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.

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.

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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