Abstract

The minimum conditions for migration of oil through water-wet carbonates are of great interest to geologists concerned with regional exploration problems. Oil can not migrate either under hydrostatic or hydrodynamic conditions unless the displacement pressure exceeds the entry pressure of the pore interconnections. Since the entry pressure is largely a function of pore size and configuration, thin sections of various carbonate rocks were studied in detail. One hundred pores and corresponding interconnections were measured, and 50 pore-throat configurations were analyzed and classified as to pore-space type. The pore-size measurements ranged from 0.02 mm. to 1.40 mm., with a logarithmic mean of 0.137 mm. The measurements of the interconnections ranged from 0.0003 mm. to 0.123 mm., with a logarithmic mean of 0.0163 mm. In the pore-space type most frequently observed, the interconnections between pores were constrictions formed by individual carbonate crystals or fragments projecting into the pore space. The logarithmic mean values for pore and throat size were used together with representative average values for crude oil and brine to estimate the minimum conditions for migration of oil through water-wet carbonates. Under hydrostatic conditions, the product of the sine of the regional dip angle (theta ) and the length of a continuous oil phase (L) must be greater than the critical height for buoyant rise (h) in order to move oil through water-wet carbonates, i.e., sin theta L=h, where h was calculated to be 231 cm. (7 1/2 ft.). Hence, the displacement pressure needed is 6 1/2 times that required for oil migration through average water-wet medium-coarse sandstone. Under hydrodynamic conditions, the interaction of permeability, porosity, viscosity, and flow rate of the water, and oil-phase length must produce a critical pressure gradient of 44,105 dynes/cm 2 /L cm. in order to move oil through average carbonate rocks. The following are tentative conclusions. 1) The dominant pore-space type influences considerably the minimum conditions for migration. 2) The average depositional dips of shelf-type carbonates are probably insufficient to cause large-scale migration under hydrostatic conditions, unless these dips are associated with hinge lines or are amplified by tectonic disturbances. 3) Even low rates of flow of water can initiate and sustain migration of short continuous oil phases. 4) In basins where both carbonates and sandstones act as carriers and reservoirs and where the dips are generally gentle, the regional concentration of oil prior to localized entrapment occurs farther downdip for the carbonates than for sandstones. 5) Exploration on a regional scale should take into account the differences in the minimum conditions for migration of oil which exist between carbonate reservoir rocks and porous and permeable sandstones.

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