Normal faults and gas migration in an active plate boundary, southern Taranaki Basin, offshore New Zealand

Abstract

The function of normal faults in upsequence flow of gas has been examined using two-dimensional and three-dimensional seismic-reflection data from the southern Taranaki Basin, New Zealand. The spatial distributions of late-stage normal faults, gas chimneys, thickness of the Oligocene mudstone-rich seal (Otaraoa Formation), and modeled hydrocarbon expulsion volumes are compared. Gas chimneys are most common above Cretaceous source rocks modeled to have expelled hydrocarbons. Most (70%) of the observed gas chimneys follow, and/or are rooted in, late-stage normal faults. These faults are the primary seal bypass mechanism for hydrocarbons, where they displace the seal (or intersect faults that displace the seal) and the seal is thick (e.g., more than 340 m [1115 ft]). Active vertical gas flow through the seal commenced after the onset of faulting (3.6 Ma), whereas subseal lateral flow started significantly earlier at approximately 15 Ma and resulted in an early charge of structural highs. Gas flow up along faults in low-permeability mudstones (1 md) is channelized with steep chimneys commonly occurring close to fault tips and relay ramps. In these cases, gas flow may be focused by the presence of high densities of open fractures locally elevating upsequence bulk permeabilities to approximately 1 to 400 md.