Overpressure prediction of the Efomeh field using synthetic data, onshore Niger Delta, Nigeria
Gabriel Efomeh Omolaiye 1 4 John Sunday Ojo 2 Michael Ilesanmi Oladapo 2 Elijah A. Ayolabi 31 Mosunmolu Limited, The Oceanview Place, Alpha Beach Road, Lekki Peninsula, Lagos 23401, Nigeria.
2 Applied Geophysics Department, Federal University of Technology, Akure 2343401, Nigeria.
3 Geoscience Department, University of Lagos 2343401, Nigeria.
4 Corresponding author. Email: gab_omolaiye@yahoo.co.uk
Exploration Geophysics 42(1) 50-57 https://doi.org/10.1071/EG09040
Submitted: 10 September 2009 Accepted: 17 November 2010 Published: 25 February 2011
Abstract
For effective and accurate prediction of overpressure in the Efomeh field, located in the Niger delta basin of Nigeria, integrated seismic and borehole analyses were undertaken. Normal and abnormal pore pressure zones were delineated based on the principle of normal and deviation from normal velocity trends. The transition between the two trends signifies the top of overpressure. The overpressure tops were picked at regular intervals from seismic data using interval velocities obtained by applying Dix’s approximation. The accuracy of the predicted overpressure zone was confirmed from the sonic velocity data of the Efomeh 01 well. The variation to the depth of overpressure between the predicted and observed values was less than 10 m at the Efomeh 01 well location, with confidence of over 99 per cent. The depth map generated shows that the depth distribution to the top of the overpressure zone of the Efomeh field falls within the sub-sea depth range of 2655 ± 2 m (2550 ms) to 3720 ± 2 m (2900 ms). This depth conforms to thick marine shales using the Efomeh 01 composite log. The lower part of the Agbada Formation within the Efomeh field is overpressured and the depth of the top of the overpressure does not follow any time-stratigraphic boundary across the field. Prediction of the top of the overpressure zone within the Efomeh field for potential wells that will total depth beyond 2440 m sub-sea is very important for safer drilling practice as well as the prevention of lost circulation.
Key words: Efomeh, Niger Delta, overpressure prediction, seismic, synthetic, velocity.
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