Publications

ExxonMobil Cepu Limited (EMCL) and partners in the Cepu PSC have successfully completed a development drilling campaign in the giant Banyu Urip field, Cepu Block, East Java. The wells will produce approximately 25% of Indonesia’s targeted crude oil production in 2016. In total, 40 development wells were drilled and key subsurface datasets such as well logs, core, pressure and dynamic data were acquired. The data confirmed several predictions including environments of deposition, reservoir quality, diagenesis and reservoir behavior that have been published at previous IPA conventions. Most of the results were as expected. However, well-test-derived average permeabilities indicated that productivity is much higher than predicted. Well tests in 18 of the 30 oil producers show excess permeability 2 to 35 times greater than measurement based on core plugs. This is attributed to major hydrothermal dissolution in the form of pervasive vug systems, dissolved fracture systems and small-scale cave collapse zones interpreted on image logs. Vugs are observed at all scales from microscopic in thin-sections, to small finger size in core, and up to 20 cm, on image logs. The low quality seismic data precluded good prediction of the top carbonate, because the basal clastic sand and top carbonate have the same seismic response. However, the clastic section that was present instead of the expected carbonate is much better quality than predicted and is, for the most part, well connected to the carbonate, so the pre-drill resource estimate is maintained. The overlying clastic units, which are also part of the trap, are nearly perfectly pressure connected and are of very high quality. These units are deposited as deepwater fans with approximately 50% silicate grains and 50% carbonate grains, making them react very similarly to hydrothermal fluids as the carbonate platform below. Therefore, both the clastic and carbonate sections display many of the same dissolution features. The dissolution occurred millions of years after the carbonates were deposited and drowned. This dissolution was the result of hydrothermal fluids as evidenced by dissolution focused along burial fractures and the presence of hydrothermal minerals. Keywords: hydrothermal dissolution, excess permeability, vug systems