Characterizing Thin Sand Reservoirs in Onshore Uganda - Challenges and Lessons Learned

The study area is located onshore North West of Uganda. It contains significant oil and gas reserves in numerous narrow river channels, whose appraisal has been realized so far using vertical wells, thus within a high risk of bypassed pay. The objective is to succeed in mapping sands down to 3–5 meter which are part of the multi-level 3D fluvial channel system and be able to identify hydrocarbon distribution within these sands. The study area presented several challenges for characterizing thin sands and also mapping the sand distribution. Despite the very shallow geological setting with several unconsolidated sand units, a poor seismic-to-well calibration, overall medium to poor quality of available log data and limited spectral overlap between angle-stacks, results of a deterministic inversion and dedicated classification could still be helpful for improving sedimentological model and reservoirs distribution understanding. By preserving the high frequency information thanks to an extensive denoise sequence it was possible to successfully delineate the geometry of thin hydrocarbon bearing sand reservoirs. Comparison with coloured inversion results led to the conclusion that a simultaneous seismic inversion should provide extra added value in terms of reservoir characterization compared to a simpler method working separately on different angle-stacks.