Relationships Between Oil and Water Composition in Niger Delta
Parke A. Dickey, Lola George, Colin Barker
In the Niger delta of west Africa, heavy, low-pour point oil is found in shallower reservoirs, and light waxy oil in deeper reservoirs. A gradation in composition exists between the two oils, and both seem to have had the same source. The shallower oil appears to have been degraded by bacteria. The change from heavy to light oil is usually abrupt, and occurs at a reservoir temperature between 150° and 180°F.
Meteoric water can easily be distinguished from connate water because it is low in total dissolved solids and high in bicarbonate, whereas connate water is saltier and contains more chloride. Meteoric water has penetrated as deeply as 7,000 ft, which is difficult to explain because no obvious discharge zone is found. The upper part of the section is extremely sandy. The entire delta is broken by growth faults, which may be seals or channels. In some fields, degraded oil is associated with connate water. In dually completed wells producing heavy oil from a shallow reservoir and light oil from a deeper reservoir, the oils differ but the waters are chemically identical.
This situation cannot be explained. Oils may have migrated secondarily, and moved into their present reservoirs after having been degraded elsewhere.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.