oil shale terminology

Courtesy of :
http://www.oilandgasevaluationreport.com/tags/toc/

What is the hydrocarbon potential of a shale? To answer this question you first have to determine if the shale contains sufficient amount of organic matter, and second, has the shale been subject to the geologic processes needed to convert organic material to oil and/or gas?

What is the total organic carbon (TOC) present in the shale? TOC is essentially the total amount of organic matter (kerogen) in a given sample of rock. Most shale plays have a TOC greater than 3%. TOC alone is only a start in evaluating a shale play.

Rock-Eval Pyrolisis evaluates the type and the maturity of the organic matter as well as determining its petroleum potential. Rock-Eval reveals the amount of generated oil and gas in the shale sample (S1), the amount of oil and gas generated through thermal breakdown of organic matter by heating (S2), the amount of CO2 released during pyrolysis(S3), and the temperature of maximum release of hydrocarbons (Tmax). Tmax indicates the stage of maturation. From this data the origin of the organic matter can be determined by the hydrogen index (HI). The oxygen index (OI) measures the oxygen richness of the sample. The production index (PI) is the ratio of generated hydrocarbons to potential hydrocarbons. Low ratios indicate immature or postmature organic matter.

Vitrinite reflectance (VF) is used for determining the thermal maturity of the shale. Vintrinite reflectance is sensitive to temperatures that correlate to hydrocarbon generation (60 to 120 degrees C). Generally, onset of oil generation in an oil-prone shale is a VF of 0.6% and a VF of >1.35%, while the onset of generation in a gas-prone shale is a VF of 0.8% and a VF of >2%.

Harry Dembicki Jr. in his article "Three common source rock evaluation errors made by geologists during prospect or play appraisals" (AAPG Bulletin, v.93, no.3 (March 2009), pp. 341-356) discusses the pitfalls of relying on only one aspect of petroleum geochemistry. Mr. Dembicki points out that it is necessary to "fully integrate TOC and Rock-Eval data with pyrolysis-gas chromatography, and using burial history diagrams to help interpret vitrinite reflectance."

Remember the petroleum geochemistry of any shale in not homogeneous vertically or horizontally in a particular unit. This heterogeneity must be taken into account when evaluating a play before and after production has been commenced. In addition, the shale must have adequate volume. That is it must be sufficiently thick and have an areal extent to generate producible hydrocarbons.