IPA17-722-G PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Forty-First Annual Convention & Exhibition, May 2017 “SOME NEW INSIGHTS TO TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE TARAKAN SUB-BASIN, NORTH EAST KALIMANTAN, INDONESIA” Sudarmono* Angga Direza* Hade Bakda Maulin* Andika Wicaksono* INTRODUCTION in Tarakan island and Sembakung and Bangkudulis in onshore Northeast Kalimantan. This paper will discuss the tectonic and stratigraphic evolution of the Tarakan sub-basin, primarily the On the other side, although the depositional setting fluvio-deltaic deposition during the Neogene time. in the Tarakan sub-basin is deltaic which is located The Tarakan sub-basin is part of a sub-basin complex to the north of the Mahakam delta, people tend to use which includes the Tidung, Berau, and Muaras sub- the Mahakam delta as a reference to discuss deltaic basins located in Northeast Kalimantan. In this depositional systems. This means that the Mahakam paper, the discussion about the Tarakan sub-basin delta is more understood than the delta systems in the also includes the Tidung sub-basin. The Tarakan sub- Tarakan sub-basin. The Mahakam delta is single basin is located a few kilometers to the north of the sourced by the Mahakam river which has been famous Mahakam delta. To the north, the Tarakan depositing a stacked deltaic sedimentary package in sub-basin is bounded by the Sampoerna high and to one focus area to the Makasar Strait probably since the south it is bounded by the Mangkalihat high. The the Middle Miocene. The deltaic depositional setting Neogene fluvio-deltaic sediment in the Tarakan sub- is confined by the Makasar Strait which is in such a basin is thinning to the north to the Sampoerna high way protecting the sedimentary package not to and to the south to the Mangkalihat high. The disperse to an open sea. No major tectonic events thickest sediment is in Bunyu, Tarakan and in the disturbed the deltaic deposition in the Mahakam Ahus structure. The sub-basin is gently deeping to delta other than sea level changes and tidal the east started from Kucing high in the west to the influences. As a result, the Mahakam delta has open sea of the Celebes sea in the east. The Kucing developed very thick sedimentary packages to be high is the hinterland and source provenance of the dumped to the Makasar strait. On the other hand, fluvio-deltaic sediment deposited eastwardly in the deltaic sediments in the Tarakan sub-basin are Tarakan sub-basin since the Early Miocene. To the sourced by several feeder rivers such as Sesayap, east of the Tarakan sub-basin is the open sea of the Sesanip and other smaller rivers. The mouth of the Celebes sea. The Nunukan PSC, operated by rivers faces the open sea of the Celebes sea which Pertamina Hulu Nunukan Company (PHENC), is means that the deltaic sediment is subject to a high located in the eastern part of the sub-basin (see dilution by waves and tidal dispersing of the Figure 1). The available data in the Nunukan PSC sediment to open sea. Consequently, the delta of the including 2D seismic of 1970 to 1991 vintages and Tarakan sub-basin is not well developed with 3D seismic data of 2010 and well location is given thickness varies across the sub-basin. Additionally, in Figure 2. 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 It has been a long time since there were any day Bunyu and Tarakan islands is probably a good publications to update the understanding of the analog to the ancient setting. Bunyu and Tarakan Tarakan sub-basin. Little interest has been shown, islands indicate a unique feature which some probably due to the low exploration success of oil geologists have used to interprete them, together with and gas in the area, although there are already oil the Ahus structure as a three arches trending NW- and gas fields in the area which have been SE. On a topography map, the Bunyu and Tarakan producing since the 1970’s, namely the Bunyu islands are positioned right in the mouth of the Nibung and Tapa fields in Bunyu island, Pamusian Sesayap river which is split into two rivers, as if * Pertamina Hulu Energi Nunukan Bunyu and Tarakan islands are sand bars deposited glidding, a couple of eastwardly normal-listric fault by the two river mouths. In fact, they act as bariers to movements in the west associated with deltaic split the current of the rivers into two main flows. deposition and toe-thrusting in the far east of a deep Before reaching the Bunyu and Tarakan islands, the water setting, both moving along a slippery Sesayap river already splits into two rivers, which decolement zone. A quite famous model illustrating may be due to the same condition as in the Bunyu W-E geological cross-section to desribe the Tarakan and Tarakan islands, i.e there is a structural high sub-basin in a regional context is the paper by Sri blocking the Sesayap river in the more upstream Hidayati (2007). In that model, the Bunyu and location (Figure 3). Tarakan islands are part of a thin skinned-induced gravity glidding up to the present day in the western This paper contributes insights and ideas about part in a pair with toe-thrusting over a major tectonic and stratigraphic evolution of the Tarakan decolment surface in the far eastern part (deep water sub-basin from the approach of the offshore data and setting). Interestingly, the gravity glidding (series of although the discussion is focused on tectonic and listric normal faults) starts from a major bounding stratigraphic evolution of the Mio-Pliocene to Recent fault to form the roll over structure of the Tarakan delta of the Tarakan sub-basin, as a basin overview a and Bunyu islands and the Paleogene presents small portion of the Paleogene setting will also be beneath the decolment surface as rift-product given. This paper will promote some evidence and described by a series of half grabens. This model has insights based on some new well and 3D seismic data not been updated since it was published. Some in addition to the existing well and 2D seismic data interesting questions about the Tarakan sub-basin to have a better understanding of the tectonic and relate to the model include defining the contact stratigraphy evolution of the Tarakan sub-basin. boundary location between the oceanic crust underneath the Celebes sea over Kalimantan and in Regional tectonic what way they are interacted. Other questions include defining what tectonic and stratigraphy is The Tarakan sub-basin is controlled by a complex underneath the Neogene delta of the Tarakan sub- tectonic interaction such as oceanic spreading in the basin and what the decolement surface as a base of Celebes Sea and subduction and collision in the the gravity glidding is. This will be discussed in the Kucing High by the South China Sea. Key tectonics Paleogene setting topic. affecting the Tarakan sub-basin is the uplift of the Of particular interest is the discussion related to the Rajang-Embaluh group during Late Oligocene to structure forming the Bunyu and Tarakan islands. form the Kucing High which then becomes a Looking at a West-East offshore seismic section sediment provenance of eastwardly deltaic across the structure of the Bunyu island, it is easy to deposition in the Tarakan sub-basin and westwardly give a general impression that the Bunyu or Tarakan to the Baram delta. The Kucing High complex was a islands is a simple roll-over anticline structure meeting point interaction of eastwardly stress by resulted by prograding delta depositional setting. subduction and collision of the South China sea to However, looking at the extensive erosion at the top Luconia and Dangerous Ground and the westwardly of the Bunyu structure, it should have been a docking of the Celebes Sea oceanic crust towards the significant sea level drop to have such an erosional northeastern margin of Sundaland. The interaction surface. Also, looking at the sesimic data, the listric has been generating some stike slip movements normal faults are limited to certain level but the afterwards which are still active until the present day, upper part of the section is a parallel bedded primarily the Sampoerna and Maratua-Mangkalihat sedimentary package which was undisturbed during strike slip faults, which are positioned in the north deposition and so the listric normal faults do not and south respectively bounding the Tarakan sub- continue to cut the parallel bedded sedimentary basin. The two strike slip faults probably have been package. This suggests that the prograding delta the main control of the tectonic events during generating the listric normal faults stopped sometime Miocene to Pleistocene in the Tarakan sub-basin. before the parallel bedded sedimentary package was The Sampoerna strike slip fault probably related to deposited. This also implies that the faults cutting the the oceanic spreading centre in the Celebes Sea while parallel bedded package in the upper part are a Maratua and Mangkalihat strike slip faults probably reactivation of the existing older faults which are the are a continuation of the Palukoro fault. normal listric faults associated to the deltaic deposition. In the Tarakan sub-basin, the deltaic package during Mio-Pleistocene time used to be interpreted by some Furthermore, an unconformity obviously appears on authors simply as a thin skinned induced gravity seismic data separating the deltaic package indicating a series of listric normal faults and the in different working areas. As an example, in the undisturbed parallel bedded sedimentary package onshore area, the characteristic of the Tabul Fm is above it (Figure 4a and 4b).