Stratigraphic and structural trapping frameworks in the central Ceduna Sub-basin

Abstract

Despite a recent renewal in exploration, the Ceduna Sub-basin is underexplored and, as is typical with frontier basins, there is considerable uncertainty regarding lithofacies distribution. As such, there is limited understanding of reservoir and seal coupling as well as structural trapping and thus fluid migration pathways within the prospective Late Cretaceous marine and deltaic intervals from the Tiger and Hammerhead supersequences.To address these uncertainties, a stratigraphic forward model using multiple sediment source points and constrained by recent marine survey data was developed to simulate the deposition and preservation of potential stratigraphic successions across the sub-basin. Pseudo-wells have been extracted from the stratigraphic forward modelling output to forecast vertical and lateral stratigraphic trends. These trends were analysed to predict fault seal and structural trapping likelihood using the Shale Gouge Ratio methodology and triangle juxtaposition diagrams.Over the study area, the modelled facies distributions and geometries are consistent with sequence stratigraphy, seismic stratigraphy and well data. The stratigraphic model shows an overall NW to SE trend of decreasing sand volume and sand interval thickness for the Tiger and Hammerhead supersequences. Although this highlights the general trend of a higher likelihood of reservoir presence to the NW of the study area, and more seal presence to the SE, the model predicts enough vertical variability within the supersequences to create ranges of stacked reservoir-seal couplets.The stratigraphic model and fault seal assessment suggest that, in the marine-influenced Tiger and lower Hammerhead supersequences, the best reservoir and top/fault seal couplets are located along a WSW-NE trend to the NW of the study area where faults with throw≥150 m are expected to yield membrane fault seal. The NW corner of the study area is predicted to have high net-to-gross ratio for the upper Tiger, too high to yield any efficient membrane fault seal. In the deltaic upper Hammerhead supersequence, the best reservoir and top/fault seal couplets are expected along an E-W trend in the central part of the study area. Along this trend faults with throw≥100 m are expected to yield membrane fault seal able to impede lateral hydrocarbon flow in modelled mid Campanian reservoirs and a nearshore interval.These inferred Late Cretaceous structural traps could have been charged by hydrocarbons expelled from local Late Cretaceous source rocks between the Cenomanian to the present-day.