G E U S
F R A
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G E O L O G I
Oil and gas exploration in the Nor th Sea:
OUR UNQUENCHABLE THIRST
FOR ENERGY
THE ABC’S OF OIL
THE ABC’S OF THE NORTH SEA
“PLAYS” AND RESOURCES IN THE
NORTH SEA
N R . 2 & 3
O K T O B E R
1 9 9 6
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The natural background and our
unquenchable thirst for energy
Kai Sørensen
25
*1
SE: Sustainable energy
NG: Natural gas
We in the western world have developed
an unrealistic attitude towards energy.
Our entire culture is based on accessible
and inexpensive energy. There is, however, a price to be paid. Combustion of
fossil fuels, particularly oil and coal produce atmospheric pollution and CO2.
Today, no one can afford to close their
eyes to the consequences.
*2
20
SE
NG
Oil
10
T.O.E.*10
6
15
5
0
50
55
60
65
70
75
2
80
85
90
94
Year
While there has been a general trend of
steadily increasing consumption since
WW II, the consumption curve fell twice
during the 1970’s (see figure 1: *1 and *2).
This was the result of the creation of OPEC together with OPEC’s aim to use oil as
an economic weapon. Most people are familiar with these events.
However, few are aware that after a steep
increase in prices immediately following
the formation of OPEC, much to the delight of oil companies and oilproducing coKuwait*
untries, the spiIran*
ralling prices
Iraq*
quickly
lost
Abu Dhabi*
momentum,
Saudi Arabia*
and in the 10
Venezuela*
years since the
Libya*
huge drop in
Nigeria*
prices at the
end of 1986,
Russia
prices actually
North Sea
fell. The expla0
10000
20000
30000
40000
nation for this
Million tons
phenomenom
* OPEC - member country
can be found in
Figure 2.A column chart depicting OPEC’s oil reserves compared to the North Sea re- the North Sea
serves. (Gas is not included). Source: BP Statistical Review of World Energy, June 1995) and Alaska. The
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Politicians attempt to alter our energy
consumption habits, with both whips and
carrots, but the results are unimpressive.
The majority of people are not prepared
to pay the price, a higher cost of fossil fuels or a change in our lifestyle. Our
consumption of fossil fuels can thus be
used as an indicator of our true will to
change our existing habits to become more environmentally conscious. In Denmark, as in other western countries, the figures for fossil fuel consumption speak for
themselves: our consumption of oil, coal
and gas has increased year by year since
the Second World War (WW II).
Coal
Figure 1.The curve depicts the Danish energy
consumption since WW II, calculated in tons of oil
equivalents t.o.e).
period during the 1970’s when the crisis
was becoming serious for non-OPEC countries coincided with increasing fossil fuel production in the North Sea and Alaska.
During the 1980’s production from these
two regions was so large that OPEC’s weapon was virtually ineffective.
In the future, however, our ability to hold
oil-producing countries and oil prices in
check may be limited, as can be observed
from figure 2 which depicts the remaining,
known oil reserves in the North Sea compared to those of the Opec countries and
Russia.
The development and expansion of the
huge North Sea oil and gas production in
the course of a decade is the subject of
this theme issue. It has a lot to do with
geology. Bear in mind that geology is “the
heart of the matter” in the world’s largest
industry.
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The ABC’s of Oil
Depth in km
oil
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Under natural conditions (figure 3) hydrocarbons in a reservoir are present in the following three ways:as a liquid capped by gas;
as a liquid; or as a gas.Liquids which are produced to the surface separate into three
components: oil, gas and water.There can be
large quantities of gas dissolved in the oil under the conditions which exist in the reservoir.When the gas in a reservoir is extracted to a surface,several kilometres above the
reservoir, some of the gas will condense to
a liquid referred to as condensate which
resembles gasoline/petrol. Gas formed from
carbon-rich source rocks, for example coal,
contains only small amounts of higher
hydrocarbons and produces only a small
amount of condensate when produced to
the surface.This is referred to as “dry gas”.
Gas which is formed in marine mudstone
(shale) is rich in higher hydrocarbons, and
large quantities of condensate are produced
at the surface. Such a gas is described as being “wet” or “rich”.Oil which is formed from
the same type of source contains a large quantity of dissolved gas (light oil).The Danish
North Sea fields contain both wet gas and
light oil.The southern part of the North Sea
contains only dry gas.
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Nature has now done its work – transforming the organic material into hydrocarbons, causing the oil to migrate and creating
a suitable reservoir to act as a trap for the
oil or gas.The rest is up to the geologist who
must find the reservoir. Once the reservoir
is discovered it is referred to as a discovery.
When production starts of a discovery, it is
then referred to as a field.
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Rocks in which oil and gas form are called
source rocks, and the sites where organic
material is transformed into hydrocarbons
are referred to as “kitchens”. The pore
spaces which are found between the sedimentary rock’s mineral particles are normally filled with groundwater.The oil and/or
gas migrate from a kitchen up into the overlying rocks by displacing this water. If the
pore spaces in the overlying rock are large,
numerous, (a porous rock type) and evenly
distributed (the rock is permeable, meaning
that fluids can flow easily through the pore
spaces/fractures) then it is possible for
hydrocarbons to accumulate, and they can
be produced (pumped or piped) to the surface. A porous and permeable rock type
where oil or gas accumulates is called a reservoir. Hydrocarbons can be trapped here
if the natural geometric configuration of the
reservoir is suitable and if the reservoir has
been sealed by an impermeable formation
called the cap rock so that the hydrocarbons cannot migrate further.
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gas
“Window" for the formation of
Oil and gas are hydrocarComposition in the well stream at the surface
bons, formed through an
Gas
anoxic transformation of
Condensate
algae and more developOil
ed plants, which, through
Water
0
fortunate circumstances,
have been preserved in
the underground’s sedi1
mentary deposits. If the
conditions are suitable,
2
if temperatures are sufficiently high and large
3
Migration route
quantities of organic material are available, first
4
oil and thereafter gas can
Sandstone
be produced. Gas can
reservoir
Marine shale
also form directly from
5
(oil source rock)
carbon-rich organic material. These transforma6
Coal (gas source rock)
tions are complicated
An oil kitchen located
Oil + condensate kitchen
Gas condensate kitchen
processes, for example:
over a dry gas kitchen
the most hydrogen-rich
organisms (algae) are
transformed into oil, the
product with the fewest hydrogen atoms (a
Figure 3.The complexity of a hydrocarbon system. Both the source rocks and the reservoirs are found at various depths.The migration route from the kitchen to a trap (or reservoir) can be short or long. Permutations
general atomic formula of CH2), while the
in the geologic configurations and hydrocarbon-water compositions presented here result in a complicated fimost carbon-rich material (land plants)
eld situation, both in terms of finding and exploiting hydrocarbons.
forms gas, the most hydrogen-rich final product (atomic formula CH4).
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The ABC’s of the North Sea
180 MaBP
It is not possible to describe the geologic
history of the North Sea briefly. Instead the
ten main events (in a petroleum geology
sense of the word) will be summarised in
the following section. The numbers in
parenthesis (1) in the text refer to the
pictograms in figure 4.
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In the early Permian period, northwest
Europe was a hot, barren landscape like the
Sahara today.Ephemeral lakes which formed
during periods of precipitation turned
saline. Briefly stated: it was a harsh environment. There was, however, desert sand (2)
which was rounded and well-sorted and
which today is preserved in the sub-surface
of the southern part of the North Sea .This
desert sand covered the remains of the
Carboniferous fern-forests (1) preserving
them as a record of more humid times and
as a reminder of the changeability of the
earth’s climate.
The early Permian landscape was later inundated by the sea. In northwestern Europe
an inland sea formed and with the continuing warm and dry climate it became hypersaline and salt precipitates formed (3).
During the end of the Permian period,
almost 1 kilometer of salt was deposited in
the central part of this basin. Coal deposits
later formed gas, which migrated upwards
and accumulated in the desert sand which
was sealed by the overlying salt deposit.This
combination of circumstances, a source
rock, a reservoir and a seal or impermeable
cap rock is called a “play”.This term will be
useful to our understanding of the geological history as well as the hydrocarbon
exploration history in the North Sea.
The saline Permian basin was connected to
the ocean.There was an ocean north of
present day Scandinavia and Greenland (remember, that the Atlantic ocean did not
exist at this time) and an ocean south of
Europe called the Tethys Sea.This sea is
another important key in understanding
the geology of the North Sea, because during much of the Mesozoic this sea inundated the North Sea area, bringing marine
sediments with it.The Permian period ended with a continent-continent collision
which formed the Ural mountain chain, and
4
165 MaBP
60 MaBP
Present
in front of this, a dry alluvial plain
towards the North.“Redbeds” were
deposited in this landscape.To the
south, marine carbonates were deposited in the shallow shelf waters of
the Tethys Sea.Throughout the Triassic period this sea sometimes inundated the land, time followed by a retreat over northwest Europe.
At the end of the Triassic period a shelf sea
spread over most of northwest Europe and
it was first in the middle of the Jurassic
period that land masses (where the North
sea is today) emerged from this sea in connection with a short volcanic pulse. Sand in
the form of huge delta complexes (4) was
deposited along the coasts of this land
which stretched from present-day Bornholm-Skåne northwestward to the Shetland
Isles. The delta plains spread seaward, only
to be inundated again by the sea.This Middle Jurassic sand is the most important reservoir for North Sea oil, particularly in the
Norwegian and English parts of the
northern North Sea.
During the Upper Jurassic period large
parts of the North Sea were again below
sea level.The region under the middle of the
North Sea became a fault zone (5) forming
a deep depression in an extensive shelf sea.
This fault zone, also called a rift zone in
analogy with the rift valley systems which
exist today in, for example east Africa, had
three main rift branches (figure 8):
• The Viking Graben
• The Central Graben
• The Moray Firth
There were, however, a large number of individual faults which played an important
role in the formation of hydrocarbon traps
of the Jurassic reservoirs, as well as indirectly forming younger traps.At the end of
the Jurassic period conditions changed in
the sea covering the three rift branches and
in the deepest, or most isolated parts mud
with a high content of marine algae was deposited. Today this mudstone (shale) is the
dominant oil source rock (6) and is known
in England as the Kimmeridge Clay. The
most important “plays” in the North Sea
north of a line from Esbjerg to Hull (figure
8) have this source rock in common.
During the Upper Jurassic period there was
rift activity, and deposits which would later
become excellent source rock were laid
down. For these two reasons the Upper Jurassic period stands out as an important time in hydrocarbon history.The story, however, is not finished. Sand, which later would
become sandstone reservoir rock (7) was
also deposited during the Upper Jurassic period. After formation of the large central
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Norwegian-Danish basin
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2
3
Norway
4
A
5
50 km
B
Tertiary
Upper Jurassic
Triassic
Cretaceous
Lower Jurassic
Upper Permian
Reflection time in seconds
0
England
In northwestern Europe this chalk deposition was suppressed during the transition
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tion of oil and gas in hundreds of established
discoveries.The fact that the eastern part of
Denmark (figure 5) did not subside during
the Neogene is the geological explanation
for the lack of success in hydrocarbon exploration in the Danish region east of the
central North Sea.With the use of models it
is possible to relatively accurately “date” the
formation of oil in the North Sea. A large
percentage of all the oil which has been found was formed in the last 10 million years.
The number of discoveries in the North
Sea (over 700) are not just the result of fortunate coincidences regarding the formation
of source rock, reservoirs and traps.The explanation lies in the fact that the North Sea
is still actively subsiding.This means that oil
and gas continue to form, also today.At the
same time, the ability of the traps to retain
hydrocarbons increases due to the continually increasing pressure. It is the history of
the Neogene which determines whether
exploration of sedimentary basins in the
North Sea area will be a success or a fiasco.
Neogene subsidence creates favorable conditions for the formation and trapping of
hydrocarbons while Neogene uplift does
the opposite. The Norwegian-Danish basin
is a fiasco of the latter type.
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During the Upper Jurassic and Lower
Cretaceous period the transport of eroded
material to the North Sea gradually decreased and finally stopped completely during
the Lower Cretaceous. At the end of the
Cretaceous period the Tethys Sea shelf extended northward,and carbonates were deposited over the majority of this shelf. In
Denmark, these deposits are referred to as
“skrivekridt” or white chalk. Parts of this
chalk series can have reservoir characteristics (8).
from the Cretaceous to the Tertiary because the initial spreading of the ocean floor in
the North Atlantic created new land along
the coasts of this newly formed ocean.The
most noticeable land mass was created near
the present-day British Islands. Large
quantities of erosional material derived
from this new land mass were washed out
into the North Sea where they formed the
North Sea’s classic reservoir: sand from the
early parts of the Tertiary (Paleocene and
Eocene). This reservoir (9) was one of the
first to be found during hydrocarbon exploration in the North Sea. Particularly the
Scandinavian peninsula, and perhaps the
distant Carpathians and the Alps supplied
huge quantities of erosional material to the
North Sea at the end of the Tertiary.Today,
these deposits are kilometer-thick and contain large quantities of sand, but because
there is no shales to seal these sands they
have no commercial interest. Nevertheless,
this late Tertiary (Neogene) history plays a
crucial role for the oil geology in the North
Sea. Subsidence in this late part of the
Tertiary brought the North Sea’s source
rocks down to depths where oil and gas
could form in large quantities (10). This
steady subsidence which continued during
the Quarternary and which still occurs
today has resulted in an effective preserva-
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land mass (where the North Sea is today)
during the Middle Jurassic,the sea inundated
the entire North Sea area. However, due to
simultaneous tectonic activity, the depositional conditions during the Upper Jurassic
were quite varied.There was a varied landscape in the area close to the present day
North Sea with land areas located close to
shallow ocean areas which were wide or
narrow. Marine sand, which later became
marine sandstone was deposited in these
“shelves”. Occasionally, sand was “pumped”
out of these shallow regions into deeper
basin regions by turbidites as a result of
tectonic activity.
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Figure 5. Profile through the Central Basin and the Norwegian-Danish Basin, based on a seismic profile.
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Plays and Resources in the North Sea
Neogene
Million years
0
10
Eocene
50
Paleogene
TERTIARY
Paleocene
Upper
9
CRETACEOUS
Lower
100
8
JURASSIC
6
Middle
150
5
Lower
Upper
7
4
Upper
Zechstein
N M
TRIASSIC
Ø
200
250
Lower
PERMIAN
300
CARBONIFEROUS
Upper
2
1
Figure 4. Episodes in the North Sea’s oil geology.
Details are found in the text.
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It’s ideas that find oil
A “play”, as defined previously, is a set of
circumstances which can lead to the formation of an extractable deposit of oil or
6
gas in the underground.A play may be speculative, meaning that it has not yet been
proven by way of a discovery, or it may be
established.The term play can also be understood as a number of prospects in which the “the set of circumstances” are assumed to be fullfilled.Thus the word play can
be used in two ways: to refer to something
concrete – a number of discoveries and/or
prospects; or something abstract – a set
of circumstances/geological conditions.
Think of Plato and his distinction between
the idea of a chair and all chairs! Once this
concept of duplicity is understood, then it
is also possible to understand the central
dictum in oil exploration: “It’s the ideas
that find oil”.
The North Sea’s exploration history can
be described with the help of half a dozen
plays, five of which contain more than 95%
of the hydrocarbon resources (figure 6).
The following discussion will concentrate
on these five plays and the regions
(“fairways”) where the “set of circumstances” of a play are met.
Since 1964, approximately 2,700 exploration wells have been drilled in the North
Sea, resulting in 800 discoveries. The
extractable oil and gas resources in these
discoveries (reserves), calculated in oil
equivalents, are equal to 100 billion barrels
(b.o.e.). Approximately 6 barrels equal 1
cubic meter. These resources are produced at a steadily increasing rate. Each year
approximately 3 billion b.o.e. of oil and gas
are produced at the surface, clearly surpassing the rate at which new resources
can be located.
The North Sea can be divided into two separate provinces, a southern province
called the “Carboniferous Gas Province”
and a northern province which will be referred to here as the “Jurassic Rift Province”. The carboniferous coal in the southern province has been a source of a large number of gas discoveries including the
region’s largest, the Dutch Groningen field.
In the northern province, which was affected by tectonic activity in the Jurassic
period, the Upper Jurassic Kimmeridge
Clay is a source of both oil and gas.
The Carboniferous Gas Province:
The Permian Play
All the plays in the Carboniferous Gas
Province in the southern North Sea and
Holland have the common characteristic
that gas was formed from Carboniferous
coal. The North Sea’s exploration history
really starts in the early 1960’s with the recognition of the immense size of the Groningen gas field and the possibility that this
play’s fairway extended out into the North
Sea. At the same time technological
advances were making oil exploration and
production at sea possible. Off-shore production of hydrocarbons was undertaken
first in the relatively protected Maracaibo
Lake in Venezula and then in the Gulf of
Mexico. Thus, hydrocarbon exploration in
the North Sea was only feasible due to the
technological advances which evolved
first in the Gulf of Mexico, and later, as initial exploration in the North Sea was
fruitful, also there. Today the challenges
associated with exploration and production of oil from the North Sea are some of
the driving forces behind technological
developments within the oil industry.
Although there are potentially more reservoirs in the southern gas province, the
Early Permian sandstone contains the
majority of the resources due to the following: its thickness (up to 300 m); its ideal
reservoir characteristics (highly porous
and permeable); and the reservoir’s
effective seal by overlying impervious
Permian salt. Although there are good
sandstones from the Triassic age in the
southern North Sea, these have been cut
off from gas migration from the Carboniferous coal by the overlying Permian salt.
The southern North Sea’s dominant play is
a Lower Permian sand with dry gas from
the Carboniferous coal which is effectively sealed by salt.This salt cap rock is so effective that gas fields in certain areas of the
southern North Sea have remained intact
during periods with significant uplift.A large part of the gas reserves in this play are
produced by a consortium composed of
Shell and Exxon, whereas BP and other
companies first became aware of this play
after the largest discoveries had been made. In the exploration history of a play, the
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largest discoveries will be made early on,
and thus it is important for oil companies
to constantly be aware of any new developments. More than half of the approximately 4,000 billion m3 of gas which has
been found in the Carboniferous Gas Province was located in the Groningen field,
and more than 95% of the gas reserves accumulated in early Permian sandstone (referred to locally as Rotliegend). Discoveries are still made in the Rotliegend sandstone, often in smaller structures which
were on the exploration “waiting list”. Exploration of new plays continues, with some success, but the gas reserves in the
Carboniferous Gas Province are declining
markedly. In the European market these
declining supplies are replaced with gas
from the northern North Sea’s Jurassic
Rift Province where the Norwegian gas fields are important.
The Jurassic Rift Province
Initial exploration in the North Sea led to
the belief that it was a gas province. This
belief sheds light on the statement “I will
drink all the oil there is in the North Sea”
– which was attributed to a number of
people including the director for N.G. U. in
Norway and BP’s head geologist in
England. The statement was decidedly
apocryphal.
A few years after the initial exploration, at
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Sum
S E A
Play
UK
N
Paleogene
1037
313
-
-
1350
97
61
826
320
-
1207
38
Upper Jurassic
1034
1468
-
-
2502
124
Middle Jurassic
1814
2237
40
-
4091
119
Permian
825
-
-
3699
4524
294
Other
353
34
-
231
618
127
Total
5324
4888
360
3930
14592
799
Cretaceous
DK
N O R T H
Other Finds
Figure 6. Resources per play per country, calculated in t.o.e..The Permian play is exclusively gas.The other
plays are gas and oil.The values are the sum of the total reserves found.The produced volumes are not deducted from the reserves (as is the case in figure 2). Source: Spencer, Leckie & Chew, 1996).
the end of the 1960’s, oil was discovered in
the northern North Sea, first in the chalk
formation in the Danish and Norwegian
regions of the North Sea, and soon after in
the Paleocene sand in the English sector.
Oil in the Jurassic sandstone was found in
1971 and in the following years. Thus, it
would be correct to state that the most
important plays in the North Sea were established within a five year period from
the end of the 1960’s to the beginning of
the 1970’s.
The reason for this order of events is due
primarily to the advances in the quality of
No seal
seismic data. In the early days of oil exploration it was not possible to “see” much
deeper than the top of the chalk, and consequently it was this formation and the
overlying Paleogene section which were
explored first. In the following section the
stratigraphy will be used as a framework
to discuss the plays, starting with the
Middle Jurassic play followed by the Upper
Jurassic, Cretaceous and the Paleogene
plays.
The Middle Jurassic Play
As mentioned previously, it was not possible to “see” the deep-lying Jurassic layer
Fairway
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Triassic
G
Cretaceous
Depth in km
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Salt
Sand
L
3
O
Coal
Dry well
Gas discovery
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Figure 7.The Permian gas play.
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Viking Graben
Magnus
Statfjord
Brent
Norway
Troll
Faults/fault zone
(Jurassic Rift)
Oseberg
Shetland
Limit of the Kimmeridge Clay
125 km
Frigg
Piper
Discovery in
Paleogene sandstone
Cretaceous Chalk
Upper Jurassic sandstone
Middle Jurassic sandstone
Permian sandstone
Miller
Brae
Morr
u
The J
ay F
irt
Rift
h
ra
ss
Forties
ic
Ula
R
if
t
P
Ekofisk
Gr
Denmark
e
l
nc
ra
vi
e
ro
C
nt
ab
en
Fulmar
Tyra
Esbjerg
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Th
Hull
e C
a
r bonife rous Ga
s Pr
o
vin
c e
rn
the
Groningen
G
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and
mian s
wer Per
of the Lo
t
i
lim
O
South
er
L
Great Britain
n limit of the Upper P
erm
ian s
a
Germany
lt
E
O
Holland
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Figure 8. Discoveries in the North Sea.The discoveries (approximately 700 in the 5 main plays)
are distributed among the main fairways: the Jurassic Rift Province and the Carboniferous Gas
Province.The source rocks are the Kimmeridge Clay and the Carboniferous coal, respectively.
Source: Spencer, Leckie & Chew, 1996
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Lyell
Ninian
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Osebjerg
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Viking Graben
Tertiary
Kimmeridge Clay
4
Depth in km
2
Cretaceous (shale)
Lower Jurassic,
Triassic & older
sediments
6
Oil and gas discovery
Oil discovery
behave so tactically. The Middle Jurassic
sandstone was named the Brent Formation (in the northern-most region of the
North Sea). In the following few years a
large number of discoveries were established in this play including the following:
Statfjord, Oseberg, Snorre, and Gullfaks in
the Norwegian region; Brent, Beryl,
Cormorant, Lyell, and Ninian in the English
region. Smaller discoveries were also made southwest of Norway in the Danish region (Harald and Lulita) and deep in the
Steep, narrow shelf
Turbidite
Wide, sanddominated shelf
Brae
Miller &
Magnus
Gr
abe
G
n
Figure 10.The Upper Jurassic play.The three main types of reservoirs are found in the fields named in the diagram.
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with the seismic techniques available in the
1960’s. In the most northern region of the
North Sea it was possible to see to the
bottom of the Cretaceous section (Cretaceous deposits in that region are composed of shale.) Although the topography of
this surface was very pronounced, there
was no indication of the nature of the underlying formations. The most widely
accepted theory was that the bottom of
the Cretaceous surface was underlain by
bedrock. In other words, the pronounced
structures were a type of Shetland islands
overlain by shale deposits. Petroleum geologists’ and oil companies’ interest in this
region was already high because there had
been some huge discoveries for example,
the Ekofisk discovery in a chalk formation,
and the Forties discovery in the Paleocene
sand. When Shell drilled an exploratory
well in 1971 and found oil, the dream of
the North Sea as a significant oil province
became a reality.The well, called the Brent
Discovery Well, was drilled through more
than 200 m of sandstone, and oil was present throughout the reservoir. Shell sealed
the well without testing it. It was not necessary, Shell knew it had a gigantic discovery.
Shell waited for the next round of licensing and applied for licences to all the
blocks into which this play might extend.
Today, no oil company involved in North
Sea exploration would be permitted to
Moray Firth. This play had the characteristic that the source rock was younger than
the reservoir rock, and it was only possible for the oil to migrate into the reservoir because fault tectonics during the Upper
Jurassic brought the source and reservoir
rocks into contact with each other, as illustrated in the Middle Jurassic play figure
(figure 9). Thirty billion b.o.e. have been
found in this play, the majority of it oil.The
two largest fields, Brent and Statfjord, contain a third of these resources.The remaining hydrocarbon resources are distributed among 117 discoveries.
NYT FRA GEUS 2&3/96
Figure 9.The Middle Jurassic play.The yellow layer
represents the “Brent” sandstone.
O I L
A N D
G A S
E X P L O R A T I O N
Paleogene
sand hinders
sealing
I N
T H E
N O R T H
S E A
Source rock
not matured
Fairway
0
gas
Tertiary
2
Gas cap
Oil-saturated
chalk
p
4
Salt
6
Triassic
8
Oil discovery
Only one percent of these resources are
Danish. Only after the Danish Underground Consortium was forced to
relinquish exploration areas in the beginning of the 1980’s exploration of the Jurassic formations in Denmark started.
Upper Jurassic Plays
Sea level rose after deposition of the
Middle Jurassic deltaic sandstone around
the uplifted central land mass (in the pres-
W. Ekofisk
Tertiary
ha
S e alin g s
Ekofisk
le
Chalk
G
E
O
2 km
200 m
O
L
Impermeable
chalk
Figure 12. High porosity and saturation under the
structural closure in one of the Norwegian chalk
fields
10
ent-day North Sea). The entire land mass
was inundated by the sea during the Upper
Jurassic period.At the same time a number
of faults were reactivated. Thus, the geologic picture of Upper Jurassic period is
complex with regions subsiding while
other areas experienced uplift. One could
refer to this time as a “geologist’s paradise” because the tectonic activity, deposition of reservoir sand as well as source
rocks all took place during the Upper Jurassic. It could also be said that this geologic time period has a special place in the
hearts of all North Sea oil geologists.
High oil saturation
G
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NYT FRA GEUS 2&3/96
Figure 11.The Chalk play.
In every oil province there is at least one
main challenge. In the Northern North
Sea the main challenge is to find reservoir
rocks. In the southern North Sea area the
challenge is to find structural closures
below the impervious cap rock of the Upper Permian salt. In the Northern North
Sea the stratigraphic focus of the main
problem is the Upper Jurassic. The reservoirs in the other plays are relatively sim-
Oil & gas
discovery
Dry well
ple to understand and thus also simple to
find and to explore.The Upper Jurassic reservoirs are complex. In fact, there is no
single Upper Jurassic play, but rather several, because the reservoirs are so different from each other.The number of plays
is the result of the geomorphology of the
Upper Jurassic.
The two main elements are the following:
1.The Upper Jurassic coastlines
2.The morphology of the sea bed
(bathymetry)
The significance of these two elements is
illustrated in the Upper Jurassic play figure
(figure 10).
Since the Upper Jurassic reservoirs are all
marine in origin, there is some justifications for regarding the Upper Jurassic
discoveries as belonging to one play. But
researchers and exploration geologists
require a more nuanced understanding of
this period. In some areas the marine shelf
A N D
G A S
E X P L O R A T I O N
was narrow and tectonically connected to
a fault.This results in one type of reservoir.
In other areas the marine shelf was wide
and experienced long-term inundations
where fluctuations in sea level produced
pronounced short-term variations in
coastline location. Finding the latter type
of sandstone reservoir is extremely difficult. In addition to the tectonic activity during the Upper Jurassic period, the North
Sea area also experienced recurring
earthquakes. Earthquake activity can be inferred from faults as well as the presence
of a particular type of sediment, called turbidites. Turbidite sediments begin as shallow water sediments which are loosened,
often by earthquakes, and transported in
the form of suspensions into deeper water. In principle then, there are three main
types of reservoirs: marine sandstones deposited on narrow shelves associated with
faults; sandstones deposited on wide shelves with labile coastlines; and turbidites
deposited in deep water. The majority of
resources in the Upper Jurassic play were
discovered in sandstone deposited on wide labile shelves. These include the following: the Norwegian Troll field, which is
the most productive field in the play and
I N
T H E
N O R T H
S E A
1500
1000
Cretaceous N, DK + UK
500
Million t.o.e.
O I L
Tertiary UK
0
65
70
75
80
85
90
95
Year
Figure 14. Discovery curve for the Cretaceous and Tertiary.The figure illustrates that the large discoveries are
made early in the history, and that the majority of discoveries are made within about five years. Politically imposed limitations on exploration can shift this picture, but at that time in the exploratory history of the North
Sea when the large chalk reservoirs in Norway and Denmark was found there was little political interference.
The Tertiary discovery curve for England is completely different from the above curve. Initially, the English
Tertiary curve has a “typical” steep shape indicating a large number of discoveries over a short period, followed
by a flattening out.Then in the 10-year period between 1985-95 the curve rose steeply as the volume of found
resources doubled, the result of a number of moderate sized discoveries.This increase in discoveries was due
to new technology (3D seismic) and an improved understanding/interpretation of the geology (seismic- and
sequence stratigraphy). Source: Spencer, Leckie & Chew, 1996.
also the world’s largest producing offshore gas field; the English Piper and Fulmar oil
fields; as well as the Norwegian Ula and
Gyda fields. These play’s total resources
are estimated to be 20 billion b.o.e., of
which almost one half, approximatley 1250
No cap rock
over reservoir
billion cubic meters of gas are found in the
Troll field. Three discoveries have been
made in Upper Jurassic sandstone within
the Danish North Sea area, the Gert, Ravn
and Elly.The reserves in these discoveries
are, however, by North Sea standards very
Fairway
No migration
0
East
NYT FRA GEUS 2&3/96
West
Depth in km
Neogene
Discovery in a
stratigraphic
trap
2
Paleogene
a la Frigg
Creraceous
I
a la Forties
Middle/Upper Jurassic
Triassic & older
G
Shetland’s
bedrock
platform
Gas discovery
Oil discovery
O
L
O
4
Dry well
G
E
Figure 13.The Paleogene play.The majority of the sand (with the exception of that
on the Shetland’s platform) was deposited as turbidites.There is also sand in Neogene deposits, but this contain no hydrocarbons due to lacking cap rock.
11
O I L
A N D
G A S
E X P L O R A T I O N
small. Many of the Brae Trend discoverys
are associated with narrow shelves. The
turbidite sediments which were transported long distances out to sea, form the main reservoirs in discoveries such as the
Claymore, Miller and Magnus Fields in the
Moray Firth and the Viking Graben.
I N
vement created a migration route through
the lower region of the chalk, and probably
also created the reservoir characteristics.
Not all the upper region of the chalk
within the play’s fairway is of reservoir
quality. Figure 12 illustrates the variation in
oil saturation and porosity in a Norwegian
chalk field.
G
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NYT FRA GEUS 2&3/96
The Upper Cretaceous Play
Oil and gas discoveries were first made in
the chalk formations in the Danish region
of the North Sea (Anne in 1966, Roar and
Tyra in 1968) and oil was found in the
Norwegian region (Valhall) in 1967. In
December 1968 the first gigantic oil field
discovery in the North Sea was made, the
Ekofisk field, located in Upper Cretaceous
chalk. These events were epoch-making in
two respects. First, the discoveries proved
that, at least in the northern part of the
North Sea, there was an oil province with
the potential to be extremely profitable.
Second, the discoveries indicated that
parts of the chalk formation has reservoir
characteristics. The reservoir in this play
was deposited after formation of the many
faults which play a definitive role in formation of traps in nearly all of the Jurassic discoveries in the North Sea. Nevertheless,
this play is dependent upon structuring
throgh salt deformation as depicted in figure 11. These strange pillar-like features
were formed out of Permian salt.This extremely mobile salt became even more so
during the intense tectonic movement during the Upper Jurassic period and was
forced upwards forming a large number of
salt pillars (diapirs). Other diapirs which
were formed during the Triassic period became active again.The rapid growth of the
diapirs ceased prior to deposition of the
chalk, but their tops ended close to the
surface.The majority of the oil discoverys
in chalk were trapped in a structure which
arose from compression over nearly immobile salt structures.The discoveries in
chalk are concentrated in two relatively
narrow fairways (see figure 8), one located
entirely in the Norwegian area and the other located entirely in the Danish region.
The chalk play’s fairway is limited to the
Central Graben where the salt was thick
enough to be deformed and the source
rocks were thick. In addition, the salt mo-
12
The oil saturation in the two fields is high
directly below the structural closures.This
indicates that the reservoir quality decreases with distance from the structure and
that this deterioration is so pronounced
that the chalk changes from being a reservoir to functioning as its “side seal” away
from the structure. The narrow fairways
and the restriction of reservoir quality
chalk to the areas over the salt diapirs can
best be explained with the model discussed in the following section.
One of the characteristics of a good chalk
reservoir is believed to be the early migration of oil into the chalk. Thus, the early
formation of oil is a prerequisite, and this is
most likely in the deepest parts of the
Central Graben. All of the commercial
discoveries in chalk are located in the immediate vicinity of the deepest parts of the
Central Basin. A good transport corridor
is also a precondition to ensure that the
chalk’s total volume (matrix) can be filled
with oil. Fracture formation in connection
with the deformation in the units over the
diapirs created this network of “highways”
for migration. Nearness to the deepest
part of the Central Graben limits this play
in the east-west direction and towards the
south. The play does not continue to the
north because a new reservoir (next section) formed on top of the chalk, so that in
this area the chalk lacks a seal.The majority of the Danish oil and gas reserves are
found in chalk.
The Tertiary Play
Until about 1995 there were no discoveries in the Tertiary formations in the
Danish region of the North Sea, but Statoil’s Siri discovery changed this situation.
Almost one hundred discoveries, located
in Tertiary sands of Paleocene and Eocene
ages have been made in the English and
Norwegian regions of the North Sea.
T H E
N O R T H
S E A
Discoveries continue to be made in this
play. Thus 4 out of 9 discoveries in the
Norwegian region in 1995 were in this
play.The most important geologic precondition for this play was uplift of the
northwestern part of England-Scotland
and the “Shetlands region” in connection
with formation of the North Atlantic. Huge
quantities of sand which were eroded from
these uplifted land masses were deposited
in a relatively narrow shelf east of the present-day Shetland islands. Sand from this
shelf was pumped out in the form of turbidites into the deep regions of the North
Sea both into the Viking Graben and
through the Moray Firth out into the Central Graben.This play’s fairway is thus easy
to understand: the area where the play
works is limited to the extent of the sand
in combination with access to a “kitchen”.
The Tertiary discoveries (see figure 13) can
be divided into three types of traps.
The first type is a structural trap where the
Paleocene sand is draped over an underlying raised feature. The gigantic Forties
field, which together with the Ekofisk field
heralded the oil bonanza of the North Sea,
are found in this type of trap. The thick
sand in the Forties field is composed of
turbidites.The entire Tertiary section compressed under its own weight and this
thick sand was folded over a underlying
bedrock ridge, in the same way that the
chalk structures were formed by compression over salt diapirs.The second type
of trap is composed of some of the Tertiary turbidites which are stacked on top of
each other, so that they, together with the
overlying clay seal form structural traps.
The largest gas field in this play, the Frigg
field, was found in this type of trap. While
the sand in Forties is from the Paleocene,
the sand in the Frigg is from the Eocene. In
the northern North Sea the quantity of
sand decreases through the oldest part of
the Tertiary (Paleogene).The third type of
discovery in the Paleogene are found in
stratigraphic traps where a lateral seal
forms via lithological changes from the reservoir (sandstone) to the non-reservoir
rock (shale). This last type of discovery is
difficult to locate, as evident from the curve of Tertiary discoveries in the English region of the North Sea (figure 14).
O I L
A N D
G A S
E X P L O R A T I O N
I N
T H E
N O R T H
S E A
Figure 15. Size of a gas field and three oil fields in the North Sea
compared to the size of the island of Funen in Denmark.
Dan
Troll
Dan
Ekofisk
Statfjord
13
NYT FRA GEUS 2&3/96
I
G
There are some areas in the North Sea
where “infrastructure” is in place. Infrastructure refers to the production platform, where the well steam can be separated into its various components (water, oil
and gas) as well as an oil and gas transport
system (for example a pipe or loading
system to a tanker vessel). If these facilities
are in place, then it may be profitable to exploit a small discovery with reserves of 1-2
million cubic meters. Thus, this discussion
makes it clear that size is a very relative
term when considering the commercial
profitability of an oil field.
O
The Ekofisk chalk reservoir contains the same volume of “in place” ressources as the
Statfjord field. The degree of recovery at
the Ekofisk field is 40%, and although this is
exceptionally high for a chalk reservoir, it
means that in practical terms the producible Ekofisk reserves are less than those
of the Statfjord field.The largest Danish oil
field, the Dan, has “in place” reserves of 500
million cubic meters, but with an anticipated degree of recovery of only 20%, the
extractable resources will be approximately 100 million cubic meters.
L
The North Sea’s largest oil field, the Statford, originally had “in place” ressources of
more than 1,000 million cubic meters and
covered an area of approximately 140 km2.
Prior to production, it was expected that
about 40% of these reserves could be extracted. However, after production commenced the expected degree of recovery
has been increased upwards to 60%
Everyday 100,000 cubic meters of oil are
produced from the Statfjord field which is
more than twice the volume of the total
Danish oil production in the North Sea.
O
reservoir thickness (200 m) and then subtracting the deduction for edge effects:
8,000,000 x 200 = 1.6 billion cubic meters
– edge effects (where the reservoir thins
towards the field limit) = 1.2 billion cubic
meters. The average porosity is over 20%,
which is a good porosity for a sandstone
field.This gives a pore volume of 250 million
cubic meters.
A good reservoir has a saturation of over
80%, meaning that more than 80% of the
fluid in the pore spaces is oil. Based on this
assumed oil saturation, the oil volume in
the reservoir would be over 200 million
cubic meters.With today’s technology it is
possible to recover more than 50% of the
original “in place” reserves. Finally, the fact
that the volume of oil shrinks during the
process of extraction to the surface must
be considered. These calculations demonstrate that the Fulmar oil field could contain 80 million cubic meters of oil even
though it’s surface area was so limited.
E
An oil field with an area of less than 10 km2
can be large if the reservoir is thick and of
good quality. Fulmar is a good example of
this type of field. Before production started,
the Fulmar field was estimated to contain
reserves of 80 million cubic meters of oil,
although the field area is less than 10 km2.
This apparently improbable situation can be
explained with a few simple calculations. If a
total reservoir volume in the field is calculated by multiplying the area (8 km2) by the
Something about Size
G
What makes an oil field large? There are
three simple elements. First, there must be
a suitable quantity of source rock present.
This condition is more than met in the central and northern North Sea and in the
southern gas province. An incompletely
filled reservoir volume within these areas
has seldom been attributed to insufficient
source rocks.A second condition of a large
oil field is that the reservoir must be thick.
The thickest reservoirs in the North Sea
are found in Upper Jurassic sandstone and
conglomerates in structural traps created
due to faulting (Brae type, figure 10). In
these fields the reservoir thickness is often
greater than the height of the column of oil.
Thus, in the fields with an exceptionally
thick reservoir the bottom reservoir regions are water-filled. The reservoirs of the
North Sea giants can be 200-300 meters
thick, while some reservoirs are only a few
meters thick. Many fields on land, where a
small discovery can be commercially viable
have a thickness under 10 meters.Additionally, the area of the trap can vary widely.
The Troll field which is the largest gas field
in the North Sea, covers an area of 700
km2.
O I L
A N D
G A S
E X P L O R A T I O N
I N
T H E
N O R T H
S E A
Oil, the sisters and …
S OCIETY
G
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NYT FRA GEUS 2&3/96
Denmark started oil production from the Dan field in 1972 and became
self sufficient early in the nineties. Coal is still
imported, but today Denmark exports both
gas and oil.The degree of total energy self sufficiency in Denmark is now over 100% of
which 8% is supplied by renewable resources,
and the remainder is supplied by oil and gas. In
terms of the national economy, Denmark’s increasing energy self sufficiency together with
falling oil prices has put Denmark in the good
company of other countries with strong economies.
One could ask why the multi-national oil companies, the survivors of “The seven sisters”
and the new oil moguls have such a prominent
role in the industry.Wouldn’t it be possible for
a country such as Denmark to create a stateowned oil company, the profits of which
would supply the national coffers? In order to
answer these questions we must understand
the undertaking of a large oil company. The
large multi-national oil companies find the hydrocarbons.They finances construction of the
production plants, where raw oil and gas are
refined, as well as construction of storage and
transportation networks. The oil companies
earn money by selling the processed hydrocarbons to consumers.This chain of activities
requires large amounts of capital and is associated with numerous financial risks. It is the
element of risk which protects the oil companies from being absorbed into the public
sector. Politicians in capitalistic countries refuse to take responsibility for the financial risks
associated with hydrocarbon exploration,
production and transportation. The multi-national oil companies distribute their risk
throughout the world, and this is an economic
necessity in the oil industry.Today, even countries with many years experience with stateowned oil companies such as Venezuela, former USSR countries and Iran, to name a few,
invite multi-national companies to participate
in exploration and production.This trend may
change, but it is unlikely until oil reserves are
severely reduced. In the meantime we must
learn to live with the multi-national oil companies.The task of each nation is to do this in
a way which is most beneficial to its citizens.
14
T HE
E NVIRONMENT
The discussion regarding the
final disposal of the Brent Spar platform, and
similar oil-related issues in the news media
may have lead to a belief that oil activities in
the North Sea are associated with huge environmental risks. In fact, the countries surrounding the North Sea have regulated activities
to such an extent that adverse environmental
effects associated with exploration and production are minuscule in comparison to many
other environmental threats. Similarly, oil contamination on beaches can, almost always, be
linked to tanker vessels which clean their
holds en route to the North Sea. Now and
then a ship runs aground, and crude oil washes
ashore.Again, the problem is not hydrocarbon
recovery, but rather the limited possibilities
available to politicians to regulate ships and
shipping traffic.The major environmental problem associated with oil and gas arises when
these hydrocarbons are combusted. In the
North Sea region and in the hydrocarbonconsuming parts of the world this environmental issue is, in reality, a lifestyle issue (see
figure 1) and not a problem which can be
solved by demanding that oil companies change their practices. In other parts of the world,
where public authorities lack the power to
impose or enforce environmental legislation,
hydrocarbons may be produced under less
stringent environmental control.Thus, we can
help the environment in two ways:
1. By decreasing our energy consumption
2. By helping hydrocarbon-producing
developing nations to formulate environmental laws and assist them in enforcing
them against oil companies.
By decreasing our energy consumption, particularly of fossil fuels we will also extend the
period in which oil can be purchased for a
reasonable price. Falling oil production in the
North Sea and Alaska must be expected in the
near future. We and our governments must
decide whether we will act now and plan for a
future with expensive oil, or whether we will
wait passively until the energy crisis comes.
R ESEARCH
It is our duty as a society to
ensure that precious non-renewable hydrocarbon resources are utilised in a way that is
best for the society as a whole.What is good
economy for an oil company is not necessarily the same for a society. It may be better to
produce 20 million tons of oil from a field with
a few production wells over 15 years rather
than to produce 30 million tons over 20 years
with more wells. In order to enter into a constructive discussion with multi-national oil
companies regarding exploitation of these resources society must develop knowledge and
competence within a number of fields. GEUS
is the Danish authorities’ “professional right
hand” in these discussions. We have earned
this role partly through work undertaken on
behalf of the Danish Energy Agency (Energistyrelsen) and through scientific investigations
and research. Furthermore GEUS provides
consulting services to oil companies.This last
activity is important because it gives GEUS the
opportunity to follow both the technological
and scientific developments within the fields
of hydrocarbon exploration and production.
The relevance of our consulting services
would quickly deteriorate without these
updates.Additionally, these consulting projects
uncover problems or points of dispute which
may be relevant research projects. Clearly the
triangle of interconnected activities - research
– expert advisor – consulting work is not just
an economic necessity, it also ensures that
GEUS’s competence evolves simultaneously
with that of the oil industry.This is crucial to
GEUS’s ability to assist authorities in regulating the hydrocarbon industry. It also means
that GEUS has a certain independence to
refuse projects which are not relevant to our
fields of interest, allowing us to select projects
which are both relevant and which will further
develop our competence. This is an ongoing
process.There are still some areas of petroleum geology, exploration and production
where our knowledge is lacking, but we are
constantly striving to keep our scientific
knowledge current, not only for ourselves, but
also for the sake of society.
O I L
A N D
G A S
E X P L O R A T I O N
I N
T H E
N O R T H
S E A
late in a reservoir rock.As a concrete
term a play refers to any number of es
tablished discoveries or prospects
where existing geological evidence in
dicates that the requirements for a play
are present, or are likely present.
The set of conditions include a source
rock, a porous and permeable reservoir covered by a seal and with a
suitable natural configuration so that
hydrocarbons cannot migrate out of
the trap.
Prospect: A hydrocarbon trap which has
not yet been investigated by exploratory drilling.
Red beds: sedimentary strata composed
primarily of sand and clay with a
characteristic red colour derived from
the presence of the iron-rich mineral
hematite, which coats the individual
grains.
Reflector: refers to a surface which separates two rock formations each with
different seismic characteristics.A reflector is found during a seismic investigation where sound waves are shot
through the underground and reflected back from these surfaces.
Reservoir: a porous and permeable
geological layer which contains oil and
gas.
Rift zone: a regional scale system of
down-throwing faults in the earth’s
crust.
Salt diapir: a column of salt which has risen through the overlying rock formations from a salt layer which is often located 2-6 km under the top of the salt
columns.The diapirs are typically 1-5
km in diameter and the driving force
for their movement is buoyancy due to
the low density of salt
Shelf: the slightly-sloping, underwater region of a continent located between
the shoreline and the continental slope.The width of the continental shelf is
normally delineated by an ocean depth
of 200 m, or by the edge of the continental slope.
Shelf sea: a shallow sea situated on the
continental shelf which rarely exceeds
a depth of 2-300 m, for example the
North Sea.
Source rock: all the types of rock in
which oil and gas can form.
Stratigraphic trap: a trap for oil or gas
which is the result of lithologic changes
in a rock formation or changes in its
extent rather than structural configuration.
Structural trap: a trap for oil or gas
which is the result of folding, faulting or
other deformation.
Tectonic activity: movement in the
earth’s crust for example earthquakes,
faulting and other deformations which
occur due to forces involved in tectonics.
Test: an experimental production of oil or
gas to the surface after a discovery
has been made during exploratory
drilling.
Trap: any type of barrier (lithological or
structural) to upward movement (migration) of oil or gas allowing these to
accumulate in underlying formations.A
trap includes reservoir rock and an impermeable cap rock.
Turbidite: a sediment deposited by a
turbidity current, which is a tongueshaped current of suspended material
which flows from shallow waters to
deeper water after being loosened,
often by an earthquake.
Wet/rich gas: a natural gas containing
liquid hydrocarbons.
L
O
G
I
Cap rock: a low permeability rock
formation through which hydrocarbons are unable to migrate.The cap
rock acts like a seal, trapping the
hydrocarbons in the underlying formation.
Deltaic: deposited in a delta environment.
Dry gas: natural gas with a low content of
liquid hydrocarbons.
Exploration well: a well which is drilled
to determine whether a prospect contains producable hydrocarbons. Often
an exploration well recovers only water.
Fairway: an area where all the components of a play are present.
Fault: a fault or fault zone along which
there has been displacement of the
sides relative to one another parallel to
the fracture.
Hypersaline: a liquid having a salt content
which is much higher than that of
normal ocean water.
Hydrocarbons: a general term referring
to organic material composed exclusively of carbon (C ) and hydrogen (H).
Kitchen: refers to the area in the underground where hydrocarbons form.
Light oil: oil with a low specific gravity.
Mesozoic: formed during the earth’s
middle age (Triassic, Jurassic and
Cretaceous).
Migration: the movement of hydrocar
bons from their source rock
(the kitchen) through permeable rocks
to a reservoir.
Migration route: the path taken by the
majority of the hydrocarbons from the
kitchen to a reservoir.
Morphology: the shape and structure of
any given surface.
Play: as an abstract concept a play refers
to a set of conditions which must met
in order for hydrocarbons to accumu
NYT FRA GEUS 2&3/96
Dictionary
G
E
O
This theme issue, Oil and Gas Exploration in the North Sea, was written by head of department Kai Sørensen. Prior
to his employment at GEUS, he was employed at Statoil, first as a senior geologist in Stavanger (Norway) and later
as Exploration Manager in Denmark. He has also taught at Århus University and Denmark’s Technical University.
He has been a visiting researcher at Imperial College in London, at M.I.T. in Boston and at Cornell University.
Kai Sørensen is the head of the geophysical department at GEUS.
15
Review of Gree
nland activities
2000
New publication from GEUS
Articles in Review of Greenland activities
bulletins are available as pdf-files from
1996 to present at the GEUS website.
www.geus.dk/publications/publ-dk.htm
OF GREEN
LAND
SURVEY B
ULLETIN
18
9 • 2001
Review of Gr
eenland activi
ties
2000
Geology of Gree
nland Survey
Bull ti
The annual Review of Greenland activities is a special bulletin regarding research in Greenland and off-shore areas,
including the north Atlantic and Arctic. It
contains review articles on primary activities written in a style that enables other
than professionals to get an all-round impression of GEUS research in Greenland.
GEOLOGY
GEOLOGICAL
SURVEY OF
MINISTRY OF
DENMARK AND
ENVIRONMEN
GREENLAN
T AND ENER
D
GY
G E U S
The following articles contain statistics regarding the number of discove ries and hydrocarbon resources:
A.M. Spencer, G.G. Leckie & K.J. Chew:
North Sea hydrocarbon plays and their resources (an article in a Geological Society
Special Publication on northwestern Europe’s hydrocarbon industry by K.W. Glennie
and A. Hurst, 1996. Figures 6, 8 and 13 are
based on numbers found in this article.
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An in-depth description of the petro-
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G
O
L
O
E
G
leum geology and exploration history
of the North Sea:
K.W. Glennie (ed.) Introduction to the Petroleum Geology of the North Sea. Blackwell Scientific Publications, Oxford 1990
(3rd edition).
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ISSN 1396-2353
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Translation: MapleLeaf Miljøtekniske
Oversættelser.
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NYT FRA GEUS 2&3/96
Further Reading
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