Publications

In normal clastic environments, the gamma ray log is fundamentally used as a sand or shale lithology indicator. This is due to the fact that shales usually emit more radiation than other sedimentary rocks – they are “radioactively hot” compared to “radioactively cool” sand. However, in certain circumstance shales will have low radioactivity. This phenomenon is observed in the Late Oligocene to Early Miocene Udang Formation at the South Belut sub-basin and surrounding area in the West Natuna Sea. Integrated log analysis and test information from the South Belut and West Belut fields has shown a prior misinterpretation and miscalculation of shale volumes in the Udang resulting in an inaccurate estimation of hydrocarbon in place. “Cool” shale may occur when clay minerals in the shale interval are dominated by constituent minerals with low radioactivity such as kaolinite or chlorite. Investigations including petrographic analysis, XRD, and SEM of side wall cores acquired in the offset wells show high kaolinite with subordinate chlorite content within the Udang Formation in the South Belut Sub-Basin. Understanding this occurrence, we introduce special methods that we used to analyze and estimate shale volume in the Udang Formation when the gamma ray response due to low radioactivity is not suitable for lithology indication. Several VShale calculation equations that could solve this problem are based on consideration of multiple “clay indicators” evaluated statistically. These log-based clay indicators include the single neutron log, density log, resistivity log and the combination of neutron-density logs (neutron-density crossplot technique) where each indicator is calibrated to the borehole environment. Further investigations on the “cool” shale were conducted to analyze the phenomenon in the regional context, suggesting that the kaolinite clays are likely coming from sediments in exposed Sundaland basement rocks. This observation helps to constrain the provenance drainage system of the Early Miocene South Belut sub-basin.