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Suppression of Vitrinite Reflectance

Within the oil window, the reflectance of vitrinite deposited in reducing marine or lacustrine environments is often lower (eg. 0.15-0.55%Ro) compared to vitrinite deposited in more oxic environments. This can be due to (1) the incorporation of lipids into the vitrinite structure, (2) to the formation of vitrinite-like macerals from algae, marine grasses, etc. (3) to the impregnation of generated bitumen or incorporation of migrated oil, or (4) to several other causes (Mukhopadhyay, 1994). If lipid reflectance suppression is not recognized, the reported vitrinite reflectance maturities will be too low and a mature source rock may be identified as immature. The problem can be recognized in a maturity profile of a well by anomalously low 'maturities' of oil source rocks that have generated oil compared to the section above and below, but is often overlooked if only the oil source rocks themselves were analyzed. Fortunately, lipid-rich vitrinite fluoresces and can easily be identified if the time is taken to analyze the samples properly. Lipid-rich, reflectance suppressed vitrinite is very common but is not always recognized due to microscopist inexperience or to failure to use fluorescence when doing reflectance work.

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Several research studies have sought to calibrate suppressed vitrinite reflectance to normal vitrinite reflectance (Mukhopadhyay, 1994). H.B. Lo of Exxon, based partially on microscopy work done by DGSI, related the degree of reflectance suppression to the hydrogen index of oil source rock and non source rock shales (Lo, H.B.,1993, Organic Geochemistry, Vol. 20, No.6, p. 653-657). He found that the degree of suppression is related both to the hydrogen index of the immature source rock and to the maturity. (Click here for the figure 432x301) Reflectance suppression increases with oil source rock quality and is greatest at mid oil window maturities. DGSI experience has shown this model to be very reliable although some modifications have been made since Dr. Los' original work was published.

The following "horror story" is an example of what can happen if reflectance suppression is not recognized. A company had a large number of wells on its concession analyzed with vitrinite reflectance but a maturity map of the oil source rock didn't fit the distribution of oil occurrences: large oil fields were often associated with "immature" source rocks. The explanation given was that "somehow" oil must have been generated "early" from the source rocks in the

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basin studied. The oils, however, were mature by every geochemical technique available (eg. gc, gc/ms, etc.), and basin geology showed that migration from more mature oil source rocks outside the concession area could not have taken place. One dimensional basin modeling was attempted on several wells but the reported maturities could not be derived from any reasonable modeling permutation: they were always too low. DGSI was asked to re-analyze some of the wells. The result was obvious: the calculated vitrinite reflectance maturities were too low in wells where the oil source rock was in the oil window because they were derived from fluorescing, lipid-rich vitrinite. Corrections were made and a new source rock maturity map was prepared which fit the distribution of oil production and oil shows almost perfectly. And best of all, it led to a new discovery that would not have been found if the original source rock maturity map had been relied upon.

The lesson here is obvious: If it's worth doing, it's worth doing right. Poor quality data can be worse that no data at all and you (almost always) get what you pay for. Keep this in mind when comparing price.



Suppression of Vitrinite Reflectance

Petroleum Alteration

Kerogen Maturity

Kerogen Types


Inorganic Compounds in Natural Gas

Solid Bitumens

What Is a Source Rock?

Depth of Dry Gas Preservation



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