The Influence of Faulting on Hydrocarbon Migration in the Kupe Area, South Taranaki Basin, New Zealand

Abstract

Faults play an important role in petroleum systems as both barriers and conduits to the flow of hydrocarbons. An understanding of the relationship between fluid and gas migration and accumulation, and faulting is often required during hydrocarbon exploration and production, and CO2 storage. While methods for predicting across-fault flow are well advanced (e.g. Yielding et al., 1997; Manzocchi et al., 1999), current geomechanical and geometrical methods for predicting the locations of up-fault (up-dip) hydrocarbon migration (and leakage) are relatively untested. This thesis investigates the relationships between up-sequence gas migration in the form of gas chimneys and Pliocene to Recent normal faults in the Kupe Area, South Taranaki Basin. It undertakes studies of the Kupe Area’s structural development, examines spatial relationships between faults and gas chimneys, tests current geomechanical and geometrical models to predict up-dip gas flow in faults, and investigates the outcrop expression of fault structure below seismic reflection data resolution and gas flux rates at an onshore site of fault-related gas leakage. Data for this study are provided by highquality 2D and 3D seismic reflection lines (tied to stratigraphy in fifteen wells), and outcrop of Miocene and Oligocene strata in coastal cliff sections, together with methane concentration and flux measurements. Structural development in the Kupe Area was complex and provides a near complete record of deformation since the Late Cretaceous (~85 Ma). Basin strata up to 9 km thick record four main periods of deformation that reflect changing plate boundary configurations. Fault reactivation was common in the Kupe Area, with the locations and orientations of pre-existing faults strongly influencing the locations and geometries of younger faults and folds. Pliocene to Recent normal faults are highly segmented with low strain, consistent with an immature fault system in which fault lengths were established rapidly and subsequent fault growth was mainly achieved by accumulation of displacement. Plio-Pleistocene to Recent reactivation of Cretaceous rift faults provides conduits for gas migration from below the regional top seal in the Kupe Area into shallow strata and results in up-dip gas migration within the Plio-Pleistocene to Recent fault zones. These late-stage normal faults (younger than 4 Ma) are shown to have a strong spatial relationship with gas chimneys suggesting that fault zones are capable of producing channelised pathways for up-dip hydrocarbon migration. Fifteen of seventeen gas iii chimneys within the study area are rooted within fault zones. All of these fifteen faultrelated gas chimneys occur at geometrical complexities in fault structure (i.e. relay zones, lateral fault tips or fault intersections). Geometrical complexities are associated with locally high throw gradients which are inferred to be accompanied by off-fault strain in the form of fractures and/or bedding rotation. Three geomechanical modelling techniques (Slip Tendency, Dilation Tendency and Fracture Stability) for predicting the locations of up-fault hydrocarbon flow (leakage) are tested using the spatial distribution of gas chimneys and Pliocene to Recent normal faults in the Kupe Area. Slip Tendency, Dilation Tendency and Fracture Stability data for all of the faults analysed predict comparable likelihoods of gas migration on chimney and non-chimney sections of the fault surfaces and therefore do not provide a robust basis for predicting where on fault surfaces channelised up-dip gas flow will occur. Field-based observations of faults show that fractures observed in outcrop and below seismic reflection data resolution are localised around bends, steps and intersections of faults and show evidence of fluid flow post fault activity. In north Taranaki these fault complexities are present in a lateral equivalent to the Otaraoa top seal and, if present in the Kupe Area, are also likely to induce up-sequence gas migration through fracture networks. Methane concentrations measured at one site (Bristol Road Quarry) along the Inglewood Fault suggest that gas flux rates up faults may not be uniform over time. Based on the measured gas flux rates gas chimneys in the Kupe Area may form in association with gas migration in a series of discrete events lasting from days to years, with possible gas flows at the seabed of ~930 ft3 per chimney per day or 0.34 million ft3 per year.