Publications

West Natuna Basin (WNB) is located in the center of the Sunda Shelf in the South China Sea, bordered by the Sunda Shelf's basement to the south, the Natuna Arch to the east, and the Khorat Swell to the north. The WNB has a complex structural history of extension, compression, and wrenching related to Cenozoic regional tectonic events. In the Early Oligocene, the major graben-bounding faults showed a significant extension. They created an extensive lacustrine system followed by the regional deposition of shales, the Benua Formation, as the primary source rocks for the WNB oil and gas. Rifting continued up to the Middle-Late Oligocene, and the basin filled with the coarse clastic material through lacustrine braided-delta systems, the Lower Gabus Formation, as one of the leading reservoir producers in the Natuna Block-A. It contains fluvio-deltaic distributary channel sands and overbank crevasse splay sands, silts, and muds. This formation is subdivided into six units with a characteristic of sandstones that are texturally very fine to fine-grained, moderately sorted, and with good silica and argillaceous cement. The end of subsidence at Late Oligocene – Early Miocene is followed by the period of tectonic quiescence, and the beginning of the post-rift phase filled the region with marine sedimentation. The post-rift was filled with the deposition of the lacustrine sediments, the Keras Shale. The deposit is followed by the Upper Gabus fluvio-deltaic distributary channel sand, which also becomes the reservoir producer for gas and oil. In the Middle Miocene, the change of regional extension to compression inverted the older graben, followed by the Arang Formation deposition. The Arang Formation depositional environments fluctuate from shallow marine to coal-swamp dominated coastal plain, tied to inversion events, and generally consist of alternating sandstone, siltstone, and shale with occasional coals. The characteristic of the Arang sandstones is very fine-grained, occasionally medium-grained, and poorly sorted with siliceous cement. This multiple sand reservoir of the Arang Formation became the key reservoir producer in the Natuna Block-A. Both core description and core data analysis from explorations wells were used to identify reservoir characteristic that gives information on facies classification. This information is used to generate Hydraulic Flow Unit (HFU) as part of development strategy optimization. Once generated, HFU is forwarded to create a porosity-permeability correlation that is used as input for 3D models that directly correlate to hydrocarbon recovery. After more than seven years of production, the production data are sufficient to conclude whether the HFU and facies classification based on core analysis accurately predicted production performance for both Gabus and Arang Formations. Some moderate modifications on some layer's properties in history matching steps show that improvement of facies analysis is essential to have representative 3D models.