ABSTRACT
The study sought to investigate and gain in-depth understanding of the regional occurrence, causes and distribution of overpressures as well as overpressure impacts on the hydrocarbons prospectivity in the Niger Delta Basin, Nigeria. Various mechanisms, notably, undercompaction disequilibrium, conversion of kerogen to hydrocarbons, long hydrocarbon columns, diagenetic transformation of clay minerals, etc have been advanced as being responsible for overpressuring in basins. Overpressure prediction tools ranging from pre-drilling geological/geophysical overpressure prognosing to monitoring/measurement while drilling have been developed for basinal overpressure evaluation.
The origin, structure and the distribution of overpressure in the Niger Delta Basin were investigated using six hundred and four (604) well pressure data, spread across 174 fields and Leak-Off Test data from 139 fields across the Niger Delta area. Net-to-gross ratio data from 270 wells spread across 92 fields were also integrated into the study to provide information on the geological control of overpressure within the basin.
A pressure-depth template consisting of overburden gradient, hydrostatic gradient and minimum horizontal stress for the Niger-Delta was developed.
The results of the study show that pressure setting in the oil and gas fields range from normal, subnormal to overpressure suggesting reservoir compartmentalization. The cyclicity of the geological processes responsible for the build-up of the Niger Delta was revealed by the repetitive pattern of the overpressure pattern from one megastructure to another. Overpressure Onset depths are shown to be shallow in the proximal edge while they get increasingly deeper in the distal area of the megastructures. The results of the study further show that the major overpressure type in the Niger Delta is undercompaction disequilibrium overpressure with additive inflationary which may be of aquathermal or long hydrocarbon column origin. Furthermore, the basement tectonics contribution to overpressuring within the basin is indicated at the Basin flanks where high overpressures were observed despite stratigraphic thinning out at the edges of the basin.
The results of trap integrity analysis within the Niger Delta Basin show a wide spectrum of near zero to high (>2000psi) trap integrity. This has great implications for hydrocarbon prospectivity and safety of human and material resources during well drilling. Several regional overpressure maps depicting aspects of the distribution of overpressure regime in the basin were generated. The aspects include shallow and deep overpressure onset depths, fluid pressure magnitudes, potential kick depth and magnitude.
Future studies focusing on the regional distribution of temperature, porosity, clay mineralogy and diagenesis should be carried out to corroborate and deepen the present findings. Regional basin modeling with geopressure development and transmission as major components should also be carried out.
