HOW WRONG CAN YOU
BE
in interpreting VR data?
VR data from different laboratories vary appreciably
in quality and reliability. Experience suggests that attribution
of modes - e.g. caved, indigenous and re-worked, solely from the
data is often erroneous. VR data produced on the basis of identifying
the indigenous vitrinite population on petrographic grounds provides
the most reliable data.
The results on the right are from a recent study
of an offshore well. VR data supplied by a client (open squares)
and new VR data generated for our report (red), are plotted against
depth. Also shown are ranges of equivalent VR levels derived from
AFTA in two samples from this well. The solid line shows the VR
profile predicted by the "Default History", i.e. the profile
expected if samples throughout the section are currently at their
maximum temperature since deposition. There is a clear mis-match
between the two VR datasets. While the original data suggest that
all units throughout the well are currently at their maximum temperatures,
the new VR data and the AFTA results suggest that most units have
been hotter in the past. |
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In the full specification of the originally supplied
vitrinite reflectance results, data were assigned to various populations
by the analysts. Mean values of different populations within each
sample are plotted here against depth. The solid line again shows
the VR profile predicted by the "Default History". Also
shown in this plot are equivalent VR values derived from SCI and
Tmax values (both also from the original analyses). These equivalent
VR values are consistently higher than the measured population assigned
to the indigenous population, but very consistent with the population
assigned to "re-worked" vitrinite. This suggests that
values originally designated as the indigenous population are too
low and that of the original dataset the VR values originally designated
as "reworked", together with the SCI and Tmax values,
provide the most reliable indication of true maturity levels in
this well. |
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In this plot, the new VR data are shown in red,
while "re-worked" vitrinite reflectance values from the
well operator's original maturity study are shown as black squares.
Equivalent VR values derived from SCI data and Tmax data (both also
taken from the operator's original maturity study) are shown in
blue and green, respectively. Also shown are ranges of equivalent
VR levels derived from AFTA in the two samples from this well. There
is clearly a high degree of consistency between all these values
through the well, which confirms that these data provide the most
reliable representation of true maturity levels in this well. The
solid line again shows the VR profile predicted by the "Default
History". All values plot well above the profile, suggesting
that units throughout the well have been hotter in the past. Synthesis
of all these data shows that maturity levels in this well are significantly
higher than previously thought. |
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