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Significance of Discoloration in Arenaceous Foraminifera

ROSEN, RASHEL N., ARCO Oil and Gas Company, Houston, TX

Many Gulf Coast biostratigraphers have observed the discoloration of arenaceous foraminifera in the subsurface Tertiary clastics of the Gulf of Mexico. This paper is a preliminary attempt to explain some possible reasons for the discoloration as well as to document formally the phenomenon. The initial interpretation was that discoloration is the result of increasing depth of burial and temperatures and is caused by progressive and irreversible fixation of carbon derived from the catagenesis of the tectin of agglutinated foraminifers and organic matter from the surrounding sediments. However, pyrolysis analyses from four wells indicate the sediments are thermally immature.

Discoloration in arenaceous foraminifera ranges from amber to amber-brown, to brown, to gray, to black, and finally to bleached white. Organically poor sediments contain amber or normal sandy-color arenaceous foraminifers, while organically rich sediments contain progressively darker color forms. The bleached opaque white forms, often also pitted, appear to be indicative of a paucity of organic matter in deeper and more heated sediments; it is speculated that the compound causing discoloration in blackened foraminifers may be driven out or volatilized at higher temperatures, giving rise to the bleached white color. Some genera of arenaceous foraminifers (i.e., those with labyrinthic internal structure) appear to be more sensitive to the color change than others. Minor color changes ar also exhibited by some calcareous forms.

Although the color changes are occurring in sediments immature for hydrocarbon generation, it is suggested that (1) heating is affecting organic matter so that some type of coloration is absorbed by the tectin from the organic matter of the surrounding sediment, (2) tectin proteins and polysaccharides may be far more temperature sensitive than plant materials, and/or (3) discoloration indicates the passage of hydrocarbons through the sediments.

A similar process has also been noted in several other microfossil groups but generally at higher temperatures and certainly over longer periods of time.

It is very important to differentiate between the organically blackened sediments and those colored by the oxidation of diffuse pyrite, as most of the producing intervals of the Gulf of Mexico do contain some pyrite.

 

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)