Publications

Several large hydrocarbon gas fields have been discovered in the South Sumatra Basin since the early 1990s. Several trillion cubic feet of gas are known to be present in the fractured Pre-Tertiary reservoirs such as granite, metamorphic quartzite, schist, phyllite, argillite, limestone and marble. However, the distribution of CO2 in these reservoirs is a key exploration uncertainty, with CO2 concentration ranging from 0 to 98 mole %. Gas composition and stable carbon isotope data help to explain the origins of the South Sumatra CO2 but 3D basin modeling is required to create predictive models for CO2 content.The CO2 concentration in study area can not be explained fully by present-day reservoir temperature or reservoir depth. The geochemical data and modeling results suggest that the CO2 is primarily of inorganic origin and that the CO2 concentration is controlled by fluid-rock reaction(s) that operate in the 150 - 300o C temperature range. Low grade metamorphism and high temperature carbonate dissociation is not required to explain the CO2 content.Two multiple linear regression equations based on 3D basin model results for field paleo-drainage area, total hydrocarbon gas charge and average temperature of the Pre-Tertiary reservoirs in the drainage area are derived here. The equations successfully predicted the CO2 concentration (±10%) in several existing South Sumatra fields that were not included in the training set from which the equations were derived. Encouragingly, the models were also successful in the first pre-drill test at the Kaliberau Dalam discovery where the predicted concentrations (24-32% CO2) compared very well with the actual result (25% CO2 ).The hydrocarbon gas charge is estimated to occur from approximately 26 to 3 mya for the fields in the study. Based on conservative estimates of the source rock thickness and richness, the hydrocarbon gas volumes available to the structurally older Pre- Tertiary traps appear to be sufficient to indicate the structures have excess charge with respect to pore volume. Hydrocarbon charge volume analysis suggests that the magnitude of the excess is an important variable in determining CO2 concentration as it aids in dilution of CO2.