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AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 3. (March)

First Page: 368

Last Page: 374

Title: Effects of Oil and Gas Accumulation on Water Movement: GEOLOGIC NOTES

Author(s): Richard E. Chapman (2)

Abstract:

The water saturation of an oil reservoir is typically 15 to 40% of the pore space and independent of elevation above a level close above the oil/water contact. This water is not produced with the oil; it is not in hydraulic continuity. Geometric considerations indicate that it is concentrated in pendular rings separated by immobile films of water adsorbed to solid surfaces of grains and any authigenic minerals present. It is water trapped during accumulation of the oil (and may therefore have a different composition from the water below the oil/water contact).

Formation water cannot flow upward through an oil or gas accumulation, nor can bacteria (which require a water substrate) penetrate above the level of irreducible water saturation. Bacterial degradation of a significant oil column and water-washing of oil most likely occur during secondary migration.

Oil is separated from some solid surfaces by an immobile film of adsorbed water no thicker than 1 mm--possibly by only a monomolecular layer--and so could be altered in the reservoir by catalysis if authigenic or detrital clay minerals are present.

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