Hydrocarbon Controls in the Southeastern Niger Delta

Paul J. Hicks and George I. Unomah
ExxonMobil, Houston, TX

Hydrocarbon distributions vary across the study area, with the discovered fields primarily in the Biafra and Qua Iboe members of the Agbada formation and the production from both deltaic and deep- water reservoirs. The structural domain is dominantly extensional and these faults are important conduits for the hydrocarbons migrating from the source intervals to the traps. In general, hydrocarbon occurrence and distribution in the area are related to its unique geology but the details of the distribution do not fit into a simple framework. What controls this variation?

The primary control on the fluid type distribution in the southeastern Niger Delta appears to be the interrelationship of the phase behavior (pressure/temperature) of the generative hydrocarbons, seal properties and the structural height (height between crest and "effective" spill point). This study indicates that the composition of fluids in higher-pressure reservoirs is similar to the composition of the generative hydrocarbons. Compositions of the lower pressure reservoirs are affected by fractionation of the oil and gas phases along the migration pathway. The split between oil and gas in these lower pressure reservoirs appears to be controlled by the properties of the seal and the structural height of the reservoir. Though the fluid and seal properties are expected to be different in deep water Niger delta, the same processes are expected to control fluid type.

The oils and gases in the study area belong to one super- geochemical family generated from mixed type III/II organofacies. Geochemical evidence indicate that variations in source facies (sub-families) occur, trending from the northwest to the southeast, but these variations appear to have minimal effect on the fluid type distributions over the study area. In-reservoir processes, such as biodegradation, have significantly altered the composition and fluid properties in relatively shallow depths.