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
Neely H. Bostick, 1979. "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", Aspects of Diagenesis, Peter A. Scholle, Paul R. Schluger
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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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There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.