Compaction and Porosity Based Pore Pressure Prediction in the “Cappe Field”, Coastal Swamp Depobelt, Niger Delta, Nigeria

Abstract

Three wells of the “Cappe” Field in the offshore part of the Coastal Swamp depobelt I, Niger Delta, were evaluated primarily to determine the impact of compaction on reservoir quality and to determine possible over-pressured zones in the Benin and Agbada formations. Sandstone porosity-depth plots of the three wells show a linear trend of gradual porosity reduction with depth for the top of the wells 1, 2 and 3 (r² = 0.26, 0.42 and 0.73 at 4500-5900ft, 3940-5000ft and 2500-5350ft respectively). Two variations from this simple trend were observed. 1: Intervals of insignificant porosity reduction (well 1; 6500-7950ft, r² = 0.00003 and well 2; 5760-7911ft, r² = 0.008), due to hydrocarbon entrapment. 2: A reversal in the trend (well 3; 5450-9658ft, r = -0.89) indicated by an increase in porosity as a result of overpressure. A number of factors such as compaction, fluid content and pore pressure affect the porosity-depth trends of the Agbada Formation. A decrease in porosity with depth generally holds true for shales (well 1: r² = 0.74 and well 2: r² = 0.81) except for an increase in porosity (r² = -0.596) observed in well 3. Compaction factor is significant in sandstone porosity reduction in the Benin Formation (well 1: 58.3% and well 2: 68.9%) than in the Agbada Formation (well 1: 25.64% and well 2: 25.29%). Sandstone porosities predicted at the base of the wells are generally low (well 1: 5.86%, well 2: 7.52%), implying uneconomical reservoirs.

KEY WORDS: Pore Pressure, Overpressure, Porosity, Compaction