Fluvial Channel Complexes in the Middle Miocene of Lower Kutei Basin, East Kalimantan – The Stacking Pattern of Sediments

A succession of middle Miocene rocks is exposed in a section near Samarinda, East Kalimantan. The succession is characterized by thick sandstone bodies alternating with shales and coal beds. The sandstones are interpreted as fluvial channels. The shales are interpreted as a flood plain setting. The coal layers are interpreted as a continental swamp environment. These three formations are vertically stacked to form fluvial parasequences. Each parasequence has a 40 to 50 m thickness. They are vertically stacked to form parasequence sets. The whole fluvial section is more than 600 m thick. At the bottom of the section a shallow marine parasequence is observed containing prograding shelf sand lobes capped by transgressive reef carbonates. These reef carbonates are interpreted as representing the maximum flooding event at 10.5 My. The transition from shelf parasequence to fluvial deposits is very abrupt and is interpreted as the sequence boundary at 10.2 My. The coarse-grained sediments deposited in fluvial environment are interpreted as being provided by the active tectonic uplift phases in the hinterland and consecutive erosion phases. INTRODUCTION AND GEOLOGICAL SETTING The Kutei Basin is the largest Tertiary basin in Western Indonesia. It is limited to the south by Paternoster Platform and the Adang flexure zone, to the north by the Mangkalihat Ridge, and to the west by the Kuching High, the source for most of the Neogene sediments. The Makasar Strait to the east shows its extension into the deep water basin. * TOTAL E&P INDONESIE Deltaic sedimentation has been continuous in the Kutei basin from late Oligocene to the present-day, as represented by the modern Mahakan delta. A fluvial plain is associated with the delta upstream in the continent and carbonate buildups are observed in the shelf offshore beyond the delta. These three environments shift continuously seaward and landward during the geological time constituting a complex geological history of the basin. We have investigated an outcrop section near Samarinda city, in the Stadion Utama, Palaran (Figure 1). The outcrops constituting the section are currently, easily accessible and are situated close to the main road to Samarinda, near and around the new stadium built in 2007. Part of this section has been previously investigated (Cibaj et al., 2007). The section is Middle Miocene in age. Our biostratigraphic studies show more precisely a Seravallian age (NN5 to NN11 nanoplancton zones, or N9 to N13 planctonic zones). There is a structural deepening of formations in the basin to the east (to the Mahakam area, where hydrocarbon fields are situated). The studied section is considered to be stratigraphically equivalent of the main producing interval in Handil and Tambora field (internal axis of Mahakam area, proximal fields, around 2000 m deep) and the deep producing interval of Tunu and Peciko fields (median axis of Mahakam area, intermediate fields, around 4000 m deep). Two main intervals can be distinguished in this section. The lower interval, from 0 to 270 m (in Cibaj et al., 2007, their Figure 7), is constituted by deltaic and shallow marine (tidal influence and shelf) parasequences. The upper interval, from 290 to 450 m, shows fluvial parasequences. A carbonate buildup, from 270 to 290 m, is observed in between the two main intervals of the section (in Cibaj et al., 2007, their Figure 7). © IPA, 2011 34th Annual Convention Proceedings, 2010