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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 69 (1985)

Issue: 2. (February)

First Page: 298

Last Page: 298

Title: Examples, Causes, and Consequences of Vitrinite Reflectance Suppression in Hydrogen-Rich Organic Matter--a Major Unrecognized Problem: ABSTRACT

Author(s): Leigh C. Price

Article Type: Meeting abstract


Vitrinite reflectance (Ro) is regarded as one of the most powerful tools available to petroleum organic geochemistry. A major limitation of this method is the severe suppression of Ro by significant exinite maceral concentrations (hydrogen-rich types I and II kerogen) in association with vitrinite macerals. This effect is not subtle, as Ro in hydrogen-rich organic matter (OM) is suppressed at least 3-5 times from what the value would be in oxygen-rich OM (type III kerogen), and the effect extends to at least Ro = 4.0. The effect has been attributed to the migration of early generated bitumen from hydrogen-rich OM into the associated vitrinite macerals with the bitumen retarding maturation of vitrinite macerals. However, this exp anation for Ro suppression is inadequate in many cases.

Suppression of Ro is due more likely to 2 factors: (1) anaerobic conditions at deposition and diagenesis when much greater amounts of hydrogen than "normal" are incorporated into the vitrinite macerals, and (2) hydrogen-rich OM requiring significantly higher burial temperatures to attain the same maturation rank as oxygen-rich OM. Thus, all maturation indices, including Ro and the threshold of intense hydrocarbon generation (TIHG), are suppressed in hydrogen-rich OM compared to oxygen-rich OM buried under the same conditions.

Ro values are primarily derived from exinite-rich sediments, leading to the establishment of the Ro value of 0.6 (± 0.1) for the TIHG in hydrogen-rich OM. Far higher Ro values are read in oxygen-rich OM at the same regional rank for the TIHG in hydrogen-rich OM. The "oil deadline" has been defined as occurring at Ro = 1.35. There is a sharp decrease in the maximal values of the hydrocarbon coefficient (mg HC/g OC) at Ro = 0.9 to the very low values at Ro = 1.35 in type III OM. This decrease of the hydrocarbon coefficient, previously assumed to result from the thermal destruction of C15+ hydrocarbons by carbon-carbon bond breakage, is actually due to a loss of C15+ hydrocarbons by intense prima y petroleum migration by gaseous solution. This general lack of recognition of Ro suppression and the necessity of higher burial temperatures to attain the same maturation rank in hydrogen-rich OM compared to oxygen-rich OM has led to a miscalibration of the regional ranks necessary for significant petroleum generation from hydrogen-rich OM and the oil deadline. Examples from the Los Angeles and Williston basins as well as other areas demonstrate these problems.

The consequences have staggering implications to petroleum exploration and to basinal and worldwide petroleum resource estimates.

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