A fundamental reappraisal of the structure of the Cormorant Field and its impact on field development strategy

It has been hypothesised that oil charging prevents
diagenesis within the oil-saturated portions of the reservoir,
or at least dramatically slows the rates of mineral reactions
(Bloch, Lander, & Bonnell, 2002; Worden, Smalley, &
Oxtoby, 1998). This has been demonstrated for individual
reservoirs, where careful measurement of cement volumes,
and modelling of thermal histories, enables the influence of
oil emplacement to be quantified (Deighton, 1996; Marchand,
Haszeldine, Smalley, Macaulay, & Fallick, 2001).
The influence of early hydrocarbon charging on reservoir
quality is still controversial (Marchand, Smalley, Haszeldine,
& Fallick, 2002). The theoretical link between oil and
diagenesis is simple: the diagenetic reactions all take place
in the water phase within the rock, and chemical species
must diffuse through the water, and/or advect, from mineral
to mineral. In an oil-filled reservoir, the residual water has a
complex 3D shape with tortuous flow paths. This restricts
the movement of the chemical species (which cannot travel
through the oil), and hence slows the reactions. Even in a
water-wet reservoir (where the grain surfaces are covered in
a thin film of water), the pathways available for diffusion
between adjacent mineral grains are tortuous, so slowing
diffusion, and the effective water permeability can be very
low (Honarpour, Koederitz, & Harvey, 1986) so slowing
advection. Thus early oil charge can prevent the mineral
reactions that lead to chemical compaction, the growth of
mineral cements during burial that occlude porosity, and
the diagenetic dissolution of feldspar and other minerals
during deep burial that generate secondary porosity
(Wilkinson, Darby, Haszeldine, & Couples, 1997). Consequently,
early oil charge can preserve high porosities to
unusual depths of burial (Bloch et al., 2002; Heasley,
Worden, & Hendry, 2000).
In this study, diagenetic evidence is described from the
Cormorant Field of the East Shetland Basin of the Northern
North Sea (Fig. 1). From the sequence of clay minerals, and
isotopic ratios, we deduce that there was an early oil charge
which leaked off, and that subsequently the reservoir refilled
with hydrocarbon. Previous work has inferred only a
single phase of oil filling, relatively late in the field history,
corresponding to peak oil generation (Taylor & Dietvorst,
1991). The Cormorant Field is an approximately north–
south oriented horst structure (Demyttenaere, Sluijk, &
Bentley, 1993), the petroleum geology of which is described
by Taylor and Dietvorst (1991). There are four major oil
pools, the largest of which is termed Cormorant IV. This is a
down-faulted anticline to the east of the main horst, with a
very complex faulted structure (Demyttenaere et al., 1993).
The Brent Group in Cormorant IV is juxtaposed with the
Permo-Triassic sediments of the main Cormorant structure,
locally called the Cormorant Group (Demyttenaere et al.,
1993). The reservoir within Cormorant IV is the Brent
Group, which has been well described in the literature as a
regressive–transgressive marginal marine-deltaic clastic