Porosity and permeability measurements can be used to calculate oil columns trapped by grain-size changes. Calculations can be useful when capillary pressure measurements are not available. Calculations are based on two major assumptions: (1) the rocks are water wet; and (2) mean effective grain size and pore size may be determined from average porosity and permeability. The first assumption is widely applicable; the second assumption has been tested and found to be reliable over a wide range of porosities and permeabilities for sandstones. Important in the calculations is that interfacial tension does not decrease to low values under subsurface conditions of higher temperatures and pressures but remains at a relatively large value of 30 dynes/cm for both gas and oil.

Calculations of oil column based on porosity and permeability data are particularly useful in evaluation of hydrodynamic flow as a trapping mechanism. Once hydrostatic or capillary-pressure oil column has been estimated, the effects of hydrodynamic flow can be evaluated as in independent variable. In many simple stratigraphic traps, the amount of oil trapped by hydrodynamic flow greatly exceeds that which can be trapped by capillary-pressure differences alone.

Studies of Recluse Muddy and Kitty Muddy fields in the Powder River basin of Wyoming indicate that hydrodynamic flow makes up a major part of the trapping element for the hydrocarbon column. Such examples show that downdip hydrodynamic flow can be an effective trapping mechanism in basins where reservoir systems are subject to recharge by meteoric waters.

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