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Radioactive sandstone reservoirs are occasionally found in Mahakam fields, especially over Shallow Zone (MSZ). It is characterized by high Gamma Ray (GR) value which indicate shale, however Neutron-Density separation indicate good quality reservoir, while both Resistivity and Total Gas has high value which indicates hydrocarbon bearing. This type of reservoir has been tested by downhole fluid analysis and perforation with proven hydrocarbon results. According to XRD and thin section data, the high value of GR reading is related to the abundance of Potassium content that occurs as micaceous fragment and K-feldspar minerals.

This reservoir is usually overlooked because of high GR value caused by radioactive mineral content. By using routine petrophysical analysis, this radioactive reservoir is usually interpreted as a non-reservoir because the shale volume (Vshale) calculation derived from GR is too high. Therefore, the Vshale calculation from GR becomes unreliable, which not only affected the reservoir identification, but also impacted the porosity and saturation calculation.

Common practical solution is to generate Vshale from Neutron-Density logs. However, in this MSZ interval, due to bigger open hole size and sliding mode of BHA, the LWD Neutron-Density logs are often unreliable. Therefore, in order to improve accuracy and reliability of Vshale calculation in radioactive reservoir of MSZ, there are two more alternative solutions which have been developed and evaluated. These solutions are Vshale from Elemental Capture Spectroscopy, and from Sigma. Both advanced measurements were acquired by the multifunction LWD technology. These solutions have been tested in 3 trial wells, yielding successful results. Radioactive (high GR) sandstone can now be recognized as a reservoir with proper netpay thickness, reliable porosity, and reliable saturation.

This paper details the alternative solutions for petrophysical analysis of radioactive sandstone reservoir in Mahakam, and further provides new alternative solution to be applied in other fields outside Mahakam which has radioactive reservoir.