The hazard associated with abnormal formation pressures are of great concern to operating companies in the oil industry worldwide.
As an effort to alleviate this problem in the Niger Delta, a spread of overpressures as they occur in the basin are shown in a map of the area. The table from which the map was produced contain the values of the top of overpressures obtained from shale resistivity, acoustic and resistivity plots against true vertical depth. The depth to the top of overpressures from the more than 230 wells studied, range from 6,500ft along the shelf, to 15,000ft at the middle belt and 16,000ft at the west-belt of die Niger Delta Basin Complex.
Shale resistivity ratios and shale transit time differences obtained from normal and observed (abnormally high pressured.) resistivity's and transit times respectively, have been used in conjunction with the fluid pressure gradient, to generate the prediction curves. Comparison is made between the predicted formation pressures using the predicted curves and actual measured pressures. The standard deviation for the resistivity method is ±0.626% psi/ft. (or about ±63 psi per 10,000ft.) and the acoustic method is ±2.058% psi/ft. (or about ±200 psi/ft. per 10,000ft.) Using data from previously published papers, correlating equations for predicting formation fluid pressure gradient are presented for some other major producing areas.
References and illustrations at end of paper.
Operators involved with the exploration, drilling and production operations, are more and more frequently confronted with complications associated, with overpressured (abnormally high fluid pressured) formations. This is found to be true in me Niger Delta Basin area of Nigeria.
Figure 1 is a location map of the Niger Delta Basin. Nigeria occupies an area of about 370,000 square miles, with the Niger Delta Basin area being the only major oil and gas productive region.
Three main areas of Basin Complex have been mapped in Nigeria.1 These are the West African Massif (eastern end), the Northern Nigerian Massif and the Eastern Nigeria Massif. These Basins and Troughs taken together with onshore part of the Niger Delta Complex, occupy about 178,000 square miles, about half the total area of Nigeria.
The Cenozoic Niger Delta Complex area today occupies around 30,000 square miles of the Southern Nigerian Sedimentary basin onshore. The total complex (onshore-offshore area) must exceed 100,000 square miles.
Details of the geology of the Niger Delta has been discussed in details by several authors.1–6 This coastal sedimentary basin originated in the early Cretaceous period as an X-shaped depression oriented NE-SW and NW-SE, and it is defined by a set of older meganestone elements which were stable during the Cenozoic. These acoustic elements include the Benin Flank to the northwest, the Cretaceous Benue Trough to the north, and the Calabar Frank to the east. An extensive alternating sequence of sand/shale deltaic complex covering over 7.50 miles in thickness was deposited within this active depression.