GEOLOGIC TIME AS A PARAMETER IN ORGANIC METAMORPHISM AND VITRINITE REFLECTANCE AS AN ABSOLUTE PALEOGEOTHERMOMETER

Abstract

Substantial published and unpublished data contradict accepted laws of organic metamorphism. Significant amounts of C15+ hydrocarbons and kerogen with significant generation potential exist in sediments which have been buried at high temperatures for long periods of geologic time. Graphite formation and greenschist metamorphism do not occur at model‐predicted burial times and temperatures. Maturation levels for sediments with long (up to 240 million years) burial times are much lower than expected, and conversely, higher than expected for sediments with short (2 million years) burial times. The threshold of intense petroleum generation is not detectable in Plio‐Miocene sediments at minimum temperatures of 160° C.Literature examination reveals that the only solid documentation of time as a controlling parameter in organic metamorphism is in the works of Karweil (1955), Lopatin (1971) and Connan (1974). Scrutiny of these authors' study areas reveals that all have had moderate to extreme paleogeothermal gradients from incipient and aborted rifting, extrusive and intrusive igneous activity, hydrothermal activity, orogenesis and/or uplift and erosion. Levels of organic metamorphism attributed to geologic time by the above authors are better explained by high paleogeothermal gradients resulting from major geologic activity.Mean vitrinite reflectance (RO) data have been compiled from different sedimentary basins currently at or near maximum geothermal gradients with sediment burial times ranging from 2–240 million years. The RO data from these basins show a strong correlation (r = 0.97) of increase in R, with increase in temperature. These same data show no correlation of increasing Ro with increasing burial times for any temperature interval. R, data from sedimentary basins affected by high‐temperature geothermal systems with heating times of only 1,000–10,000 years overlie R, data from these petroleum‐bearing sedimentary basins with burial (heating) times of 2–240 million years.Contrary to accepted thought, there is no evidence from the natural system that petroleum generation‐maturation reactions have first‐order reaction kinetics. By contrast, a substantial body of evidence from petroleum generation‐maturation laboratory studies carried out in closed water‐ wet systems show that these reactions are characterized by multiple‐order reaction kinetics.Geologically older sediments clearly tend to be more organically mature than younger sediments at the same burial temperatures. However, this correlation ismorea reflection that older sediments have had a greater chance of being affected by a major geologic event with an accompanying high heat flow. The major conclusions of this paper are that: (1) geologic time has no observable effect on organic metamorphism; (2) vitrinite reflectance can be used as an absolute paleogeothermorneter; and (3) the accepted models of organic metamorphism at the very least need substantial revision.