Where hydrocarbons exist today in the subsurface depends upon the juxtaposition of source rocks, reservoir fluids, and reservoir traps. Source rocks and reservoir fluids have already been discussed, reservoir traps have not.

There are two types of migration when discussing the movement of petroleum, primary and secondary. Primary migration refers to the movement of hydrocarbons from within source rock and into reservoir rock. Secondary migration refers to the subsequent movement of hydrocarbons within reservoir rock; the oil and gas has vacated the source rock and has entered the reservoir rock.

Question: Why hydrocarbons moved into the Cretaceous reef structure was covered in the discussion on reservoir fluids, but this does not explain why these same hydrocarbons stopped flowing (moving) and accumulated in the reef; why didn't they continue to move through the reef complex and stop somewhere else?
Answer: The reef complex has now become a reservoir trap blocking further movement of the hydrocarbons causing them to pool at this location. The forward flow was impeded by some sort of restriction forming a hydrocarbon trap that prevented the hydrocarbons from reaching the surface of the earth where they would be lost.

Question: What is a hydrocarbon trap?
Answer: A trap consists of an impervious stratum that overlies the reservoir rock thereby prohibiting hydrocarbons from escaping upward and laterally. This impervious stratum is called a roof rock; it intervenes to collect and hold hydrocarbons underground. The roof forms a seal, or a barrier, which creates the needed conditions for a pool. Trap material must have a lower permeability than the existing rock material though which the hydrocarbons are flowing. The rock forming the seal is also referred to as a capping bed.

We need to reiterate two important concepts, porosity and permeability and how they relate to source rocks and reservoir rocks. Porosity controls the volume of hydrocarbons present in a given trap while permeability controls the volume of hydrocarbons that can be extracted from the trap.

Most of the hydrocarbons that forms in sediments does not find a suitable trap and eventually flows to the surface along with the formation water. It is estimated that less than 0.1% of all organic matter originally buried becomes trapped in an oil pool. The greatest ratio of hydrocarbon pools to volume of sediment is found in rock no older than 2.5 million years and almost 60% of all oil discovered thus far is found in strata of Cenozoic age.

Question: Why do you think 60% of the oil found is in strata of Cenozoic age?
Answer: Age implies the length of time the hydrocarbons have been subjected to temperature variations. The longer hydrocarbons are pooled in the subsurface, the more susceptible they are to high temperatures & tectonic processes. All of these tend to vaporize the hydrocarbons or breech cap rock allowing product to vent to the surface and be lost.

There are three basic types of hydrocarbon traps:

1. Structural traps are created when the seal or barrier is concave upward (looking from below). The geometry is formed by tectonic processes AFTER deposition of the reservoir beds involved.

   There are two types of structural traps:

   1. Folds result in the physical bending (deformation) of the rock units without breaking. The simplest form is an anticline -- creating an Anticline trap. The rock units undergo bending very slowly over long periods of geologic time. These types of traps are often found adjacent to mountain ranges.
   2. The second type of Structural trap is a Fault trap. These are the result of fractures (breaks) within the rock units where one side has moved relative to the other side. Faulting may be the sole cause of the formation of a trap or more commonly, faults form traps in combination with other structural features such as folding

      Click to view an example of folding due to faulting and a dome formation.

      which is common along the Gulf Coast region of Louisiana. Often an oil field is the result of multiple faulting forming many structural traps containing hydrocarbons as in the case of the Jennings Field near Jennings, Louisiana. Click for more details about the types of faults.

2. Stratigraphic traps are created when the seal or barrier is formed by changes in lithology or rock type (which also controls porosity and permeability), DURING the deposition of the reservoir beds involved. Lithological variations may be depositional, as in the case of reefs , channels, and sand bars. A Louisiana example of this type of is (Echols) Lithological variations may also be post-depositional in nature; truncations occur where erosion has removed a significant portion of an existing tilted structure followed by deposition of another lithologic rock unit, the latter forming the roof rock and thus becoming the capping bed. An example is the Monroe gas field in Morehouse Parish.

3. The third type of hydrocarbon trap is called a Combination Trap. The geometry of this type of trap is the result of a combination of tectonic processes and changes in lithology. A common trap that would be an example of a Combination Trap is a salt dome. A salt dome is a mass of NaCl (Sodium Chloride) generally of a cylindrical shape and with a diameter of about 2 km near the surface, though the size and shape of the dome can vary. This mass of salt has between pushed upward from below through the surrounding rock and sediments into its present position. The source of the salt lies as a deeply buried layer that was formed in the geologic past. Salt is an evaporite. Salt beds were formed by the natural evaporation of sea water from an enclosed basin; in Louisiana, this occurred in Permian or Jurassic time. Subsequently, the precipitated salt layer is buried by successive layers of sediments over geologic time until segments of it begin to flow upward toward the surface of the earth.

   Review Question: Question: There are three basic types of hydrocarbon traps: Structural, Stratigraphic, and Combination traps. What is the difference between each of the three trap types in terms of how they were formed? Answer: A Structural trap is formed by tectonic processes AFTER deposition of the reservoir beds involved while a Stratigraphic trap is created during deposition of the reservoir beds. A Combination trap is formed by a combination of processes present in the sediments DURING the time of deposition of the reservoir beds AND by tectonic activity that occurred in the reservoir beds after their deposition.

   The origin of salt domes is best explained by the plastic-flow theory. Salt has a density of 2.2 under standard conditions. But at a depth of about 12,000 feet, the mass of the overlying sediments exerts a compressive, downward force, density decreases and salt begins to flow like a plastic substance. A small fracture in the overlying, higher density sediments or a slightly elevated mass of salt above its surroundings would trigger the upward movement. Once this upward salt movement begins, salt from elsewhere in the salt bed moves into the region surrounding the salt plug to replace the salt that is flowing upward to form the salt plug. The upward movement of the salt plug, or dome, continues as long as there is sufficient source of salt "feeding" the dome OR until the upward movement is halted by a more rigid formation.   Once equilibrium is reached, upward movement of the salt dome ceases, but may begin again if sufficient sediments are added to the weight of the overburden which again increases the load pressure on the parent salt mass. In Louisiana, the age of the salt domes is dependent upon which side of the Cretaceous reef structure you are on. The domes are oldest on the north side and youngest on the south side. This also corresponds to the age of the hydrocarbon deposits discussed earlier.

   Question: Why, in Louisiana, would the age of the salt domes approximate the age of the hydrocarbon deposits?
   Answer: Louisiana was formed over geologic time with the older sediments located in the north and progressively younger sediments building southward. It would follow that the age of the salt domes would also reflect this trend.

   Salt domes come in all sizes and shapes, but all have certain characteristics. For example, they have a cap rock. The cap rock is composed of limestone located at the top of the dome followed by, in descending order, gypsum, anhydrite, and finally rock salt. The average cap rock is between 300 to 400 feet thick, but cap rock up to 1,000 feet thick are known to exist.

   NOTE: Cap rock was first thought to form during the initial precipitation of the evaporite, the rock salt, and that it was a top layer that was the last unit to form in the enclosed basin [rock salt overlain by limestone]. This theory has been discarded in favor of one that considers cap rock as a secondary product that is created as the salt dome ascends through the overlying materials. The top of the dome dissolves as it rises through the sediments and the residual constituents of the salt become concentrated at the top of the rising plug becoming three layers: limestone, gypsum, and anhydrite.

   The cap rock of some salt domes, and sometimes the rock salt itself, is found to overhang, or drape down, the side of the main salt mass. It is commonly found to overhang on one or two sides. The reason for the overhang probably is dependent upon the nature of circulating waters in the sediments through which the dome is ascending. Salt is soluble and dissolves, but the cap rock is not very soluble and is left more intact. Thus the appearance of the cap rock "overhanging" the main salt body is the result of disolutioning of the salt body within the subsurface, but not dissolving the cap rock.

   Cap rock has all the characteristics of a petroleum reservoir and often contains hydrocarbons. But cap rock also contains free sulfur. Free sulfur is present in cap rock of almost all salt domes, but it occurs in commercial quantities only in salt domes that have a thick limestone cap. The free sulfur forms after the limestone is created and the sulfur is probably derived from the destruction of the anhydrite.

   Louisiana has many salt domes, over 100 that have been identified and mapped. In Louisiana and the Gulf Coast region, the salt has moved up through thousands of feet of sediments and the upward motion of the salt is still going on especially in South Louisiana.

Question: Are salt domes source materials; i.e., do they have anything to do with the origin of hydrocarbons?
Answer: No! Salt domes account only for creating reservoir traps.

Before leaving this discussion, it is important to understand that in the early days of petroleum exploration in the U.S.A., there was no specific legislation governing it. Legal battles often ensued regarding ownership issues, so the courts applied wildlife game rules to hydrocarbons. The justification for this was that oil and gas were considered fugacious, likely to flee, and could move from property to property. Law was eventually established that the petroleum was owned by the person under whose land they were trapped. This perception as set forth over a century ago, is adhered to today.

REFERENCES

DOTT, R. H., and M. J. REYNOLDS, 1969, Source book for Petroleum Geology: Am. Assoc. Petrol. Geol., Mem. No. 5, 471 pp.

JUDSON, SIDNEY A. and R. A. STAMEY, 1933, Overhanging Salt On Domes of Texas and Louisiana: Bull. Am. Assoc. Petrol. Geol., Vol. 17, pp. 1492-1520.

Other Materials Utilized in Preparation of this Section

BATES, R. L. AND J. A. JACKSON, 1980, Glossary of Geology: American Geological Institute, Falls Church, VA., 2nd Ed., 749 pp.

BERGER, B. D. and K. E. Anderson, 1992, Modern Petroleum - A Basic Primer of the Industry: PennWell Books, 3rd Ed., Tulsa, OK., 517 pp.

LEVORSEN, A. I., 1967, Geology of Petroleum: W. H. Freeman and Company, San Franciso, CA., 2nd Ed., 724 pp. SELLEY, R. C., 1985, Element of Petroleum Geology: W. H. Freeman & Co., New York, 449 pp.

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA., 607 pp.