Exploration geophysics uses seismic, gravity, magnetic, electrical, and electromagnetic methods to search for oil, gas, minerals, and water underground. Seismic data is collected using vibrator trucks on land or seismic vessels at sea, and reflected waves are received by geophones or hydrophones. The data is processed to create 2D and 3D images of underground strata. Wells are drilled using a rig and drill string to further explore promising areas identified on seismic maps. If hydrocarbons are found, economic analysis is conducted to determine if developing the reservoir will cover costs and be profitable.

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Exploration geophysics
is the use of seismic,
gravity, magnetic,
electrical,
electromagnetic, etc.,
methods in the search
for oil, gas, minerals,
water, etc.,

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1. Signal emitted by
vibrator truck
2. Reflected waves
received by
geophones
3. Data transmitted
to laboratory truck

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1. Isochrones
2. 3D seismic Maps
The geophysicist's seismic
recordings are fed into powerful
computers. The terrain is mapped
by means of isochronic lines
linking points on the ground at
which the waves take exactly the
same length of time to be
reflected back to the surface. This
method yields two and three-
dimensional images of the
underground strata, and the
resulting seismic maps serve to
determine whether certain strata
are likely to contain
hydrocarbons.

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1. Seismic vessel
2. Hydrophones
In the oil man's jargon,
exploration and production at
sea is known as "offshore."
Because it is not practicable to
survey the terrain at sea, seismic
methods are used systematically.
And since ships can travel easily
in all directions, seismic
measurement is in fact easier at
sea than on land.
The geophysicist can thus obtain
more data offshore than onshore
and a more precise three-
dimensional image, once the
data have been processed.

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All these results are
aggregated and studied.
Geologists, geophysicists,
petroleum architects,
together with drilling,
production and reservoir
engineers all supply data to
economists and financial
planners. By juggling figures,
parameters and
probabilities, they seek to
work out a possible strategy
for developing the reservoir
in the event of confirmation
of the presence of
hydrocarbons.

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1. Geophysicist
2. Geologist
Each member of the exploration
team has contributed to the
performance of the mission. By
collating and comparing their
experience, know-how and findings,
their ultimate conclusions are the
result of a team effort. Those
conclusions are stated briefly:
No: the chances of a result are too
slim; or...
Yes: the "prospect", i.e. this highly
promising reservoir, is worth taking a
gamble. The team is prepared to
"pay to see," making the decision to
drill.

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FINAL DECISION. .
Geologists, geophysicists
and reservoir engineers
have concluded there is a
"prospect" or possible
producing zone. But to find
out whether there really
are hydrocarbons trapped
in the rock, they are going
to have to drill down to
that zone.

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1. Usually set up directly over the
thickest layer of hydrocarbons.
Some fields lie at depths
equivalent to twelve times the
height of the Eiffel Tower...
the siting of the drill rig is
determined based on the
existing state of knowledge of
underground conditions and the
topography of the terrain. This is
generally sited vertically above
the thickest part of the stratum
thought to contain
hydrocarbons. The drilling team
often operates under difficult
conditions. This narrow-bore
hole (with a diameter of 20-50
centimeters) is generally sunk to
a depth of between 2,000 and
4,000 meters. In a few cases it
may go beyond 6,000 meters,
and one has even gone to a
depth of 10 kilometers, or
30,000 feet.

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1. Hoist attachment
2. Derrick (mast)
3. Traveling block
4. Hook
5. Injection head
6. Mud injection column
7. Turntable driving the
drilling pipes
8. Winches
9. Motors
10. Mud pump
11. Mud pit
12. Drilling pipe
13. Cement retaining the
casing
14. Casing
15. Drill string
16. Drilling tool
The derrick, or "mast" in
oil slang, is the visible
part of the well. This is a
metal tower several tens
of meters tall, and its
serves to lower the "drill-
string" vertically into the
ground. This drill string is
in fact a set of drill pipes
screwed end-to-end. In
rotary drilling, this string
transmits the rotating
movement to the drilling
tool (of drill-bit) and
channels mud down to
the well-bottom as the
drilling progresses.

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1. Three-cone rock bit
2. Diamond drill bit
The drill assembly consists of
a derrick, drill-string, drive-
shaft, and the drill-bit itself.
The commonest kind of drill-
bit consists of three cones
made of extremely tough
steel capable of eating into
the rock face. When the rock
is very hard, a diamond-
tipped monobloc drill-bit is
used

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1. Mud pit
2. Pump
3. Injection line
4. Injection head
5. Drilling pipes
6. Descending mud (in pipes)
7. Returning mud (in annular
space)
8. Filter
9. Mud return for recycling
Specially-formulated mud,
prepared under the
supervision of the hoghead
(oil man's slang for the mud
engineer) is injected through
the hollow drill-string in
order to cool the drill-bit and
consolidate the walls of the
hole. The mud also helps
prevent the oil, gas or water
found in the strata crossed
from gushing out at the
surface. Finally, the mud
cleans the well-bottom and
carries the rock cuttings back
along the pipes to the
surface. The geologist
analyzes these cuttings to
understand the nature of the
rocks traversed and detected
signs of hydrocarbons

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Well Logging:
1. Well casing
2. Cable retaining the
down hole probe
3. Down hole Probe
4. First probe sensor
5. Second probe sensor
6. Third probe sensor
7. Measurements obtained
by the sensors
Once a certain depth has been reached, the
exploration crew conducts a series of
measurements known as well-logging. An
electronic probe is lowered into the well to
measure the physical properties of the rocks
traversed. These actual measurements either
confirm or disprove the hypotheses formulated
prior to drilling, and generally provide more
accurate data. The sides of the well are then
consolidated by means of steel tubes screwed
together, and the casing is cemented to the
terrain to keep the strata separate from each
other.

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1. Coring tool
2. Core sample
3. Indications
concerning height of
beds
4. Clues concerning type
of rock
The cuttings brought up the surface do
not supply sufficient information for a
thorough understanding of the rocks
traversed: that's where core sampling
comes in. The drill-bit is replaced by a
hollow bit called a coring tool, which
extracts a cylindrical sample of rock
several meters long. A study of the
resulting core sample yields information
about the nature of the rock, its slope,
structure, permeability, porosity, fluid
content, fossils present, etc.

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1. In this example,
one hole in five is
positive
Drilling progresses very gradually, at a
speed of a few meters per hour, slowing
to just one meter an hour by the time one
is down to 3,000 meters below the
surface. Snags are encountered from time
to time, and the entire drill-string has to
be pulled out regularly for a change of
drill-bit.
An exploratory well takes from three to
six months to drill. Four wells out of five,
or even six out of seven in pioneer zones,
fail to yield commercially viable
quantities of oil or gas. Sometimes,
though, the drill-bit strikes a
hydrocarbon-impregnated rock, in which
case the drilling crew conducts extensive
well-logging to find out more.

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1. Economic data
2. Choice of operating methods
3. Geological data
The exploratory phase has been
successful: a reservoir has been
identified, with the prospect of
producing profitably. Based on
assumptions as to future oil or gas
prices, the next step is to determine
whether sales of products extracted
from the reservoir will be sufficient to
cover the high cost of studies,
development, construction and
funding, as well as production costs
proper. The decision to bring a
reservoir on-stream is a major one, as
the investment outlay can run into
several hundred million, indeed a
billion, dollars

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