Prodigy Oil And Gas Science
Gaining an understanding of the complex work that goes into the exploration and production of oil and gas can be a hefty job. While hardly a comprehensive list of the elements this works entails, the links and stories below are provided to give you a general understanding of how the petroleum industry functions and how the advances in the technology available for implementation has changed this occupation for the better.
How and Where Oil Was Formed
Geologists generally agree that crude oil was formed over millions of years from the remains of tiny aquatic plants and animals that lived in ancient seas. There may be bits of brontosaurus thrown in for good measure, but petroleum owes its existence largely to one-celled marine organisms. As these organisms died, they sank to the sea bed. Usually buried with sand and mud, they formed an organic-rich layer that eventually turned to sedimentary rock. The process repeated itself, one layer covering another.
Then, over millions of years, the seas withdrew. In lakes and inland seas, a similar process took place with deposits formed of non-marine vegetation.
In some cases, the deposits that formed sedimentary rock didn't contain enough oxygen to completely decompose the organic material. Bacteria broke down the trapped and preserved residue, molecule by molecule, into substances rich in hydrogen and carbon. Increased pressure and heat from the weight of the layers above then caused a partial distillation of the organic remnants, transforming them, ever so slowly, into crude oil and natural gas.
The oldest oil-bearing rocks date back more than 600 million years; the youngest, about 1 million. However, most oil fields have been found in rocks between 10 million and 270 million years old.
Trap, Reservoirs, Seal, Source and Timing
The physical geology of the earth has gone through several changes over time and many of these changes can create oil and gas. Considering that marine environments are prime geologic areas for the sedimentation needed to produce oil and considering that the surface of the planet is roughly 75% ocean covered it would seem that the production of oil would be prolific. The good news is that it is. The bad news is that during the times of oil formation most oil is not trapped. Understanding this, we can see that in oil exploration finding productive traps is the name of the game.
The 5 components of oil and gas traps vary in complexity but not in importance. Each one of them is mandatory in order to be successful. The 5 components are that you have to have biogenic source rocks that are deposited with significant organic material that doesn't have enough access to oxygen to decompose. These rocks must be covered until the overburden pressure and heat cause the break down of the molecular matter in the source rock in a geothermal environment that will form oil and gas.
As this oil and gas migrates upward along cracks and fissures, due to it’s lighter density, it must encounter an impermeable cap rock or seal. In that area, there must also be a reservoir rock that has porosity and permeability characteristics that will allow the oil to rest there instead of seeping around the seal. The geometry of the reservoir rock has to be as such that the reservoir and the sealing rock form a trap. Finally, all of this has to take place with a concurrent timing where the trapping seal is in place over the reservoir at the time of oil migration and the structure must keep each of these things in place until the time the prospect is discovered.
So to have a trap you need to have Source rocks, Reservoir rocks, a sealing impermeable overburden, a trapping mechanism and the correct sequence of timing.
Most geologists agree that oil and gas form from the preserved soft parts of ancient organisms that were buried, and then broken down and converted into petroleum by the combined effects of heat and time. Buried organic matter is called kerogen, and a petroleum source is any rock that contains enough kerogen to generate oil or gas. Most source rocks are shales with a total organic content (TOC) of at least 3%.
Kerogen is converted into oil when exposed to heats above 50 to 70 degrees Celsius (122 - 158 degrees Fahrenheit). Around 120 -150 degrees Celsius (248 - 302 degrees Fahrenheit) the oil is cracked to become gas. This “oil window” of temperature generally lies between the burial depths of 1000 to 3000 feet. As the oil and gas are formed the hydrocarbons are expelled to lower pressure areas. This expulsion of fluids and gas is called a migration and the importance of the time of migration will be covered in a little bit.
As oil migrated from high-pressure, high-heat, high-density areas it will make its way upward until it reaches the surface unless it encounters a seal. Seals are only common in the feature that they are impermeable. Permeability is a concept that is key to understanding in both regards to seals and reservoirs. Permeability is the ability for fluids or gases to flow through a substance. In the case of a seal, the migrating oil comes in contract with a layer that it can’t flow through as a result it is forced to move around the seal, if it can.
Seals are normally rock layers with low permeability. In most cases these are sedimentary layers that have undergone some degree of metamorphism. In some cases, igneous layers, sediments with smectite, or other clays, or the dynamic metamorphosed faces of faults can serve as an impermeable seal. In some cases, the very shale that functions as a source rock can serve as a seal for it’s self.
If a rock has enough porosity, the ability to hold fluids, and permeability to flow oil or gas, then it is a potential reservoir. Although it may not be very much, most rocks, in particular sandstones and conglomerates contain pore space. If enough pores are present, the pores are large enough, and they are interconnected so that fluids flow through them (i.e., the rock is permeable), then the rock is a potential petroleum reservoir.
With sandstones, a porosity of 18% or more is usually needed for an economic oil reservoir, and 12% or more for a gas reservoir. Less porosity, perhaps as little as 9%, is needed if the sandstone is also fractured. Because of fracturing, limestone and dolomite reservoirs can have much lower porosities than sandstone reservoirs.
Porosity and permeability are important, but a petroleum reservoir needs to contain hydrocarbons as well. In most rocks, the pores are filled entirely with a salty solution called formation water, but in a few some oil or gas is present as well. A general rule of thumb is that 40% or more of the pore fluids must be hydrocarbons (i.e., the water saturation is less than 60%). If the water content is greater, then oil tends to stay behind and the reservoir produces only water. These types of reservoirs are said to be "wet". If the water saturation is less, then the reservoir may be "productive".
Traps are the mechanism that allow for the accumulation of migrated oil and gas in reservoirs under seals. There are several types of traps but the general designations are Structural traps, stratigraphic traps and combination traps.
Structural traps form due to changes in the earth’s crustal composition as result of the pressures and stresses of tectonic movement. In a simplified form, as the earth’s crust moves, forces cause different reactions in the land due to its differing composition and some of this works to form traps.
Some common types of structural traps are anticlines, domes and fault blocks. Stratigraphic traps are formed by layers of deposition that were formed on the surface and have been covered by other layers over the years. Some common types of stratigraphic traps are lenses, depositional or erosional pinch-outs and carbonate reefs. Combination traps are a manifestation of both structure and stratigraphy being in play. Some common examples are eroded or deformed anticlines and salt dome traps.
All of the elements come together in the concept of timing. In looking at a play, the order of the other concepts as well as understanding the geographic history is very important. All of these elements have to be in place at the proper time in order to trap and fill a reservoir, as well as keep it in one piece until it is tested.