Clastic Plays in The North East Natuna Basin

The East Natuna Basin comprises several sub-basinal areas which formed during the Tertiary as a result of extension relating to the opening of the South China Sea during the Oligocene and Early Miocene. Several small Oligocene half-grabens occur in the northern part of the area close to where the Tuna Block is located. They have a NE-SW trend and the sediments are relatively uniform in thickness, less than 2,600 ft thick, with regional thickness growth to the east towards the basin center. However in the Lower Miocene a more pronounced extensional episode enlarged the existing half grabens and generated large growth fault related sediment packages consisting predominantly of clastic sequences. Marine conditions in the Mid Miocene gave rise to carbonate development which continued to the latest Miocene with some very large reef related carbonate build-ups observed on seismic. In Late Miocene this area underwent further deformation caused by right lateral transtension which produced numerous NE-SW normal faults. This was then followed by a period of quiescence and deposition of the overlying Pliocene Muda Formation marine shales and clastics. The tectonically controlled, locally thick sequences of the Oligocene and Lower Miocene sediments indicate good potential for source and reservoir/seal pair rocks. Exploration in the East Natuna Basin began in the 1970’s, mainly targeting the Terumbu Formation carbonate buildups, which were easily mapped on the available 2D seismic data. Some discoveries were made and the most significant of which was the supergiant D Alpha gas field discovery by AGIP in 1973 containing over 220 TCF including +/- 71% CO2. However, the underlying clastics were not well imaged by the 2D seismic data and many of the wells drilled were not optimally located for testing the underlying Arang and Gabus clastic sections. The discovery of hydrocarbons in the Gabus and Lower Terumbu clastics in the Kuda Laut-1 and Singa Laut-1 wells by Premier Oil in 2014 confirmed the exploration potential of these intervals. The discoveries now known as the Tuna Field were appraised by two additional wells in Q3-Q4 2021 confirming the presence of rich wet gas with a high condensate yield, but also encountering oil rims, to the previously discovered gas columns. The main reservoirs are Lower Terumbu shallow marine bioturbated shaly sandstones in the Kuda Laut hanging wall structure and Gabus fluvial channel sandstones in the Singa Laut footwall structure. These two reservoirs exhibit very different properties as indicated by conventional cores, extensive wireline data and the DST’s. Post well analysis is ongoing to further characterize these reservoirs and the fluids contained, the results of which will be critical for the commercial development of the Tuna field.