Uncovering Evidence for Late Cretaceous Extension on the Sunda Shelf in the Vera Zaitun Basin Region, Offshore Northwest Java

Abstract

The Vera Zaitun Basin, located in offshore northwest Java and spanning 600km2, represents one of the most under-explored areas of the southeastern Sunda Shelf. The geological history of this region is relatively unknown. The objectives of this research are three-fold, including; uncovering the structural and stratigraphic evolution of the Vera-Zaitun Basin; presenting evidence for a previously overlooked sub-basin to the north of the Vera-Zaitun Basin, and importantly, to postulate Late Cretaceous extension along the southeastern margin of the Sunda Shelf. The research also aims to examine these findings and their implication for petroleum prospectivity in the area. The accepted understanding of the region’s development is that isolated sub-basins present on the Sunda Shelf formed no earlier than the Eocene. However, observations from new, proprietary 2D 2017 seismic data, combined with those from petroleum wells provide evidence for deep, rift-related sediments that have not been previously recognized in the basin or region. Based on palynological and stratigraphic information from petroleum wells, we propose that these sediments may be Late Cretaceous in age. The availability of the new seismic data has provided a unique opportunity to image deeper sediments on the Sunda Shelf that may have otherwise been interpreted as ‘basement’ in vintage and older datasets. Interpretation of this newer seismic data and available potential fields data has revealed that these sediments are contained within a NE-SW oriented extensional system, located to the north of the Zaitun depression. We propose that this rift system could contain a previously overlooked petroleum system, currently untested by exploration drilling. Our structural interpretation of this deep basin will be supported by 2D structural modelling and section balancing. The findings presented here have implications for the Vera-Zaitun study region, as modern seismic indicates the presence of a series of sub-basins that might have formed earlier than the surrounding basins, such as the nearby Biliton Basin. Therefore, while other regional basins on the Sunda Shelf such as the Jatibarang, Ardjuna and Biliton Basins are accepted in existing literature to be Tertiary in age, these findings indicate that newer seismic in these regions may reveal overlooked, deeper sediments on the Sunda Shelf, preserved in Late Cretaceous depocenters. Hence, understanding what sediments may be present in the Vera Zaitun basin, analyzing the structural evolution of this area and determining how these sub-basins have evolved will provide an improved insight into how this margin of the Sunda Shelf developed. As such, these depocenters could form previously un-mapped hydrocarbon kitchens, and key structural features present in this region could form the basis for structural hydrocarbon traps. Hence, these findings may act to enhance the petroleum prospectivity of the Vera Zaitun Basin region and provide new insights into the complex geologic history of the southeastern Sunda Shelf.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019