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MIGRATION
The preceding sections discussed the basic formula for the origin of petroleum. We examined the raw material, its accumulation and preservation in sediments, and we just finished examining its transformation first into kerogen and then into hydrocarbons. The last section deals with migration: how and why fluid hydrocarbons migrate from a source rock (rock material where they formed) to the reservoir rock (rock material where they are found). We continue to use the basic formula:
Petroleum End Product = [Raw Material+Accumulation+Transformation+Migration] + Geologic Time
There are two types of migration when discussing the movement of petroleum, primary and secondary. Primary migration refers to the movement of hydrocarbons from source rock into reservoir rock and it is this type that the following discussion refers to. Secondary migration refers to the subsequent movement of hydrocarbons within reservoir rock; the oil and gas has left the source rock and has entered the reservoir rock. This occurs when petroleum is clearly identifiable as crude oil and gas although the gas may be dissolved in the oil. Buoyancy of the hydrocarbons occurs because of differences in densities of respective fluids and in response to differential pressures in reservoir rock.
There are two important concepts that must be understood and how they relate to source rocks and reservoir rocks in order to discuss migration. They are porosity and permeability.
Porosity refers to the percentage of total volume of a material that is occupied by voids or air spaces that exist between the rock grains. The more porous a material is, the greater the amount of open space, or voids, it contains. Stored in these voids are liquids and gases. Porosity differs from one material to another. Unconsolidated deposits of clay have the greatest porosities because of their crystallographic structure; they are comprised of parallel sheets of clay minerals. Unconsolidated deposits of sand have lower porosities because of the nature of the sand grains to each other. Source rocks have high porosities; the best source materials are clays & shales, but these same materials make poor reservoir rocks.
Permeability (measured in centimeters per second) refers to the ability of a material to transmit [fluid or gas]. The rate at which a material will transmit a fluid or gas depends upon total porosity, number of interconnections between voids, and size of interconnections between voids. For example, although clay has a higher porosity than sand (clay has a greater number of voids), the voids that make up the clay are not interconnected and therefore can not transmit the fluid or gas out of it. The permeability of a typical clay in Louisiana would be 1 x 10-7 cm/sec, or a movement of about 3 feet in 30 years. Therefore movement of a fluid or gas out of a clay is very difficult. Sand on the other hand has a typical permeability of 1 x 10-5 cm/sec, or a movement of about 300 feet in 30 years. Therefore sand has greater permeability than clay.
Answer: Reservoir rocks have higher permeabilities. The best are generally sandstones and carbonates, but this type of rock make poor source rocks.
There is considerable disagreement regarding why and how migration occurs and the theories are many and varied; it is the last problem in petroleum geology to be solved. Most scientists agree that the vast majority of petroleum hydrocarbons are generated by thermal processes from organic material contained in certain types of sedimentary rocks referred to as source rocks. Oil and gas generally do not originate in the reservoir rock in which they are found. There are 3 primary observational evidence that suggests that the hydrocarbons migrated into reservoir rocks at considerable depth below the surface and at some time after burial:
All of the observations suggest that hydrocarbons migrate into reservoir rocks at considerable depth below the surface and at some time after burial. But these three observations create a major paradox to resolve. Oil and gas are trapped in permeable reservoir rock, yet the source rock from which they came are shale (clay) that is essentially impermeable. So how did the fluids move from the source rock to the reservoir rock? Furthermore, why does it leave the source rock and how? There are a number of ideas, but no definitive answers.
One idea is that it is squeezed from the source material (organic rich clay) before compaction destroys permeability, but there are a couple of problems with this. First, most water (pore water) expulsion by compaction occurs in the upper 2 km of burial before hydrocarbons have a chance to form. And second, this compaction occurs at temperatures lower than that required for the creation of hydrocarbons. So it would appear that migration of hydrocarbons from source rock by flushing of pore water is not viable option. Or is it? Could hydrocarbons be flushed from source rock AFTER compaction has destroyed most of the permeability? But how can this be? Compaction reduces permeability, yet permeability is needed to move liquids and gases through a material. If compaction reduces permeability, how can you push a fluid or gas through it?
The answer lies in the fact that there are two types of water in clays, pore water which is water trapped in sediments at time of deposition and structured water or water bonded to the layers of the clay minerals. The process of hydrocarbon expulsion works something like this. Organic-rich clay is deposited on the sea floor as montmorillonitic-rich muds. The expulsion of water occurs in two phases at different periods of time. The early phase is the time frame in which pore water is removed by compaction that occurs in upper 2 kilometers (km) of sediments. A second phase occurs later in time where structured water is de-linked from the clay lattice and expelled by further compaction. This phase occurs at deeper depths and at temperatures that vary from 100 - 110o C. If you refer back to Figure (pg 198, Selley), you will see that temperatures ranging from 100 - 110o C occur in the middle of the oil window, meaning it is this temperature at which hydrocarbon production is in full swing. The second phase is also a time where compaction causes the collapse of the montmorillonite lattice which forms a new clay mineral called illite. The whole process of water expulsion is called clay dehydration.
But how are the hydrocarbons expelled from the sediments? The exact physical and chemical process not clear, but all theories postulate that oil migrates from the source bed as a discrete oil phase. The hydrocarbons are attached to the structured water in the montmorillonite clay lattice. The structured water is de-linked from the clay lattice and expelled during the second phase of expulsion. Upon further dehydration, hydrocarbons become detached from the structured water before illite is formed and are in a free-phase as discrete hydrocarbons. But there are some other important factors besides dehydration. Differential pressure creates gradients and therefore induces flow of the discrete hydrocarbons, temperature may heat the hydrocarbons which induces them to move more freely in the subsurface, and the presence of wetting agents (natural soaps) enhance solubility of hydrocarbons which also aids in their migration elsewhere. Also important in expulsion of hydrocarbons from the sediments are micro fractures in the source rock because they create pathways for migration, and the presence of gases. Natural gas goes into solution at great depths because of pressure, but condenses and becomes a free-phase component at shallow depths. And the presence of other gases such as CO2 lowers viscosity thus increasing mobility.
Sources:
"Geology of Oil," Steven Cooperman, Ph.D.
"Understanding Petroleum Exploration and Production," National Energy Foundation, Student Activity Guide
"The Upstream: A Guide to Petroleum Exploration and Production," Exxon Corporation Informational Brochure
NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester,
MA.
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Question: Which would have higher permeabilities; Reservoir rocks or source rocks?