Faujin The Plover Formation source rock was a poor-to-good hydrocarbon generative potential and reached the geolohy to late mature oil window in the Sunset-Loxton Shoals field whereas in the Chuditch field, it was an overall fair-to-good hydrocarbon generative potential, and attained the Late mature oil window. Amazon Inspire Digital Educational Resources. Browse titles authors subjects uniform titles series callnumbers dewey numbers starting from optional. Scientific Research An Academic Publisher. More tools Find sellers with multiple copies Add to want list.

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Get Permissions Abstract Geologic framework. Short or long migration. Primary migration: water squeezed out of clays — normal water circulation — sedimentary oil — recycled oil.

Secondary migration: entrained particles — capillary-pressure — displacement-pressure phenomena — buoyancy — dissolved gas effects — accumulation — tilted oil-water contacts — stratigraphic barriers — vertical migration — time of accumulation — petroleum supply.

Once petroleum has reached the reservoir rock, it must be concentrated into pools if it is to be commercially available. As we have seen, it was deposited in the nonreservoir shales and carbonates as disseminated soluble hydrocarbon particles associated with the nonsoluble organic matter. By the time diagenesis was complete most of the petroleum hydrocarbons were probably in the form of petroleum.

During and after diagenesis, water was squeezed out of the nonreservoir rocks into the reservoir rocks, and a fraction of the petroleum and petroleum hydrocarbons was entrained in the water. Petroleum and petroleum hydrocarbons probably continued to come out of the nonreservoir rocks after diagenesis and some may have been deposited directly in the reservoir rocks.

The movement from the nonreservoir rocks to the reservoir rocks is called the primary migration to distinguish it from the concentration and accumulation into pools of oil and gas, the secondary migration. There are many facets to the problem of the migration and accumulation of oil and gas into pools. Because of the apparent complexities of the problem and the many opportunities for speculative and imaginative analysis, many theories Geologic framework.

Because of the apparent complexities of the problem and the many opportunities for speculative and imaginative analysis, many theories have been devised to explain the phenomena that have been observed.

We must admit that we do not have a completely satisfactory answer to the problem of the migration and accumulation of petroleum, and in this connection, we should again remind ourselves that no one has ever seen an oil or gas pool forming, nor has anyone ever seen an oil pool in place. All of our information is from well and production records.

As drilling continues, a vast amount of new data is continually being added, and this eventually should clarify many ideas that are now in doubt. Presumably the petroleum entrained in the water squeezed out of the water-saturated nonreservoir rocks was dispersed in minute particles, possibly of colloidal or microscopic size, and some might even have been in solution in the water.

In the absence of any unbalanced forces, such petroleum might remain more or less stationary for a long time and might become deeply buried. Local fluid potential or temperature gradients might cause local movements, but it would require some regional disturbance or change to cause regional movement.

This might be regional folding, tilting, mountain building, or warming—possibly due to igneous activity or to various changes in the hydrodynamic gradients. The normal geologic history of most sedimentary regions includes many such episodes that could upset the equilibrium of the reservoir fluids, and each may have caused some increment of fluid movement.

Before we discuss the theories and many facets of the problem, it would be useful to point out two of the fundamental aspects of the problem that should be kept in mind: 1 the geologic framework within which petroleum migration and accumulation must have occurred, and 2 the distances through which petroleum has migrated. Are we limited to short distances of migration—say less than a mile? Such a frame can at best be only general, for it includes many variables, both known and unknown.

The sides of the frame might be summarized as follows: Nearly every petroleum pool exists within an environment of water— free, interstitial, edge, and bottom water. This means that the problem of migration is intimately related to hydrology, fluid pressures, and water movement.

The interstitial water content of the reservoir rocks generally shows a hydrodynamic fluid pressure gradient, as between wells, and this indicates that the water, which is confined and in phase continuity throughout the reservoir rock, is in motion. The water moves in the direction of the lower fluid potential, and the rate varies with the magnitude of the difference in fluid potential and the transmissibility of the aquifer.

The rate may be small and measured in inches or a few feet per year, but the effect of the hydrodynamic conditions may be extremely important to the movement of the petroleum. The gas and oil are chiefly immiscible in the water, and both are of lower density than the surrounding water. Reservoir rocks that contain petroleum differ from one another in various ways. They range in geologic age from Precambrian to Pliocene, in composition from siliceous to carbonate, in origin from sedimentary to igneous, in porosity from 1 to 40 percent, and in permeability from one millidarcy to many darcies.

There is a wide variation also in the character of the trap or barrier that retains the pool. The trap may have been chiefly due to structural causes, to stratigraphic causes, or to combinations of these causes. Where there is a fluid potential gradient in the reservoir rock unit, it may form a barrier to the movement of petroleum and become an important trapping agency; it is a trapping element that combines readily with the structural and stratigraphic elements.

The microscopic shapes and sizes of the porosity, the tortuous paths of permeability, and the chemical character of the reservoir rock may vary widely in complexity. It is within these pore spaces and chemical environments that the migration and accumulation must take place. The minimum time for oil and gas to originate, migrate, and accumulate into pools is probably less than one million years.

See p. The evidence for this lies in the fact that in some pools the trap was not formed until Pleistocene time. An example is the Kettleman Hills pool in California; the oil and gas of this pool are in the Miocene Temblor formation, but the fold that forms the trap cannot be earlier than Pleistocene, for the Temblor formation fold is parallel to the Pleistocene rocks at the surface of the ground.

An illustration of the short time necessary for a pool to adjust itself to a change in conditions may be seen in the tilting of the Cairo pool in Arkansas. The tilt occurred within a period of 10—12 years; if it had gone on for a few years more, at the same rate, the oil would probably have moved completely out of the trap.

Thus the time it takes for oil to accumulate into pools may be geologically short, the minimum being measured, possibly, in thousands or even hundreds of years. The exceptions to such a statement are not important and might be only apparent if all the facts were known.

The fluid pressures within the reservoir rock may also fluctuate during the life of the reservoir, depending on the geologic history of the region.

They have been observed to range from one atmosphere up to 1, atmospheres or greater, and they may have fluctuated up and down many times during the geologic life of the rock. The geologic history of the trap may vary widely—from a single geologic episode to a combination of many phenomena extending over a long period of geologic time.

Pools trapped in limestone and dolomite reservoir rocks, moreover, have the same relations that pools trapped in sandstone rocks have to such things as the reservoir fluids, oil-water and oil-gas contacts, and trap boundaries. Yet the chemical relations of the reservoir rock and the effects of solution, cementation, compaction, and recrystallization are quite different in sandstone and carbonate reservoirs. Some believe that migration was negligible and that petroleum was formed virtually where it is now found.

Probably the best evidence for migration from a nearby source are the pools in isolated porous and permeable lenses formed by facies changes, reefs, and sand patches. Reservoir rocks such as these are commonly formed in a highly organic shore environment and are contemporaneous with the source rocks; they represent the nearest reservoir to the richest source for the longest time.

Petroleum particles entrained in the water squeezed out of nearby shales and other nonreservoir rocks would tend to flocculate in the larger pore spaces of the lens but would not develop enough capillary pressure to re-enter the fine-grained sediments. The petroleum, in effect, would be screened or filtered out and retained in the local reservoir rock.

There are several good reasons, on the other hand, for believing that some oil and gas may have migrated for relatively long distances to accumulate into pools. Several kinds of evidence direct support to the view that oil may migrate through permeable reservoir rocks from a source of supply to become concentrated in a distant area of accumulation, where pools may be formed.

The common occurrence of oil and gas seepages and springs is direct evidence that the movement of petroleum is possible. In some of these the petroleum may be observed being carried along with the water; in others it escapes independently of any water movement. The production of oil and gas from pools demonstrates that oil and gas may move through the permeable rocks and into the bore holes. The distance they move depends on the well spacing, which normally ranges from one-eighth to one-half mile and may be as much as a mile, being wider in gas pools than in oil pools.

Given a longer time and a wider spacing, the distances through which oil or gas might be shown to move would no doubt be considerably greater. The movement of oil and gas into the well bore may accompany movement of water, or it may be independent of any water movement. Reservoirs with as much as 50 percent interstitial water, for example, are known to produce only clean oil and gas. A structural trap may not begin to form until long after the reservoir rock is in place.

Pools that accumulate in late-forming traps such as these are generally in reservoir rocks of regional extent. The oil in a late-forming trap should, then, have traveled a much greater distance than that in an isolated lens within a rich organic shale.

Another factor that argues for free movement of petroleum in a reservoir rock is that very few traps have remained unchanged in size, character, and effectiveness since they were first formed. As a result of the repeated folding, faulting, tilting, erosion, uplift, deposition, solution, and cementation that affect the average sedimentary basin or province, they have undergone numerous modifications, in hydraulic gradients, temperatures, and pressures, and in many physical properties.

Each change that a trap undergoes either decreases or increases its capacity to hold oil and gas, or modifies the position of the oil and gas within the trap. We find that the fluids in oil and gas pools today are in density adjustment to the present shape and nature of the trap, including the present hydrodynamic gradients. If this adjustment is true now, it must have been true throughout the life of the pool. In other words, the petroleum content has been at all times in gravity adjustment to the changing position of the trap.

In order that a pool may maintain itself in a gravity equilibrium when trap conditions change, there must be a movement of the oil and gas within the reservoir rock. Such movements of the petroleum can in some cases extend for miles. The regional westward tilt of the Paleozoic rocks across Texas, Oklahoma, and Kansas provides an example; many pools in that region have shifted their position within their traps in order to maintain their gravity adjustment with the changing structure.

The tilting is illustrated in Figure , in which A shows the structure across northern Oklahoma at the time the Permian was being deposited; the base of the Permian is taken as a level reference plane, and it is seen that the bedding of the Mississippian limestone diverges from it toward the east-southeast.

When the Permian was being deposited, presumably as a level or nearly level plane, therefore, the intervening sands of the Pennsylvanian were dipping toward the east and southeast.

Any oil or gas present in these sands would be expected to move to the highest side of the sand, and consequently would be found along the western, or up-dip, edge. At some time after the deposition of the Permian formations, the region was tilted down toward the west. The resulting structural section is shown at B. This is the present situation, and it can be seen that some of the Pennsylvanian sands the Burbank-Bartlesville-Glenn sands , which originally dipped toward the east, now dip slightly toward the west; in these sands the westward tilting exceeded earlier tilting toward the east.

The oil pools now occur along the eastern, or up-dip, edge of these Pennsylvanian sands; so they presumably migrated from the western edge to the eastern edge between Permian time and the present because of the reversal in the direction of dip.

The lower Pennsylvanian sands, on the other hand, called the Dutcher sands in Oklahoma, were not tilted enough to reverse their dip; so their dip is still slightly toward the east, and the oil pools are generally found along their western edge. During Permian time, the Pennsylvanian sands all dipped toward the east-southeast because the interval from the Permian to the Mississippian increases in that direction.

The oil accumulations would be expected to be along the up-dip western edges. After the west-northwest dipping homocline was formed, probably before Cretaceous time, the dip of some of the Pennsylvanian sands, notably the Bartlesville and Glenn sands, was reversed, and the oil moved from the west end of the sand to the up-dip east end, where it is now found. This adjustment of the oil pools to the changing structure is taken as evidence of the movement of oil through the reservoir rocks, a movement which may extend for many miles.

The length of the section above is approximately seventy-five miles. Petrol Geol. Obviously, if petroleum will migrate a distance measured in feet, it can, by increments, multiply the distance into miles and tens or even hundreds of miles during geologic time.

The distance it may migrate is determined by the distance from the source area to the nearest trap; if this distance is short, as it usually is, the distance the petroleum has moved is short; but, if there is no obstruction to migration through the reservoir rock, we may expect the petroleum to keep on moving until it reaches a trap or a barrier that can hold it or until it is lost through escape at the surface. It may have to move tens or even hundreds of miles, but the chances of its having to go so far without finding a trap are slight.

Some of the oil and gas is in the form of submicroscopic and colloidal particles and some is in solution, the volumes being on the order of 10 to 50 parts per million.

A quite different means by which large quantities of oil may have been moved is by a recycling of oil released during the erosion of oil pools and of nonreservoir rocks carrying petroleum hydrocarbons. See pp. Some of the recycled oil might be deposited directly in the potential reservoir rocks, such as reefs, sand bars, and sand deposits. Several summaries of theories based primarily on the movement of petroleum along with moving water are available.

These include the hydraulic theory of Munn, 6 the hydraulic-buoyancy theories of Rich, 7 Mrazec, 8 and Daly, 9 the sedimentary compaction theories of King, 10 Monnett, 11 and Lewis, 12 and the compaction-hydraulic theory of Cheney.


Geology of Petroleum

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Pratt Reprinted from AAPG Bulletin, volume 50 , number I January It is with a sense of inestimable privilege that I speak on this occasion in acknowledgment of the debt I know every member of this audience feels all of us owe, and this Association owes, to our cherished friend, Honorary Member and former President-the late Arville Irving Levorsen. I do not exaggerate when I assert that all around the earth petroleum geologists held him in highest esteem. Among us who knew him personally, a similar high esteem blended with our warm affection. Auden insists that among criteria of worth in a man, "no documents, no statistics, no objective measurements can ever compete with the single intuitive glance. Are his values to be discerned only in the personality of the man himself? Or are they reflected also by the fruits of his labors?

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