OUTCROPS IN NATUNA ISLAND: NEW INSIGHTS OF RESERVOIR POTENTIAL AND SEDIMENT PROVENANCE OF THE EAST NATUNA BASIN

摘要

Although few hydrocarbon discoveries from Tertiary intervals have been made in East Natuna Basin, the exploration stage of this basin is still considered immature due to its remoteness. As a consequence there is limited available well data and geological information to perform a comprehensive geological and geophysical evaluation. This poster highlights the Tertiary reservoir potential and probable sediment provenance in East Natuna based on outcrop observation. Some of interesting features and potential hydrocarbon plays are seen on seismic data in offshore East Natuna. Unfortunately due to limited well control, those seismic features cannot be calibrated. In order to develop a reliable geological model of the subsurface a field visit to Natuna Island was conducted mid-2017 to integrate surface and subsurface data, assess some of key petroleum system elements such as reservoir presence and their corresponding depositional environments, and assess the potential provenance for the expected reservoir intervals. The oldest outcrops consist of Jurassic to Early Cretaceous ophiolite and melange sediments of Bunguran Formation that were intruded by Late Cretaceous Granodiorite in Ranai and surrounding areas. This assemblage and their stratigraphic reconstruction can be explained easily by the "Oceanic Plate Stratigraphy" concept. The Bunguran sediment outcrops tie to the interval that is considered as economic basement on offshore seismic data which was previously interpreted as an Oligocene interval. Those Pre-Tertiary intervals were overlain by basal conglomerates, stacked sandstones and interbedded siltstone and claystone. The exposed stacked sandstones generally have good net to gross ratio. Under thin-section, these sandstones are mostly classified as sub-litharenite dominated by quartz, chert and metamorphic fragments possibly derived from the Bunguran Formation and Ranai Granodiorite as sedimentary-source provenance. The depositional environments of these Tertiary sediments range from fluvial to shallow marine environments that have good potential for reservoir.