Depositional Model of Muda Formation, East Natuna Basin

The current exploration in the Natuna Basin is focusing on the Muda Formation, as there have been shallow gas discoveries such as Dara and Tuna in the East Natuna Basin as well as Kaci and Mako in the West Natuna Basin. Unfortunately, only limited studies or publications are available because the Muda Formation has not been the main exploration target in the past. This study attempts to build a series of regional Gross Depositional Environment (GDE) of the Muda Formation in the southeastern part of the East Natuna Basin that has never been published before. The present-day bathymetric and ancient river network maps of the Natuna region were integrated into the interpretation to complement limited seismic and well data. The present-day bathymetric map was adjusted to the Haq’s eustatic sea level curve and biostratigraphic data to depict ancient water depth, shoreline, and basin configuration maps. Meanwhile, the ancient river network map from Molengraaff (1921) was also used to establish the locus of the sediment influx into the basin. The depositional history of the Muda Formation began in the Late Miocene until the present-day during a period of tectonic quiescence. The continental slope gradient in the Natuna region was very gentle and hence the sedimentary process in the basin was strongly controlled by sea level changes. The Muda Formation has been subsequently divided into three sequences based on the eustatic sea level curve by Haq et al (1987), namely: the Lower Muda, Middle Muda, and Upper Muda sequences. Meanwhile, the paleobathymetric map and paleoriver network modeling suggests that there were two major estuaries, namely Maludam and Molengraaff that were influenced by tidal-dominated processes during the deposition of the Muda interval in the nearshore area. The Lower Muda deposition began in the Late Miocene, following the Middle Miocene tectonic uplift in the Natuna region. Its paleobathymetry is interpreted to be approximately 50 meters deeper than the present-day bathymetry. The sediment provenance of Lower Muda derived from onshore NW Borneo through the Maludam Estuary in the southeastern part of the basin. The Middle Muda Sequence marked a maximum transgressive event during the Early Pliocene period. The interpreted Middle Muda water depth was approximately 80 meters deeper than the present-day water depth due to global sea level rise. The Upper Muda Sequence was deposited during Plio-Pleistocene. A rapid and significant change of water depth, shoreline and sediment influx were controlled by the sea level change due to glaciation during the Ice Age. Its main sediment source was coming from Sumatera through the Molengraaff River toward estuary in the southwestern part of the basin during events when sea level dropped and alternately shifted through Maludam River and estuary during interglacial. This study proves that the eustatic curve, ancient river network and present-day water depth maps are valuable data to interpret the depositional model of the Muda Formation. The same data integration can be applied to other cases, which have equivalent age ranges in a similar geological setting. Overall, the Muda Formation is interpreted as clastic sediment that was deposited in fluvial to marine shelf environments.