Syn-rift drainages and sedimentary fill architecture: a case study in the Jurassic of the Dampier Sub-basin

Abstract

Ancient rift basins host important hydrocarbon-bearing systems worldwide. A review of these systems indicates that the syn-rift reservoirs are commonly clastic, fluvio-deltaic facies with marine-related seals. Although the passive margin systems overlaying many of these basins have been well-studied and exploited, exploration into the deeper rift strata is riskier and relys on seismic surveys with little well or core data. Modern analogues, like the East African Rift System, provide insights into the early stages of rift development, but seismic analysis of ancient rifts is needed to analyze an entire rift cycle. Seismic surveys of the Jurassic sediments of the Dampier Sub-basin image the complete rift stratigraphy and contain well data through most of the syn-rift sediments. The purpose of this study is to better characterize the location and interaction of the reservoir and seal facies within the Dampier Sub-basin using seismic geomorphological techniques. This study focused on the regional play intervals (RPI) within the syn-rift of the Dampier Sub-basin: the J20 RPI, J30 RPI, J40 RPI, and the J50 RPI. The J20 RPI contained an axially prograding delta system, the J30 RPI contained a transverse prograding, coarse-grained delta system, the J40 contained deepwater fans and debrite deposits, and the J50 RPI contained a mix of delta and submarine fan systems. The interpreted syn-rift stratigraphy of the Dampier Sub-basin shows a switch from axial deposition during rift initiation to transverse deposition during rift climax. This change in dominant sediment drainage direction during rift evolution is important for exploration into syn-rift plays, especially since targeting high quality reservoir sands can be a challenge in these systems. Though transverse systems may contain immature sediments, this study shows that they can be significantly sized features and, when sealed in source rock quality shales, potentially productive plays.