the rock fragments, largely microfossils, is used to
determine the age of the strata (see Biostratigraphy).
Not all the cuttings which come up with the drilling
mud have necessarily come from precisely the strata
being drilled through at that time. There may also be
contamination due to the caving in of overlying strata
into the rising drilling mud. This means we can find
material from younger rocks with a different composition mixed in with the formation being drilled,
together with younger fossils. This demands considerable care when making stratigraphic interpretations
based on microfossils identified in cuttings. The safest
way is to register the first occurrences of a species
when proceeding downwards from the top in the well.
The last occurrence of a fossil may be the result of
cave-ins from younger strata.
Since the pressure of the drilling mud is being
monitored and adjusted to prevent oil and gas from
penetrating into the well, significant oil and gas
occurrences may be drilled through without being
registered. This should be detected on the logs but it
may be advisable to carry out special tests in the most
promising strata to find out if there is petroleum present, and in what quantities.
As drilling proceeds, the well is lined with steel
casing to prevent rock and loose sediment falling into
it, but prior to casing, each section of the well has to be
logged with different logging tools which require
physical contact with the wall of the well. Radioactive
logs, however, can also be run after the casing has
been installed. It is useful to note when the different
casings are installed, because that limits the strata
which could have “caved in” and contaminated the
cuttings. If the well is going to produce oil, a production pipe is used and installed running through the
petroleum-bearing strata. It is then perforated by
shooting holes in the steel casing (in the oil column)
so that petroleum can flow into the well.
For an oil field to be capable of full production,
several wells are normally required.
Up to 1990–1995 most of the exploration wells and
production wells were nearly vertical. Horizontal
barriers due to changes in facies or faults could then
be critical barriers during production. It was difficult
to produce oil from thin sandstones or carbonate beds
because of vertical flow of water from below or gas
from above. Onshore this could be compensated for by
having a dense well spacing but offshore that would be
too expensive. Horizontal drilling has revolutionised
oil production.
It is now possible to follow thin oil columns
laterally and to make complex wells to drain different
compartments in the reservoir. Large oil fields with
thin oil columns like the Troll field in the North Sea
would have been difficult to produce without horizontal drilling. Horizontal and deviated wells may extend
8–9 km from the drilling platform, enabling production from relatively small reservoir compartments
away from the platform.
Earlier, geophysics was used mainly to define
structures but the quality of the seismic data was not
good enough to provide much detail. 3D seismic based
on 50 m line spacing allows the construction of a
three-dimensional cube of geophysicial data, which
provides much more detailed information.
By repeating seismic surveys during production at
2–5 year intervals, the effect of changes in the fluid
composition on the seismic data can be seen, thus
adding the time dimension to 3D seismics. 4D
seismics has made it possible to follow the depletion
of an oil or gas field by monitoring the GWC and often
also the OWC during production. By this means, parts
of a reservoir that have not been drained by the production wells can be detected. If the isolated reservoir
compartments are large it may be economic to drill
additional wells to drain them.
1.15 Oil Reserves – How Long Will
Conventional Oil Last?
In the last 40–50 years we have had a discussion about
how long the oil reserves will last and the famous
geologist M. King Hubbert predicted in 1956 that oil
production would peak in the United States between
1965 and 1970, and later Colin Campbell predicted
that world production would peak in 2007 (see the
Peak Oil movement). There is, however, a great deal
of uncertainty in the estimation of reserves since this
will depend on advances in exploration technology as
well as production efficiency.
The price of oil and taxation policies will also
determine which type of oil accumulations can be
exploited economically and thus reckoned as reserves.
At present the world reserves are estimated to be
1:2 Â 10
12 bbl 2 Â 10
11 m
3
À
Á
and nearly 60% of this is
1 Introduction to Petroleum Geology
23
determine the age of the strata (see Biostratigraphy).
Not all the cuttings which come up with the drilling
mud have necessarily come from precisely the strata
being drilled through at that time. There may also be
contamination due to the caving in of overlying strata
into the rising drilling mud. This means we can find
material from younger rocks with a different composition mixed in with the formation being drilled,
together with younger fossils. This demands considerable care when making stratigraphic interpretations
based on microfossils identified in cuttings. The safest
way is to register the first occurrences of a species
when proceeding downwards from the top in the well.
The last occurrence of a fossil may be the result of
cave-ins from younger strata.
Since the pressure of the drilling mud is being
monitored and adjusted to prevent oil and gas from
penetrating into the well, significant oil and gas
occurrences may be drilled through without being
registered. This should be detected on the logs but it
may be advisable to carry out special tests in the most
promising strata to find out if there is petroleum present, and in what quantities.
As drilling proceeds, the well is lined with steel
casing to prevent rock and loose sediment falling into
it, but prior to casing, each section of the well has to be
logged with different logging tools which require
physical contact with the wall of the well. Radioactive
logs, however, can also be run after the casing has
been installed. It is useful to note when the different
casings are installed, because that limits the strata
which could have “caved in” and contaminated the
cuttings. If the well is going to produce oil, a production pipe is used and installed running through the
petroleum-bearing strata. It is then perforated by
shooting holes in the steel casing (in the oil column)
so that petroleum can flow into the well.
For an oil field to be capable of full production,
several wells are normally required.
Up to 1990–1995 most of the exploration wells and
production wells were nearly vertical. Horizontal
barriers due to changes in facies or faults could then
be critical barriers during production. It was difficult
to produce oil from thin sandstones or carbonate beds
because of vertical flow of water from below or gas
from above. Onshore this could be compensated for by
having a dense well spacing but offshore that would be
too expensive. Horizontal drilling has revolutionised
oil production.
It is now possible to follow thin oil columns
laterally and to make complex wells to drain different
compartments in the reservoir. Large oil fields with
thin oil columns like the Troll field in the North Sea
would have been difficult to produce without horizontal drilling. Horizontal and deviated wells may extend
8–9 km from the drilling platform, enabling production from relatively small reservoir compartments
away from the platform.
Earlier, geophysics was used mainly to define
structures but the quality of the seismic data was not
good enough to provide much detail. 3D seismic based
on 50 m line spacing allows the construction of a
three-dimensional cube of geophysicial data, which
provides much more detailed information.
By repeating seismic surveys during production at
2–5 year intervals, the effect of changes in the fluid
composition on the seismic data can be seen, thus
adding the time dimension to 3D seismics. 4D
seismics has made it possible to follow the depletion
of an oil or gas field by monitoring the GWC and often
also the OWC during production. By this means, parts
of a reservoir that have not been drained by the production wells can be detected. If the isolated reservoir
compartments are large it may be economic to drill
additional wells to drain them.
1.15 Oil Reserves – How Long Will
Conventional Oil Last?
In the last 40–50 years we have had a discussion about
how long the oil reserves will last and the famous
geologist M. King Hubbert predicted in 1956 that oil
production would peak in the United States between
1965 and 1970, and later Colin Campbell predicted
that world production would peak in 2007 (see the
Peak Oil movement). There is, however, a great deal
of uncertainty in the estimation of reserves since this
will depend on advances in exploration technology as
well as production efficiency.
The price of oil and taxation policies will also
determine which type of oil accumulations can be
exploited economically and thus reckoned as reserves.
At present the world reserves are estimated to be
1:2 Â 10
12 bbl 2 Â 10
11 m
3
À
Á
and nearly 60% of this is
1 Introduction to Petroleum Geology
23