Suppression of Vitrinite Reflectance
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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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).
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.
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 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.
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.