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The Oil & Gas Industry. 4. Seismic Survey. 6. How does a drilling rig work?. What happens if we make a discovery?. 1. Where does Oil and Gas come from?.

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The Oil & Gas Industry

4. Seismic Survey

6. How does a drilling rig work?

What happens if we make a discovery?

1. Where does Oil and Gas come from?

Oil and gas are hydrocarbons that were formed from the remains of plants and other organisms between layers of sediment around 300 million years ago. These liquid hydrocarbons gradually seeped through porous rock until they reached an impervious layer and became trapped, forming an oil or gas field. Oil in different parts of the world contains different proportions of the various hydrocarbons.

A land seismic survey involves firing pulses of sound energy down through the layers of rock beneath the Earth and recording the energy that is bounced back by the rock layers below the surface. The way in which the sound energy travels back depends on the kinds of rocks and the structures they form.

Think of it as a hand drill . . but 10,000 times bigger!The well is started by rotating the bit into the ground Drilling can continue to depths of several miles into the earth.

Well Log

1. Source Rocks – THE INGREDIENTS FOR OIL & GASThese contain organic material from plants or algae. They are buried and cooked below theearth’s surface for millions of years.2. MigrationOnce the oil is formed, it is forced by gravity to move out of the source rock and upwards towards the surface. This is a very slow process travelling only a few kilometres over millions of years.3. ReservoirThe oil or gas will flow until it collects in a reservoir rock, which has pores (holes) that acts like a sponge. 4. TrapTo trap the oil and gas the reservoir rock needs to have an impermeable rock, called a ‘caprock’ above it and around it. This caprock will not allow oil or gas to travel through it. If there is no caprock the oil and gas will reach the surface as an oil or gas seep.

Land Rig

Drill ship

Offshore rig

Seismic data acquisition on land

Signal emitted to vibrator truck

Data transmitted to a recording truck

Reflected waves received by geophones

Our activities in Uganda

How do we know if we have found oil or gas?

Once the well reaches its target depth, electric logging tools are lowered down the hole on a cable.

These logs can tell us about the quality of the rocks and the reservoir fluids.

To do a seismic survey at sea, a survey ship fires blasts of compressed air into the water. A long cable with receivers called hydrophones, which are sensitive listening devices, like microphones, is towed behind the ship to detect the echoes from the layers of rocks beneath the seabed.


If the well finds oil or gas - it can be tested to estimate the production rate.

The fluid in the reservoir is at a high pressure and will flow naturally to surface through tubing set in the wellbore.

If the well test is successful then a production platform will be installed to develop the field.

2. How do we know where the Oil and Gas is?

Seismic survey at sea

How does the search begin?

Rock structures that contain oil or gas are hidden from view by the sea or other rock layers but we can't just go around drilling lots of holes and hoping to strike lucky. We have to be more precise and start by doing a full survey of the area. This is where geologists and geophysicists, the scientists who study rocks and their formations, are really valuable.

A geologist looks at the makeup of rocks - where they are and the formations they make. A geophysicist, as the name implies, uses the physical characteristics of rocks - their magnetic and gravitational properties, and very importantly, how sound waves travel through different kinds of rocks - to help understand the structures below the Earth's surface.

Different surveying techniques can be used to locate the particular rock formations that might contain trapped oil. Geologists and geophysicists can build up a detailed picture of the rock structures, even though these are often many hundreds of metres beneath the Earth's surface using the results from these techniques.

7. The Crude Oil cycle

An Oil Refinery

Source Rock

An oil refinery is an industrial processing plant where crude oil is processed and refined into more useful petroleum products such as Gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas. Oil refineries are typically large sprawling industrial complexes with extensive piping running throughout, carrying streams of fluids between large chemical processing units.

Transporting Petroleum to Refinery

Seismic Vessel

Seismic section for interpretation

General survey techniques cover large areas quite quickly and easily and help to build a broad picture of the underlying rock structures. If the results are encouraging, and a promising area located, a second stage of exploration is undertaken and a more detailed seismic survey is carried out.

3. Why do we carry out an EIA Assessment?

A computer is used to measure and record the echoes, or pulses, that are reflected back to the surface. The geophysicists then have to interpret the data, which is displayed as a series of wavy lines, to build up a detailed picture of the rock formations. Seismic data can now be shown as a three dimensional picture using the latest computer technology.

An Environmental Impact Assessment (EIA) is an assessment of the possible impact-positive or negative - that a proposed project may have on the environment; considering natural, social and economic aspects.

The International Association for Impact Assessment (IAIA) defines an environmental impact assessment as "the process of identifying, predicting, evaluating and mitigating the biophysical, social, and other relevant effects of development proposals prior to major decisions being taken and commitments made."

The purpose of the assessment is to ensure that those proposing to undertake certain projects must show that their projects will not significantly affect the environment.

The result of an EIA is assembled in a document known as an Environmental Impact Statement (EIS) which looks at all the positive and negative effects of a particular project on the environment.

This is just one component of the information required to aid decision makers in making their ultimate choices about a project.

Railroad Tank cars

Tank truck

5. If we make a discovery?


Once the environmental and seismic surveys are completed, and if the results look good, drilling an exploratory well is considered. If large deposits of crude oil and gas are found, the company must apply to the Government to develop the oil field and bring it into production. The average time between discovering an oil field and piping oil ashore is four to five years. During that time various stages before production are gone through.

Crude oil is brought to a refinery by either tanker or pipeline. The refined products are then stored before local distribution by truck or flowlines to industrial users

The Geology team

8. Usage of oil and gas

Crude oil is separated by a heat process into many substances that we rely on for our normal daily lives.

For example road surfaces, coatings for medicines in the pharmaceutical industry, petrol for boats and cars and the building industry for roofing and piping.