Publications

This paper will discuss the tectonic and stratigraphic evolution of the Tarakan sub-basin, primarily the fluvio-deltaic deposition during the Neogene time. The Tarakan sub-basin is part of a sub-basin complex which includes the Tidung, Berau, and Muaras sub-basins located in Northeast Kalimantan. In this paper, the discussion about the Tarakan sub-basin also includes the Tidung sub-basin. The Tarakan sub-basin is located a few kilometers to the north of the famous Mahakam delta. To the north, the Tarakan sub-basin is bounded by the Sampoerna high and to the south it is bounded by the Mangkalihat high. The Neogene fluvio-deltaic sediment in the Tarakan sub-basin is thinning to the north to the Sampoerna high and to the south to the Mangkalihat high. The thickest sediment is in Bunyu, Tarakan and in the Ahus structure. The sub-basin is gently deeping to the east started from Kucing high in the west to the open sea of the Celebes sea in the east. The Kucing high is the hinterland and source provenance of the fluvio-deltaic sediment deposited eastwardly in the Tarakan sub-basin since the Early Miocene. To the east of the Tarakan sub-basin is the open sea of the Celebes sea. The Nunukan PSC, operated by Pertamina Hulu Nunukan Company (PHENC), is located in the eastern part of the sub-basin (see Figure 1). The available data in the Nunukan PSC including 2D seismic of 1970 to 1991 vintages and 3D seismic data of 2010 and well location is given in Figure 2. It has been a long time since there were any publications to update the understanding of the Tarakan sub-basin. Little interest has been shown, probably due to the low exploration success of oil and gas in the area, although there are already oil and gas fields in the area which have been producing since the 1970’s, namely the Bunyu Nibung and Tapa fields in Bunyu island, Pamusian in Tarakan island and Sembakung and Bangkudulis in onshore Northeast Kalimantan. On the other side, although the depositional setting in the Tarakan sub-basin is deltaic which is located to the north of the Mahakam delta, people tend to use the Mahakam delta as a reference to discuss deltaic depositional systems. This means that the Mahakam delta is more understood than the delta systems in the Tarakan sub-basin. The Mahakam delta is single sourced by the Mahakam river which has been depositing a stacked deltaic sedimentary package in one focus area to the Makasar Strait probably since the Middle Miocene. The deltaic depositional setting is confined by the Makasar Strait which is in such a way protecting the sedimentary package not to disperse to an open sea. No major tectonic events disturbed the deltaic deposition in the Mahakam delta other than sea level changes and tidal influences. As a result, the Mahakam delta has developed very thick sedimentary packages to be dumped to the Makasar strait. On the other hand, deltaic sediments in the Tarakan sub-basin are sourced by several feeder rivers such as Sesayap, Sesanip and other smaller rivers. The mouth of the rivers faces the open sea of the Celebes sea which means that the deltaic sediment is subject to a high dilution by waves and tidal dispersing of the sediment to open sea. Consequently, the delta of the Tarakan sub-basin is not well developed with thickness varies across the sub-basin. Additionally, uplifting by some tectonic events and erosion have also contributed to make the deltaic deposition to be not focused in one area. The setting of the present day Bunyu and Tarakan islands is probably a good analog to the ancient setting. Bunyu and Tarakan islands indicate a unique feature which some geologists have used to interprete them, together with the Ahus structure as a three arches trending NW-SE. On a topography map, the Bunyu and Tarakan islands are positioned right in the mouth of the Sesayap river which is split into two rivers, as if Bunyu and Tarakan islands are sand bars deposited by the two river mouths. In fact, they act as bariers to split the current of the rivers into two main flows. Before reaching the Bunyu and Tarakan islands, the Sesayap river already splits into two rivers, which may be due to the same condition as in the Bunyu and Tarakan islands, i.e there is a structural high blocking the Sesayap river in the more upstream location (Figure 3). This paper contributes insights and ideas about tectonic and stratigraphic evolution of the Tarakan sub-basin from the approach of the offshore data and although the discussion is focused on tectonic and stratigraphic evolution of the Mio-Pliocene to Recent delta of the Tarakan sub-basin, as a basin overview a small portion of the Paleogene setting will also be given. This paper will promote some evidence and insights based on some new well and 3D seismic data in addition to the existing well and 2D seismic data to have a better understanding of the tectonic and stratigraphy evolution of the Tarakan sub-basin.