Authors : Muhammad Syafiq Zainal Malek, Lo Shyh-Zung and Abdul Ghani Md Rafek
Abstract: Investigations in Niger delta adjacent to Termit basin, Niger has widespread occurrence of radioactive sands. About 34-45% of producing wells contains this anomaly which is misinterpreted as shales in petrophysical analysis, specifically from Gamma Rray (GR) log. This causes potential pay zones to be overlooked unless revaluation of conventional logs is done. A four-step approach is established to define existence of radioactive sands and identifying source of radioactivity based only on conventional wireline logs. Targeted radioactive sands for the research are hydrocarbon bearing. Methods starts with identifying mudcakes in shale lithology layers detected by GR log and cross-referencing with neutron-density (NPHI-RHOB) crossover. By interpretation of NPHI-RHOB crossover and identifying high values in resistivity log, hydrocarbon zones are identified focussing specifically on shale layers with mudcake. Hydrocarbon zones detected as shales contradicts with interpretation as mudcake only sticks on porous and permeable layers specifically on sandstone not shale. To confirm interpreted radioactive sands, NPHI vs. RHOB cross plot is done to identify lithology. This is followed by radioactive source identification by evaluating spectral gamma ray to determine potential source. Lastly, thorium vs. potassium cross plot is used to identify specified minerals present which contributes to high radioactivity. Approximately 25-30 m of accumulated pay zones were found within all five wells. Methods mentioned is to re-evaluate wells in the system without any additional logging tools. The study is to facilitate the development and improve interpretative procedures that might lead to cost-effective re-completion of hidden reservoirs within the same producing system.
Muhammad Syafiq Zainal Malek, Lo Shyh-Zung and Abdul Ghani Md Rafek, 2018. Petrophysical Analysis on Radioactive Sands for Koala Field in Termit Basin, Niger. Journal of Engineering and Applied Sciences, 13: 5122-5130.