Publications

Offshore area of North Sumatra Basin is believed to have a good potential of hydrocarbon accumulation and remain challenging for exploration. Several exploratory wells had been drilled in this area with major discoveries mainly in Tertiary age reservoir like Baong and Belumai Formation. The Pre-Tertiary plays in North Sumatra Basin are underexplored and we believe that they play an important role in future success of hydrocarbon exploration in this area. The Pre-Tertiary play is the main exploration target in West Glagah Kambuna in offshore North Sumatra Basin. Previous regional studies suggested that the basements in this area are mainly the Tampur Formation of Eocene - Early Oligocene age. Based on our present study, however, basement in West Glagah Kambuna area mainly consists of Batumilmil, Kaloi and Kualu Formation of Permian - Triassic age that are parts of the Peusangan Group. The offshore seismic data clearly depicts angular unconformity contact between the Pre-Tertiary and the Tertiary stratigraphy. In addition to the clearly defined Tertiary sedimentary-fills section, the seismic also shows good images of the Pre-Tertiary packages which can be differentiated based on their internal-external reflection characteristics and reflectivity. By correlating the pre-rift seismic section with onshore geology which based on outcrop data, it is interpreted that the Pre-Rift stratigraphy found in the offshore area belongs to Mesozoic Peusangan Group. The Peusangan Group in the offshore area can be divided into three different formations, from the oldest to the youngest are Batumilmil, Kaloi, and Kualu Formations. The Batumilmil Formation is represented by the chaotic seismic character, the Kaloi Formation is represented by high amplitude seismic reflection, while the low amplitude seismic reflection above Kaloi Formation is interpreted as Kualu Formation. The good match between seismic and well data in WGK block and surrounding area strongly support the interpretation. The outcrop study in Aceh & Sumatra Utara province has confirmed that the pre-Bampo/Bruksah section of the North Sumatera Basin comprising not only the Dolomite Tampur Formation, but also several formations from Meureudu and Peusangan Groups which are Pre-Tertiary in ages. The study also suggested that the pre-rift dolomitic carbonate lithology are not just belong to the Tampur Formation since the Peusangan Group also shown similar lithology, particularly in Batumilmil and Kaloi Formations. The extensive tectonic and subaerial exposure has promoted fractures and caving in those formations. Beside the dolomitic carbonate, the study also found clastic rock within the Pre-Tertiary stratigraphy. The clastic rocks are black shale in Batumimil and mudstone in Kualu Formation. Several fossils could still be found in Batumilmil carbonate and Kualu mudstone. The outcrop sedimentology characteristic are still showing sedimentary rock character where bedding and layering is clear and common. The Pre-Tertiary formations that developed in WGK block and surrounding area are parts of the Peusangan Group, which based on our field observation and laboratory analysis of Kaloi, Kualu, and Batumilmil Formation, they show good reservoir properties and also with high carbon content in their shales in some places. It shows a possibility of the Pre-Tertiary formations to have its own petroleum system, separated from the Tertiary formations. This promotes new exploration opportunity in southern part of North Sumatera Basin, particularly in WGK block.