A reservoir rock is a subsurface body of rock having sufficient porosity and permeability to store and yield hydrocarbons at a commercial rate. It contains little, if any, insoluble organic matter. Therefore, it is unlikely that the vast quantities of oil now present in some reservoir rocks could have been generated from material of which no trace remains. Therefore, the site where commercial amounts of oil originated apparently is not always identical to the location at which they are ultimately discovered. (Gas/Oil Traps).
Porosities in reservoir rocks usually range from about 5 to 30 percent, but not all the available pore space is occupied by petroleum. A certain amount of residual formation water cannot be displaced and is always present.
The porosity and permeability of carrier and reservoir beds are important factors in the migration and accumulation of oil. Most petroleum accumulations have been found in clastic reservoirs (sandstones and siltstones). Next in number are the carbonate reservoirs (limestones and dolomites). Accumulations of petroleum also occur in shales and igneous and metamorphic rocks because of porosity resulting from fracturing, but such reservoirs are relatively rare.
Reservoir rocks may be divided into two main types: (1) those in which the porosity and permeability is primary, or inherent, and (2) those in which they are secondary, or induced. Primary porosity and permeability are dependent on the size, shape, and grading and packing of the sediment grains and on the manner of their initial consolidation.
Secondary porosity and permeability result from post-depositional factors, such as solution, recrystallization, fracturing, weathering during temporary exposure at the Earth’s surface, and further cementation. These secondary factors may either enhance or diminish the inherent conditions.
A petroleum trap concentrates the petroleum fluids at particular locations, allowing commercial exploitation. Petroleum traps provide what is known as closure at the level of the petroleum-bearing reservoir.
There are two main types of traps:
– Structural traps
– Stratigraphic traps
In reality most traps are formed by more complex sequence of events, and cannot be classified so rigidly. For example, in (e) the reservoir rock was first folded and eroded, then sealed by an impermeable rock that was deposited later over the eroded structure.
In order to trap migrating oil and gas, structures must exist before hydrocarbon generation ceases. It could take, for example that sea trap structures can exist 125 million years ago, but the were not filled with oil until 100 million years later.
Structural traps are traps formed by Earth movements. Examples of structural traps are:
– Fault traps
– Anticlinal traps (80% of the world’s oilfields are in this category)
– Rocks are domed over rising salt masses
A fault trap occurs when the formations on either side of the fault have been moved into a position that prevents further migration of petroleum. For example, an impermeable formation on one side of the fault may have moved opposite the petroleum-bearing formation on the other side of the fault. The impermeable layer prevents further migration of petroleum.
An anticline is an upward fold in the layers of rock, much like an arch in a building. Petroleum migrates into the highest part of the fold, and an overlying bed of impermeable rock prevents its escape. (80% of the world’s petroleum fields are in this category).
Stratigraphic traps are traps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself. Examples of stratigraphic traps are:
- A tilted or inclined layer of petroleum-bearing rock is cutoff or truncated by an essentially horizontal, impermeable rock layer (unconformity). An unconformity is a surface of erosion. If reservoir beds beneath an unconformity are tilted, and impermeable beds above the unconformity form a seal, then a trapping geometry results. The giant Midway-Sunset oil field in the San Joaquin Valley is an example of an unconformity trap. Prudhoe Bay field in Alaska is another.
- Petroleum-bearing formation pinches out. If a porous reservoir rock is encased within an impermeable seal, such as shale or salt, then a trap may form at the updip pinchout of the reservoir (i.e., where the reservoir thickness decreases to zero). The Gatchell sand in the East Coalinga Extension field in the northern San Joaquin Valley is an example of an updip pinchout.
- Reservoir bed is surrounded by impermeable rock.
- Change in porosity and permeability in the reservoir itself. If a reservoir rock suddenly loses porosity and permeability in the updip direction and becomes a seal, then a trap may result. The updip change may result from decreasing grain size, or from more clay or cement in the pore spaces. North Coles Levee field in the San Joaquin Valley is a small anticline, some might say just a wrinkle, on the east side of the giant Elk Hills field anticline. Some geologists think that a permeability barrier prevents the oil at North Coles Levee from leaking updip into Elk Hills.
A Final Word
Only about 2% of the organic matter dispersed in sediments becomes petroleum! Of this only 0.5 % of it gets accumulated in reservoirs!