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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 69 (1985)

Issue: 2. (February)

First Page: 285

Last Page: 285

Title: Burial Thermal Histories, Vitrinite Reflectance, and Laumontite Isograd: ABSTRACT

Author(s): Thane H. McCulloh, J. J. Fan

Article Type: Meeting abstract


The optical reflectance of vitrinite has become the standard basis for quantitative judgments of integrated temperature-time (burial) histories. Inferences about the crystallization temperature of the calcium zeolite laumontite also have been used repeatedly for such purposes. In a few cases, these 2 approaches have been combined or their results compared. As generally employed, neither approach has quantitative validity.

Factors other than temperature and time play roles in the way that burial history affects vitrinite reflectance (Ro). In particular, the organic geochemical environment exerts a strong and variable local-to-regional influence on the rate of increase of Ro versus temperature. Hydrocarbon-rich environments retard the rate of Ro increase; hydrocarbon-deficient environments accelerate it. Local (interbed) Ro divergencies up to 0.5% result, and regional (interbasin) divergencies are equal or possibly greater. Much of the scatter in compilation plots of Ro versus TTI may result from such divergencies.

Laumontite, where it can crystallize at all, precipitates according to specific stringent requirements of fluid pressure and temperature. The crystallization temperature at the laumontite isograd ranges from 32°C (1 atm) to 193°C (Pf = 1,325 atm). The crystallization rate is geologically instantaneous, completely unlike the time-dependent organic reactions responsible for the progressive aromatization of coal macerals during kerogen maturation, providing an instantaneous pressure-sensitive maximum-recording "thermometer." Paleotemperatures to constrain interpretations of Ro data may be one of the greatest values to be gained from studies of diagenetic laumontite.

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