Abstract
The deep drilling campaign in the Niger Delta has demonstrated the need for a detailed geopressure and trap integrity (drilling margin) analysis as an integral and required step in prospect appraisal. Pre-drill pore pressure prediction from 3-D seismic data was carried out in the Greater Ughelli depobelt, Niger Delta basin to predict subsurface pressure regimes and further applied in the determination of hydrocarbon column height, reservoir continuity, fault seal and trap integrity. Results revealed that geopressured sedimentary formations are common within the more prolific deeper hydrocarbon reserves in the Niger Delta basin. The depth to top of mild geopressure ( 0.60 psi/ft) ranges from about 10 000 ftss to over 30 000 ftss. The distribution of geopressures shows a well defined trend with depth to top of geopressures increasing towards the central part of the basin. This variation in the depth of top of geopressures in the area is believed to be related to faulting and shale diapirism, with top of geopressures becoming shallow with shale diapirism and deep with sedimentation. Post-depositional faulting is believed to have controlled the configuration of the geopressure surface and has played later roles in modifying the present day depth to top of geopressures. In general, geopressure in this area is often associated with simple rollover structures bounded by growth faults, especially at the hanging walls, while hydrostatic pressures were observed in areas with k-faults and collapsed crested structures.
Highlights
The quantity of hydrocarbon accumulation is a function of generation, migration, entrapment, sealing and preservation
The fluid pressures in offset wells used in the calibration process were predicted using the calibration parameters A and B. This process was repeated in the wells not used in the calibration process for the purpose of validation. The accuracy of this prediction is noticed in the correlation of the predicted pressures with the measured Repeat Formation Test (RFT) data in a correlation well (Oo-2 well); see Figure 4
The Root Mean Square (RMS) velocity data was converted to interval velocity using appropriate smoothing parameters as shown in Figure 7 below
Summary
The quantity of hydrocarbon accumulation is a function of generation, migration, entrapment, sealing and preservation. All of the deep wells (“HPHT” wells) drilled by Shell Petroleum Development Company of Nigeria Limited in the basin were overpressured (Opara, 2008) In most of these cases, the wells were abandoned, drilled without reaching the objective sequence, or with drilling prolonged unnecessarily to sometimes as long as seven months leading to astronomical rise in drilling cost. This work is aimed at developing a robust geopressure and trap integrity prediction and analysis of the Greater Ughelli depobelt using 3-D seismic velocity data This is very important considering the current campaign for deeper wells, as the economic consequences of exploitation in areas with an unspecified risk of abnormal pressure profiles may range. By comparing case histories, one can obtain valuable clues for estimating current and paleo-geopressure conditions in the frontier basins that can be assessed prior to drilling (Yu and Lerche, 1996)
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