Publications

The North Sumatra Offshore area (NSO) is part of the highly prolific North Sumatra Basin, which has experienced a complex tectonic history since the Eocene, including phases of rifting, wrenching, and compression. Historical exploration efforts in NSO have focused on the post-rift basin-fill stratigraphy: carbonate build-ups of the Middle Miocene Peutu Formation and faulted structures with Late Miocene turbidite objectives (Keutapang and Seurula Formations). Together, these post-rift plays are responsible for more than 90% of the discovered resources to date. In comparison, exploration to date for syn-rift objectives has been modest, and discovered volumes to date are small. However, modern 3D and 2D marine seismic surveys reveal significant inverted syn-rift structures, with the potential for good-quality syn-rift clastics to be in structural closure in close proximity to excellent hydrocarbons kitchens. This paper describes the key geological elements of the emerging syn-rift play. N-S trending horsts and grabens developed in the south of study area, and SW-NE trending horsts and grabens developed in the north. The basinal lows were filled by a vertical succession of continental clastics (lower Parapat Formation), followed by transgressive marine muds, silts, and sands (upper Parapat Formation), and deep-marine shales and sands (Bampo Formation). Several offset exploratory wells that penetrated the syn-rift succession have been used to analyze the petroleum systems and stratigraphy of the syn-rift play. The presence of good quality sandstone is proven regionally in the ITU-1, BLD-1 and JAU-1 wells. Sandstones in the Parapat Formation have good reservoir potential: porosity reaches 20-30% and permeability can exceed 100 mD. The best reservoir quality is found in sands in the hanging wall's lower slope. The basins were inverted during late-stage back-arc compression to form high-relief, structures in southern NSO and lower-relief structures in northern NSO. Potential hydrocarbon traps consist of fault-bounded three-way closures and faulted four-way closures. Intraformational Parapat or overlying Bampo shales are interpreted to be effective seal rocks. Petroleum is likely to have been generated from lacustrine and restricted marine (Type II and III ) shales of the Parapat Formation. A 3D thermal model of the NSO area indicates that Parapat source rocks have been generating hydrocarbons that have migrated into structures since the middle Miocene.