Abstract
Geopressured sedimentary formations are common within the more prolific deeper hydrocarbon reserves in the Niger Delta basin. While overpressured zones could serve as tools for hydrocarbon prospectivity evaluation, they are significant safety concern during drilling. Post-drill pore pressure prediction using wireline log and mudlog was carried out in the X-field, Onshore Niger Delta basin to predict the depth of abnormal pressure occurrence, estimate the amount of pore pressure encountered and its trend within the field. The results obtained from the compaction trend analysis method reveal that overpressures in this field are associated with simple rollover structures bounded by normal faults, and are caused mainly by undercompaction of Akata shale. The depth to top of mild overpressures (<0.71 psi/ft) in this field ranges from 8,000 to 11,000 ft. Similarly, the depth to top of severe overpressures (>7.10 psi/ft) ranges from about 12,000 to over 13,000 ft. Post-depositional faulting is believed to have controlled the configuration of the overpressure surface and has played later roles in modifying the present day depth to top of overpressures. Abnormal pressure trend within this field is observed to be trending towards the N–E direction.
Highlights
The pore spaces of sedimentary rocks below the water table filled with fluids of varying salinities are normally linked through interconnected network of pores, to pore fluids at the surface
The plot shows that D-exp increases with depth until the top of overpressure is reached, at which it starts to decrease with increasing depth
700 2700 4700 6700 8700 10700 12700 1500 hys pp est pp litho resistivity data is 0.552 psi/ft (5,622 psi) of mild pressure, to 0.8756 psi/ft (11,720 psi) of overpressure. This depth, the estimated pore pressure trend diverts from the hydrostatic pressure trend, increasing toward the lithostatic pressure (Fig. 8). This is due to the fact that the bulk volume is taken over by pore fluids resulting to decrease in resistivity data and slight increase in sonic data and a decrease in bulk density
Summary
The pore spaces of sedimentary rocks below the water table filled with fluids of varying salinities are normally linked through interconnected network of pores, to pore fluids at the surface. The pressure of the pore fluids at depth is due to a continuous column of fluids which extends to the surface. This pressure invariably is a function of column height and the density of the fluids. The pore pressure exerted by one foot high column of fresh water and very saline water are 0.433 and 0.47 psi/ft and their densities are 1.0 and 1.09 g/cm, respectively. Fluid pressure gradient outside the range of 0.433–0.47 psi/ft in sedimentary sequence is termed abnormal. The unique characteristics of abnormal pressure zone are the development of abnormal porosities in shale sections, leading to abnormal pore fluid content
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