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

摘要

Faults play an important role in petroleum systems as both barriers and conduits to theflow of hydrocarbons. An understanding of the relationship between fluid and gasmigration and accumulation, and faulting is often required during hydrocarbonexploration and production, and CO2 storage. While methods for predicting across-faultflow are well advanced (e.g. Yielding et al., 1997; Manzocchi et al., 1999), currentgeomechanical 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 formof gas chimneys and Pliocene to Recent normal faults in the Kupe Area, South TaranakiBasin. It undertakes studies of the Kupe Area’s structural development, examinesspatial relationships between faults and gas chimneys, tests current geomechanical andgeometrical models to predict up-dip gas flow in faults, and investigates the outcropexpression of fault structure below seismic reflection data resolution and gas flux ratesat an onshore site of fault-related gas leakage. Data for this study are provided by highquality2D and 3D seismic reflection lines (tied to stratigraphy in fifteen wells), andoutcrop of Miocene and Oligocene strata in coastal cliff sections, together with methaneconcentration and flux measurements.Structural development in the Kupe Area was complex and provides a near completerecord of deformation since the Late Cretaceous (~85 Ma). Basin strata up to 9 kmthick record four main periods of deformation that reflect changing plate boundaryconfigurations. Fault reactivation was common in the Kupe Area, with the locationsand orientations of pre-existing faults strongly influencing the locations and geometriesof younger faults and folds. Pliocene to Recent normal faults are highly segmented withlow strain, consistent with an immature fault system in which fault lengths wereestablished rapidly and subsequent fault growth was mainly achieved by accumulationof displacement.Plio-Pleistocene to Recent reactivation of Cretaceous rift faults provides conduits forgas migration from below the regional top seal in the Kupe Area into shallow strata andresults in up-dip gas migration within the Plio-Pleistocene to Recent fault zones. Theselate-stage normal faults (younger than 4 Ma) are shown to have a strong spatialrelationship with gas chimneys suggesting that fault zones are capable of producingchannelised pathways for up-dip hydrocarbon migration. Fifteen of seventeen gasiiichimneys within the study area are rooted within fault zones. All of these fifteen faultrelatedgas chimneys occur at geometrical complexities in fault structure (i.e. relayzones, lateral fault tips or fault intersections). Geometrical complexities are associatedwith locally high throw gradients which are inferred to be accompanied by off-faultstrain in the form of fractures and/or bedding rotation.Three geomechanical modelling techniques (Slip Tendency, Dilation Tendency andFracture Stability) for predicting the locations of up-fault hydrocarbon flow (leakage)are tested using the spatial distribution of gas chimneys and Pliocene to Recent normalfaults in the Kupe Area. Slip Tendency, Dilation Tendency and Fracture Stability datafor all of the faults analysed predict comparable likelihoods of gas migration onchimney and non-chimney sections of the fault surfaces and therefore do not provide arobust basis for predicting where on fault surfaces channelised up-dip gas flow willoccur.Field-based observations of faults show that fractures observed in outcrop and belowseismic reflection data resolution are localised around bends, steps and intersections offaults and show evidence of fluid flow post fault activity. In north Taranaki these faultcomplexities are present in a lateral equivalent to the Otaraoa top seal and, if present inthe Kupe Area, are also likely to induce up-sequence gas migration through fracturenetworks.Methane concentrations measured at one site (Bristol Road Quarry) along theInglewood 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 inassociation 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 ft3per year.