Classification of Petroleum Reservoirs  

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Petroleum reservoirs exist in many different sizes and shapes of geologic structures. It is usually convenient to classify the reservoirs according to the conditions of their formation as follows:  1. Dome-Shaped and Anticline Reservoirs: These reservoirs are formed by the folding of the rock layers as shown in Figure 1. The dome is circular in outline, and the     anticline is long and narrow. Oil and/or gas moved or migrated upward through the porous strata where it was trapped by the sealing cap rock and      the shape of the structure.   Fig.1 A reservoir formed by folding of rock layers Fig.2 a cross section of a faulted reservoir     2. Faulted Reservoirs: These reservoirs are formed by shearing and offsetting of the strata (faulting), as shown in Figure 2. The movement of the nonporous rock opposite        the porous formation containing the oil/gas creates the sealing. The tilt of the petroleum-bearing rock and the faulting trap the oil/gas in the reservoir.   3. Salt-Dome Reservoirs: This type of reservoir structure, which takes the shape of a dome, was formed due to the upward movement of large, impermeable salt dome that     deformed and lifted the overlying layers of rock. As shown in Figure 3, petroleum is trapped between the cap rock and an underlying impermeable rock layer, or between     two impermeable layers of rock and the salt dome.   4. Unconformities: This type of reservoir structure, shown in Figure 4, was formed as a result of an unconformity where the impermeable cap rock was laid down across the       cutoff surfaces of the lower beds. Fig.3 Section in a salt-dome structure Fig.4 a reservoir formed by unconformity.   5. Lense-Type Reservoirs: In this type of reservoir, the petroleum bearing porous formation is sealed by the surrounding, nonporous formation. Irregular deposition of       sediments and shale at the time the formation was laid down is the probable cause for this abrupt change in formation porosity.   6. Combination Reservoirs: In this case, combinations of folding, faulting, abrupt changes in porosity, or other conditions that create the trap, from this common type of        reservoir.  References:  1. Petroleum & Gas Field Processing, H K. Abdel-Alal and Mohamed Aggour,  King Fahd University of Petroleum & Minerals  2. Petroleum Engineering Handbook, L.W.Lake, Vol.1 "General Engineering"