Stratigraphic traps in the Tertiary rift basins of Indonesia: case studies and future potential

Abstract

AbstractStratigraphic traps are often regarded as one of the most risky exploration targets and are frequently the most difficult trap type to secure management support for drilling. This is despite their frequent occurrence in many basins, in particular the mature onshore basins of North America. The critical factors required for the development of stratigraphic traps are the presence of a fully charged petroleum system, favourable basin and reservoir architectures, low dips in the accumulation zone and good seal integrity. These factors commonly occur in the Palaeogene rift basins of Indonesia and consequently, since the early 1990s stratigraphic traps have often accounted for the majority of recently discovered fields.The Palaeogene rifts of Indonesia are prone to the stratigraphic trapping of hydrocarbons simply because they possess an almost perfect petroleum system in which traps of this type can form. Every rift has a similar history beginning with a syn-rift phase, which primarily provides source rocks of various types as well as reservoir sandstones. This is followed by a thermal sag phase (‘early post-rift’) where better quality reservoir sandstones and sometimes reef carbonates are found. The ensuing transgressive phase (‘late post rift’) guarantees a marine shale regional seal. This seal is most impressive at the Minas Field in Central Sumatra where 6 billion barrels of oil are trapped at only 600 metres depth with no surface oil seeps. The various late Tertiary ‘orogenic’ phases trigger migration of hydrocarbons generally up the flanks of the rifts as well as creating structures at shallower levels. Sometimes this structural activity is so intense that the regional seal is breached causing hydrocarbons to migrate into these shallower structures where major accumulations have been found. However, where the seal is not breached the hydrocarbons must still be trapped below it on the flanks of the rift. The giant Widuri and Kaji Semoga fields are perfect case history examples.There is no doubt that in Indonesia the ‘easy oil’ has already been found in large 4-way dip closures or classic Sunda Fold inversion structures. However, it is contended that there is still a huge potential for finding large reserves in stratigraphic traps in basins with the right characteristics. In this context, two largely unexplored basins within the Asahan Offshore PSC, North Sumatra and the Biliton PSC, West Java are discussed. Both these areas exhibit all the ingredients required for successful stratigraphic trap discoveries but both remain at the present day undrilled for this play type.