THE ROCK beneath the North Sea has been a rich source of oil. But even after
30 years of exploration and hundreds of wells drilled, why does only one well in
three find oil or gas? And why鈥攚hen the Northern North Sea has been
producing oil in abundance for more than 20 years鈥攊s it only in the past
year that the rich deep fields in the Central North Sea have begun to produce
anything at all?
The simple answer is that the geology there is so complicated. The stretching
and tearing of the tectonic plate has created an extraordinary jumble of
different faults and rock types. Squeezed between impermeable layers of rock,
the oil and gas can be held at pressures as high as you would find at the bottom
of the Marianas Trench鈥攖he ocean鈥檚 deepest point鈥攁nd at temperatures
of more than 150 degrees Centigrade. Even if you find and tap into such a field,
drilling becomes much harder, since you have to avoid any chance of a blowout.
And if you do produce oil, the sand grains that are mixed in can end up slamming
into the well at high speed. Add to that the difficulty of maintaining offshore
rigs and platforms in a harsh physical environment, and it鈥檚 amazing there are
any wells at all.
But in the past few years new deep fields in the rich central region between
Scotland and Norway have opened up, driven by important technological
developments and a deeper understanding of the complexities of North Sea
geology. At the end of last year, Shell-Esso and BP opened up three new deep oil
fields there, and three more will follow next year, operated by Shell and Elf.
What鈥檚 more, as the search for oil continues around the world, what we鈥檝e
learned from the North Sea鈥攃oupled with a little ingenuity鈥攕hould
make a big difference to finding and exploiting new fields of black gold.
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Oil forms when rocks that are rich in carbon are buried and heated. But
that鈥檚 only part of the story: though it seems thick and viscous, crude oil is
very buoyant, seeping upwards through pores and cracks until it reaches a
鈥渢rap鈥�. Here impermeable rocks block off the of the oil, forcing it to remain in
porous reservoir rock beneath鈥攋ust as an upturned bell jar traps gas. The
simplest traps are found in places where the geology has created large domes of
rock beneath the surface of the seabed. Most of the earliest and most accessible
oilfields are like this鈥攖he ones in Texas, for instance, and the Middle
Eastern fields which are among the largest oilfields ever found.
However, as oil production from some of these early fields declined, and as
high oil prices and the difficult political situation of the early 1970s
demanded oil supplies from stable parts of the world, oil companies began to
turn to harsher environments, such as Alaska, the Northern North Sea and the
shallow fields in the Central North Sea. There, too, production is declining,
and the quest for oil continues in more challenging areas, like the deep fields
of the Central North Sea.
The North Sea contains several 鈥済rabens鈥濃�攂uried valleys bounded by
faults in the rock. Together they make up a large basin formed when movement in
the tectonic plates stretched the whole region between about 200 and 130 million
years ago. Then the stretching stopped in this area, and moved on to the west of
Britain, eventually forming the Atlantic Ocean about 60 million years ago. The
grabens are now buried by sediments that are 4 to 6 kilometres thick.
Around 250 million years ago, the Central North Sea and surrounding area was
a large, shallow, salty sea, which dried out periodically, leaving behind layers
of salt. Tectonic stretching gradually caused the region to subside and at this
stage, the Central North Sea was very similar to the Northern North Sea. Then,
something happened that changed everything.
About 140 million years ago, a new set of faults appeared at a different
angle to the first set, breaking up and displacing the previous simple blocks of
rock in the area, and making the patterns of rock types vastly more
complicated鈥攁nd the oil-bearing traps much harder to detect. What鈥檚 more,
the rocks where the oil is found were sandwiched in between two impermeable
layers of rock: a salt layer below left behind from the shallow sea, and a layer
of shale above. As the rocks were buried to depths of more than 4000 metres,
enormous pressure built up鈥攖rapped as they were between salt, shale and
faults. Because there were no pores and cracks in the rock layers, the pressure
rose to as much as 800 atmospheres.
These very high pressures have important implications for finding oil. In
places where the pressure is high enough, the oil breaks through the overlying
shale and escapes. So you can drill a well into what looks like a very promising
structure, only to find that there鈥檚 nothing there.
All of this makes the Central Graben a daunting place to drill for oil and
gas. But, fortunately, geologists have a formidable tool at their disposal:
seismic waves. Set off a large air gun just below the sea surface and you can
watch how the seismic waves reflect back from the underlying rock, and work out
the location of faults and different types of rock. This much has been possible
for years. But now you can build up a complete 3D picture of the rocks from
closely spaced vertical reflections. In recent years, oil companies have
acquired almost blanket 3D coverage of the Central North Sea. It is probably the
most complete picture of an ancient rift valley anywhere in the world. Using the
Global Positioning Satellite systems for pinpointing the exact positions of the
airgun and the seismic receivers has also improved the quality of the data
tremendously, and faster computers have made the analysis much more
efficient.
High precision
Three-dimensional imaging and virtual reality let geologists visualise the
rocks as never before. Some companies have even installed virtual reality
鈥渃aves鈥� where geologists can wander through the rocks, identifying potential
structures.
These new developments mean that鈥攅ven in the complicated geological
environment of the Central North Sea鈥攊t is now possible to pinpoint where
to drill with much greater accuracy. And each well that is drilled provides more
information on how to interpret the seismic data and locate the oil.
The pressure in the Central North Sea is a more difficult problem to crack,
but there have been hints recently that seismic information could be the key to
that too. In principle, changes in pressure should create changes in the speed
of the seismic waves as they pass through the rock. And sensitive seismic
equipment should be able to detect these changes and allow oil companies to make
accurate maps of the pressure variations. Drilling will be safer and you鈥檒l be
less likely to bore and find your oil has vanished.
Another clue to solving the pressure problem could come from work by Dick
Swarbrick and his team at Durham University. They have been working on the
origin of the abnormally high pressures. One of the ways that high pressures can
be created is the formation of hydrocarbons. So the researchers hope that by
modelling the timing of hydrocarbon generation and how this varies across the
Central Graben, they might be able to predict where pressures are likely to be
less severe.
More clues to help understand the geology of areas like the North Sea come
from looking at modern basins such as the Aegean Sea, the Red Sea and the
western US. Researchers from a number of universities including Edinburgh,
Norwich and Cambridge have been studying these regions for the past 10 years,
and applying models they have developed to older basins such as the Central
North Sea. Instead of being buried under several kilometres of sediment, the
faults and rocks in these modern basins are there for anyone to see. So
geologists can travel round the basins and look at the faults in 3D without
having to rely on seismic waves.
These technological advances and new ideas, developed for the Central North
Sea, can be applied to other complex areas. This means that the success rates of
exploration wells, in the Central North Sea and elsewhere, should improve. What
the North Sea has taught oil companies is that they may have to drill a lot of
wells, and spend a lot of money, before they find outwhat is really going on and
make the oil discoveries pay.