Effects of episodic fluid flow on hydrocarbon migration in the Newport‐Inglewood Fault Zone,Southern California

Fault permeability may vary through time due to tectonic deformations, transients in pore pressure and effective stress, and mineralization associated with water‐rock reactions. Time‐varying permeability will affect subsurface fluid migration rates and patterns of petroleum accumulation in densely faulted sedimentary basins such as those associated with the borderland basins of Southern California. This study explores the petroleum fluid dynamics of this migration. As a multiphase flow and petroleum migration case study on the role of faults, computational models for both episodic and continuous hydrocarbon migration are constructed to investigate large‐scale fluid flow and petroleum accumulation along a northern section of the Newport‐Inglewood fault zone in the Los Angeles basin, Southern California. The numerical code solves the governing equations for oil, water, and heat transport in heterogeneous and anisotropic geologic cross sections but neglects flow in the third dimension for practical applications. Our numerical results suggest that fault permeability and fluid pressure fluctuations are crucial factors for distributing hydrocarbon accumulations associated with fault zones, and they also play important roles in controlling the geologic timing for reservoir filling. Episodic flow appears to enhance hydrocarbon accumulation more strongly by enabling stepwise build‐up in oil saturation in adjacent sedimentary formations due to temporally high pore pressure and high permeability caused by periodic fault rupture. Under assumptions that fault permeability fluctuate within the range of 1–1000 millidarcys (10^?15–10^?12 m²) and fault pressures fluctuate within 10–80% of overpressure ratio, the estimated oil volume in the Inglewood oil field (approximately 450 million barrels oil equivalent) can be accumulated in about 24 000 years, assuming a seismically induced fluid flow event occurs every 2000 years. This episodic petroleum migration model could be more geologically important than a continuous‐flow model, when considering the observed patterns of hydrocarbons and seismically active tectonic setting of the Los Angeles basin.     

An exhumed palaeo-hydrocarbon migration fairway in a faulted carrier system, Entrada Sandstone of SE Utah, USA

The Moab Anticline, east‐central Utah, is an exhumed hydrocarbon palaeo‐reservoir which was supplied by hydrocarbons that migrated from the Moab Fault up‐dip towards the crest of the structure beneath the regional seal of the Tidwell mudstone. Iron oxide reduction in porous, high permeability aeolian sandstones records the secondary migration of hydrocarbons, filling of traps against small sealing faults and spill pathways through the Middle Jurassic Entrada Sandstone. Hydrocarbons entered the Entrada Sandstone carrier system from bends and other leak points on the Moab Fault producing discrete zones of reduction that extend for up to 400 m from these leak points. They then migrated in focused stringers, 2–5 m in height, to produce accumulations on the crest of the anticline. Normal faults on the anticline were transient permeability barriers to hydrocarbon migration producing a series of small compartmentalized accumulations. Exsolution of CO₂ as local fault seals were breached resulted in calcite cementation on the up‐dip side of faults. Field observations on the distribution of iron oxide reduction and calcite cements within the anticline indicate that the advancing reduction fronts were affected neither by individual slip bands in damage zones around faults nor by small faults with sand: sand juxtapositions. Faults with larger throws produced either sand: mudstone juxtapositions or sand: sand contacts and fault zones with shale smears. Shale‐smeared fault zones provided seals to the reducing fluid which filled the structural traps to spill points.     

Influence of fault map resolution on pore pressure distribution and secondary hydrocarbon migration; Tune area,North Sea

Pressure and hydrocarbon migration modelling was carried out in the Tune Field area, Viking Graben, offshore Norway. The pressures are considered to be controlled by compartments bounded by mapped faults. Two different interpreted fault maps at the top reservoir level (Brent Group) are used as input to the modelling. First, a low‐resolution fault map is used, with only the large faults interpreted, and next, both large and small faults are included. The simulations show high overpressures generated in the western area, in the deeper part of the Viking Graben, and hydrostatic in the eastern areas. A sharp transition zone results from using the low‐resolution fault map in the simulations. Small N–S striking faults situated in between the wells have to have higher sealing capacity than expected from juxtaposition analysis alone, to be able to match the overpressures measured in well 30/5‐2 and 30/8‐1S in the Tune Field, and well 30/8‐3 east of Tune. The intermediate pressure in the western part is probably related to flow in the deeper parts of the sedimentary column in the compartment, where well 30/8‐3 is situated. The secondary oil migration models show that overpressures have major effects on the migration pathways of hydrocarbons. The level of detail in the fault interpretation is important for simulation results, both for pressure distribution and for hydrocarbon migration.     

Downward hydrocarbon migration predicted from numerical modeling of fluid overpressure in the Paleozoic Anticosti Basin,eastern Canada

The Anticosti Basin is a large Paleozoic basin in eastern Canada where potential source and reservoir rocks have been identified but no economic hydrocarbon reservoirs have been found. Potential source rocks of the Upper Ordovician Macasty Formation overlie carbonates of the Middle Ordovician Mingan Formation, which are underlain by dolostones of the Lower Ordovician Romaine Formation. These carbonates have been subjected to dissolution and dolomitization and are potential hydrocarbon reservoirs. Numerical simulations of fluid‐overpressure development related to sediment compaction and hydrocarbon generation were carried out to investigate whether hydrocarbons generated in the Macasty Formation could migrate downward into the underlying Mingan and Romaine formations. The modeling results indicate that, in the central part of the basin, maximum fluid overpressures developed above the Macasty Formation due to rapid sedimentation. This overpressured core dissipated gradually with time, but the overpressure pattern (i.e. maximum overpressure above source rock) was maintained during the generation of oil and gas. The downward impelling force associated with fluid‐overpressure gradients in the central part of the basin was stronger than the buoyancy force for oil, whereas the buoyancy force for gas and for oil generated in the later stage of the basin is stronger than the overpressure‐related force. Based on these results, it is proposed that oil generated from the Macasty Formation in the central part of the basin first moved downward into the Mingan and Romaine formations, and then migrated laterally up‐dip toward the basin margin, whereas gas throughout the basin and oil generated in the northern part of the basin generally moved upward. Consequently, gas reservoirs are predicted to occur in the upper part of the basin, whereas oil reservoirs are more likely to be found in the strata below the source rocks. Geofluids (2010) 10 , 334–350     

Unravelling the petroleum system by enhancing fluid migration paths in seismic data using a neural network based pattern recognition technique

Understanding the hydrocarbon migration system in the sub‐surface is a key aspect of oil and gas exploration. It is well known that conventional 3D seismic data contains information about hydrocarbon accumulations. Less known is the fact that 3D seismic data also contains information about hydrocarbon migration paths in the form of vertical noise trails. A method has been developed to highlight vertical noise trails in seismic data semi‐automatically, using assemblies of directive multi‐trace seismic attributes and neural network technology. The results of this detection method yield valuable information about the origin of hydrocarbons, about migration paths from source to prospect and about leakage or spillage from these prospects to shallow gas pockets or to the sea bed. Besides, the results reveal the sealing quality of faults, provide information on overpressure and whether prospects are charged or not. All these aspects are useful information for basin modelling studies and for an increased understanding of the petroleum system.     

Equations of state for basin geofluids: algorithm review and intercomparison for brines

Physical properties of formation waters in sedimentary basins can vary by more than 25% for density and by one order of magnitude for viscosity. Density differences may enhance or retard flow driven by other mechanisms and can initiate buoyancy‐driven flow. For a given driving force, the flow rate and injectivity depend on viscosity and permeability. Thus, variations in the density and viscosity of formation waters may have or had a significant effect on the flow pattern in a sedimentary basin, with consequences for various basin processes. Therefore, it is critical to correctly estimate water properties at formation conditions for proper representation and interpretation of present flow systems, and for numerical simulations of basin evolution, hydrocarbon migration, ore genesis, and fate of injected fluids in sedimentary basins. Algorithms published over the years to calculate water density and viscosity as a function of temperature, pressure and salinity are based on empirical fitting of laboratory‐measured properties of predominantly NaCl solutions, but also field brines. A review and comparison of various algorithms are presented here, both in terms of applicability range and estimates of density and viscosity. The paucity of measured formation‐water properties at in situ conditions hinders a definitive conclusion regarding the validity of any of these algorithms. However, the comparison indicates the versatility of the various algorithms in various ranges of conditions found in sedimentary basins. The applicability of these algorithms to the density of formation waters in the Alberta Basin is also examined using a high‐quality database of 4854 water analyses. Consideration is also given to the percentage of cations that are heavier than Na in the waters.     

Massive dolomitization of a Messinian reef in the Great Bahama Bank: a numerical modelling evaluation of Kohout geothermal convection

The hypothesis that Kohout thermal convection may have induced the massive dolomitization of the 60 m thick lowest more reefal unit in well Unda [top of Great Bahama Bank (GBB)] is evaluated through numerical modelling. A two‐dimensional (2‐D) section, including lithological and petrophysical data, together with datings for the sediments of the GBB, was used in the basin model TEMISPACK to reconstruct the history of the whole platform, with a focus on the reef unit. Simulations showed that during high sea‐level periods, Kohout convection is a valid mechanism in the settings of the GBB, although the convection cell remains flat in most cases because of high permeability anisotropy. This mechanism induces rapid fluid flow in the superficial as well as in the deeper parts of the platform, with velocities of at least two orders of magnitude higher than with compaction alone. Lithology appears as a strong control of fluid circulations at the margin scale through the permeability anisotropy, for which a critical value lies between values of 10 and 100. The reefal unit in Unda is part of a larger area determined by the lithologic distribution, in which flow velocities are significantly higher than in the rest of the platform. These velocities are high enough to bring the magnesium necessary to precipitate the observed amounts of dolomite, within durations in agreement with the available time of post‐reef deposition high sea level(s). However, neither fluid flow pattern nor flow velocities are able to explain the preferential massive dolomitization of the lower reef unit and the complete absence of dolomite in the upper one.     

Seismic evidence for fluid migration pathways from an overpressured system in the South China Sea

Overpressured systems and intense, anomalously hot fluid expulsion in the Yinggehai Basin of the South China Sea offer an opportunity to understand the history of fluid flow and the process of hydrocarbon accumulation in overpressured environments. Fluid migration pathways from overpressured compartments in the basin are largely controlled by the distribution of faults and fractures. Episodic opening of these faults are related to the dynamics of an overpressured system and tectonic movements during basin evolution. At the crests of diapiric structures, fluid expulsion is seismically imaged as chimney‐ or plume‐like features, low to middle seismic amplitudes, and intermittently chaotic and blank reflecting seismic facies. These fluid pathways are controlled by vertical faults, which commonly penetrate overpressured and overlying normally pressured zones. Fluid expulsion is also observed near the main faults, such as the No. 1 Fault at the north‐eastern margin of the basin. Investigation by sidescan sonar on onshore and offshore Hainan Island indicates that there are more than 100 gas seepages adjacent to the No. 1 Fault. Migration pathways in the diapiric structures are controlled by three types of fault and fracture. Penetrative faults formed by dextral strike‐slip movement of the Red River faults commonly occur in the centre of the diapirs, and may have been a triggering factor for the diapirism, and controlled their distribution. Hydrofractures occur in certain mud‐rich layers and may have been generated by hydraulic fracturing. Radial normal faults occur at the top of diapirs and were formed by the intrusive process. These fluid migration pathways played an important role in regional hydrocarbon accumulation.     

Significance of large-scale sand injectites as long-term fluid conduits: evidence from seismic data

Sand injectites and related features that are interpreted to have formed by large‐scale, often sudden, fluid escape in the shallow (typically <500 m) crust are readily imaged on modern seismic data. Many of the features have geometrical similarity to igneous dykes and sills and cross‐cut the depositional stratigraphy. Sand injectites may be multiphase and form connected, high‐permeability networks that transect kilometre‐scale intervals of otherwise fine‐grained, low‐permeability strata. North Sea examples often form significant hydrocarbon reservoirs and typically contain degraded, low‐gravity crude oil. Fluid inclusion and stable isotope data from cements in sand injectites record a mixing of aqueous fluids of deep and shallow origin.     

Topography‐driven flow versus buoyancy‐driven flow in the U.S. midcontinent: implications for the residence time of brines

Topography‐driven flow is normally considered to be the dominant groundwater flow system in uplifted sedimentary basins. In the U.S. midcontinent region east of the Rocky Mountains, the presence of brines derived from dissolution of halite suggests that significant topography‐driven flushing has occurred to remove older brines that presumably formed concurrently with Permian evaporites in the basin. However, the presence of evaporites and brines in the modern basin suggests that buoyancy‐driven flow could limit topography‐driven flushing significantly. Here we used numerical models of variable‐density fluid flow, halite dissolution, solute transport, and heat transport to quantify flow patterns and brine migration. Results indicate the coexistence of large‐scale topography‐ and buoyancy‐driven flow. Buoyancy‐driven flow and low permeability evaporites act to isolate brines, and the residence time of the brines was found to be quite long, at least 50 Myr. The modern distribution of salinity appears to reflect near‐steady‐state conditions. Results suggest that flushing of original evaporatively‐concentrated brines occurred tens of millions of years ago, possibly concurrent with maximum uplift ca. 60 Ma. Simulations also suggest that buoyancy‐driven convection could drive chemical exchange with crystalline basement rocks, which could supply significant Ca²⁺, Sr²⁺, and metals to brines.     

Physical properties of carbonate fault rocks,fucino basin (Central Italy): implications for fault seal in platform carbonates

We documented the porosity, permeability, pore geometry, pore type, textural anisotropy, and capillary pressure of carbonate rock samples collected along basin‐bounding normal faults in central Italy. The study samples consist of one Mesozoic platform carbonate host rock with low porosity and permeability, four fractured host rocks of the damage zones, and four fault rocks of the fault cores. The four fractured samples have high secondary porosity, due to elongated, connected, soft pores that provide fluid pathways in the damage zone. We modeled this zone as an elastic cracked medium, and used the Budiansky–O'Connell correlation to compute its permeability from the measured elastic moduli. This correlation can be applied only to fractured rocks with large secondary porosity and high‐aspect ratio pores. The four fault rock samples are made up of survivor clasts embedded in fine carbonate matrices and cements with sub‐spherical, stiff pores. The low porosity and permeability of these rocks, and their high values of capillary pressure, are consistent with the fault core sealing as much as 77 and 140 m of gas and oil columns, respectively. We modeled the fault core as a granular medium, and used the Kozeny–Carmen correlation, assigning the value of 5 to the Kozeny constant, to compute its permeability from the measured porosities and pore radii. The permeability structure of the normal faults is composed of two main units with unique hydraulic characteristics: a granular fault core that acts as a seal to cross‐fault fluid flow, and an elastic cracked damage zone that surrounds the core and forms a conduit for fluid flow. Transient pathways for along‐fault fluid flow may form in the fault core during seismic faulting due to the formation of opening‐mode fractures within the cemented fault rocks.     

Hydrologic and geochemical controls on soluble benzene migration in sedimentary basins

The effects of groundwater flow and biodegradation on the long‐distance migration of petroleum‐derived benzene in oil‐bearing sedimentary basins are evaluated. Using an idealized basin representation, a coupled groundwater flow and heat transfer model computes the hydraulic head, stream function, and temperature in the basin. A coupled mass transport model simulates water washing of benzene from an oil reservoir and its miscible, advective/dispersive transport by groundwater. Benzene mass transfer at the oil–water contact is computed assuming equilibrium partitioning. A first‐order rate constant is used to represent aqueous benzene biodegradation. A sensitivity study is used to evaluate the effect of the variation in aquifer/geochemical parameters and oil reservoir location on benzene transport. Our results indicate that in a basin with active hydrodynamics, miscible benzene transport is dominated by advection. Diffusion may dominate within the cap rock when its permeability is less than 10^?19 m². Miscible benzene transport can form surface anomalies, sometimes adjacent to oil fields. Biodegradation controls the distance of transport down‐gradient from a reservoir. We conclude that benzene detected in exploration wells may indicate an oil reservoir that lies hydraulically up‐gradient. Geochemical sampling of hydrocarbons from springs and exploration wells can be useful only when the oil reservoir is located within about 20 km. Benzene soil gas anomalies may form due to regional hydrodynamics rather than separate phase migration. Diffusion alone cannot explain the elevated benzene concentration observed in carrier beds several km away from oil fields.     

Rural Youth Migration Trends in Australia: an Overview of Recent Trends and Two Inland Case Studies

Much of what has been written on the topic of Australian rural youth migration trends and processes has often proceeded from data‐free, or data‐poor grounds. In this context, this paper analyses recent trends in youth (15 to 24 years of age) migration for a temporally‐consistent set of Statistical Divisions (SDs) in inland rural Australia, and for local government areas within the Northern Tablelands and Slopes and Ranges of northern New South Wales and the Western Australian Central Wheatbelt. The paper finds that rates of youth loss from rural regions have increased over the past twenty years. Yet the patterns, processes, causes and impacts of rural youth migration are distributed in a spatially‐uneven fashion. Some remote areas are receiving net migration gains while booming ‘sea change’ coastal regions have experienced heavy losses. While the ‘flight to the bright city lights’ syndrome is evident, relatively high proportions of young people in the Northern SD of NSW move within their immediate region. Nevertheless, some common understandings concerning youth mobility were also confirmed. Gender differentials in migration propensity between women and men are evident even at quite local scales. Young people are also more likely to search out capital cities than the rest of the population. Most inland areas still continue to experience heavy losses of local youth. A more precise understanding of rural youth migration trends is an important stepping stone in the establishment of a reinvigorated research effort into young rural people's perspectives of their changing life chances in their home communities.     

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin,UK

Calcite veins at outcrop in the Mesozoic, oil‐bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin‐scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO₂ from petroleum source rocks. Veins become progressively enriched in source‐rock‐derived CO₂ from the outer margin towards the middle, indicating a growing influence of external CO₂. The carbon isotope data suggest large‐scale migration of substantial amounts of CO₂ around the whole basin. Fluid inclusion salinity data and interpreted water‐δ¹⁸O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite‐rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large‐scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source‐rock‐derived CO₂ seems to have permeated the entire section, water movement has been more restricted. Oil‐filled inclusions in vein calcite have been found within dominant E‐W trending normal faults, suggesting that these may have facilitated oil migration.     

Migration of Mn-rich fluids through normal faults and fine-grained terrigenous sediments during early development of the Neogene Vallès-Penedès half-graben (NE Spain)

The Miocene siliciclastic sediments infilling the Vallès‐Penedès half‐graben are affected by two sets of structures developed during the extensional tectonics that created the basin. The first set, represented by extension fractures infilled with mud and sands, is attributed to seismically induced liquefaction. The second set, represented by normal faults, corresponds to a high‐permeability horsetail extensional fracture mesh developed near the surface in the hanging walls of normal faults. The incremental character of the vein‐fills indicates episodic changes in the tectonic stress state and fault zone permeability. Two episodes of fluid migration are recorded. The first episode occurred prior to consolidation and lithification when shallow burial conditions allowed oxidizing meteoric waters to flow horizontally through the more porous and permeable sandy layers. Development of clastic dikes allowed local upward flow and dewatering of the sandy beds. Liquefaction and expulsion of fluids were probably driven by seismic shaking. During the first episode of fluid migration there was no cementation of the sandstone or within the fractures, probably because little fluid was mobilized by the predominantly compaction‐driven flow regime. The second episode of fluid migration occurred synchronously with normal fault development, during which time the faults acted as fluid conduits. Fluids enriched in manganese, probably leached from local manganese oxyhydroxides soon after sedimentation, moved laterally and produced cementation in the sandstone layers, eventually arriving at the more porous and permeable fault pathways that connected compartments of different porosities and permeabilities. Carbonate probably precipitated in fractures saturated with meteoric water near the ground surface at a transitional redox potential. Once the faults became occluded by calcite cement, shortly after fault development, they became barriers to both vertical and horizontal fluid flow.     

Hydrothermal fluid flow models of stratiform ore genesis in the McArthur Basin, Northern Territory, Australia

This paper explores the role of basin‐scale fluid migration in stratiform Pb–Zn ore formation in the southern McArthur Basin, Australia. Mathematical models are presented for coupled brine migration and heat transport in the basin. The models account for: (i) topographically driven flow (forced convection) during periods when parts of the McArthur Basin were subaerial and elevated above the central Batten Fault Zone; (ii) density‐driven flow (free convection) during periods when the basin was mostly submarine; and (iii) transient flows associated with fault rupture during periods of transpression. These hydrologic models help to compare and contrast a variety of hypotheses concerning deep fluid migration and the origin of base metal ores in the McArthur Basin. The numerical results exhibit a strong structural control on fluid flow caused by the north‐trending fault systems that characterize the Batten Fault Zone. As a result, fluids descend to depths of a few kilometers along the western side, migrate laterally to the east through the clastic and volcanic aquifers of the upper Tawallah and lowest McArthur Groups, and then ascend along the eastern side of the fault zone. This recharge–discharge pattern dominates all of the hydrogeologic models. The basin‐wide flow pattern suggests that Na–Ca–Cl brines acquired base metals in the deepest levels of the basin stratigraphy as the fluids migrated eastwards through the aquifer system. Upward flow was relatively rapid along the Emu Fault Zone, so much so that fluid temperatures likely approached 130°C in the muddy sediments near the sea floor due to upward flow and venting at the HYC (‘Here’s Your Chance'). Transient pulses of flow characterized periods of transpressional stress and subsequent faulting may have punctuated the basin history. Large‐scale free convection, however, characterized notably long periods of diagenesis and ore mineralization during the Proterozoic in the McArthur Basin.     

Faults and fault properties in hydrocarbon flow models

The petroleum industry uses subsurface flow models for two principal purposes: to model the flow of hydrocarbons into traps over geological time, and to simulate the production of hydrocarbon from reservoirs over periods of decades or less. Faults, which are three-dimensional volumes, are approximated in both modelling applications as planar membranes onto which predictions of the most important fault-related flow properties are mapped. Faults in porous clastic reservoirs are generally baffles or barriers to flow and the relevant flow properties are therefore very different to those which are important in conductive fracture flow systems. A critical review and discussion is offered on the work-flows used to predict and model capillary threshold pressure for exploration fault seal analysis and fault transmissibility multipliers for production simulation, and of the data from which the predictions derive. New flow simulation models confirm that failure of intra-reservoir sealing faults can occur during a reservoir depressurization via a water-drive mechanism, but contrary to anecdotal reports, published examples of production-induced seal failure are elusive. Ignoring the three-dimensional structure of fault zones can sometimes have a significant influence on production-related flow, and a series of models illustrating flow associated with relay zones are discussed.     

Fluids,basin analysis,and mineral deposits

Sedimentary basins are the largest structures on the surface of our planet and the most significant sources of energy‐related commodities. With time, sedimentary successions in basins normally are subjected to increasingly intense diagenesis that results in differential evolution of basin hydrology. This hydrologic structure is in turn vitally important in determining how and where deposition of metals may occur. Fluids in all basins originate and flow as a result of sedimentological and tectonic events, so that fluid histories should reflect the control of both lithology and tectonism on ore deposition. Sandstone lithologies, in particular, reflect fluid‐flow events because they are normally the major aquifers in basins. However, early cementation results in occlusion of primary permeability in some facies (diagenetic aquitards) whereas in others, permeability develops due to the dissolution of unstable grains (diagenetic aquifers). Particularly for ore deposits in Precambrian basins, identification of paleohydrologic systems during basin evolution requires the integration of data derived from tectonics, sedimentology, stratigraphy, diagenesis, geochemistry and geology. Assessment of all these data is a prerequisite for the ‘holistic basin analysis’ needed to guide the search for basin‐hosted ores. Recent results from the Paleoproterozoic Mt Isa and McArthur basins in northern Australia serve as a template for exploring for mineral deposits in basins. Basinal fluids were saline, 200–300°C and evolved primarily from meteoric water in the Mt Isa Basin and from seawater in the McArthur Basin during burial to depths of 4–12 km. The δD_fluid and δ¹⁸O_fluid values in these brines were isotopically identical to those in the Zn‐Pb, Cu and U deposits. Geochemical changes of various lithologies during alteration support detrital minerals as the major source of the U, and volcanic units proximal to diagenetic aquifers as a source for the transition metals. Ages of diagenetic phases extracted from aquifer lithologies reveal that fluid migration from the diagenetic aquifers effectively covers the period of formation for U, Zn‐Pb and Cu mineralization, and that the deposits formed in response to tectonic events reflected in the apparent polar wandering path for the area. Sequence stratigraphic analysis and models of fluid flow also indicate that basinal reservoirs were likely sources for the mineralizing fluids. Thus, diagenetic aquifer lithologies were being drained of fluids at the same time as the deposits were forming from fluids that were chemically and isotopically similar, linking diagenesis and fluid events within the basin to the formation of the deposits.     

Asian migration to Australia: Changing trends and implications

International migration between Asia and Australia has undergone profound change in the last two decades. This paper outlines the major changes which have occurred in this movement. The proportion of Australia's resident population born in Asia has increased from 1.1 % in 1976 to 5.5 % in 2001. Asians now make up more than a third of new settlers to Australia but there has been an increased diversity in the flow of Asian settlers to Australia as well as the substantial increase in numbers. Australian international migration has entered a new era since the mid‐1990s with a greatly increased flow of temporary workers to Australia. The inflow of these workers is less dominated by Asia‐born people than is the permanent flow except for student migration. There is also an increasing flow of movers from Australia to Asia. Much current knowledge and research remain focused on Asian permanent settlement in Australia. While this is important and worthwhile, there is a need to broaden the work to encompass the full range of population movements between Asia and Australia if the impact of international migration is to be adequately assessed.     

Modeling CO2 generation,migration, and titration in sedimentary basins

High mole fraction CO₂ gases pose a significant risk to hydrocarbon exploration in some areas. The generation and movement of CO₂ are also of scientific interest, particularly because CO₂ is an important greenhouse gas. We have developed a model of CO₂ generation, migration, and titration in basins in which a high mole fraction CO₂ gas is generated by the breakdown of siderite (FeCO₃) and magnesite (MgCO₃) where parts of the basin are being heated above approximately 330°C. The CO₂ reacts with Fe‐, Mg‐, and Ca‐silicates as it migrates upward and away from the generation zone (CO₂‐kitchen). Near the kitchen, where the Fe‐, Mg‐, and Ca‐silicates have been titrated and destroyed by previous packets of migrating CO₂, gas moves upward without lowering its CO₂ mole fraction. Further on, where Fe‐ and Mg‐silicates are still present but Ca‐silicates are absent in the sediments, the partial pressure of CO₂ is constrained to 0.1–30 bars and reservoirs contain a few mole percent CO₂ as described by Smith & Ehrenberg (1989) . Still further from the source, where Ca‐silicates have not been titrated, partial pressure of CO₂ in migrating methane gas are orders of magnitude lower. A 2D numerical model of CO₂ generation, migration, and titration quantifies these buffer relations and makes predictions of CO₂ risk in the South China Sea that are compatible with exploration experience. Reactive CO₂ transport models of the kind described could prove useful in determining how gases migrate in faulted sedimentary basins.