1. INTRODUCTION
Course objectives
This short course in Applied Reservoir Geology aims at giving the students some basic skills
in the understanding of the geology of hydrocarbon reservoirs. Furthermore, it is
demonstrated how these basic skills and this reservoir understanding can be applied to the
planning of new wells, and optimize the recovery of oil.
The course is focused on the Statfjord Field, which is in the tail-end phase of production, and
where geology has significant influence on the oil recovery work processes.
The course curriculum (pensum) includes the following material : 1) the following text and
accompanying figures ; 2) course lectures (most, if not all, of the lecture material is included
below) ; 3) core workshop and excursion material (will be handed out at the excursion /
workshop day, March 29th 2004.
Definitions, principles
Oil and gas fields are located on every continent, with the largest fields occurring in super
provinces found in North America, Saudi Arabia and Russia (Fig. 1.1). Other significant oil
provinces not appearing on this map have also been discovered since 1980.
Petroleum occurrences and associated oil fields are not only well spread out geographically,
they also appear in a range of different tectonic settings and sedimentary basins, such as
intracratonic basins (ie. Russia), pericratonic basins (ie. eastern margin of North America), rift
basins like pull-aparts and troughs (ie. North Sea), and other types of basins (Fig. 1.2).
Within a sedimentary basin, hydrocarbons need not only be generated from a source rock, but
they must also migrate within permeable layers, and become trapped and sealed. There are
several different trapping mechanisms possible, and which lead to petroleum accumulations
that we call gas and/or oil fields. Figure 1.3 shows the different types of hydrocarbon traps
existing. Most traps are structural in the form of domes / anticlines and tilted fault blocks, but
some are also stratigraphic (ie. sandstone pinch-outs). Traps combining both mechanisms are
called structural-stratigraphic traps. Within a field, individual stratigraphic units that contain
hydrocarbons are often referred to as reservoirs.
Case examples
The Western Canadian / Williston Basin is an intracratonic foreland basin centered on the
Alberta Province of Canada (Fig. 1.4). Both limestones and sandstones form the producing
reservoirs, in structural and stratigraphic traps.
The North Sea is another mature and well-developed petroleum province. As seen in Figure
1.5, this basin is a faulted rift basin, with individual fields being located in tilted structural
blocks. The main trapping mechanism in the North Sea is structural, and most accumulations
are in sandstone reservoirs, with limestone reservoirs occurring in addition in the Southern
North Sea area (ie. Ekofisk). Figure 1.6 shows the British fields in the Tampen area, west of
the Statfjord Field. Basically all fields are associated with major faults, and this is also the
case in the Norwegian sector of the North Sea (east of the fields shown in Fig. 1.6).
Statfjord Field
The Statfjord Field is located some150km northwest of Bergen, halfway between Norway and
Great Britain (Fig. 1.7). The field was discovered in 1974, production started in 1979, and to
now more than 300 wells have been drilled, largely producers (Fig. 1.8). Approximately 625
million standard cubic meters of oil (Fig. 1.9) has been recovered to date (3.9 billion barrels).
Gas is also being produced, with much of it being re-injected to provide pressure support and
maintain voidage. A fair amount is also exported, and minor volumes are used for fuel and
flare (Fig. 1.10).
As seen in Figure 1.11, the Statfjord Field was on plateau production between 1985 and 1995,
and is now producing 4-5 times less than what it once was (tail-end production). The
hydrocarbons are concentrated within three separate reservoirs: the Brent Group, the Cook
Formation (of the Dunlin Group), and the Statfjord Formation (Fig. 1.12). In addition to the
so-called Main Field reservoirs, hydrocarbons are also being recovered from a slumped area
above and to the east of these reservoirs (East Flank area). Of the 1 000 million Sm3 oil
originally in place, 625 million Sm3 have been produced, and only 35 million Sm3 are
believed to be recoverable, giving a recovery factor approaching 70% at the end of
production. This remaining recoverable oil is present is isolated “pockets” both upflanks (attic
oil, East Flank oil, undrained Cook) and downflanks (bypassed Brent oil, Statfjord Fm wedge
zone), as illustrated in Figure 1.13.
The Brent Group / Dunlin Group / Statfjord Formation were deposited soon after the break-up
of Pangea, at a time when the North Atlantic was not yet an open ocean system like it is today
(Fig. 1.14). At the end of the Triassic and Early Jurassic, the Statfjord Formation was
deposited in mainly alluvial (river) environments. Figure 1.15 shows this river system (look
for the star symbol). New research shows that the rivers didn’t flow northwards towards the
Arctic Ocean, but southwards towards the Tethys Ocean (was to later become the Indian
Ocean). Later in the Early Jurassic and in the early part of the Middle Jurassic (Fig. 1.16),
rifting created a sea between Norway and Greenland, and the Dunlin Group was deposited
largely in deep marine environments (except for the Cook Formation, which is a tidaldominated, shallow marine environment). Late in the Middle Jurassic, a large area between
Denmark and England (called the Mid North Sea dome) was uplifted, and the Brent delta was
initiated and prograded northwards (Fig. 1.17). Significant amounts of sands were deposited
in alluvial, deltaic, and shoreface environments over much of today’s North Sea area. During
the Late Jurassic, a new period of rifting and faulting led to the rapid southwards
retrogradation of the Brent delta, and segmentation of the North Sea basement into the rotated
fault block geometries that we observe today on seismic. In addition, deep marine shale
deposition was also established in the Late Jurassic, leading to the formation of the
Kimmeridge Clay, which is the main source rock in the North Sea (Fig. 1.18).