Reconstruction of rapid charge of a giant gas field across a major fault zone using 3D petroleum systems modelling (Taranaki Basin, New Zealand)

Abstract

The timing of petroleum charge, especially in structurally complex basins, is often poorly defined, with few data to constrain petroleum systems models. In this study we reconstruct in detail the evolution of the >6 Tcf Maui Field, a mature gas-condensate field in the southern Taranaki Basin, northwestern New Zealand. Structural restoration suggests that closure developed between 6 and 5.3 Ma. Maturation modelling shows that charge of the main gas reservoir is only feasible from the nearby Maui sub-basin kitchen, across a major fault zone, the Cape Egmont Fault Zone (CEFZ). We apply petroleum systems modelling to predict timing and amount of charge using a reconstructed distribution of shaly fault rock as an approximation of fault permeability. Models reproducing the variable oil and gas accumulations in different areas of the field suggest that the field was charged within 4 Myr. This supports the notion that connected permeability exists even across large fault zones, allowing charge in geologically short times frames. Models using a range of fault and carrier bed properties suggest that presence of connected sandy carrier beds linking the fault zone with reservoirs is essential for petroleum migration. Predicted migration pathways occur through outer parts of the CEFZ characterized by greater architectural complexity as well as through the central area of the CEFZ with the largest offset. This suggests that the impact of shale-rich fault rock on petroleum migration across large fault zones is smaller than commonly expected. In particular, in deeper parts of the basin, a major fault zone can provide sufficient permeability to be the preferred pathway for vertical migration. These results illustrate that despite the complexity of migration pathways, petroleum migration can occur in geologically short time.