Fluid dynamic potentials are critical to the entrapment, distribution, phase, leakage and migration of hydrocarbons. The classification system combines two cap rock properties (static and dynamic) with five seal and leakage modes (capillary seal, pressure seal, hydraulic resistance seal, water drive leakage and hydraulic fracturing leakage) and two hydrocarbon migration mechanisms into and out of a trap (fill/spill and fill/leak). Static cap rocks, such as thick shales, usually seal more than the vertical closure of the trap, as a consequence of the critical pore throat capillary pressure (capillary seal). In two-phase hydrocarbon systems fill/spill tends to concentrate gas. Hydraulic fracturing is usually required to generate significant leakage through static cap rocks. Then fill/leak tends to concentrate oil. Dynamic cap rocks, such as inhomogeneous sand/shale sequences and faults have less sealing capacity than the static cap rocks, but are more widespread. They are strongly affected by variations in the fluid dynamic properties and are less dependent on capillary pressure. Water potentials are often critical, for seal (pressure seal) or for leakage and redistribution (water drive leakage). Hydraulic resistance seal is important for trapping of oil and crucial for concentration of oil relative to gas (fill/leak).

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