A source rock is a rock that can generate natural gas and/or crude oil. As sediments are deposited, both inorganic mineral grains, such as sands and mud, and organic matter (dead plants and animals) are mixed.The black color in sedimenary rocks comes primarily from its organic content.
When woody plant material is buried, it is transformed into coal and methane gas (CH4) by temperature and time.This is why coal mines are dangerous; they contain methane gas and sometimes explode.Shale is the most common sedimentary rock, and many are black. Black shale commonly has 1 to 3% organic matter by weight and can have up to 20%. Green or gray shale has only about 0.5% organic matter.
The most important factor in the generation of crude oil from organic matter in sedimentary rocks is temperature.The deeper the depth, the higher the temperature.At relatively shallow depths, the temperature is not sufficient to generate oil. There, just a few feet below the surface, bacterial action on the organic matter forms large volumes of biogenic or microbial gas. Biogenic gas is not commonly trapped and usually leaks into the atmosphere in enormous volumes.
In a typical sedimentary basin, oil generation starts at about 150˚F (65˚C) and ends at about 300°F (150˚C). If the source rock is buried deeper, where temperatures are above 300°F (150°C), thermogenic gas is generated. It is the gas that is often trapped.The zone in the earth’s crust where the oil is generated is called the oil window.Crude oil generated in the oil window is originally good oil with ˚API gravities between 30 and 40. Where does heavy oil come from? It is formed later when bacteria, along with chemical and physical processes, degrade the good oil to form heavy oil.Wet gas is formed under more shallow depths and cooler temperatures. Under higher temperatures at deeper depths, dry gas is formed.At temperatures higher than about 300°F (150˚C), crude oil is irreversibly transformed into graphite (carbon) and natural gas.
Maturity is the degree to which petroleum generation has occurred in a source rock. A mature source rock has experienced the temperature and time to generate petroleum in contrast to an immature source rock.
After gas and oil are generated in shale source rock, some is expelled from the impermeable shale. The generation of a liquid (crude oil) or gas (natural gas) from a solid (organic matter) causes a large increase in volume. This stresses the source rock and fractures the shale. The hydrocarbons escape through the fractures. After the pressure is released, the fractures close, and the shale becomes impermeable again.
Because gas and oil are light in density compared to the water that also occurs in the pores of the subsurface rocks, petroleum rises. Oil and gas can also flow laterally and upward along unconformities and through carrier beds.Carrier beds are rock layers that are very permeable and transmit fluids. The vertical and lateral flow of the petroleum from the source rock is called migration. If there is no trap on the migration route, the gas and oil will flow out onto the surface as a gas or oil seep. If there is a trap along the migration route, the gas and oil can accumulate in the trap. Of all the gas and oil that form in sedimentary rock basins, only from 0.3 to 36% is ever trapped.The rest of the gas and oil did not get out of the source rock, was lost during migration, or seeped into the earth’s surface.
The trap must be in position before the gas and oil migrate through the area. If the trap forms after the migration, no gas or oil will occur in the trap.Once the gas and oil migrate into the trap, they separate according to density. The gas, being lightest, goes to the top of the trap to form the free gas cap, where the pores of the reservoir rock are occupied by gas. The oil goes to the middle of the trap, the oil reservoir. Saltwater, the heaviest, goes to the bottom.
The most common trap is a saturated pool, which always has a free gas cap on top of the oil reservoir. The oil in the reservoir has dissolved all the natural gas it can hold and is saturated. An unsaturated pool lacks a free gas cap. The oil has some dissolved gas, but it can hold more and is unsaturated. Sometimes there is only a gas reservoir on water.
The boundary in the reservoir between the free gas cap and the oil is the gas-oil contact.The boundary between the oil and water reservoir is the oil-water contact. The first exploratory well is usually drilled on-structure, on the crest of the structure where the probability is highest that petroleum will be encountered. If a well is drilled too far off-structure, it might not encounter commercial amounts of oil or gas and is called a dry hole, duster, or wet well.
In a trap, the reservoir rock must be overlain by a caprockor seal, an impermeable rock layer that does not allow fluids to flow through it. Two common caprocks are shalesand salt layers.
A field is the surface area directly above one or more producing reservoirs on the same trap, such as an anticline. A reservoir is a subsurface zone that produces oil and gas but does not communicate with other reservoirs. Fluids cannot flow from one reservoir to another. The oil or gas in a single reservoir has the same characteristics throughout the reservoir but can be very different between reservoirs in the same field.