Groundwater age,brine migration,and large‐scale solute transport in the Alberta Basin,Canada

There is a great contrast in geochemical and hydrogeologic estimates of the residence times of pore fluids in sedimentary basins. This contrast is particularly evident in the Alberta Basin, Canada, which has served as the study area for important studies of long‐term fluid flow and transport. To address these differences, we developed two‐dimensional simulations of groundwater age, constrained by both hydrogeologic and geochemical observations, to estimate the residence time of fluids and the amount and timing of flushing by meteoric waters in the Alberta Basin. Results suggest that old, residual brines have been retained in the deepest parts of the basin since their formation ca. 400 Ma, but significant dilution by younger waters has reduced the age of these pore waters to no more than approximately 200 My. Shallower formations have been flushed extensively by fresh, young waters. Loss of brines and dilution of older pore waters occurred primarily after the uplift of the Rockies with the introduction of the gravity‐driven flow regime. Despite these large changes in flow regime, solute exchange between deep saline aquifers and the overlying vigorous freshwater flow system was found to be consistent with long‐term dispersive mixing across subhorizontal concentration gradients rather than by direct flushing. Sensitivity studies using an analytic solution supported the use of 100 m for transverse dispersivity in large‐scale numerical models. These simulations confirm that the age and origin of brines are in many cases poor indicators of long‐term solute transport rates in sedimentary basins, but the geochemical indicators that are used to determine the origin of brines can provide useful constraints for calculating groundwater age and are far more commonly available than isotopic groundwater age tracers.     

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.     

Impacts of Pleistocene glaciation on large‐scale groundwater flow and salinity in the Michigan Basin

Pleistocene melting of kilometer‐thick continental ice sheets significantly impacted regional‐scale groundwater flow in the low‐lying stable interiors of the North American and Eurasian cratons. Glacial meltwaters penetrated hundreds of meters into the underlying sedimentary basins and fractured crystalline bedrock, disrupting relatively stagnant saline fluids and creating a strong disequilibrium pattern in fluid salinity. To constrain the impact of continental glaciation on variable density fluid flow, heat and solute transport in the Michigan Basin, we constructed a transient two‐dimensional finite‐element model of the northern half of the basin and imposed modern versus Pleistocene recharge conditions. The sag‐type basin contains up to approximately 5 km of Paleozoic strata (carbonates, siliciclastics, and bedded evaporites) overlain by a thick veneer (up to 300 m) of glacial deposits. Formation water salinity increases exponentially from <0.5 g l^?1 total dissolved solids (TDS) near the surface to >350 g l^?1 TDS at over 800 m depth. Model simulations show that modern groundwater flow is primarily restricted to shallow glacial drift aquifers with discharge to the Great Lakes. During the Pleistocene, however, high hydraulic heads from melting of the Laurentide Ice Sheet reversed regional flow patterns and focused recharge into Paleozoic carbonate and siliciclastic aquifers. Dilute waters (<20 g l^?1 TDS) migrated approximately 110 km laterally into the Devonian carbonate aquifers, significantly depressing the freshwater‐saline water mixing zones. These results are consistent with ¹⁴C ages and oxygen isotope values of confined groundwaters in Devonian carbonates along the basin margin, which reflect past recharge beneath the Laurentide Ice Sheet (14–50 ka). Constraining the paleohydrology of glaciated sedimentary basins, such as the Michigan Basin, is important for determining the source and residence times of groundwater resources, in addition to resolving geologic forces responsible for basinal‐scale fluid and solute migration.     

Modeling thermal convection in supradetachment basins: example from western Norway

The juxtaposition of fault‐bounded sedimentary basins, above crustal‐scale detachments, with warmer exhumed footwalls can lead to thermal convection of the fluids in the sediments. The Devonian basins of western Norway are examples of supradetachment basins that formed in the hanging wall of the Nordfjord‐Sogn Detachment Zone. In the central part of the Hornelen and Kvamshesten basins, the basin‐fill is chiefly represented by fluvial sandstones and minor lacustrine siltstones, whereas the fault margins are dominated by fanglomerates along the detachment contact. Prominent alteration and low‐greenschist facies metamorphic conditions are associated with the peak temperature estimates of the sediments close to the detachment shear zone. Fluid circulation may have been active during the burial of the sediments, and we quantify the potential role played by thermal convection in redistributing heat within the basins. Different models are tested with homogeneous and layered basin‐fill and with material transport properties corresponding to sandstones and siltstones. We found that thermally driven fluid flow is expected in supradetachment basins as a transient process during the exhumation of warmer footwalls. We demonstrate that the fluid flow may have significantly affected the temperature distribution in the upper five kilometers of the Devonian basins of western Norway. The temperature anomaly induced by the flow may locally reach about 80°C. The sedimentary layering formed by sand‐ and siltstones strata does not inhibit fluid circulation at the scale of the basin. The presence of fluid pathways along the detachment has an important impact on the flow and allows an efficient drainage of the basin by channelizing fluids upward along the detachment.     

Basin‐Wide Sediment Grain‐Size Numerical Analysis and Paleo‐Climate Interpretation in the Shiyang River Drainage Basin

Basin‐wide sediment transport affects estimates of basin sediment yield, which is a fundamental scientific issue in drainage basin studies. Many studies have been conducted to examine erosion and deposition rates in drainage networks. In this study, we proposed a new approach using grain‐size standard deviation model of sedimentary samples from different geomorphological units for numerical analysis and paleo‐climate interpretation in the Shiyang River drainage basin, arid China. 1043 sedimentary samples were obtained from the upper reaches, the midstream alluvial plain and the terminal lake area; chronological frames were established based on 58 radiocarbon ages. Grain‐size standard deviation model was introduced to examine sediment components according to grain‐size and transport forces. In addition, transient paleo‐climate simulations, including the Community Climate System Model version 3 and the Kiel models, were synthesized, as well as the results from PMIP 3.0 project, to detect the long‐term climate backgrounds. Totally, we found four major common components, including fine particulates (<2 μm), fine silt (2–20 μm), sandy silt (20–200 μm), coarse sand (>200 μm), from basin‐wide sedimentary samples. The fine particulates and fine silt components exist in all the sedimentary facies, showing long‐term airborne aerosol changes and its transport by suspended load. There are some differences in ranges of sandy silt and coarse sand components, due to lake and river hydrodynamics, as well as the distance with the Gobi Desert. Paleo‐climate simulations have shown that the strong Asian summer monsoon during the transition of the Last Deglaciation and Holocene was conducive to erosion and transport of basin‐wide suspended load, also enhancing sediment sorting effects due to strong lake hydrodynamics. Our findings provide a new approach in research of long‐term basin‐wide sediment transport processes.     

Abnormally pressured beds as barriers to diffusive solute transport in sedimentary basins

Diffusion can drive significant solute transport over millions of years, but ancient brines and large salinity gradients are still observed in deep sedimentary basins. Fluid flow within abnormally pressured beds may prevent diffusive transfer over geologically significant periods, if the abnormally pressured bed is surrounded by normally pressured beds. Analytic solutions based on sediment loading and unloading demonstrate that this effect should be considered in beds with a compressibility exceeding 10^?8 Pa^?1, with a thickness of 100 m or more, or a sedimentation rate exceeding 10^?5 m year^?1. Conditions favourable for our model of abnormally pressured beds appear common in sedimentary basins. Large salinity gradients associated with clay beds have previously been attributed to membrane effects, but flow patterns associated with abnormally pressured beds appear more robust in the presence of heterogeneity and discontinuities than membrane effects. Calculations suggest that thick underpressured shales in the Alberta basin may have allowed ancient evaporatively concentrated brines to be preserved beneath a vigorous topography‐driven flow system over the last 60 My. In the Illinois basin, drained overpressured beds may have limited solute transport across the New Albany shale until approximately 250 Ma. It is unlikely, however, that overpressures could have persisted long enough to explain concentration gradients observed in the modern basin. These gradients may instead reflect relatively recent halite dissolution above the New Albany shale.     

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.     

Diverse origins and timing of formation of basinal brines in the Gulf of Mexico sedimentary basin

A review of five different field areas in the Gulf of Mexico sedimentary basin (GOM) illustrates some of the potentially diverse chemical and physical processes which have produced basinal brines. The elevated salinities of most of the formation waters in the GOM are ultimately related to the presence of the Middle Jurassic Louann Salt. Some of these brines likely inherited their salinity from evaporated Mesozoic seawater, while other saline fluids have been produced by subsequent dissolution of salt, some of which is occurring today. The timing of the generation of brines has thus not been restricted to the Middle Jurassic. The mechanisms of solute transport that have introduced brines throughout much of the sedimentary section of the GOM are not entirely understood. Free convection driven by spatial variations in formation water temperature and salinity is undoubtedly occurring around some salt structures. However, the driving mechanisms for the broad, diffusive upward solute transport in the northern Gulf rim of Arkansas and northern Louisiana are not known. In the Lower Cretaceous of Texas, fluid flow was much more highly focused, and perhaps episodic. It is clear that many areas of the Gulf basin are hydrologically connected and that large‐scale fluid flow, solute transport, and dispersion have occurred. The Na‐Mg‐Ca‐Cl compositions of brines in the areas of the Gulf Coast sedimentary basin reviewed in this article are products of diagenesis and do not reflect the composition of the evaporated marine waters present at the time of sediment deposition. Large differences in Na, Ca, and Mg trends for waters hosted by Mesozoic versus Cenozoic sediments may reflect differences in: (i) the sources of salinity (evaporated seawater for some of the Mesozoic sediments, dissolution of salt for some of the Cenozoic sediments); (ii) sediment lithology (dominantly carbonates for much of the Mesozoic sediments, and dominantly siliciclastics for the Cenozoic sediments); or (iii) residence times of brines associated with these sediments (tens of millions of years versus perhaps days).     

A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA

This paper presents a suite of two‐dimensional mathematical models of basin‐scale groundwater flow and heat transfer for the middle Proterozoic Midcontinent Rift System. The models were used to assess the hydrodynamic driving mechanisms responsible for main‐stage stratiform copper mineralization of the basal Nonesuch Formation during the post‐volcanic/pre‐compressional phase of basin evolution. Results suggest that compaction of the basal aquifer (Copper Harbor Formation), in response to mechanical loading during deposition of the overlying Freda Sandstone, generated a pulse of marginward‐directed, compaction‐driven discharge of cupriferous brines from within the basal aquifer. The timing of this pulse is consistent with the radiometric dates for the timing of mineralization. Thinning of the basal aquifer near White Pine, Michigan, enhanced stratiform copper mineralization. Focused upward leakage of copper‐laden brines into the lowermost facies of the pyrite‐rich Nonesuch Formation resulted in copper sulfide mineralization in response to a change in oxidation state. Economic‐grade mineralization within the White Pine ore district is a consequence of intense focusing of compaction‐driven discharge, and corresponding amplification of leakage into the basal Nonesuch Formation, where the basal aquifer thins dramatically atop the Porcupine Mountains volcanic structure. Equilibrium geochemical modeling and mass‐balance calculations support this conclusion. We also assessed whether topography and density‐driven flow systems could have caused ore genesis at White Pine. Topography‐driven flow associated with the Ottawan orogeny was discounted because it post‐dates main‐stage ore genesis and because recent seismic interpretations of basin inversion indicates that basin geometry would not be conductive to ore genesis. Density‐driven flow systems did not produce focused discharge in the vicinity of the White Pine ore district.     

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.     

Palaeo‐formation water evolution in the Latrobe aquifer,Gippsland Basin,south‐eastern Australia continental shelf

Offshore fresh or brackish groundwater has been observed around the globe and represents an interesting but unusual freshwater reserve. Formation waters in sedimentary basins evolve at geological time through fluid–rock interactions and water movements in aquifers. However, the mechanism and timing of freshwater displacing and mixing with pre‐existing formation water offshore under the seafloor has not been investigated in many cases. The growing need for developing freshwater resources in deeper parts of sedimentary basins that have not been economic or technically feasible in the past, may potentially lead to an increasing conflict with petroleum production or injection of carbon dioxide. For being able to assess and mitigate possible impacts of fluid production or injection on groundwater flow and quality, a better understanding of the natural history of the interaction between fresh meteoric water and deep basin formation water is necessary. A low‐salinity wedge of meteoric origin with less than 5000 ppm currently extends to about 20 km offshore in the confined Latrobe aquifer in the Gippsland Basin (Australia). The Latrobe aquifer is a freshwater resource in the onshore, hosts major petroleum reservoirs and has been considered for carbon dioxide storage in the offshore parts of the basin. The objective of this study is to constrain the evolution of formation water in the Latrobe aquifer by investigating the water naturally trapped in fluid inclusions during burial. The measured palaeo‐salinities from onshore and offshore rock samples have a minimum of about 12 500 ppm (NaCl equivalent) and a maximum of about 50 000 ppm. Most of the salinities are in the 32 000–35 000 ppm range. There is no evidence for freshwater in fluid inclusions and the variation in palaeo‐salinity across the basin is consistent with the palaeogeography of deposition of the sedimentary rocks. The current low‐salinity water wedge must have started to form recently after most of the diagenetic processes that led to the trapping of water in fluid inclusions happened. The minimum homogenisation temperatures (T_h) recorded are consistent with current formation temperature. However, they are generally higher than present day suggesting that hotter temperatures were attained in the past. The T_h and salinity data together suggest that the fluid inclusions record the diagenetic modification of connate water to higher salinities over a time period that was accompanied by an increase in temperature, consistent with a westward palaeo‐fluid flow from the deeper part of the basin through the aquifer. Subsequent pore‐water evolution from palaeo‐ to current day conditions is consistent with an influx of fresher and cooler meteoric water into the Latrobe Group. The meteoric recharge originates from the area of the Baragwanath anticline in the onshore part of the basin where the Latrobe Group subcrops at high elevations.     

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.     

Stress‐dependent transport properties of fractured arkosic sandstone

We measure the fluid transport properties of microfractures and macrofractures in low‐porosity polyphase sandstone and investigate the controls of in situ stress state on fluid flow conduits in fractured rock. For this study, the permeability and porosity of the Punchbowl Formation sandstone, a hydrothermally altered arkosic sandstone, were measured and mapped in stress space under intact, microfractured, and macrofractured deformation states. In contrast to crystalline and other sedimentary rocks, the distributed intragranular and grain‐boundary microfracturing that precedes macroscopic fracture formation has little effect on the fluid transport properties. The permeability and porosity of microfractured and intact sandstone depend strongly on mean stress and are relatively insensitive to differential stress and proximity to the frictional sliding envelope. Porosity variations occur by elastic pore closure with intergranular sliding and pore collapse caused by microfracturing along weakly cemented grain contacts. The macroscopic fractured samples are best described as a two‐component system consisting (i) a tabular fracture with a 0.5‐mm‐thick gouge zone bounded by 1 mm thick zones of concentrated transgranular and intragranular microfractures and (ii) damaged sandstone. Using bulk porosity and permeability measurements and finite element methods models, we show that the tabular fracture is at least two orders of magnitude more permeable than the host rock at mean stresses up to 90 MPa. Further, we show that the tabular fracture zone dilates as the stress state approaches the friction envelope resulting in up to a three order of magnitude increase in fracture permeability. These results indicate that the enhanced and stress‐sensitive permeability in fault damage zones and sedimentary basins composed of arkosic sandstones will be controlled by the distribution of macroscopic fractures rather than microfractures.     

Spatial variations in the salinity of pore waters in northern deep water Gulf of Mexico sediments: implications for pathways and mechanisms of solute transport

Spatial variations in the salinity of pore waters in sedimentary basins can provide important insight into basin-scale hydrogeologic processes. Although there have been numerous studies of brine seeps in the deep water Gulf of Mexico, much less is known about porewater salinities in the vast areas between seeps. A study has been made of spatial variation in pore water salinities in sediments in an approximately 500 km by 200 km area of the northern deep water (water depth >500 m) Gulf of Mexico sedimentary basin (GOM) to provide insight into pathways and mechanisms of solute transport in this portion of the basin. A second objective was to document salinities in the upper 500 m of the sedimentary section, the approximate depth to which methane hydrates, a potential future energy resource, may be stable. Elevated salinities would reduce the P – T stability range of hydrates. Salinities were calculated from borehole logs using a dual electrical conductivity model. Even though much of the northern GOM is underlain by allochthonous salt most of the undisturbed shallow sedimentary section has not been permeated by hypersaline waters. These waters are limited to areas near brine seeps. Hypersaline waters having salinities in excess of 100 g l⁻¹ become more common at subseafloor depths of 2 km and greater. A field study at Green Canyon 65 and published numerical simulations of fluid flow above tabular salt bodies suggest that brines produced by salt dissolution migrate laterally and pond above salt and/or within minibasins and that the dominant mechanism of vertical solute transport is a combination of compaction-driven advection and diffusion, not large-scale thermohaline overturn. Superimposed on this diffuse upward flux of dissolved salt is the more focused and localized expulsion of saline fluids up faults.     

The Retreat of the North American Ice Sheet and Shifts in Cyclone Tracks

The alternating retreats and readvances of the North American ice sheet from 18,000 years to 6000 years ago are attributed to shifts in storm tracks. According to the author's theory, proglacial lakes formed during the retreat of the ice sheet became the paths of cyclone tracks whose precipitation led to an accumulation of ice and a readvance of the ice sheet. The readvance into the proglacial lake basins resulted in a reduction or southward shift of cyclone tracks, and this in turn led to a halt of the readvance and a retreat of the ice sheet. As soon as the ice had vacated the Great Lakes basin, the cyclone tracks would once again shift northward and the process of readvance would start all over again.     

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.     

Linked thermal convection of the basement and basinal fluids in formation of the unconformity‐related uranium deposits in the Athabasca Basin,Saskatchewan, Canada

World‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.     

Factors controlling free thermal convection in faults in sedimentary basins: implications for the formation of zinc–lead mineral deposits

Stratiform sediment‐hosted Zn–Pb–Ag mineral deposits constitute about 40% of the Earth's zinc resources ( Allen 2001 ), and in most cases their genesis involves the discharge of basinal brines near or on the seafloor through syndepositional faults ( Sangster 2002 ). From the point of view of base metal exploration, it is therefore essential to identify all possible faults that formerly carried the upwelling ore‐forming solutions during mineralising events. This paper presents a numerical investigation of the relative importance of various physical parameters in controlling fluid discharge, recharge and heat transport in faults. A two‐dimensional, free convection of pure water, hydrogeological model is developed for the McArthur basin in northern Australia based on the surface geology, known stratigraphic and structural relationships and regional geophysical interpretations. Numerical experiments and sensitivity analyses reveal that faults with strong initial heat input, due to depth of penetration or magmatic activity, are the most likely candidates to carry discharge fluids to the sites of metal precipitation. Deeper, wider and more permeable faults are more likely to behave as the fluid discharge pathways, whereas shallow, narrow or less permeable faults act as marine water recharge pathways. Compared with these fault‐related factors, aquifer physical properties are less important in determining fluid flow patterns and the geothermal regime. These results are an important step in understanding hydrothermal fluid flow in sedimentary basins in order to develop effective exploration criteria for the location of stratiform Zn–Pb–Ag deposits.     

Fluids and halogens at the diagenetic–metamorphic boundary: evidence from veins in continental basins, western Norway

Seven vein types are recognized in three continental Devonian molasse basins (the Hornelen, Kvamshesten and Solund basins) in western Norway. These include calcite‐, quartz‐ and epidote‐dominated veins. The salinities of fluid inclusions from quartz‐dominated veins in the Hornelen and Kvamshesten basins are close to or slightly higher than those for modern seawater, whereas the fluids from quartz‐ and calcite‐dominated veins in the Solund basin range from seawater values to 20 wt % NaCl equivalent. Minerals such as biotite, amphibole, titanite, chlorite and epidote are abundant in the latter veins, and are important constituents of the authigenic mineral assemblages. A combination of fluid inclusion and petrological data suggest that at least some of the veins formed at depths around 12–14 km. The Cl/Br ratios and the salinity of the fluid inclusions can be explained by interactions with evaporites, implying that the sedimentary environment forming the basin fill had the strongest influence upon low‐grade metamorphic fluid Cl and Br contents. Differences in the Cl/I and Na/Br ratios between the Solund basin and the Hornelen and Kvamshesten basins are best explained by local mass transfer between pore fluids and the surrounding rock matrix during burial and increasing temperatures.     

Geophysical and geochemical evidence of large scale fluid flow within shallow sediments in the eastern Gulf of Mexico,offshore Louisiana

We analyse the fluid flow regime within sediments on the Eastern levee of the modern Mississippi Canyon using 3D seismic data and downhole logging data acquired at Sites U1322 and U1324 during the 2005 Integrated Ocean Drilling Program (IODP) Expedition 308 in the Gulf of Mexico. Sulphate and methane concentrations in pore water show that sulphate–methane transition zone, at 74 and 94 m below seafloor, are amongst the deepest ever found in a sedimentary basin. This is in part due to a basinward fluid flow in a buried turbiditic channel (Blue Unit, 1000 mbsf), which separates sedimentary compartments located below and above this unit, preventing normal upward methane flux to the seafloor. Overpressure in the lower compartment leads to episodic and focused fluid migration through deep conduits that bypass the upper compartment, forming mud volcanoes at the seabed. This may also favour seawater circulation and we interpret the deep sulphate–methane transition zones as a result of high downward sulphate fluxes coming from seawater that are about 5–10 times above those measured in other basins. The results show that geochemical reactions within shallow sediments are dominated by seawater downwelling in the Mars‐Ursa basin, compared to other basins in which the upward fluid flux is controlling methane‐related reactions. This has implications for the occurrence of gas hydrates in the subsurface and is evidence of the active connection between buried sediments and the water column.