Secondary Migration In secondary migration, the oil droplets are moved about within the reservoir to from pools. Secondary migration can include a second step during which crustal movements of the earth shift the position of the pool within the reservoir rock. Accumulations can be affected by several, sometimes conflicting, factors. e.g. a. Buoyancy causes oil to seek the highest permeable part of the reservoir, capillary forces direct the oil to the coarsest grained area first then successively into finer grained areas later. b. Any permeable barriers in the reservoir channel the oil into somewhat random distribution. c. Oil accumulations in carbonate rock are often erratic because part of the original void spaces have been plugged by minerals introduced from water solutions after rock is formed. d. In large sand bodies, barriers formed by thin layers of dense shale may hold the oil at various levels. With crustal movement of the earth, accumulations are shifted away from where they were originally placed. e. Faults sometimes cut through reservoirs destroying parts or shift them to different depths. f. Uplift and erosion bring accumulations nearer to surface where lighter hydrocarbons may evaporate. g. Fracturing of the cap rock may allow accumulations to migrate vertically to much shallower depth. Wherever differential pressures exist and permeable openings provide a path, petroleum will move. Once the water, oil and gas migrates into the trap, it separates according to density. Gas being the lightest, goes to the top of the trap to form the free gas cap. Oil goes to the middle and water that is always present, on the bottom. The oil portion of the trap is saturated with a certain percentage of oil and water. Vertical migration via faults and fractures is also possible, this has led to many of the large oil accumulations, such as that found at shallow depths in Bolivar, Venezuela, and Northern Iraq. In other cases, such as the Khurais filed in Arabia, migration over relatively long distances has to take place by movement up dip within a porous bed until a trap was encount
Posted on: Tue, 08 Jul 2014 00:34:04 +0000
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