Publications

Betara structure is located in the northern part of South Sumatra Basin (SSB). It is formed by the mega sequence tectonic processes of rifting and inversion with various stratigraphic sequences from land to marine. Referring to the schematic map of global overpressure occurrences, most of Indonesia including Sumatra is an overpressure region. The indications of overpressure zones in the Betara structure are in Gumai and Talang Akar Formations where the syn-rift to post-rift sequence is dominated by fine grained sediments (shale) with low permeability. A preliminary study has been conducted with available data to predict the type of overpressure mechanism in the Betara structure. Existing data such as XRD, pressure (RFT/MDT), drilling parameters, wireline log suites and geochemistry are used to determine the overpressure zones. The presence of overpressure zones is marked by a significant reversal of sonic log with increasing depth from the normal compaction trend on several existing wells. Cross-plot analysis of Th/K (clay typing), Sonic vs. Density, and vitrinite reflectance are used to analyze the causative overpressure mechanism that occurs in the Betara structure. Results showed overpressure indications in the Betara structure with low overpressure characteristic in the Gumai interval and with increasing depth overpressure decreased to almost equal hydrostatic pressure in the lower part of near Baturaja Formation. High overpressure occurs in the Upper Talang Akar interval and it decreased or back to hydrostatic pressure in the Lower Talang Akar interval. Based on preliminary study analysis, it can be predicted that the overpressure mechanism in the Gumai interval is possibly caused by disequilibrium compaction of thick shale layer whereas in Talang Akar it is caused by clay minerals transformation and hydrocarbon maturity as a result of the temperature influence.