no more favorable place can be found for examining the conditions presumed to precede the accumulation of organic matter in sedimentary deposits.

The lake appears to have been fresh in the recent past. Sea water gained access to the basin when the rising sea level drowned its approaches to a present depth of 60 feet. Since then marine sediments have greatly reduced the sill depth. Escaping fresh water limits the entrance of sea water, which occurs only during three months of the dry season.

The lake water contains about one part in thirty of sea water, uniformly distributed by an active wind-driven circulation. In the deeper basin, with depths of 100 feet, the salt content is substantially higher. The oxygen content of this "deep water" is reduced and over a large area of bottom is anaerobic.

The fauna of the lake on superficial observation is limited in variety and numbers. Marine forms are restricted to species adapted to life at very low salinities. The significant bulk of organic matter is produced by fresh-water species of planktonic algae. These at times produce prominent surface blooms.

The potential fertility of the lake water is revealed by its phosphorous content. The total phosphorous is about 1.4 microgram atoms per liter in the mixed layers above the "deep water." This is more than commonly found in surface waters of the sea, but is less than that of the deep water of the Pacific and Indian Oceans. The phosphorus, and presumably other nutrients, are derived from land drainage and are accumulated in the lake by evaporation and by the sinking of organisms into the deep water.

One third of the phosphorus is present in inorganic form available for plant nutrition. Consequently, growth is not limited by its availability in the photic zone. In the "deep water" the phosphorus content is manifoldly increased by the sinking of organisms. The oxygen available is insufficient to oxidize this accumulation, and conditions unusually favorable for the entrapment of organic matter in sediments result.

Sedimentation is active in the lake, particularly in the southwest quarter, where the Catatumbo River is building an extensive delta. Its water discolors the lake for a great distance along the shore. The lake bottom consists of soft blue mud which, in the anaerobic region, is black for several inches below the surface and smells strongly of H2S. The mud is sufficiently unconsolidated to permit a test pile to sink 30 feet under its own weight.

It is concluded that Lake Maracaibo is an unusually favorable place to study the processes which lead to the entrapment and the subsequent transformation of organic matter in sediments.

"Petroleum in its natural occurrence is closely associated with marine or semi-marine nearshore sediments. The relation is practically universal and has persisted since Cambrian time, very probably without material alteration. The study of the depositional zones of present-day seas and oceans therefore appears essential to an understanding of oil occurrence, particularly in its initial stages." Eugene Stebinger (1950).

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Contents

AAPG Special Publication

Habitat of Oil

Lewis G. Weeks
Lewis G. Weeks
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American Association of Petroleum Geologists
ISBN electronic:
9781629812434
Publication date:
January 01, 1958

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