How Is a Potential Oil Deposit Located?

Useable hydrocarbons are located within geological formations made up of rocky layers called sedimentary basins. Seismic reflection- a technique that provides 3D maps of the substratum- is used to find deposits. Before prospecting an area, oil companies must obtain an oil drilling permit from the appropriate authorities.

Gas wellhead on the L7CC offshore platform in the North Sea, off the coast of the Netherlands (June 2010).
© Total / Dufour Marco

Oil-Bearing Regions: Sedimentary Basins

Above ground, there are many sedimentary basins, where superimposed rocky layers made up of sediments have accumulated over millions of years. Sedimentary basins are located beneath oceans, but also under continents, in regions that used to be under the sea. Oil and methane gas form within these basins, which is why oil companies are interested in them.

Oil companies seek to prospect recent basins where they hope to discover large quantities of useable hydrocarbons.

In oil exploration, there are different types of basins depending on their hydrocarbon resources and exploration history:

   • Prolific basins, such as the Niger delta, are rich in oil and gas or both.

   • Sterile basins either do not contain any workable hydrocarbons, or are depleted.
An example of the latter is the French Lacq basin, where all the natural gas has already been extracted.

   • Virgin basins are basins that have not yet been explored. Once there were many, but today they are rare. They are located in regions that are difficult to access because of hostile geography or climate. Some of them, such as the Antarctic, are protected for environmental reasons.

   • Immature basins, such as the Arctic Ocean, are those that have been relatively untouched by exploration. Oil companies seek to prospect these regions where they hope to discover large quantities of extractable hydrocarbons.

   • Mature basins, such as the North Sea, are those that have been known about for a long time. There is little chance of finding sizeable new deposits because they have already been drilled many times. However, companies continue to explore these areas in the hope of finding smaller or hard-to-find deposits.

Obtaining an Oil Drilling Permit Prior to Prospecting

In regions with high oil or gas potential, oil companies compete with each other to explore the sedimentary basins.

Oil companies can drill anytime, anywhere.

False. Before drilling underground in a country, oil companies must obtain an oil drilling permit authorizing them to explore an area with significant hydrocarbon potential.

This also applies to areas within the a country's territorial waters, i.e. less than 200 nautical miles (370 km) from the coastline. This is why defining sea borders can sometimes cause disputes when there is oil beneath the sea bed.

To unlock the value of their underground resources, states seek to grant permits to the highest bidder. This is why they auction off these permits through international tenders. Interested companies send in detailed proposals that include a commitment to:

   • The total amount they intend to invest in prospecting the area

   • The volume of work they intend to carry out to explore this region

Their bids cover a specific prospection period, generally 2 to 5 years.

By investing in this type of project, these companies are taking a risk. if their proposals are selected, they will have to carry out very expensive research operations, with no guarantee that they will find any extractable hydrocarbons. This is why two or three companies often work together. If they do not discover oil or gas, they share the expenses.

Once all the tenders have been received, the country looks at the bids and selects a company or group of companies to explore the relevant site. The winning company is granted a permit authorizing it to start work. Once they have this document, the companies can also sell some or all of their interests in the area to be prospected. This has given rise to a kind of permanent global market in regions with exploration potential.

The market value of these sites changes over time, depending on the results of the drilling operations.
Each oil company has its own team of specialists who continuously monitor all offers for sale of interests worldwide (i.e. all offers for sale and exchange of permits to work a potential
oil-bearing site). These transactions give rise to hotly contested negotiations.

Using Seismic Reflection to Explore the Earth's Depths

Once they have their permit, companies start to look for potential oil traps buried underground. Those located near the earth's surface were already explored a long time ago. These reservoirs were easy to locate - hydrocarbons seeping out of a rock face or underground geological layers bulging above the surface gave them away.

But oil companies quickly realized that this visual detection method was not enough because:

   • Many potential traps are hidden by thick sedimentary deposits.

   • Traps under the seabed are invisible to the naked eye.

In the 1930s, engineers developed a technique that uses a form of underground echolocation to locate deposits  called seismic reflection. This involves several stages:

   • First, vibrator trucks send sound waves underground by dropping a heavy object on the ground or with an explosion.

   • Each time they encounter a rocky layer, the waves' propagation speed changes. Some of them go deeper underground while others are reflected as if by a mirror and return to the surface.

   • Sound waves returning to the surface are recorded with highly sensitive receptors, called geophones. The first of these are reflected by the first geological layer, the next ones are reflected by the second layer, and so on. At sea, seismic recording is done from a boat that tows a stream of floating receivers, called hydrophones. This operation is easier because there are no natural obstacles to stop the transmitters or receivers from being moved around to take all the required measurements.

Interpreting Seismic Data to Obtain a Picture of the Substratum

The data gathered using reflection seismology is highly complex and can only be interpreted by geophysicists. Significant computing power similar to that used in weather forecasting is required to process this data. With these measurements, several types of pictures of the substratum are obtained, revealing more about its physical properties, such as:

   • The speed at which the waves reflected by the geological layers move between their place of transmission and their receiver varies for each rocky layer, depending on the type and shape of the rocks. When this speed is known, a 2D image that provides information on the substratum's morphology (i.e. its shape and the type of rock it contains) is put together.

   • Even more information can be obtained using 3D seismic imaging, which is more accurate and more reliable than 2D seismic imaging. A 3D image of the site is then built, providing information on volume. This image is used to directly identify hydrocarbons in the geological layers

   • The 4D seismic method is even more sophisticated, as it uses the fourth dimension, i.e. time. This involves carrying out a number of successive 3D recordings on a producing deposit at regular intervals. By comparing these recordings, the deposit's development can be monitored during extraction.

Unfortunately, seismic imaging is not 100% accurate or reliable, for various reasons:

   • Problems can arise when recording waves, compromising quality.

   • Soft or uneven ground can cause alterations in the waves that are often difficult to correct when processing the data.

   • For regions located far underground, waves reflected by rocky layers get weaker as they have further to travel before returning to the surface. This sometimes results in miscalculations that can produce fake images just like mirages.

   • In regions that have not been extensively drilled, wave speed is often unknown and this can lead to errors in the seismic sections and substratum maps.