Publications

Detailed seismic structural analysis of the Terang- Sirasun feature, offshore Bali, has revealed a complex history of alternating long-term subsidence and episodic uplift. These fluctuations have had a significant impact on gas volume expansion and gas producibility due to differences in drainage rather than imbibition reservoir saturation history. A drainage rather than imbibition saturation history should significantly improve total ultimate production from this reservoir.The Sirasun structure was charged very late and is filled with biogenically sourced gas. The bulk of the gas volume entered the trap since the initiation of the ongoing period of uplift, and charge continues at present. Although the structure overall has subsided since the Late Cretaceous, this subsidence has been punctuated by periods of pronounced uplift, with the initial phase of uplift occurring in the early Miocene, and additional phases in the Plio-Pleistocene and present. Mapping of truncated seismic reflections around the crest of the Terang-Sirasun structure indicates that more than 1,000 feet of uplift has occurred during the last 1.5 mybp (million years before present) and that the bulk of that uplift is due to the current phase of inversion.The current uplift is causing the biogenic gas to expand and displace water within the transition zone and at the gas water contact (GWC). Because the expanding gas is displacing water, the reservoirs saturation history is due to drainage conditions. A gas pool which has experienced a drainage saturation history will result in reduced water production, a higher initial gas saturation, and larger recoverable gas volumes as compared to a pool that has experienced an imbibition saturation history. ARCOs previous reservoir model was calibrated to imbibition, consistent with a depositional model that involved continuous subsidence over the life of the gas volumes. ARCOs new depositional model that involves episodic uplift and a drainage saturation history therefore should result in improved reservoir performance.