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

<p>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.</p>

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

<p>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.</p>

Assessing the spatial and temporal variability of GHG emissions from different configurations of on-site wastewater treatment system using discrete and continuous gas flux measurement

Global emissions linked to wastewater treatment are estimated to account for up to 1.5 % of total greenhouse gas (GHG) emissions globally. However, few studies have measured GHG emissions from domestic on-site treatment systems (DWWTSs) directly. In this study, two DWWTSs were monitored for 446 days and > 42,000 gas flux measurements were conducted using both discrete spot measurements and continuous flux chamber deployments. The observed GHG fluxes from biological activity in the soil and water phase were found to be highly spatially and temporally variable and correlated to environmental factors, water usage patterns and system design. In total, the results show that a septic tank discharging effluent into a well-designed soil treatment unit is estimated to emit a net 9.99 kg-CO2eq cap−1 yr−1, with approximately 63 %, 27 % and 10 % of the total CO2-equivalent net emissions in the form of CO2, CH4 and N2O, respectively. Emissions from the septic tank surface contributed over 50 % of total emissions and tended to be strongly underestimated by one-off discrete measurements, especially when episodic ebullitive events are to be considered. Fluxes from the soil treatment unit (STU) stemmed from both the soil surface and the vent system, but were also found to be periodically negative, i.e. net uptakes. Soil fluxes were mostly influenced by temperature but peaked regularly under conditions of rapidly changing soil water content. Vent fluxes were mostly governed by effluent quality and a low number of high emission events was responsible for the majority of total observed vent emissions. Owing to the strong overall spatial and temporal heterogeneity of observed fluxes from DWWTSs across all modules, future studies should focus on continuous deployments of a number of flux chambers over discrete measurements to accurately assess GHG emissions from on-site systems. This study also provided insights into managing GHG emissions from DWWTSs by different system configuration design, as well as indicating that the current IPCC emission factors for CH4 and N2O are significantly overestimating emissions for on-site wastewater treatment systems.

Effect of elevation, season and accelerated snowmelt on biogeochemical processes during isolated conifer needle litter decomposition

Increased drought and temperatures associated with climate change have implications for ecosystem stress with risk for enhanced carbon release in sensitive biomes. Litter decomposition is a key component of biogeochemical cycling in terrestrial ecosystems, but questions remain regarding the local response of decomposition processes to climate change. This is particularly complex in mountain ecosystems where the variable nature of the slope, aspect, soil type, and snowmelt dynamics play a role. Hence, the goal of this study was to determine the role of elevation, soil type, seasonal shifts in soil moisture, and snowmelt timing on litter decomposition processes. Experimental plots containing replicate deployments of harvested lodgepole and spruce needle litter alongside needle-free controls were established in open meadows at three elevations ranging from 2,800–3,500 m in Crested Butte, Colorado. Soil biogeochemistry variables including gas flux, porewater chemistry, and microbial ecology were monitored over three climatically variable years that shifted from high monsoon rains to drought. Results indicated that elevation and soil type influenced baseline soil biogeochemical indicators; however, needle mass loss and chemical composition were consistent across the 700 m elevation gradient. Rates of gas flux were analogously consistent across a 300 m elevation gradient. The additional variable of early snowmelt by 2–3 weeks had little impact on needle chemistry, microbial composition and gas flux; however, it did result in increased dissolved organic carbon in lodgepole porewater collections suggesting a potential for aqueous export. In contrast to elevation, needle presence and seasonal variability of soil moisture and temperature both played significant roles in soil carbon fluxes. During a pronounced period of lower moisture and higher temperatures, bacterial community diversity increased across elevation with new members supplanting more dominant taxa. Microbial ecological resilience was demonstrated with a return to pre-drought structure and abundance after snowmelt rewetting the following year. These results show similar decomposition processes across a 700 m elevation gradient and reveal the sensitivity but resilience of soil microbial ecology to low moisture conditions.

Analysis of Natural and Power Plant CO2 Emissions in the Mount Amiata (Italy) Volcanic–Geothermal Area Reveals Sustainable Electricity Production at Zero Emissions

Geothermal energy is a key renewable energy for Italy, with an annual electric production of 6.18 TWh. The future of geothermal energy is concerned with clarity over the CO2 emissions from power plants and geological contexts where CO2 is produced naturally. The Mt. Amiata volcanic–geothermal area (AVGA) is a formidable natural laboratory for investigating the relative roles of natural degassing of CO2 and CO2 emissions from geothermal power plants (GPPs). This research is based on measuring the soil gas flux in the AVGA and comparing the diffuse volcanic soil gas emissions with the emissions from geothermal fields in operation. The natural flux of soil gas is high, independently from the occurrence of GPPs in the area, and the budget for natural diffuse gas flux is high with respect to power plant gas emissions. Furthermore, the CO2 emitted from power plants seems to reduce the amount of natural emissions because of the gas flow operated by power plants. During the GPPs’ life cycle, CO2 emissions in the atmosphere are reduced further because of the reinjection of gas-free aqueous fluids in geothermal reservoirs. Therefore, the currently operating GPPs in the AVGA produce energy at a zero-emission level.