Publications

The Semai area of Eastern Indonesia became highly prospective when Fugro shot a multi-client survey in 2006. Over a period of 8 years almost $ 600 MM has been invested in the offshore Semai Basin by oil companies exploring for hydrocarbons with no commercial success to date. The question is whether there is any remaining prospectivity for this area. This talk aims to summarize the known petroleum elements in this area and review remaining potential and let the audience decide. The primary reservoir objective for this area was considered to be the Lower and Middle Jurassic Sandstone, broadly equivalent to the main producing reservoirs found in the Tangguh area to the North East, and considered to be a regional analogue to the Middle Jurassic Plover Formation sandstones of the Australian offshore continental shelf. The Jurassic shale and Permian coals are considered to be the main Pre-Tertiary source rocks in the Bird's Head region. Greater than 500 meters of Early to Middle Jurassic section that is predominantly shale were penetrated by Murphy’s first well Lengkuas-1. The well encounter interpreted thermogenic gas from headspace gas and carbon isotope analysis. Gas shows were also reported in the Jurassic interval. A basin model predicts that the Jurassic shales and Permian coals source rock reached the gas generation window during the Pliocene as the result of thick Late Miocene to Pliocene sedimentation, therefore the timing of gas expulsion, migration and charging is anticipated to have taken place from Pliocene to present day. Trap generation timing could be as early as Late Miocene but the main phase of regional structural inversion and uplift took place very recently in the Pleistocene as evidenced by the inversion along the Misool-Onin-KumawaAnticline. The Upper Jurassic and the Lower Cretaceous shales are proven to provide a good top seal for the Jurassic sandstone reservoirs in the Bird’s Head Region. A similar deep marine facies shale of Upper Jurassic to Lower Cretaceous was penetrated in Lengkuas-1 well, as part of a regional deposition and therefore top seal presence and quality risk is considered low in the Semai Basin. The Lower Jurassic sandstone is the main reservoir objective in the Semai basin to date. It was penetrated in the Lengkuas-1 well with a gross thickness in excess of 63metres (not fully penetrated). In contrast the main reservoirs to the North East, in Tangguh fields are slightly younger, of Middle Jurassic age. The presence of the Lower Jurassic sandstone at the Lengkuas-1 well location could support a West-East progressively back-stepping deltaic model of the Lower to Middle Jurassic sandstone with deposition in a transgressive depositional setting. Reservoir quality was defined as the primary failure mode of the Lengkuas-1 well. The reservoir was encountered very deep at Lengkuas-1, at a depth of 6,500 m TVDSS. Same age reservoir was encountered much shallower at Bawang Putih-1 and Serai-1 wells to the East, but also with very poor reservoir quality. These two wells were drilled in a location that has suffered very significant late structural uplift and inversion, of perhaps more than 3 km, resulting in a heavily reduced porosity related to both compaction and also the onset of diagenesis prior to uplift. The challenge for the future exploration of the Semai area is to find good quality reservoir, it could be the less deeply buried Jurassic sandstone