Identifying source rocks and reservoir elements of South China Sea grabens using 3D seismic data, West Natuna Basin, Indonesia

Burton, Darrin
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Reservoir quality and source potential are significant risk factors when exploring for hydrocarbons in Southeast Asian rift basins (Sladen, 1997b). In the West Natuna Basin, Indonesia, wells infrequently penetrate most of the synrift section, and as a result source and reservoir potential of the lower synrift is under-evaluated. This study was designed to evaluate the source and reservoir potential of half grabens in the Gabus and Belanak surveys in the West Natuna Basin by reconstructing the stratigraphy. The stratigraphic architecture of the folded, asymmetric fill can be imaged by stratal slicing. Utilizing 3D seismic geomorphology from 150 stratal slices in conjunction with available logs, rift analogues, and rift models, depositional systems were interpreted at 20-40 meter vertical resolution. The observed stratigraphic architecture, when compared with models, shows a classic, tectonically-influenced, half graben sedimentary fill in the West Natuna rift (Schlische & Olsen, 1990; Schlische, 1993; Gawthorpe & Leeder, 2000; Lambiase et al., 1999). Half grabens evolve in four stages; fault initiation, fault linkage, through-going fault, and fault death stages. Half graben fill consists of basal alluvial fans and fluvial deposits during fault initiation, lacustrine flooding and deposition of lacustrine shales during fault linkage, and fluvial-lacustrine deltaic deposits in the through-going fault stage. During fault death shallow lacustrine shales, as well as fluvial and shorelines sands were deposited. While there is generally predictable stratigraphy associated with each stage, but sediment thickness and character varies from one graben to the next. Thus, it is important to assess the source rock and reservoir potential of each sub-basin. The detailed stratigraphy imaged in stratal slices allows potential source rocks and undrilled reservoir targets to be identified. Two potential source rocks facies were recognized in the survey areas in the synrift stratigraphy, a deep lacustrine facies and a coal/coaly shale facies. Lower synrift flexural margin deltas form potential reservoirs that have yet to be tested. This study shows that in exploration situations where poor well control exists, stratal slicing of large 3d seismic volumes can be used to make detailed stratigraphic interpretations