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In 1960, there were 5724 members of the SEG.

	In ancient times, oil was collected at the surface. In the 19th century, holes were drilled to depths of several dozen meters. Today, drillholes reach as far down as several thousand meters.
From the time of high antiquity, in ancient Mesopotamia, oil that had seeped to the surface was collected for medicinal use, as well as lighting fuel and caulking for boats. Today, now that we have been producing from accessible reservoirs for 150 years, it is increasingly hard to find hydrocarbon-impregnated rock. Explorers now have to look hundreds and even thousands of meters below ground.

	Map Hammer Acids Magnifying glass Logbook
The geologist's job is to observe, explore and scrupulously record any clue to the possible presence of hydrocarbons below ground. Geologists are people of action and naturalists. They examine rocks and take samples to ascertain their nature and date the strata from which they were taken. They then seek to reconstitute a scenario that may have been written 4 billion years ago.

	Aerial shot. Satellite shot.
Combined with aerial and satellite photographs, the geologist's observations then serve to formulate initial hypotheses: yes, there could be oil down there, below ground, and it could be worthwhile looking further.

Now it's the geophysicist's turn to study the physical properties of the subsoil. A variety of methods are used at this stage, and a comparison of their results serves to enrich the geologist's findings. Gravimetry measures gravity, to give some idea of the nature and depth of strata depending on their density. Magnetometry (generally performed from the air) measures variations in the magnetic field. This gives an idea of the depth distribution of crystalline terrains which have no chance of containing any oil.

	Signal emitted by vibrator truck Reflected waves received by geophones Data transmitted to laboratory truck
A surface shock generates sound waves which are refracted and reflected underground. The way in which the waves are propagated varies as they pass through the different strata. Using a highly-sensitive microphone known as a "geophone," the geophysicist at the surface listens to the echo of these waves and records them.

Isochrons 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.

Seismic vessel 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.

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.

Geophysicist 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.

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.

Drilling is 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 site 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.

Hoist attachment Derrick (mast) Traveling block Hook Injection head Mud injection column Turntable driving the drilling pipes Winches Motors Mud pump Mud pit Drilling pipe Cement retaining the casing Casing Drill string 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.

	Three-cone rock bit 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.

Mud pit Pump Injection line Injection head Drilling pipes Descending mud (in pipes) Returning mud (in annular space) Filter 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.

Well casing Cable retaining the downhole probe Downhole Probe First probe sensor Second probe sensor Third probe sensor 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.

Coring tool Core sample Indications concerning height of beds 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.

In this example, one hole in five struck oil

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.

Economic data Choice of operating methods 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 onstream is a major one, as the investment outlay can run into several hundred million, indeed a billion, dollars.

The content and images on this page are courtesy of ELF Aquitaine

Fifteen Steps
to
Finding Oil

#1. Surface Inspection

#2. Geologic Evaluation

#3. Satellite Imagery

#4. Gravity - Magnetic Interpretation

#5. Seismic Prospecting

#6. Seismic Interpretation

#7. Data Synthesis

#8. Decision Making

#9. Prospect Proposal

#10. Drill Site Determination

#11. Wildcat Drilling

#12. Well Logging

#13. Core Sampling

#14. Prospect Confirmation

#15. Economic Feasibility