Sedimentology and reservoir characteristics of syn-rift to syn-inversion succession in Anoa half-graben, West Natuna basin, Indonesia

Abstract

West Natuna Basin (WNB) has been a target area for hydrocarbon exploration for more than half a century. However, the geological history in this area is not comprehensively understood because the complexity of tectonic evolution and depositional history of each sub-basin separated by a basement high are unique and distinctive from one another. This condition leads to difficulty in determining the reservoir age and stratigraphic correlation of sub-basins in the syn-rift intervals due to significant inversion, erosion, and lack of fossils for dating. Furthermore, due to the fast-rifting process, the reservoir distribution, quality, and variation of syn-rift sediments in the WNB also significantly changed laterally and vertically. These conditions were also reflected in Anoa Hal-Graben (AHG) in the northern part of WNB, where three big oil fields were discovered, i.e., Belumut, Anoa, and Ande-Ande Lumut (AAL) Field. Therefore, this study performed a comprehensive data integration from biostratigraphy, cutting, well logs, and seismic to develop an updated stratigraphy in AHG. In addition, this study also conducted reservoir characterization and gross depositional environment modeling. Sediment deposition in the AHG respectively starts from the Middle Eocene syn-rift, Late Oligocene post-rift, Early to Middle Miocene syn-inversion, and Late Miocene post-inversion. The depositional environment varies from fluvial plain, fluvial channel, lacustrine delta, and open marine. The reservoir quality, including reservoir Net to Gross & porosity, depends on the depositional environment and the effects of compaction controlled by the inversion of the basin.