Chapter 1: Where do oil and gas come from?

Combination of Conditions

A combination trap has a combination of stratigraphic and structure element traps as seen in the photo.

Permeability

A rock is permeable, or has permeability, when the pores of the rock are connected. Permeability provides a pathway for any oil and gas in the rock to move or flow from pore to pore.

A rock with pores

A rock with pores is porous and has porosity. Any oil and gas in the rock resides in the pores.

Structural trap

A type of geological trap that forms as a result of changes in the structure of the subsurface, due to tectonic, diapiric, gravitational, and compactional processes. They are the most important type of trap as they represent the majority of the world's discovered petroleum resources. The three basic forms of structural traps are the anticline trap or folding, the fault trap, and the salt dome trap.

Fracture porosity

A type of secondary porosity produced by the tectonic fracturing of rock. Fractures themselves typically do not have much volume, but by joining preexisting pores, they enhance permeability significantly.

5 main types of reservoir fluids

BLACK OILS, VOLATILE OILS, CONDENSATE GAS, WET GAS & DRY GAS

BLACK OILS

Black oils are made up of a variety of large, heavy, and non-volatile hydrocarbons. When the reservoir pressure is anywhere along line 1 → 2, the oil is said to be undersaturated - meaning the oil could dissolve more gas if more gas were present. If the pressure is at 2, the oil is at its bubble point, and is said to be saturated - meaning the oil contains the maximum amount of dissolved gas and can't hold any more. A reduction in pressure at this point will release gas to form a free gas phase inside the reservoir. Additional gas evolves from the oil and causes some shrinkage of the oil, That is why Black oil is often called low shrinkage crude oil.

Hydrocarbons

Both oil and natural gas are called hydrocarbons because they are made up of two elements: hydrogen and carbon

Salt Dome Trap

Caused by salt formations allowing the hydrocarbons to migrate on both sides or above the salt column because salt is impervious, meaning it does not permit passage.

Conventional Resources

Concentrations of oil or gas that occur in discrete accumulations or pools. Conventional oil and gas pools fall into several categories based on the mechanism responsible for the trapping or pooling of the hydrocarbon: 1. Structural traps 2. Stratigraphic, 3. And a combination of conditions

Fold Trap

Constitute as the twists and bends in rocks without fracturing at high confining pressures. This is known as ductile deformation and the rock is said to behave plastically. Traps formed because of folding include an anticline trap.

Volatile oils

Contain fewer heavy molecules and more intermediate components (ethane through hexane) than black oils. The phase envelope for a Volatile oils tend to cover a much narrower temperature range when compared to a black oil; but like a black oil, the reservoir temperature is always lower than the critical temperature for the fluid. As the reservoir temperature approaches the critical temperature a volatile oil will become more gas-like and will have a relatively low liquid content.

Paleogeomorphic traps

Controlled by buried landscape. (hills); (valleys) and unconformities.

Earthquakes

During the same time kerogen was transformed into oil and gas, there was a lot of plate tectonic movement and earthquakes that changed the face of the Earth. As the plates moved, the layers of rocks were folded into arches and troughs. The plate tectonic movement forced the newly formed oil and gas upwards through the pores of the source rock, which resulted in the formation of hydrocarbon traps. The overlying rock beds created extreme pressures that forced the newly formed petroleum to flow through cracks and fissures in rocks and ooze from pore to pore. The fluids constantly moved in a twisting path, seeking the surface by way of moving up an arch. Sometimes the petroleum actually reached the surface. But sometimes, its upward movement was blocked by an impermeable rock layer deep below the surface. This type of oil and gas trapped below the surface is what we typically drill for today.

Impermeable Rock

Fluids move up until they reach this layer (tight, clay, shales) also known as a caprock. Forms above porous/permeable rock layer

Stratigraphic traps

Formed as a result of lateral and vertical variations in the thickness, texture, and porosity of the reservoir rock. Examples of this type of trap are an unconformity trap, a lens trap and a reef trap.

Igneous rock

Formed from magma when it cools and can be found closer to the mantle in thinner parts of the crust, as well as around volcanoes

Vugular porosity

Formed in sandstones when water flows through it to make more holes and pathways

Sedimentary rock

Formed through erosion and deposition, and there are a few types of sedimentary rocks that we deal with in particular, such as shale, sandstone, limestone, dolomite, and anhydrite. These are the ones that are more petroleum-rich.

Hydrocarbon Traps

Happen in rock layers that have moved and trapped the oil and gas under a roof of impermeable rock

Sedimentary Processes

In order for oil and natural gas to form the sedimentation process must occur. The sedimentation process starts with weathering, which is the physical breakdown of rocks. Next, these broken-down rocks, algae, sand, silt, and mud are transported down rivers through the process of erosion. The organic material is then deposited into the sea and as the microscopic organisms die, they sink to the seafloor and are buried by many layers of organic material. *** → The fraction of organic material that is transformed into oil and gas is called Kerogen. The weight of all the layers of sediment caused a great amount of pressure and heat which compacted the bottom layers of organic material into rock through the process of lithification.

Metamorphic rock

Is created when other rock formations undergo changes due to heat and pressure to form new rocks

Oil Pool

It is not an open pool underground that stores oil, rather a portion of rock that contains fluids, called a reservoir

Unconformity

Lack of continuity between rock beds, or strata. Impermeable rocks laid down on top of the weathered surface of the lower beds stop hydrocarbons from escaping upward. Unconformities occur over long period of time from multiple layers of rock continuously deposited on the bottom of the sea. Eventually, the rocks stopped depositing, perhaps due to the sea's retreat or evaporation. Over time, the earth's movements caused the layers to gradually tilt. The layers exposed tot he atmosphere were eroded by wind, water, or ice. After millions of years, environmental changes deposited new beds on top of old, weathered ones. The final result can be seen today as a series of beds in contact with one another but with a gap in the sequence of layers. The gap was caused by the period of time when no rock-forming sediment was deposited.

Strata

Layer of sedimentary rock. The crust is made up of layers, called strata, which signify a change. Mississippian + Pennsylvanian 65 million years This is when we believe oil and gas reservoirs were formed

Physical Structure of the Earth

Lithosphere - the entire physical body of the earth, excluding the hydrosphere Crust - land and ocean floor; average width is 20 miles Mantle - Ductile mass; about 1,800 miles wide Outer Core - Liquid mass; 7,000-9,000 degrees F Inner Core - Solid mass made mostly of iron; 8,500 degrees F

High permeability

Making it easy for fluids such as oil and gas to move through the rock's pores

Low permeability

Making it harder for fluids to move about within the rock

Petroleum

Mixture of hydrocarbons forms an oily, thick, flammable liquid Called crude oil, or petroleum. Word comes from greek word for rock, Petra, and Latin word for oil, oleum - combined they mean Rick oil

Intergranular porosity

Most common type found between grains of sediment. Limestone and dolomite have intergranular porosity. Fracture porosity comes from fractures formed from stress and forces on the rock.

Wet gas

Natural gas that contains significant heavy hydrocarbons such as propane and butane. Wet gas exists solely as a gas in the reservoir throughout the reduction of pressure. Unlike retrograde condensate, no liquid is formed inside the reservoir. However, separator conditions lie within the phase envelope, causing some liquid to be formed at the surface. This surface liquid is normally called condensate, and the reservoir gas is sometimes called condensate-gas, which can cause confusion between condensate and wet gas.

Dry Gas

Natural gas that occurs in the absence of condensate or liquid hydrocarbons, or gas that had condensable hydrocarbons removed Its is primarily methane, AND this hydrocarbon mixture is solely gas in the reservoir As you can see as the pressure declines the pressure depletion path never enters the 2 phase region.

Low porosity

Other rocks have few pores or very small pores and, therefore, have low porosity.

Earthquake - Fault Plane

Planar (falt) surface along which there is a slip during an earthquake

Reservoir

Porous rock that contains hydrocarbons (oil)

Secondary stratigraphic traps

Result from variations that developed after sedimentation.

Fault Trap

Result in a fracture or break in rock layers caused by the earth's movements and external force applied to buried rocks under low confining pressures. This is known as a brittle deformation.

Where oil and gas is stored

Rocks contain many holes, or openings; some are so tiny they can be seen only under strong magnification called pores or pore spaces

Connate Water

Salt water. No matter what type of trap, oil and gas seldom fill all the rock in the reservoir, Salt water usually takes up some space. Connate water lies underneath the oil and gas in small pore spaces of the rock. The salt water often coats the grains of the rock where the oil and gas occur. This water is actually the ancient seater that existed when the formation was originally laid down. As hydrocarbons move into the reservoir, they move or displace much of the salt water in the rock pores. Still, pore spaced could contain from 10-to 50 percent or more salt water, even int he midst of oil and gas

Directional permeability

Some rocks are not vertically permeable, usually when there are layers of shale in between the other rock (called shale breaks). Therefore, we have to drill through them horizontally in order to avoid the shale.

High porosity

Some rocks are quite porous and are labeled as having high porosity

Organic Theory

States that oil and gas come from the remains of plant organisms that were microscopic in size. The plants in rivers were carried along with river silts and mud down to the seas, which were also full of microscopic plant life. As these small organisms died, they fell to the ocean floor where they mixed with the silt, sand, and mud. As a result, a rich, organic mixture called kerogen, formed. This mixture was cut off from any oxygen dissolved in the water. With no oxygen, the organic material could not decay as it normally would on land. As thousands of years passed, the silt, sand, mud, and organic material continued to build up. Eventually, a thick body of sediments enriched by organic remains accumulated on the bottom of the ocean. As more time passed, sediments continued to be deposited on top until thick beds of sand and mud lay above the body of sediments containing organic remains. The great weight of the overlying sediments pushed the lower sediments deep into the earth and caused changes to occur within the lower layer. As a result, the bottom beds turned into rock. The high heat and intense pressure also worked on the organic substances locked in the lowest layer of rock, Bacteria, chemical reactions, and other forces were hard at work, too. The end result: the organic remained were transformed into oil and gas.

Primary stratigraphic traps

Stratigraphic traps result from variations in characteristics of the rock that developed during sedimentation.

Tectonic plates

The crust consists of 8-9 large tectonic plates with some smaller ones on top of the mantle. Plates drift about ½ inch each year to form mountains and valleys over time. They also cause natural disasters like earthquakes, tsunamis, and volcanoes

Kerogen

The environment for kerogen to transform into oil and gas must have immense pressure, high heat, lack of oxygen, and have the right chemical environment. When all of these criteria are met, oil and natural gas are formed.

Source bed

The layer of rock that was the origin of a valuable material (hydrocarbon place of origin)

Oil, gas & salt water densities

The oil, gas, and water in a reservoir are generally presented in an organized manner due to their different weights, or densities. Gas occurs int he highest part, oil occurs below the gas, and most of the salt water lies below the oil and coats the pore spaces with the gas and oil. However, sometimes other combinations are present, only oil and water, only gas and water, or only water.

Earthquake - Epicenter

The point on Earth's surface directly above the focus.

Earthquake - Hypocenter

The point within the earth where earthquake rupture (extent of slip that occurs during earthquake) starts

Unconventional Resources

They are oil or gas bearing units where the permeability and porosity are so low that the resource requires a horizontal wellbore followed by multistage hydraulic fracturing and is primarily composed of shale

Opportunity to produce oil is promising when...

When a rock has pores containing oil and gas, and when the rock is permeable. This means it might be possible to drill a well down into the rock to let the oil or gas flow out of the pores through the rock's permeable channels, and bring the oil and gas up the well

Shale Oil

When some petroleum bearing rocks are porous, but not permeable. Because the rock is not permeable, the hydrocarbons have no way to flow out of the rock pores, so wells cannot produce them. The hydrocarbons cannot be drawn into the hole drilled for the well, or the wellbore. To get the hydrocarbons out of this type of rock, advanced mining techniques and other innovative technology are required.

Condensate gas

is very similar to volatile oils in terms of the colour and gravity. However, the reservoir temperature of a condensate gas is greater than the critical temperature of the fluid, and so where a volatile oil is a liquid at original reservoir pressure and temperature, a condensate gas is a gas But as pressure drops, liquid condenses from the gas to form a free liquid inside the reservoir. In the diagram to the left, the condensate gas exists completely in a gaseous state inside the reservoir at point 1. As the pressure decreases, the condensate exhibits a dew point at point 2. As the reservoir pressure drops, liquid condenses from the gas to form a free liquid inside the reservoir.