|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Chapter||Predicting preservation and destruction of accumulations|
|Author||Alton A. Brown|
As the pore volume within a trap decreases due to burial cementation, several phenomena combine to destroy the economic accumulation.
- Petroleum is displaced from the trap at the spill point as the pore volume within the structural closure decreases. If porosity is lowered sufficiently, the accumulation may be subeconomic in size.
- Reduced porosity may result in lower permeability so that production rates are subeconomic, even where economic quantities of petroleum are still trapped.
- As pore size decreases in a petroleum-filled reservoir, the capillary pressure of the petroleum phase must increase to occupy the pore spaces (assuming no change in wettability). In low-permeability tight sands or carbonates, the capillary displacement pressure in the reservoir rock may approach that of moderate-quality seals. As a result, a lithology that could seal an accumulation at shallow depth may no longer be effective at deeper depths because it differs little from the reservoir rock.
Predicting spillage by cementation
Reduced porosity can be predicted from empirical or statistical evaluation of porosity data for reservoir intervals with similar composition and burial history as the prospect. In general, porosity decreases with increasing age, depth, and temperature. Numerical modeling techniques are not yet refined enough to quantitatively predict prospect reservoir quality loss.
- Bloch, S., 1991, Empirical prediction of porosity and permeability in sandstones: AAPG Bulletin, vol. 75, p. 1145–1160.
- Schmoker, J. W., and D. L. Gautier, 1988, Sandstone porosity as a function of thermal maturity: Geology, vol. 16, p. 1007–1010, DOI: 10.1130/0091-7613(1988)016<1007:SPAAFO>2.3.CO;2.