Cement distribution in a carbonate reservoir: recognition of a palaeo oil

Hamilton et al. (1992) made a detailed study of illite K–
Ar ages in the Brent Group, and concluded that illite ages
are related to oil filling. Ages from the water-leg of the
reservoir were thought to immediately post-date filling.
Hence, we use K–Ar ages of illite from Cormorant as
probable indicators of the cessation of field filling and use
the single oil-leg age (this study) as an indication of time of
oil-filling (60.3 ^ 2.0 Ma). Using the burial history of
Hamilton et al. (1992) implies oil emplacement at maximum
temperatures of 70–80 8C. This is also the maximum
temperature at which biodegradation can occur, to form the
bitumen observed in thin-sections. Fig. 9 shows that
temperatures of 70–80 8C are consistent with kaolin
recrystallisation from Jurassic meteoric porewaters
(d18O ¼ 25 to 27‰ V-SMOW), and fit well with data
from siderite (Wilkinson et al., 2000) and quartz overgrowths
(Tables 3 and 4). The porefluid evolution shown is
comparable to that of Haszeldine et al. (1992), which was
proposed for the Dunlin, Thistle and Murchison fields.
These fields lie some 30–40 km to the east of Cormorant
IV. Note that the present-day porewater in Cormorant IV
has considerably heavier d18O values than Jurassic meteoric
water. Haszeldine et al. (1992) proposed that the East
Shetland Basin had a zoned paleo-hydrogeology, with
active meteoric water recharge down to a depth corresponding
to a temperature of 80–90 8C. Below this were static
‘basinal’ porewaters with d18O of greater than 0‰ SMOW.
Our results support this model, and imply that the first
hydrocarbon charge would have been emplaced within the
actively recharging, meteoric water aquifer, portion of the
basin. This might be expected to lead to the formation of
bitumen, due either to water washing, or to bio-degradation
of the oil due to the low ambient temperatures (Lomando,
1992). Bitumen is observed within thin-sections, even
within the present-day water-leg, and is especially prominent
within secondary pores developed in potassium
feldspars.