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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 441

Last Page: 441

Title: Vitrinite Reflectance--What, How, and Why?: ABSTRACT

Author(s): Wallace G. Dow, Dolores I. O'Connor

Article Type: Meeting abstract

Abstract:

Vitrinite reflectance is a proven technique for determining the thermal evolution of sedimentary organic matter. The reflectance power of the coal maceral, vitrinite, increases with depth of burial in response to increasing temperature and time. Because most coals and sedimentary kerogens contain vitrinite, the technique has universal application in determining rank or degree of catagenesis.

Kerogens are concentrated by acid leaching, then are mounted in epoxy and polished, and the degree of reflectance (Ro) is determined with a reflecting-light microscope. Most kerogens contain mixtures of primary and recycled materials commonly with various contamination products such as drill-bit cavings and mud additives. Vitrinite reflectivity can also be affected by chemical or physical weathering, bitumen or pyrite inclusions, natural coke and, in low-rank rocks, by variations in various vitrinite subgroups. Semifusinite, pseudovitrinite, and some solid bitumens are often misidentified as vitrinite. All of these variables must be dealt with before the maturity (average Ro) of a sample can be determined.

A sequence of Ro values in a well or exposed sedimentary section results in a maturation gradient, the slope of which depends on the geothermal gradient and the sedimentation rate. Because both reflectance increase and oil and gas generation are time- and temperature-dependent chemical reactions, maturation gradients based on vitrinite reflectance can be used to determine whether hydrocarbons have been generated in source beds or preserved in reservoirs. Maturation gradients can also be used to recognize major faults and unconformities and to estimate the amount of section lost, proximity to igneous activity, the rank of recycled material when deposited, geothermal history, and other features useful in understanding the geologic history of basins. If properly measured and i terpreted, vitrinite reflectance can be a powerful exploration tool.

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Copyright 1997 American Association of Petroleum Geologists