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Residual oil zones (ROZs) could present a new, potentially large and commercially viable oil resource for Australia and provide an avenue for geological storage of carbon dioxide (CO2) through CO2 enhanced oil recovery (CO2-EOR). These reservoirs, which can contain a moderate amount of residual oil and resemble water-flooded oil fields, can be associated with conventional fields (brownfields) or occur with no associated main pay zone (greenfields). Both types of ROZ are currently produced commercially through CO2-EOR in the Permian Basin, USA, and are of growing interest internationally, but our understanding of the occurrence and economic viability of oil production from ROZs in Australia is limited. We are employing geological and petrophysical methods to identify, map and quantify the potential oil resources of ROZs, initially in central Australian basins. Complementing this, we are conducting a series of CO2 core-flooding experiments combined with reservoir modelling to investigate the techno-economic feasibility of producing oil and storing CO2 in these formations. We aim to establish and test a workflow for characterising and evaluating ROZs in Australia. ROZs could prove to be good targets for CO2-EOR+, potentially even producing carbon-neutral or carbon-negative oil by using CO2 from anthropogenic sources, such as from blue hydrogen production.

Western Australia’s Canning Basin is an underexplored prospective basin with proven petroleum systems and small-scale production. Recently, several formations within deeper depocentres have been in...

Middle Cambrian sediments in the southern Georgina Basin contain multiple organic-rich source rocks and have been suggested to be prospective for both conventional and unconventional hydrocarbons. ...

We present results from a multidisciplinary rock mechanics study designed to develop a high resolution mechanical framework of the lower Paaratte Formation, a reservoir used for CO2 storage at the CO2CRC Otway Research Facility, Otway Basin Victoria. A continuous high-resolution record of unconfined compressive strength (UCS) was determined through the scratch testing technique along approximately 80 m of CRC-2 core. Multiple correlations between UCS and several different wireline log suites resulted in the development of a strength proxy that can accurately predict strength and lithologic transitions. The strength proxy was then applied to a section of the CRC-1 well which traverses the Naylor South splay fault. The proxy predicts a UCS of approximately 20 MPa for the splay fault, which is a central learning as it constrains a key parameter that is used in fault stability modelling. A number of triaxial rock mechanical tests were also conducted to complement the scratch testing and also characterise the poroelastic properties of the various units. UCS and stiffness of the rocks appear to be positively correlated, albeit with significant data scatter and data gaps at moderate strength values. The workflow developed in this study provides a more accurate and higher resolution mechanical framework to be developed at potential CCS sites and provide a predictive methodology for mechanical properties when actual mechanical tests may be absent.

CO2CRC has made a significant investment into establishing the feasibility of conducting a CO2 injection experiment into a shallow fault. This world-leading experiment, located at the CO2CRC Otway International Test Centre in Victoria, Australia, would seek to improve our understanding of the conditions necessary for CO2 to move vertically up faults. The work undertaken during Phase 2 of the Otway Fault Project confirms the experiment is technically feasible and can be done safely. Two appraisal wells drilled and cored through the shallow Brumbys Fault indicate the fault extends to the base of the upper 2 m thick Hesse Clay layer, which forms the seal to the underlying Port Campbell Limestone aquifer. The fault does not have a defined core but an approximately 6-10 m wide fault cataclastic zone. Permeability within the Port Campbell Limestone is variable, ranging from tens to thousands of millidarcies. The rock strength is low, however, and it is recommended to conduct the experiment at approximately 80 m depth rather than the 40 m originally proposed. This provides more confining pressure and will ensure that the injection pressure does not exceed the fracture pressure. A deeper injection also provides better spatial and timing conditions for geophysical monitoring and tracking of the CO2 plume. Simulations indicate only a small 10 tonne CO2 injection experiment would be required to monitor CO2 migration using geophysical techniques. In addition to providing an opportunity to demonstrate semi-continuous, near real-time monitoring of CO2 migration up a fault, the planned CO2 injection experiment presents a unique opportunity to obtain field measurements on vertical fault permeability. It also provides an opportunity to predict fluid flow and potential metal remobilisation through comprehensive reactive transport modelling, collection and analysis of core post CO2 injection, and evaluate the effectiveness of the modelling versus field observations.

A fully cored sequence of Hesse Clay, Port Campbell Limestone and uppermost Gellibrand Marl in the onshore Otway Basin, southeastern Australia, offers new insight into the evolution of the middle Miocene Port Campbell Limestone. The Port Campbell Limestone comprises grey unconsolidated to semi-consolidated and rarely lithified bioclastic muddy carbonate sands in a stack of thin repetitive cycles within cycles of predominantly shoaling-upward character. A glauconitic band with a distinctive mollusc–echinoderm–bryozoan fauna provides a distinctive marker interval in the sequence. In mineralogy, the Port Campbell Limestone is predominantly calcite with traces of remnant aragonite in muddier low-permeability sands, and with dolomitic zones in permeable intervals. The small non-carbonate component of the Port Campbell Limestone is between 3 and 15 wt% and comprises quartz silt with minor clay, feldspar and mica. Dissolution overprints are prominent throughout the carbonate sequence. Three distinct geochemical signatures of provenance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contributions with felsic sources. Foraminifera are common and generally well preserved. Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma. Shallowing after 14 Ma indicates variable paleodepths of ca 13.2 Ma when the sequence was emergent for a brief but undetermined period, corresponding with sharp changes in geochemical ratios. Observed cyclicity in these mid-shelfal, cold-water carbonates is strongly correlated with orbital forcings—eccentricity and obliquity. Sedimentation rates determined from cyclostratigraphic analysis indicate 4–6 cm/kyr at the end of the MMCO, diminishing to 1.5–3 cm/kyr during the MMCT and the subsequent accumulation of the Port Campbell Limestone.KEY POINTSThe Port Campbell Limestone is a stack of thin repetitive depositional cycles within cycles. The cyclicity is strongly correlated with orbital forcings—eccentricity and obliquity—and this is reflected in the geochemistry.Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma.The sequence was emergent for a brief but undetermined period at ca 13.2 Ma, corresponding with sharp changes in geochemical ratios.Three distinct geochemical signatures of provenance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contributions with felsic sources. The Port Campbell Limestone is a stack of thin repetitive depositional cycles within cycles. The cyclicity is strongly correlated with orbital forcings—eccentricity and obliquity—and this is reflected in the geochemistry. Foraminiferal data suggest a depositional transition from outer shelf conditions in the Gellibrand Marl at ca 15 Ma to middle shelf environments in the lower part of the Port Campbell Limestone during the Middle Miocene Climatic Optimum (MMCO) at ca 14.24 Ma. The sequence was emergent for a brief but undetermined period at ca 13.2 Ma, corresponding with sharp changes in geochemical ratios. Three distinct geochemical signatures of provenance are evident in the Port Campbell Limestone sequence, including possible volcanogenic contributions with felsic sources.

CO2CRC and its partners are undertaking a feasibility study for a planned CO2 controlled release and monitoring experiment on a shallow fault at the CO2CRC Otway Research Facility. In this project we plan to image, using a diverse range of geophysical and geochemical CO2 monitoring techniques, the migration of CO2 up a fault from a controlled release point at approximately 30 m depth. This paper describes the results of site characterisation and modelling work undertaken to date. It also includes a description of the activities planned that will enable for a more detailed characterization of the fault and proposed injection interval. Together these results will enable an assessment as to whether the planned injection experiment is feasible and how it can be optimally designed.

This paper presents the pre-injection fault characterisation that has been conducted at the CO2CRC Otway Project in Victoria, Australia, aimed at understanding the response of faults to pressure increases in the formation, fault transmissibility horizontally and vertically, and also presents a methodology to extract key rheological fault properties using novel rock mechanical techniques. The initial modelling results show that the likelihood of injection induced fault reactivation is exceedingly small, due to the very small pressure increases associated with injection (∼ 0.05 MPa after about 100 days of injection during the Stage 2C experiment). The potential for CO 2 flow across the main reservoir-cutting faults was quantified using the shale gouge ratio algorithm. Results indicate that the faults should be sealing to some degree and should therefore restrict the lateral movement of CO 2 . Despite these positive results with regard to fault behaviour during current operations, there remains significant uncertainty with regard to certain fault properties such as friction and cohesion that have a large effect on fault stability. Here we develop a workflow which involves several rock mechanical testing techniques to gain information regarding a key fault at the Otway site. One relatively novel technique that is used is the scratch test, which measures strength heterogeneity of the host rock at the centimetre scale. The scratch data can be correlated to certain well logs via a multivariate analysis in order to develop proxies for rock strength that can be applied to other wells and also to wells that traverse fault zones, thereby providing some estimate of fault strength. This scratch analysis is complemented by a series of triaxial experiments designed to measure poroelastic properties and also assess the rate and state frictional parameters of potential fault gouge materials. While the work presented here is aimed specifically at understanding the properties of key faults at the Otway Project, it is hoped that the knowledge gained and the transforms developed here can be used as a guide to constrain fault properties at other CCS sites.

Future CO 2 storage projects will require Monitoring and Verification (M&V) operations at the CO 2 storage site to understand the behavior of the CO 2 plume, including the assurance that leakage of the CO 2 has not occurred. Current surface based monitoring technologies may be unable to yield sufficient resolution or accuracy. CO2CRC, in conjunction with its Australian partners, is developing the Otway Stage 3 Project to identify and validate sub-surface monitoring techniques and configurations as a key element of a risk-based M&V program in large scale CCS projects. Subsurface monitoring approaches will be tested on a plume of CO 2 from an array of monitoring wells. Primary monitoring methods will be pressure tomography and downhole seismic, although other modalities (gravity, electromagnetic) are also being considered.

The CO2CRC Otway Project “Stage 3” is planned to be an injection of between 15,000 and 30,000 tonnes, to be carried out at the Otway site in South Western Victoria, Australia. The objective of the project is to test unobtrusive, continuous monitoring methods that would operate with a small environmental footprint and limited impact on other land use activities such as farming. Progress since GHGT13 falls into three broad areas; reservoir characterisation, dynamic modelling, and feasibility and design of the monitoring methods. The appraisal well CRC-3 was successfully drilled and a wide range of core and log data was collected. While only 600-700 metres from existing logged wells, it was crucial to confirm that the injection concept was feasible and that the plume would migrate towards the proposed monitoring wells. In addition, injection tests were performed that confirmed pressure continuity to the existing wells. As there are many small faults in the area this was an important outcome. The geological and dynamical models for the experiment have been updated with the data from CRC-3; and model predictions have benefited from the ongoing performance history match to the stage 2C time-lapse seismic monitoring experiment. Currently the Stage 3 project is in its Define phase, having completed its Opportunity Definition and Evaluate phases. By the time of GHGT14 we expect to be commencing the Execution phase, with designs finalised and the drilling programme imminent.

CO2CRC is undertaking a feasibility study for a planned CO2 controlled release and monitoring experiment on a shallow fault at the CO2CRC Otway Research Facility. In the first phase of the project, a series of geophysical surveys and groundwater permeability assessments were conducted at the Otway Research Facility to characterise the prospective experimental site and confirm the presence of a shallow fault. The data from the surveys have been integrated into a 3D model of the upper faulted shallow geology of the CO2CRC Otway site. Five properties (hydrostratigraphic unit, horizontal permeability, vertical permeability, fault permeability and P-velocity from refraction tomography) were populated in the 3D grid to produce the final static model. The model has been developed to assist with designing the experiment and conceptualising the shallow stratigraphy and fault structure at the Otway site. Dynamic modelling simulations based on the static model suggest that under certain scenarios it is possible that CO2 could migrate up the fault to the base of the Hanson Plain Sand if injected at approximately 30 m depth. This suggests that the planned fault injection experiment is feasible but additional data is required to validate the geological model and better constrain the modelling simulations.

The petroleum industry, through the production and consumption of oil and gas, contributes to global greenhouse gas emissions. However, the industry’s leadership and experience in underground injection and storage of CO2, especially through CO2 enhanced oil recovery (CO2-EOR), which has been proposed as a possible solution to reducing atmospheric CO2 levels, has not been well acknowledged. Unlike traditional CO2-EOR, which tends to be a net carbon emitter due to the use of predominantly natural CO2, rather than anthropogenic, CO2-EOR+ focuses on storing a larger volume of CO2. Thus CO2-EOR+ not only provides a potential solution to dispose of anthropogenic emissions but at the same time reduces reliance on imported oil through increased domestic production. Increased industry interest and energy policy strategies directed at reducing and/or removing emissions from industry processes reflect the growing social and economic impetus to improve operation practices and the petroleum industry’s reputation. Residual oil zones (ROZs) below identified oil–water contacts provide an excellent target for the application of CO2-EOR+. They offer a producible residual oil resource accessible through CO2-EOR, as well as a large pore volume for CO2 storage, with efforts focused on converting ROZs into resources and reserves. Existing fields in the Surat and Cooper-Eromanga Basins are already well placed to utilise anthropogenic CO2 sources to achieve conventional CO2-EOR metrics. The ROZs in these basins will hopefully allow potential EOR projects to increase the CO2 volumes stored, per incremental barrel of oil, well past traditional levels (0.2–0.3 tCO2/bbl), and in doing so, potentially achieve net negative-emission oil.

Polygonal fault systems are observed in over 100 sedimentary basins worldwide where they are confined to fine-grained strata and have the potential to impact on seal integrity for CO2 storage and hydrocarbon reservoirs. 3D seismic reflection and well data have been used to characterise a layer-bound polygonal fault system and gas chimneys in the Bathurst Island Group regional seal, Petrel Sub-basin, offshore NW Australia. Segmented fault arrays extend through the entire faulted interval which contains at least 10 tiers of polygonal faults each spanning ∼4 Myrs of sediment deposition. Polygonal fault densities and intersections reach a maximum at 1000–1100 ms TWT and in some cases extend into the Sandpiper Sandstone reservoir at >1200–1300 ms TWT. Down-dip displacement profiles are symmetrical and increase progressively across tier boundaries towards maxima of ∼25–47 ms between 600 and 1100 ms. These systematic displacement variations are similar to those of tectonic faults and suggest that fault segments in different tiers develop synchronously as kinematically coherent arrays. Three-dimensional imaging of the polygonal fault system indicates that throughout the Bathurst Island Group the fault network is well connected vertically and horizontally. Polygonal faulting may locally promote gas flow through the seal to the seabed suggesting that, where faulted, the Bathurst Island Group may not be an effective seal for CO2 storage in the study area.

The Otway Basin in the south of Victoria, Australia underwent three phases of deformation during breakup of the southern Australian margin. We assess the geometry and kinematics of faulting in the basin by analysing a 3‐D reflection seismic volume. Eight stratigraphic horizons and 24 SW‐dipping normal faults as well as subordinate antithetic faults were interpreted. This resulted in a high‐resolution geological 3‐D model (ca. 8 km × 7 km × 4 km depth) that we present as a supplementary 3‐D PDF (Data S1). We identified hard‐ and soft‐linking fault connections over the entire area, such as antithetic faults and relay ramps, respectively. Most major faults were continuously active from Early to Late Cretaceous, with two faults in the northern part of the study area active until at least the Oligocene. Allan maps of faults show tectonic activity continuously waned over this time period. Isopach maps of stratigraphic volumes quantify the amount of syn‐sedimentary movement that is characteristic of passive margins, such as the Otway Basin. We show that the faults possess strong corrugations (with amplitudes above the seismic resolution), which we illustrated by novel techniques, such as cylindricity and curvature. We argue that the corrugations are produced by sutures between sub‐vertical fault segments and this morphology was maintained during fault growth. Thus, they can be used to indicate the kinematics vector of the fault movement. This evidences, together with left‐stepping relay ramps, that 40% of the faults had a small component (up to 25°) of dextral oblique slip as well as normal (dip‐slip) movement.

Mechanical damage (e.g. faults and fractures) related to tectonic forces and/or variations in formation pore pressures may enable the leakage of fluids through otherwise effective seal rocks. Characterisation of faults and fractures within seals is therefore essential for the assessment of long-term trap integrity in potential CO2 storage sites. 3D seismic reflection data are used to describe a previously unrecognised network of extensive, small Miocene-age faults with displacement of generally

Structural and geomechanical models of the CO2CRC Otway Project's Paaratte Formation are presented in advance of the proposed Otway Project time-lapse seismic monitoring of plume stabilisation test. The main goal of this experiment is to detect and monitor a CO2 plume in a saline aquifer using 4D seismic techniques and subsequently confirm plume stabilisation. The modelling results show that the likelihood of injection induced fault reactivation is exceedingly small, due to the very small pressure increases associated with injection (∼0.05 MPa at the faults after about 100 days of injection). The potential for CO2 flow across the main reservoir-cutting faults was quantified using the shale gouge ratio algorithm. Results indicate that the faults should be sealing to some degree and should therefore restrict the lateral movement of CO2. A dynamic simulation characterizing the potential for CO2 flow vertically through the Naylor South splay fault indicates that a very limited volume of CO2 gas would migrate up the fault. According to dynamic simulations of a deliberately leaky fault, CO2 is only able to migrate 10 s of metres vertically after 100 years, which is still more than 350 m below the top of the Paaratte Formation. This study is not only important as it provides a peer reviewed workflow for risking a carbon capture and storage (CCS) experiment, but geophysical observations during the experiment will also allow the assumptions and models to be tested in the field and various model parameters to be calibrated. Structures sealed by faults are extremely common for hydrocarbon accumulations, and will also be for CCS, and thus fault modelling will be a vital part of site characterisation.

This paper presents a 3D geomechanical modelling study of the Iona gas storage facility in the state of Victoria, Australia. The results provide important information pertaining to gas storage, which can then be used to understand certain geomechanical aspects of CO2 storage. A key finding in this paper is that significant changes to the horizontal stress magnitudes are imparted by changes to the fluid pressure due to gas injection or withdrawal. This effect, known as the reservoir stress path, significantly influences fault stability by counteracting the changes to effective stress. In the case of Iona, pressurisation of the field results in a stress path which is parallel to the failure criterion rather than towards it, as would be expected in a classical treatment which does not incorporate complex poro-elastic effects. Another output of interest relates to reservoir deformation, which would be manifested at the ground surface as heave or subsidence. During periods of peak gas withdrawal and injection, surface ground movement is predicted to be on the order of −9 mm and +2.5 mm, respectively. These numbers are similar to the surface deformation observed at the In Salah CO2 injection project, but much smaller than the subsidence observed in some producing hydrocarbon fields.

The successful commercial scale deployment of carbon capture and storage (CCS) requires assurance of the confinement of the injected CO 2 at each potential storage site. The critical elements of the confinement of CO 2 are the caprock overlying the storage formation, and any faults or fractures which occur within the caprock. The most significant aspect of containment is the seal potential of the caprock, defined as the seal capacity, seal geometry and seal integrity. The seal capacity refers to the CO 2 column height that the caprock can retain before capillary forces allow the migration of the CO 2 into and possibly through the caprock. Determination of capacity is achieved primarily through petrophysical analyses such as mercury injection capillary pressure (MICP) tests. For storage in depleted fields, assessments of seal capacity can be made from empirical observations of actual hydrocarbon column heights and converting these to CO 2 physical properties (density, temperature, pressure). Where these data sources are unavailable, the use of analogs can be a viable alternative. Seal geometry refers to the thickness and lateral extent of the caprock. The caprock must have sufficient lateral extent to cover whatever structural, stratigraphic or hydrodynamic storage reservoir in which the CO 2 is trapped. In addition, its thickness should exceed the throw of any faults that cut so as to maintain an effective barrier despite faults through it. Seal geometry is evaluated through well data (stratigraphic, sedimentological and wireline log analyses) and seismic surveys, which are pre-requisites to any viable storage project. Seal integrity refers to the geomechanical properties of the caprock. These properties are controlled by caprock mineralogy, regional and local stress fields as well as any stress changes induced by injection or withdrawal of water or CO 2 . The modification of the stress field within a storage formation during and after injection of CO 2 can lead to reservoir and caprock mechanical failure. This failure can result in the generation of new faults and fractures, reactivation of existing faults and/or bedding parallel slip. The key parameters determining whether faults might act as conduits or as seals are the juxtaposition relationships of rocks on either side of a fault plane, the properties of the fault zone itself or the reactivation potential of the fault. The greatest likelihood of fluid migration up faults is during or immediately after reactivation. Thus, the mere existence of faults does not automatically rule out a site for geological storage of carbon dioxide. On the contrary, sealing faults commonly trap hydrocarbons and compartmentalize oil and gas reservoirs and could also form suitable confining barriers at CO 2 storage sites. Seal capacity, geometry and integrity interpretations must be tempered by the potential geochemical reactions between fluids and rocks and injected CO 2 as well as by the hydrodynamic environment above and below the seal which may modify the calculated pressure regimes.

Underground gas storage (UGS) facilities provide a wealth of information, which can be used to better understand various aspects of CO2 storage in depleted reservoirs. In some cases UGS facilities can provide important site specific information for carbon storage projects that are planned in similar formations in close proximity. In this paper, we discuss the various ways in which UGS facilities can be used to extract important information, and when possible we draw upon information from the Iona gas storage facility in Australia’s Otway basin. The Iona facility is located 20 km away from the CO2CRC Otway Project, in which CO2 65445 tonnes of 77 mole% carbon dioxide, 20 mole% methane and 3 mole% other gas components (containing about 58000 tonnes of carbon dioxide) was injected into the Waarre C formation over a 17 month period. In this paper, we compare the factors that control CO2 seal capacity and discuss how UGS facilities can provide information on sustainable column heights either limited by faults or by cap rocks. We also present dynamic modeling results in which information is gained regarding injectivity, pressure evolution of the reservoir, storage capacity and maximum fluid pressures sustained by the faults. Understanding such parameters is important for the safe operation of any carbon storage project, be it on a demonstration or industrial scale.

The Tuna field, located on the northern margin of the offshore Gippsland Basin, has a wide range of natural CO 2 accumulations and provides a natural analogue that can be used to assess reservoir sealing capacity. A holistic approach is assumed for this study, which incorporates analysis of the hydrodynamic system in conjunction with evolutionary history, CO 2 and mineralogical distribution, fault mechanical stability and sealing potential. Integration of these parameters for each reservoir unit within the field produces potentially conflicting results regarding the West Tuna fault. Hydrodynamic evaluation has shown that the fault has a complex fluid flow history with significant variation at different reservoir levels, pressures suggest it is currently leaking across certain horizons in the intra-Latrobe while fault seal analysis predicts good sealing properties at those depths. The CO 2 distribution varies with depth, with higher concentrations of CO 2 found in the deeper, overpressured reservoirs of the early Latrobe. This in conjunction with the basin history and mineral studies suggests that the CO 2 distribution may be controlled by the fault and fault reactivity. Geomechanical studies show that pressures in these deeper reservoirs are insufficient to generate shear failure, while the likelihood for extensional fracturing, and thus the possibility for it to act as a pathway for vertical fluid flow, is very high. This study shows that an integrated approach can lead to a better understanding of the current day reservoir and fault properties, even if not all components resulted in the same conclusions. Moreover, it reduces the risk of mis-interpreting fault-seal properties based on less parameters.

A geomechanical assessment of the Naylor Field, Otway Basin, Australia has been undertaken to investigate the possible geomechanical effects of CO2 injection and storage. The study aims to evaluate the geomechanical behaviour of the caprock/reservoir system and to estimate the risk of fault reactivation. The stress regime in the onshore Victorian Otway Basin is inferred to be strike–slip if the maximum horizontal stress is calculated using frictional limits and DITF (drilling induced tensile fracture) occurrence, or normal if maximum horizontal stress is based on analysis of dipole sonic log data. The NW–SE maximum horizontal stress orientation (142°N) determined from a resistivity image log is broadly consistent with previous estimates and confirms a NW–SE maximum horizontal stress orientation for the Otway Basin. An analytical geomechanical solution is used to describe stress changes in the subsurface of the Naylor Field. The computed reservoir stress path for the Naylor Field is then incorporated into fault reactivation analysis to estimate the minimum pore pressure increase required to cause fault reactivation (ΔPp). The highest reactivation propensity (for critically-oriented faults) ranges from an estimated pore pressure increase (ΔPp) of 1 MPa to 15.7 MPa (estimated pore pressure of 18.5–33.2 MPa) depending on assumptions made about maximum horizontal stress magnitude, fault strength, reservoir stress path and Biot's coefficient. The critical pore pressure changes for known faults at Naylor Field range from an estimated pore pressure increase (ΔPp) of 2 MPa to 17 MPa (estimated pore pressure of 19.5–34.5 MPa).

A laboratory study of simulated quartz gouges was conducted to investigate how solution transfer processes influence the mechanical behaviour of fault wear products at high temperature, hydrothermal conditions. Experiments were performed under nominally dry conditions, as well as in the presence of an aqueous pore fluid, at elevated temperatures (500 to 927 °C), and at effective confining pressure conditions ( σ 2 ′ = σ 3 ′ = 100 MPa) to simulate, on a laboratory timescale, processes that may be important in fluid-active fault zones at depth in the continental crust. The mechanical data and microstructural analysis indicate that the kinetics of solution transfer processes can exert a fundamental control on the mechanical behaviour of fault wear products. It is found that, at nominally dry conditions, gouges deform by cataclastic creep and distributed shear, with strength and microstructures being relatively unaffected by temperature. At moderately chemically reactive, hydrothermal conditions (500–600 °C, coarse grain size, or fast deformation rate), the presence of a reactive pore fluid slightly reduces the shear strength with respect to dry conditions. However, at highly chemically reactive, hydrothermal conditions (600–927 °C, small grain size, and slow deformation rate), rapid porosity reduction is accommodated by dissolution–precipitation processes. Deformation under such conditions results in a fast increase of grain contact area and the development of cohesive bonds between adjacent particles, which in turn inhibits cataclastic granular flow. With increasing displacement and compaction of the quartz gouge, there is a sudden transition from distributed cataclastic flow, to slip localization at the interface between the gouge and one of the forcing blocks. This deformation mode switch is associated with dramatic weakening (up to 50% drop in shear resistance, and changes in the apparent coefficient of friction from > 0.7 to ≈ 0.4). Stress drop occurs over many minutes in the laboratory. It is speculated that solution-assisted gouge compaction, and consequent slip localization with associated slow, yet dramatic stress drop, could provide a mechanism for the occurrence of slow earthquakes.

In this paper, we present experiments that explore the interaction between crustal deformation, hydrothermal reaction and permeability evolution. We postulate that deformation should result in changes to mineral surface area, which will either favour or inhibit water–rock reactions involving the formation of secondary minerals. Mineral precipitation can greatly affect hydrologic properties such as permeability, so it is therefore crucial that the controlling parameters are well constrained. Experiments were conducted using a Paterson gas apparatus capable of independently controlling confining pressure (P c ), pore pressure (P p ) and axial load. Most experiments were conducted at P c = 100 MPa and P p = 50 MPa with temperatures of 200–600 °C, using a fine-grained granitic rock. In the experiments, permeability evolution was measured under different stress regimes, ranging from the isostatic to the shear failure case. We find that permeability decreases with time by a well-defined exponential function, with a rate constant, r , representing the duration of the transient in permeability. The rate constant is lowest when the specimen is in the compactional regime, where mineral surface areas are minimized. Conversely, the permeability reaction rate constant changes at the greatest rate when the specimen is stressed into the dilatant regime or undergoes shear failure. These stress states result in the generation of fresh, reactive mineral surface areas leading to enhanced reaction and a rapid associated change in permeability. Results from this study suggest that in natural systems, a negative feedback effect might be operative, in which enhanced mineral precipitation moderates permeability generated during episodes of deformation. This feedback is likely to be important in dynamic environments such as fault zones or injection sites intended as emerging hydrothermal energy sources.

Veined lithologies are formed by fracturing and sealing processes, with the veins representing former fluid conduits through the rock. Although detailed fieldwork and numerical simulations have provided a better understanding of vein growth, few studies have attempted to seal fractures and generate veins experimentally. In this pilot study, we subjected fractured quartzite to temperature gradients of 45–125 °C under hydrothermal conditions in a static fluid, with the aim of precipitating secondary quartz in the cooler portions of the fracture. Results show that secondary quartz precipitates due to the imposed temperature gradient, causing the initial fracture to seal locally. Although no systematic sealing pattern was observed along the fracture, samples subjected to higher temperatures exhibit a smaller fracture width and appear to have reacted more extensively. Electron microprobe mapping visualizes the spatial distribution of secondary quartz, which contains elevated concentrations of aluminium.

Physical and chemical fluid–rock interactions are implicated in controlling earthquake nucleation and recurrence. In particular, interseismic compaction, sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones. In contrast, these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults. Here, we present high-temperature, hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock, and also characterise the associated changes to porosity and permeability. Cores of Fontainebleau sandstone were initially loaded to failure in a high-pressure gas–medium apparatus. The failed specimens were then hydrothermally reacted at 927°C for variable duration under isostatic conditions, and subsequently re-fractured to determine the ‘interseismic’ strength recovery. In the most extreme case, hydrothermally induced gouge compaction, cementation and crack healing resulted in 75% strength recovery after reaction for 6 h. Isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability. Strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity, consistent with previous studies on brittle failure of porous aggregates. The experimental results show how hydrothermal reactions in fault zones may lead to two competing, time-dependent effects; fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented, low-permeability gouge layer.

During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low-temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow-through experiments using deionized water and labradorite/quartz sand. All experiments were conducted in a modified triaxial apparatus, configured to allow continuous permeability measurements. Under most of the conditions tested, significant permeability reduction is observed with no concurrent decrease in porosity. The overall permeability reduction sometimes exceeds 1 order of magnitude over 4 days and is positively correlated to temperature and deviatoric stress. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. Si and Al concentrations in the postexperiment fluids are also correlated to temperature and stress, confirming the link between the chemical state of the system and permeability behavior. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times while dissolution of quartz and labradorite is most active. As the system approaches equilibrium, the components necessary for secondary mineral formation are liberated at a lower rate, thereby causing precipitation to slow. Although authigenic mineral formation does not reduce total pore space in these experiments, there is a reduction in effective porosity, which results in permeability reduction.

High-CO2 gas fields serve as important analogues for understanding various processes related to CO2 injection and storage, including assessing the relative importance of mineral precipitation and/or solution trapping efficiency. In this paper, we present a high resolution study focused on the Gorgon gas field and associated Rankin Trend gases on Australia’s North West Shelf. The gas data we present here display clear trends for CO2 abundance (mole %) CO2 and d13C CO2 both areally and vertically. Generally, CO2 mol % decreases and becomes depleted in d13C with decreasing depth and towards the north; a pattern which also holds true for the greater Rankin trend gases in general. We propose that these variations were driven by the precipitation of a carbonate phase, namely siderite, which is observed as a common late stage mineral. The results from this study have important implications for carbon storage operations and suggest that significant volumes of CO2 may be sequestered from a gas plume over short migration distances.

[1] The permeability (k) of fine-grained quartz aggregates were measured in situ during hot pressing (HPing) experiments to explore the evolution of fluid transport properties of fault zones during the interseismic period. Experiments were conducted at temperatures of 150°C and between 700 and 850°C, with confining and pore water pressures of 250 and 150 MPa, respectively. Significant permeability reduction was observed between 700 and 850°C, with permeability reduction rates (r = (1/t) ln (kto/kt)), ranging from approximately 6 × 10−5 s−1 at 700°C to a maximum of approximately 7.4 × 10−4 s−1 at 850°C. Permeability decreased exponentially with time, and the permeability reduction rate increased with increasing temperature, increasing differential stress, and decreasing grain size. Analysis of the permeability-porosity relationships indicates that permeability in the simulated gouge at high temperature shuts off at a critical porosity of 0.045 ± 0.004. The presence of microstructures, such as grain interpenetration, grain shape truncation, arrays of fluid inclusions, and development of quartz overgrowths on grains, indicate that k reduction was controlled by dissolution-precipitation creep processes. Extrapolation of the permeability reduction rates, measured in this study, to temperatures typical of the continental seismogenic regime highlights the strongly time-dependent nature of permeability in natural fault wear products at depths of nucleation of major earthquakes. Within the recurrence time of large earthquakes, quartz-rich fault zones in the fluid-active midcrustal to lower continental crustal regimes can evolve from high-permeability conduits to low-permeability seals. Episodic changes in the fluid transport properties of faults during the interseismic period are likely to impact on the pore pressure evolution of fault wear products.

[1] There is widespread evidence indicating that faults regain a portion of their strength during the interseismic period. Here, we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid-rock reactions in fault zones. The triaxial experiments consisted of fracturing cores of Fontainebleau sandstone under dry conditions, forming a localized shear failure zone (stage 1). The specimens were then reacted hydrothermally under isostatic conditions, allowing the fault damage zone to compact, consolidate and strengthen (stage 2). Following reaction, the specimens were then reloaded to failure under nominally dry conditions, so that the increase in cohesive strength of the fault could be measured (stage 3). Experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction. After 6 hours of reaction at the highest temperatures (927°C) and pore pressures (200 MPa), cohesion increases by as much as 35 MPa. Microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together, exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed. A theoretical treatment of the data was conducted using these experiments in combination with results on time-dependent changes in fault cohesion presented by Tenthorey et al. (2003). We find that the rate-controlling process in our experiments has an activation energy (Q) of approximately 70 kJ mol−1. We use this information to develop a model for time-dependent cohesive strengthening in fault zones within the continental seismogenic regime. We conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids.

We experimentally investigated the trace element compositions of fluids released during breakdown of subducted serpentinites. Serpentinites contain significant amounts of fluid-mobile elements such as B, Cs, As, and Ba, which during seafloor alteration are incorporated into mantle rocks. During the later high-pressure breakdown of the serpentinites, these trace elements are redistributed among the residual olivine, orthopyroxene, minor chlorite, and fluid. We find that B is far more compatible in these minerals than previously assumed; it has a fluid/residue partition coefficient (F/R D ) of 3–5. Most other fluid-mobile elements (Cs, As, Ba, Pb) are strongly enriched in the fluid and exhibit expected F/R D values of 30–250. Serpentinites are possibly the most important sink for B in subduction zones; the experimental results suggest that significant B is recycled into the deep mantle. Furthermore, high B concentrations in mantle olivines might be a fingerprint for previous metasomatism or serpentinization.

Fluid mobilization during prograde metamorphic reactions is a poorly understood, yet crucial, phenomenon that has implications for a number of geologic problems. Here we present the first experiments to characterize permeability evolution during dehydration of a natural rock. Dehydration of serpentinite specimens resulted in the generation of pore space and the rapid development of an interconnected pore network. During breakdown of antigorite to olivine + talc + H 2 O, permeability increased rapidly by at least three orders of magnitude. In natural systems, such an increase in permeability would be transitory, because high confining stresses would rapidly eliminate the porosity generated during reaction. Such transitory, reaction-enhanced permeability of serpentinite provides a mechanism by which fluids can migrate and possibly facilitate subduction-zone seismicity and contribute to partial melting of the mantle wedge. The results presented also have implications regarding middle- to deep-crustal fluid flow.

[1] A new technique is presented which allows easy identification of secondary minerals, formed during experimental diagenesis. Reacting fluids are doped with Ba and Sr; heavy metals which substitute into the alkali-bearing authigenic minerals that precipitate because of alteration of primary labradorite and quartz. The main secondary phases observed are a Ba-rich zeolite mineral, strontianite, and a Ba-rich calcite. When polished sections of postexperiment specimens are viewed using scanning electron microscope backscatter imaging, secondary phases appear as bright patches due to their higher density and average atomic number. The more difficult method of X-ray elemental mapping confirms that these zones represent secondary minerals rich in Ba and Sr. This technique allows the spatial distribution of secondary minerals to be mapped out at the specimen scale in a short time frame. Image analysis shows that observed reductions in permeability are caused primarily by mineral precipitation in narrow pore throats and in areas containing an abundance of fine-grained material. Secondary mineral formation is concentrated near the top of the sample and is interpreted to migrate through the sample as a reaction front. Mineral coverage is then quantified and used to explain the observed permeability evolution.

High- CO 2 gas fields serve as important analogues for understanding various processes related to CO 2 injection and storage. The chemical signatures, both within the fluids and the solid phases, are especially useful for elucidating preferred gas migration pathways and also for assessing the relative importance of mineral precipitation and/or solution trapping efficiency. In this paper, we present a high resolution study focused on the Gorgon gas field and associated Rankin Trend gases on Australia’s North West Shelf. The gas data we present here display clear trends for CO 2 abundance (mole %) and δ 13 C CO 2 both areally and vertically. The strong spatial variation of CO 2 content and δ 13 C and the interrelationship between the two suggests that processes were active to alter the two in tandem. We propose that these variations were driven by the precipitation of a carbonate phase, namely siderite, which is observed as a common late stage mineral. This conclusion is based on Rayleigh distillation modeling together with bulk rock isotopic analyses of core, which indicates that the late stage carbonate cements are related to the CO 2 in the natural gases. The results suggest that a certain amount of CO 2 may be sequestered in mineral form over short migration distances of the plume.

We propose a new model to describe precipitation-induced permeability reduction during chemical diagenesis of sandstones. The model has two parts: (1) geochemical model for determining the expected mass of precipitates and (2) physical model relating how a given mass of precipitates affects permeability. The model is used to study the experiments reported by Tenthorey et al. [this issue], where reactions involving dissolution of quartz and feldspar and precipitation of authigenic minerals were observed to cause drastic permeability reduction in feldspathic sandstone. We find that the geochemical evolution of the system dictates permeability evolution, and thus permeability dependence on temperature and stress is related to geochemical responses to these variables. The model predictions for the experimental system of Tenthorey et al. provide a very good fit to the experimental permeability curves over all times, stresses, and temperatures within the experimental range. Following the experiments and model, we suggest that precipitation of secondary mineral phases is an extremely efficient method for permeability reduction but one that depends critically on the system's approach to equilibrium. The reaction products of chemical diagenesis observed in rocks are the consequence of chemical reactions that occur in natural systems as they approach equilibrium, and we predict extensive and rapid permeability reduction as a result of such reactions.