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 (r 2 = 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, r 2 = 0.00003 and well 2; 5760-7911ft, r 2 = 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: r 2 = 0.74 and well 2: r 2 = 0.81) except for an increase in porosity (r 2 = -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
Highlights
Porosity loss in sandstones is of concern, because it affects the nature of oil and gas reservoirs.Shale porosity – depth plots are important because fluid pressures are reflected in shale, pressures cannot be measured directly but can be estimated from shale porosity-depth trends
The Benin Formation was identified by a continuous sand succession with rare intervals of shale indicated by low gamma ray readings
Sandstone porosity-depth plots show a linear trend of gradual porosity reduction with depth, at the top
Summary
Porosity loss in sandstones is of concern, because it affects the nature of oil and gas reservoirs. Shale porosity – depth plots are important because fluid pressures are reflected in shale, pressures cannot be measured directly but can be estimated from shale porosity-depth trends. The compaction coefficient “C” governs the compaction of sediments and it is a function of the depositional environment (Luo and Vassuer, 1992). The compaction coefficient changes from field to field and even from one sediment section to another in the same field. For a given section, it does not change in a normal compaction zone during burial. The coefficient represents the characteristics of sediment fabrics. The larger the compaction coefficient, the greater the decrease in porosity with burial (Luo and Vassuer, 1992). According to Weber and Daukoru (1988), compaction in the Niger Delta varies from one depobelt to another
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