Publications

The gamma ray log signatures of the South Sumatra Basin Tertiary section have a distinct trend correlated to geological markers. Low gamma ray reading is correlated to younger sediment packages i.e. Lower Palembang, Telisa and Baturaja Equivalent, whilst higher gamma ray readings are correlated to oldest sediment packages i.e. Pendopo and Talang Akar Formations. The low gamma ray reading in younger rocks is contradictive to well-known regional geology understanding that they were deposited in marine environment based on its biostratigraphy data. To understand the cause of this unique gamma ray signature, a combination of XRD and spectral gamma ray analyses were performed on fourteen wells to generate a clay mineralogy vertical profile. The data indicates that the amount of kaolinite content is abundant in the younger strata, and it is decreasing toward the older strata. Large amount of kaolinite in marine sediment, such as in Telisa Formation, is an uncommon phenomenon that should be related to a geological event. Volcanic activity is postulated as the cause of the abundant amount of kaolinite in this formation. This hypothesis is supported by fossil amber from Miocene rocks found in Jambi. The large amount of kaolinite in younger sediment creates a challenge for reservoir characterization. Low gamma ray shale baseline makes it difficult to separate sand from shale. Therefore, an improvement in the petrophysical analysis is required to accurately evaluate the younger strata. Instead of solely using gamma ray log, a combination of neutron-density, resistivity and gas logs analysis is proven effective for identifying reservoir intervals.