Bypassed Shallow Gas Interval And The Challenge To Develop The Potential In Matured Oil And Gas Fields Of Malacca Strait, Central Sumatra Basin

Abstract

Malacca Strait PSC has been producing oil since 1984 and gas since past decade, which make the fields categorized as brownfield. Major reservoirs for oil and gas fields in Central Sumatra Basin, Malacca Strait area in particular, are sandstone intervals from Early Miocene Sihapas Group and the Early Oligocene Lower Pematang Formation. Formations that responsible in sealing those reservoirs are shale dominated Petani and Telisa formations that act as regional seal across the basin. Both formations were deposited in two different tectonic phases. The marine Telisa Formation was deposited in a post-rift sagging phase from Early to Middle Miocene that marked as the end of transgressive sequence of Sihapas Group deposition. Tectonic inversion followed the event that marked the starting of regression sequence in the basin, resulted in deposition of the fluvio-deltaic Petani Formation. These formations are shallow in depth and have indications of gas accumulations as encountered by wells in the area. The wells that encountered significant high gas readings are currently producing oil. The high gas indications in Petani and Telisa Formations are identified from gas reading from mudlog data, which have significant difference between peak and background gasses. This interval does not have complete wireline surveys, as the main objective of every wells in the area is oil accumulations in the proven major reservoirs below. The gas zone occurred between depth of 1700 ft SS to around 3000 ft SS with the composition of 98% methane. With geothermal gradient in the area ranging 3.2° - 3.4°F/100 ft, the zone has current temperature around 120° - 180°F. Even though the data is limited, wells correlation shows that the gas occurrence strongly relates to depth than stratigraphic layers and spreads across the Malacca Strait Block. The condition stated above is favorable for biogenic gas generation in general. Regional mapping of the formations indicated that the gas occurrences correlate to closures. Bentu PSC, which has the same entity with Malacca Strait PSC, is one of the company that already producing biogenic gas from the equivalent formations with depth around 1000 – 2000 ft SS and lies on the same basin of Central Sumatra. Identified initially as potential shallow hazardous gas intervals in drilling, they tested it to get the economic rate and successfully produced the gas accumulations. From this case, Malacca Strait PSC sees the same opportunity with potential biogenic gas accumulations despite the limitation of data for the shallow target. The best shallow gas zone area within the Malacca Strait PSC currently lies on one of matured oil field in the block, the first producing oil field owned by the PSC. The well still producing oil and the aging facility become obstacles in order to prove the gas occurrence. Challenges arise in order to develop the shallow gas potential because the development of this gas accumulation can reduce operational cost as part of efficiency and increases revenue of the PSC. Once succeed, this shallow gas potential can change the exploration and development scenarios of the Malacca Strait PSC in the future.

AAPG Datapages/Search and Discovery Article #90331©2018 AAPG Asia Pacific Region GTW, Back to the Future – The Past and Future of Oil and Gas Production in the Asia Pacific Region, Bangkok, Thailand, September 26-27, 2018