Hydrocarbon filling history from diagenetic evidence: Brent Group, UK North Sea

Could the kaolin d18O-depth correlation be due simply to
the ambient geothermal gradient at the time of kaolin
recrystallisation? Calculation shows that if the entire
variation in d18O were due to temperature change, then a
geothermal gradient of ca. 180 8C/km would be implied. A
high geothermal gradient ‘event’ within the East Shetland
Basin can be dismissed conclusively. The present-day
geothermal gradient is only 30–35 8C/km (unpublished
sub-surface temperature measurements, Shell Expro UK).
There is no evidence of any igneous activity within the area,
and no previous reports of anomalously high isotopic
mineral ‘temperatures,’ fluid inclusion values, or vitrinitereflectance
data. As the East Shetland Basin has been
exhaustively explored for its hydrocarbon resources, it is
unlikely that such evidence could have been missed entirely.
Any igneous intrusion underlying the Cormorant reservoir
would probably have been revealed by seismic imaging of
the oil-bearing structures. Moreover, the offset in the d18Odepth
correlation at ca. 3015 m could not be easily
explained by the geothermal gradient model. We hence
reject the possibility that the d18O-depth correlation for
kaolin was due to an enhanced geothermal gradient at the
time of kaolin recrystallisation.