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An up-hole seismic refraction survey was carried out in OPL-135 in the Western part of the Niger Delta Basin, (SS) Nigeria. A total of thirty seven (37) up-hole locations in a (4.03.750) km grid configuration were drilled and logged over the prospect. The focal objective for the study was to assess the weathering layer characteristics over the prospect, specifically, to estimate the weathering and sub-weathering layer thicknesses and velocities, in a bid to better understand the near-surface properties and to enhance the subsequent acquisition and processing of 3 D seismic reflection data of the prospect. From the data acquired, time-depth plots for each up-hole shot point were derived, which was used to determine the velocities of the underlying layers for each point. Depth to the refractor distance were computed to determine the thickness of the weathered layers. The result obtained gave an overview of the lateral variation in the thicknesses and velocities of the near subsurface over the prospect. The thickness of the weathering layer obtained ranged from 3.1 m at up-hole point 21 and 28 to 6.8 m at up-hole point 12, with an average value of 4.7 m across the prospect. The weathering layer velocity ranged from as low as 210 m/s at up-hole point 4 to as much as 593 m/s at up-hole point 29, with an overall average value of 361 m/s. The sub-weathering layer velocities ranged from 1131 m/s at up-hole point 32 to a maximum of 1987 m/s at up-hole point 3, with an overall average of 1707 m/s across the prospect. The results from this study would aid proper planning/implementation of the 3 D seismic acquisition program for the prospet and would as well be an indispensible tool in the pre/main and post processing of the acquired seismic reflection data.

Seismic refraction was used to image the near-surface, using a 3D seismic data acquired from an onshore Niger Delta field. The objective of the study was to build a near-surface model of the earth in terms of sub-weathering layer thicknesses and seismic wave velocities and highlight the near-surface seismic wave velocity variation in the region. A velocity-depth model consisting of the weathering layer and three consolidated (sub-weathering) layers was first estimated. The first-breaks were picked and used as input for this study. The models were generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general. Results support confidence in the reliability and robustness of the refraction inversion method. The method of analysis and resolution for 3-layer models adopted in this study may be used in resolving such cases observed during processing of seismic reflection data from the Niger-Delta Basin.

Prediction and evaluation of overburden pressure are critical for the exploration and production of hydrocarbon reservoirs. Overburden pressure was estimated using well log (density and sonic) data obtained from two wells (B1 and B2) of an X -Field within the Niger Delta basin. Overburden pressure depends primarily on the bulk density data. Bulk density was extracted from density and sonic logs based on the log signatures. The bulk density was then used to determine overburden pressure using Eaton's equation. The results reveal that overburden pressure increases linearly with depth, and an overburden gradient of 1.0 psi/Ft. was obtained. The overburden pressure was used to estimate pore pressure and vertical effective stress and thus enabled the determination of overpressure zones within the well.