Assessment of Weathering Layer Characteristics over OPL-135 in the Western Part of the Niger Delta Basin, South-South, Nigeria

Abstract

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.