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September/October 2009
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A dispersed vitrinite fragment of 1.2 percent reflectance is located in a potential gas shale reservoir rock that is part of the Lower Cretaceous Pearsall Formation in south Texas.

Reflectance of Vitrinite

Measuring the reflectance of vitrinite dispersed in sedimentary rocks is one of the most commonly used tools for determining thermal maturity in hydrocarbon exploration, basin modeling and other geological applications. Despite this, there currently is not a consensus standard for making these measurements.

ASTM International Committee D05 on Coal and Coke is developing a proposed standard to cover this issue, WK24192, Test Method for Microscopical Determination of the Reflectance of Vitrinite Dispersed in Sedimentary Rocks. The proposed standard is under the jurisdiction of Subcommittee D05.28 on Petrographic Analysis of Coal and Coke.

The proposed new standard is being developed by a task group that includes experts from both the International Committee for Coal and Organic Petrology and the Society for Organic Petrology.

Paul Hackley, a research geologist, U.S. Geological Survey, and the chair of Subcommittee D05.28, notes that vitrinite is the coalified remains of woody plant material, which is found in nearly all sedimentary rocks formed since the evolution of vascular land plants about 400 million years ago. The vitrinite reflectance changes systematically with the amount of burial and heating that the rock experiences.

“Therefore, vitrinite reflectance can be used as a thermometer by which to infer the thermal maturity of the rocks it is contained in,” says Hackley. “This has very important practical implications in hydrocarbon exploration as economic accumulations usually are found only within certain ranges of thermal maturity, often called oil and gas windows.”

According to Hackley, petrographers can generally measure similar values of dispersed vitrinite reflectance in interlaboratory exercises, but disparities exist and a consensus standard will improve agreement by providing a common methodology and a set of guidelines for analysts to follow.

All interested parties who have expertise in the measurement of reflectance of dispersed vitrinite are encouraged to participate in the ongoing development of WK24192.

CONTACT

Technical Information: Paul Hackley, U.S. Geological Survey, Reston, Va.

Phone: 703-648-6458

ASTM Staff: Scott Orthey

Phone: 610-832-9730