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Microscopic Measurement of the Level of Catagenesis of Solid Organic Matter in Sedimentary Rocks to Aid Exploration for Petroleum and to Determine Former Burial Temperatures—A Review

By
Neely H. Bostick
Neely H. Bostick
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Published:
January 01, 1979

Abstract

Dispersed solid organic matter occurs as a minor constituent in most sedimentary rocks. It consists of diverse materials that are like the macerals in coals (though usually in different proportions than in normal coals), and its maturation is chemically and physically like coalification. Reflected-light microscopy enables one to recognize the different organic grains and to select the best type for optical measurement to indicate the indigenous maturation of organic matter in the rock. The organic constituent selected should have the following characters. It (1) is virgin when deposited with the sediment, (2) matures regularly, (3) is not subject to retrograde alteration, (4) resists reaction with adjacent fluids and solids, (5) is not significantly affected by pressure, (6) occurs widely in rocks of diverse lithology and facies, (7) is distinguishable from pre-altered and redeposited material, (8) can be analyzed separately, (9) persists through a broad range of catagenesis and metamorphism, and (10) has properties that can be analyzed throughout the alteration range by a relatively inexpensive technique.

Vitrinite grains that are not recycled from previous rocks satisfy the above requirements, and reflectance is normally the property measured. Data from experimental studies in the laboratory and from a number of sedimentary basins show that, for the most part, temperature and duration of heating determine the progress of catagenesis.

Regional and vertical studies of organic catagenesis indicate a correlation between rank of solid organic matter and occurrence of oil and gas—even though the fluids migrate extensively. Oil is generated first at about 0.5% vitrinite reflectance (oil immersion) and occurs last, associated with gas condensate, at about 1.3%. Above that rank, abundant methane can be generated from types of kerogen that do not yield oil. Petroleum occurrence is limited in much of the eastern United States because the sedimentary rocks have been too hot in the past (mostly as a result of former deep burial). The regions that are favorable or unfavorable for exploration for petroleum or gas, from the point of view of level of organic maturation, are indicated on a map. On the continental shelf, especially where sediments are less than two kilometers thick, it is most important to determine whether burial temperature has been adequate for petroleum formation.

Determination of actual past temperatures is not required for correlation with petroleum occurrence. Measured level of organic catagenesis can be used, however, to estimate actual former temperatures from our knowledge of the general time-temperature-rank function of the reactions and from geologic information on the burial history of the rocks in question.

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Contents

SEPM Special Publication

Aspects of Diagenesis

Peter A. Scholle
Peter A. Scholle
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Paul R. Schluger
Paul R. Schluger
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SEPM Society for Sedimentary Geology
Volume
26
ISBN electronic:
9781565761568
Publication date:
January 01, 1979

GeoRef

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