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  • Author: Etim D. Uko x
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Abstract

Three dimensional (3D) seismic survey was acquired and processed in Bornu-Chad basin, Nigeria with the aim of detecting and attenuating multiples to aid proper imaging of the subsurface. The 25.5km2 volume was processed using SeisUp processing software on a 32-Node Cluster Infrastructure (CI) hardware. Considering the imaging objectives and depth of interest of 1.2s-4.5s, the minimum, middle and maximum offsets were set at 500-2300m, 2500-4300m and 4500-7300m respectively. Since the study area comprised of dry open land and swampy Lake Chad, vibroseis and dynamite sources were used respectively. Charge depth was 0m (surface) for vibroseis and 25m dynamite. The dataset was first pre-conditioned, normalized, regularized before application of demultiple process. The detection and demultiple processes based on multiple characteristics of periodicity and velocity discrimination were applied as the multiples have comparable velocity with the primaries. The near-surface reverberations and short-period multiples were attenuated using predictive deconvolution and radon transform algorithms. High resolution radon was performed on post-migrated common-mid-point (CMP) gathers and stacked with 1kmx1km target line velocities. Internal multiples were detected and attenuated using data-driven methodology of extended internal multiple prediction (XIMP). Multiples detected were short, long period and interbed multiples on all frequencies ranges of 0 – 90Hz but useful seismic frequency range was between 20Hz and 70Hz. The frequencies and amplitudes of the primaries and multiples were very comparable, therefore great care was taken in the attenuation processes. The results of this work has produced better seismic section for interpretation of subsurface geology in the study area.

Abstract

Geophysical well logs were used to delineate the stratigraphic units and system tracks in the XYZ Field of the Niger Delta. The gross percentages for sand levels range from 93-96% in the shallow levels to 60-66% in the deeper levels. Porosity values ranged between 27% at shallower sections and 9% at deeper depths. Six depositional sequences were identified and categorized into their associated system tracts. Porosity decreases with depth in normal compacted formation for both sandstone and shale units. Surface porosity for sandstone is 42%, and for shale it is 38.7% from extrapolation of sub-surface porosity values to the surface. The depth to the base of Benin Formation is highly variable ranging between 1300 and 2600m. This study reveals the possibility to correlate sand levels over long distances which enables inferring porosity values laterally. The knowledge of the existent stratigraphic units, the Benin, Agbada and Akata Formations and their petrophysical parameters such as porosity, lateral continuity of the sands and shales, the variation of the net-to-gross of sands with depth, enables the reservoir engineer to develop a plan for the number and location of the wells to be drilled into the reservoir, the rates of production that can be sustained for optimum recovery. The reservoir engineer can also estimate the productivity and ultimate recovery (reserves) using the results on this work.