Publications

North Sumatra Basin located on northern area of Sumatra Island, including offshore area of it. North Sumatra Basin is combination between pull-apart basin and half-graben basin. North Sumatra Basin formed on Late Eocene when Australian plate collided with Eurasian plate. It is influenced by two major faults, Sumatra fault system and Malaka fault system, which caused block faulting become pull apart basin. In Early Oligocene, velocity of subduction decreased and made the plate more stabil, resulted compressional deformation in oblique direction that changed from extentional deformation. Motion of Indian plate on northwest of Andaman Sea toward north direction in Early Miocene, changed the tectonic from subduction became dextral strike slip. Released of energy from changing tectonic made rifting occured in Andaman Sea. Continuing of subduction made heat motion upward and resulting uplift of Barisan Mountains in Plio-Pleistocene.Oligocene black shale of Bampoo Formation recognized as a good source. This formation was resulted by sea transgression, so there is possibility that it has amount of marine organic content. It also has 2000 m in thickness. Another source rock is Peutu Shale Formation (Early Miocene). Here, trangression also occured after development of Peutu reefal. Most reservoirs in this Basin are recognized as Belumai and Peutu reefal limestone. They have porosity of 18% in average. But there is another potential reservoir, i.e. Lower Keutapang clastics formation with reefal limestone in spot area. Sediment was supplied from Barisan Mountains. Great seal rock formed as Baong Shale Formation that covered almost all of Early Miocene reefal limestone. Upper Keutapang and Seurula Formations taken place as seal rock of lower Keutapang reservoir. Both of primary and secondary migrations occured in this basin. Primary migration occured on Peutu shale formation that directly migrated to reefal of Peutu (Arun Reef). While secondary migration occured on black shale Bampoo which migrated through horst and graben fracture and fault system around basement. Oil trap developed as an anticline on reefal reservoir, resulted from basement stress upward, combinating with fault system and stratigraphic trap by Baong shale Formation. Reefal of Peutu was trapped on upper part of Basement highs.Generally, sequence stratigraphy application in Keutapang Formation consists of 3 sequences. First sequence is characterized by progradational regressive tract, overlain by transgressive system tract. TST sediment is recognized as a fining upward sediment, particularly shale sediment in great thickness. Those sediments become seal rock of hydrocarbon play in Keutapang formation. The last sequence is HST sediment that is recognized as an aggradational sequence continued by coarsening upward sediment. HST sediment would be a good clastic reservoir. Based on those sequences, it could be interpreted that sedimentation in Keutapang occurred in three phase sequences. Trangression phase is identified by present of TST sediment. While regression phase in Keutapang is identified by present of HST sediment. In this paper, sequence stratigraphy will be described based on several well log data from Keutapang Formation.