Time as Factor in Organic Metamorphism and Use of Vitrinite Reflectance as an Absolute Paleogeothermometer: ABSTRACT

Abstract

The only evidence that time is a factor in organic metamorphism lies in the works of Karweil, Lopatin, and Connan. All three attributed the high degree of organic diagenesis in geologically older areas to today's presently low geothermal gradients End_Page 619------------------------------ and long burial (cooking) times. Yet the heat flows in all areas had been much higher in the geologic past due to volcanism, igneous intrusion, orogeny, metamorphism, and/or uplift and erosion. Mean random vitrinite reflectance (Ro) is an indicator of organic metamorphism. A plot of Ro versus present temperature from a number of areas that have not undergone major geologic mutilation, increases in tight (R = 0.97) linear fashion. Yet burial times for these different areas range from 0.3 to 240 m.y. These same data, when plotted against increasing burial time at constant temperature, do not show the expected trend of increasing Ro values with increasing burial time. Vitrinite reflectance data from a geothermal (rift valley) area with a maximum heating age of 10,000 years, directly overlie the preceding plot, which suggests the time needed for full organic maturation is 10,000 to 300,000 years, a geologic instant. Geochemical data from deep (up to 9 km), high-temperature (up to 300°C) wells having long burial times (up to 240 m.y.), suggest that some geochemical postulates are in error and that time has little effect on organic maturation. It appears that vitrinite reflectance can be used as an absolute paleogeothermometer from 20° to at least 400 + 20°C. End_of_Article - Last_Page 620------------