Hydrocarbon Potential of Early Syn-Rift Plays in the Flank of Continental Rift Basin: A Case Study in the Eastern Edge of Cuu Long Basin, Offshore Vietnam

Abstract Structural syn-rift reservoirs in faulted margins of continental rift basins are usually disregarded in exploration consideration due to concerns of absence of reliable top seal and diagenetic alterations that significantly reduce reservoir rocks’ porosity/permeability. Not an exception to this trend, Early Oligocene reservoirs in the eastern margin of Cuu Long Basin, offshore Vietnam were overlooked in the past since they were interpreted as small and scattered structures without top seals. This paper presents the re-evaluation of hydrocarbon potential of the early syn-rift plays in the flank of continental rift basins based on a case study in the eastern edge of Cuu Long Basin in light of the new seismic data as well as drilling results. Recent exploration results in the eastern edge of Cuu Long Basin differ from past assessment regarding early syn-rift reservoirs of the area. Re-processed and new seismic data supported by latest well data put top of granitic basement at a deeper depth than previously interpreted. As a result, syn-rift reservoirs which were small and insignificant have become large scale and two- or three-way faulted structures. Key reservoirs discovered is the basal sandstones which deposited right on top and shared a similar topography with the underlying Pre-Cenozoic granitic basement, mainly composed of near source sediment, mostly granitic fragments. These basal sandstones’ porosity and permeability have been preserved better than shallower reservoirs thanks to the unaltered Feldspars in granitic fragments. Analyses of new wells data also indicate the existence of thick and extensively distributed organic-rich shale formations on top of the basal sandstones. These black shale layers not only act as top seals to hydrocarbon migration for underneath reservoirs but also play the role of a "stress shield" that absorbs and dissipates vertical compaction and helps preserve porosity and/or permeability. The results of this study coupled with important drilling discoveries have confirmed the hydrocarbon potential in the eastern edge of Cuu Long Basin. This leads to a revived petroleum exploration and development activities in this area where early syn-rift prospects have been overlooked. Furthermore, these findings, applicable to the eastern edge of Cuu Long Basin, can be used as principal guidance for hydrocarbon evaluation and exploration strategy in similar syn-rift basins.