into deeper water as turbidites and other slope
deposits, but the sorting and reservoir quality is then
reduced.
Ooids are rather stable mechanically during burial,
but at 2–3 km porosity may be strongly reduced by
cementation in the intergranular pore space. The calcite cement may be derived by dissolution along
stylolites. Stylolites have a thin layer of clay and
other minerals that are not soluble and may present a
barrier during oil migration and production.
Carbonate muds are very fine grained and do not
have high enough primary porosity and permeability
to form reservoir rocks, but may gain porosity and
permeability by fracturing and become fractured
reservoirs. They can also have mouldic porosity from
dissolved aragonite fossils.
1.14 Drilling for Oil and Gas
Drilling for oil is a costly process, especially offshore.
The object of a well is to prove the presence of, or
produce, oil or gas. Sometimes wells are also drilled to
inject water, chemicals or steam into the reservoir
during production. Even a well which fails to find
hydrocarbons (a dry well) is still of great value,
because of the information it provides about the
rocks in the area. This information forms part of the
basis for the geological maps and profiles which are
used in further exploration for oil and can be sold or
exchanged for data from other companies. This is the
reason why oil companies wish to keep the geological
results of oil drilling confidential for some years after a
well has been completed.
The first well in a new area is called a wildcat well,
while appraisal wells are drilled to estimate the extension of an oil field. They may also become production
wells. Stratigraphic wells are drilled mainly to obtain
stratigraphical information from the basin.
Oil drilling used to be carried out largely on land,
but now offshore drilling takes place not only on our
continental shelves, but also in deep water (1–3 km).
This type of drilling is many times more costly than
drilling in shallow water or on land. This has led to
increased efforts to gain maximum information from
wells. The cost of analysing samples and logs is small
in relation to the cost of drilling the well. We shall not
go very deeply into the technical aspects of drilling for
oil here, but merely look briefly at some of the most
important principles.
When drilling commences at the surface, the diameter of the well may be 20"–30" (50–75 cm), but
decreases downwards to 3"–6" (7–15 cm) at great
depths. Normally a roller bit is used, which crushes
the rock into small pieces (about 2–5 mm) called
cuttings. Core samples are only taken when drilling
through especially important rock strata (usually reservoir rocks) where large intact samples are needed for
detailed examination. A circular diamond core drill bit
must then be used. This takes time and costs a lot more
per running metre, as the entire drill string has to be
recovered to get each core section to the surface. Only
the most critical sections are therefore cored.
Drilling mud is pumped down through the drill
string into the well during drilling. This mud has
several functions. When one drills several hundred or
a 1,000 m down into rock, one encounters water, gas
or oil which may be under high pressure. The drilling
mud acts as a counterweight to prevent the uncontrolled gush of water or petroleum into the well and
up to the surface in a blow-out. The pressure exerted
by the drilling mud must exceed the pressure of oil and
water in the surrounding formation. Heavy minerals
such as barytes are frequently added to increase density; the main components of drilling mud are montmorillonite (smectite) containing clays, with a large
number of different additives. The drilling mud also
serves to cool the drill bit, and cuttings are brought
back to the surface suspended in the circulating mud.
The cuttings are then washed out from the drilling mud
onto a sieve (shale shaker) and the mud can be used
again.
The cuttings are continuously analysed on the drilling platform by a geologist who logs the composition
of the cuttings, making a preliminary description of
the rocks which are being drilled, and their mineralogy. Samples of cuttings are usually taken every
10–30 ft drilled. More detailed analyses are carried
out in the laboratory. The cuttings are often poorly
washed and need extra cleaning to get rid of the
drilling mud; in weakly indurated mudstones it may
be difficult to separate the drilling mud from the soft
cuttings. Organic additions to drilling mud may cause
problems when analysing cuttings, and sometimes oilbased rather than water-based drilling mud is used,
which confuses analyses for oil. The fossil content of
22
K. Bjørlykke
deposits, but the sorting and reservoir quality is then
reduced.
Ooids are rather stable mechanically during burial,
but at 2–3 km porosity may be strongly reduced by
cementation in the intergranular pore space. The calcite cement may be derived by dissolution along
stylolites. Stylolites have a thin layer of clay and
other minerals that are not soluble and may present a
barrier during oil migration and production.
Carbonate muds are very fine grained and do not
have high enough primary porosity and permeability
to form reservoir rocks, but may gain porosity and
permeability by fracturing and become fractured
reservoirs. They can also have mouldic porosity from
dissolved aragonite fossils.
1.14 Drilling for Oil and Gas
Drilling for oil is a costly process, especially offshore.
The object of a well is to prove the presence of, or
produce, oil or gas. Sometimes wells are also drilled to
inject water, chemicals or steam into the reservoir
during production. Even a well which fails to find
hydrocarbons (a dry well) is still of great value,
because of the information it provides about the
rocks in the area. This information forms part of the
basis for the geological maps and profiles which are
used in further exploration for oil and can be sold or
exchanged for data from other companies. This is the
reason why oil companies wish to keep the geological
results of oil drilling confidential for some years after a
well has been completed.
The first well in a new area is called a wildcat well,
while appraisal wells are drilled to estimate the extension of an oil field. They may also become production
wells. Stratigraphic wells are drilled mainly to obtain
stratigraphical information from the basin.
Oil drilling used to be carried out largely on land,
but now offshore drilling takes place not only on our
continental shelves, but also in deep water (1–3 km).
This type of drilling is many times more costly than
drilling in shallow water or on land. This has led to
increased efforts to gain maximum information from
wells. The cost of analysing samples and logs is small
in relation to the cost of drilling the well. We shall not
go very deeply into the technical aspects of drilling for
oil here, but merely look briefly at some of the most
important principles.
When drilling commences at the surface, the diameter of the well may be 20"–30" (50–75 cm), but
decreases downwards to 3"–6" (7–15 cm) at great
depths. Normally a roller bit is used, which crushes
the rock into small pieces (about 2–5 mm) called
cuttings. Core samples are only taken when drilling
through especially important rock strata (usually reservoir rocks) where large intact samples are needed for
detailed examination. A circular diamond core drill bit
must then be used. This takes time and costs a lot more
per running metre, as the entire drill string has to be
recovered to get each core section to the surface. Only
the most critical sections are therefore cored.
Drilling mud is pumped down through the drill
string into the well during drilling. This mud has
several functions. When one drills several hundred or
a 1,000 m down into rock, one encounters water, gas
or oil which may be under high pressure. The drilling
mud acts as a counterweight to prevent the uncontrolled gush of water or petroleum into the well and
up to the surface in a blow-out. The pressure exerted
by the drilling mud must exceed the pressure of oil and
water in the surrounding formation. Heavy minerals
such as barytes are frequently added to increase density; the main components of drilling mud are montmorillonite (smectite) containing clays, with a large
number of different additives. The drilling mud also
serves to cool the drill bit, and cuttings are brought
back to the surface suspended in the circulating mud.
The cuttings are then washed out from the drilling mud
onto a sieve (shale shaker) and the mud can be used
again.
The cuttings are continuously analysed on the drilling platform by a geologist who logs the composition
of the cuttings, making a preliminary description of
the rocks which are being drilled, and their mineralogy. Samples of cuttings are usually taken every
10–30 ft drilled. More detailed analyses are carried
out in the laboratory. The cuttings are often poorly
washed and need extra cleaning to get rid of the
drilling mud; in weakly indurated mudstones it may
be difficult to separate the drilling mud from the soft
cuttings. Organic additions to drilling mud may cause
problems when analysing cuttings, and sometimes oilbased rather than water-based drilling mud is used,
which confuses analyses for oil. The fossil content of
22
K. Bjørlykke