Abstract
Pore pressure prediction in parts of the Onshore Ni ger Delta Sedimentary Basin using seismic reflectio n data has been carried out. The Bower’s technique of velocity-to-pore pressure transform was adopted. T he seismic velocities were derived from the seismic da ta using layer cake velocity model and were later compared with the available sonic logs velocities o btained from the area. Parameters in the velocity-t o-pore pressure transform were estimated using the seismic interval velocities. The top of overpressure zone is about 3500 m which is in agreement with available d ata. The overpressure occurs within the shale rich region of the Agbada and top of Akata Formations in the basin. Comparison of the overpressure zone wit h available geophysical logs shows that the interval has lower bulk density, lower interval velocity and high porosity. With regard to the young age of the basin , the factors that might be responsible for the overpressure are mechanical compaction, chemical compaction and hydrocarbon generation. The results of the analysis revealed that the overpressure in the area is associated with simple rollover structures bounded by growth faults, especially the hanging walls. Thi s method of pore pressure prediction is very useful because it helps to predict pore pressure ahead of the bit.
Highlights
The Niger Delta Basin is the most prolific and economic sedimentary basin in Nigeria by the virtue of the size of petroleum accumulations, discovered and produced as well as the spatial distribution of the petroleum resources
The results of the analysis revealed that the overpressure in the area is associated with simple rollover structures bounded by growth faults, especially the hanging walls
Seismic interval velocity- pore pressure transform has been derived for part of Onshore Niger Delta Sedimentary basin
Summary
The Niger Delta Basin is the most prolific and economic sedimentary basin in Nigeria by the virtue of the size of petroleum accumulations, discovered and produced as well as the spatial distribution of the petroleum resources. The understanding of overpressure in the subsurface is important for drilling safety, exploration risk and reservoir depletion studies (Opara and Onuoha, 2009). Accurate pore pressure prediction is vital for successful and safe drilling of wells and the Deep- and Ultra-Deep offshore area of the Niger Delta is no exception. In the Niger Delta kicks have been observed in permeable zones within the Early Miocene shale, suggesting mud-weights have been set too low as a result of inaccurate predrill pressure prediction (O’Connor et al, 2011). The prevention of drilling mud losses or kicks, the drilling risk assessment in connection with expected formation pressure, the casing design and so on are typical industry problems associated with prediction and estimation of pore pressure before and during exploration well drilling (Bell, 1994; Huffman, 2002a; 2002b). Detection of overpressures before drilling is most useful as precautions can be taken and planning can be done
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