Upstream Oil and Gas

Exploration

In this module, we’ll discuss the origins of oil and gas, the importance of depth and temperature, and we’ll talk a bit about oil and gas migration.

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Origin of Oil and Gas

The oil and gas industry is also referred to as the petroleum industry. “Petroleum” comes from the Latin terms “petra”, meaning rock, and “oleum”, meaning oil. The name is a perfect description, as oil and gas deposits are trapped in tiny openings in buried layers of rock, thousands of feet beneath the earth’s surface.

Oil and gas are also termed hydrocarbons because they are chemically made up of Hydrogen and Carbon atoms.

Almost all oil forms from the buried remains of minute oceanic algae and bacteria.

With the passage of time, enormous quantities of the dead organic material mixed with the mud and sand on the seafloor building layers of sediment that became hundreds or even thousands of feet thick.

Over the course of millions of years, the deep layers of sediment were transformed by heat and pressure into rock, and the dead organic material in the layers transformed into crude oil and natural gas.

Geologists are the people who study and understand these activities.

Sedimentary layers are commonly formed from large river deltas, like the Mississippi in the US or the Niger River in Africa. Sea levels have changed in some regions by over 300 feet, flooding the land with organically rich plankton and algae over the millennia.

By numerous geological forces, these sediments are buried deeply and slowly ‘cooked’ to yield oil or gas. Heat is available because the Earth’s temperature rises by 72 degrees Fahrenheit for every mile of subsurface depth.

Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks. The most common and desirable hydrocarbon source rock is black shale sedimentary rock.

Most oil and gas is found in and produced from what are called sedimentary basins. Sedimentary basins are wide geographic areas of these sedimentary rocks, in some cases with layers 10,000 to 50,000 feet thick.

Importance of Temperature and Depth

Whether oil or gas is formed depends primarily on two things:

The depth that the sediments are buried and
The length of time that they are under pressure.

Gas forms if these remains are deeply buried. The stems and leaves of buried land plants most often become coal. One exception is methane, generated at the surface through decomposition of plants in places such as swamps and forests.

The processes of oil and gas formation resemble those of a kitchen where the sedimentary rocks are slowly cooked. Since temperatures within the Earth’s crust increase with depth, sediments and contained hydrocarbons warm up as they become buried under thick piles of new sediments.

Again, this is a very slow process taking millions of years.

As a source rock deposited under the sea, river delta or lake, becomes hot (typically 50oC or 150oF) long chains of hydrogen and carbon atoms form a waxy and viscous heavy oil. These temperatures are achieved at depths of about 7,000 feet.

At higher temperatures, (typically up to 150oC, 300oF) and 18,000 feet deep, shorter hydrocarbon chains break away to give light oil.

Above 150oC, 300oF and below 18,000 feet, gas is formed.

Oil and Gas Migration

Another critical factor in understanding where to drill for oil and gas is the scope, extent and timing of oil and gas migration.

Oil and gas are not found in easy-to-access underground pools or puddles but, instead, are trapped in various rock formations and geological structures in quantities that can range from minute to massive.

Natural gas is sometimes mixed with oil but often found in separate reservoirs.

There are four key types of geological formations needed to have oil and gas reserves in place:

Source rock – where the oil or gas was formed

A Seal – which is impermeable and stops the migration of the oil and gas

Reservoir – where oil or gas reside, having migrated through microscopic structures in the surrounding rock

A Trap – which is the combination of the reservoirs and seals
Oil, gas and water migrate through permeable rocks in which the cracks and pore spaces between the rock particles are interconnected and are large enough to permit fluid movement. Fluids cannot flow through rocks where these spaces are very small or are blocked; such rocks are called impermeable.

Oil and gas are less dense than the water which also is present in the pore spaces, so they tend to migrate upwards. Oil and gas also migrate along large fractures and faults which may extend for great distances, as a result of underground movement of the strata.

For example, in the Gulf of Mexico, vertical migration pathways have been shown to extend for more than 20,000-40,000 ft (or 5-8 miles). Lateral migration can extend for more than 15 miles across multiple offshore blocks.

One of the main reasons for the acceptance of the basin-play model, is the realization that the history of oil and gas, especially its movements or migrations, is as important as the drilling history on an individual trap and reservoir. Any wildcat prospect inside or near a preferred migration pathway will carry a higher chance of success than one outside.

We discuss the basin/play model in more detail in subsequent lessons.