ESTIMATION OF MULTIPLE SOURCES OF OVERPRESSURES USING VERTICAL EFFECTIVE STRESS APPROACH: CASE STUDY OF THE NIGER DELTA, NIGERIA

The relationship between formation overpressures and porosity, and overpressures and stress regime can differ significantly depending on the geological setting and mechanisms responsible for generating overpressures. Therefore, the most important aspect in modeling over-pressures becomes a thorough study of the mechanisms and geological settings of a particular basin. A new velocity–effective stress model capable of estimating other sources of overpressures besides under compaction from velocity data was proposed and used in this paper with the objective of determining and accounting for overpressures from multiple sources. Data obtained from wells with a variety of geological settings within the basin were used to achieve a better understanding of the major causes of overpressures in the Niger Delta basin. This study has revealed the presence of velocity hysteresis (i.e., unloading effects) in several wells in the basin. Most overpressures in the basin occurred within the depth range of 6000 ftss to 13000 ftss with top of hard overpressures (>0.60psi/ft) having a regional mean of 12000 ftss. Similarly, results revealed a correlation of depth to high overpressures with zones of hydrocarbon maturation and smectite- to –illite transformation. The hard overpressures observed at depth in the Niger delta coincided with the time- temperature related fluid expansion mechanisms resulting principally from unloading. These findings therefore have shown that under compaction is not the sole overpressure generation mechanism in the basin as has been widely believed.