Lithology and fluid prediction from amplitude versus offset (AVO) seismic data

Seismic reflection data as used in the oil industry is acquired and processed as multitrace data with source‐receiver offsets from a few hundred metres (short offset) to several kilometres (long offset). This set of data is referred to as ‘pre‐stack’. The traces are processed by velocity analysis, migration and stacking to yield a data volume of traces with ‘zero‐offset’. The signal‐to‐noise enhancement resulting from this approach is very significant. However, reflection amplitude changes in the pre‐stack domain may also be analysed to yield enhanced rock physics parameter estimates. Pre‐stack seismic data is widely used to predict lithology, reservoir quality and fluid distribution in exploration and production studies. Amplitude versus offset (AVO) data, especially anomalous signals, have been used for decades as indicators of hydrocarbon saturation and favourable reservoir development. Recently, enhanced quantification of these types of measurement, using seismic inversion techniques in the pre‐stack domain, have significantly enhanced the utility of such measurements. Using these techniques, for example, probability of the occurrence of hydrocarbons throughout the seismic data can be estimated, and as a consequence the many pre‐stack volumes acquired in a three‐dimensional (3D) can be survey, reduced to a single, more interpretable volume. The possibilities of 4D time lapse observation extend the measurements to changes in fluid content (and pressure) with time, and with obvious benefits in establishing the accuracy of dynamic reservoir models and improvements in field development planning. As an illustration, recent results from the Nelson Field (UK North Sea), are presented where we show the method by which probability volumes for oil sands may be calculated. The oil–sand probability volumes for three 3D seismic datasets acquired in 1990, 1997 and 2000 are compared and production effects in these data are demonstrated.     

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.     

High‐Resolution 3D Marine Seismic Investigation of Hedeby Harbour,Germany

Offshore 3D‐seismic acquisition has been a standard for high‐precision structural imaging in the oil and gas industry for many years. Recently this technique has been adapted by only a few teams to the resolution required for archaeological marine investigation. In contrast to sonar techniques, the 3D‐seismic method produces images below the sea‐floor. We investigate the harbour of the Viking age proto‐town of Hedeby in Northern Germany with the SEAMAP‐3D system. SEAMAP‐3D allows for rapid acquisition and employs an automated data processing sequence. We observe a wealth of archaeologically relevant detail and compare our results with previous work.     

Geophysical signatures associated with fluid flow and gas hydrate occurrence in a tectonically quiescent sequence,Qiongdongnan Basin,South China Sea

Interpretation of high‐resolution two‐dimensional (2D) and three‐dimensional (3D) seismic data collected in the Qiongdongnan Basin, South China Sea reveals the presence of polygonal faults, pockmarks, gas chimneys and slope failure in strata of Pliocene and younger age. The gas chimneys are characterized by low‐amplitude reflections, acoustic turbidity and low P‐wave velocity indicating fluid expulsion pathways. Coherence time slices show that the polygonal faults are restricted to sediments with moderate‐amplitude, continuous reflections. Gas hydrates are identified in seismic data by the presence of bottom simulating reflectors (BSRs), which have high amplitude, reverse polarity and are subparallel to seafloor. Mud diapirism and mounded structures have variable geometry and a great diversity regarding the origin of the fluid and the parent beds. The gas chimneys, mud diapirism, polygonal faults and a seismic facies‐change facilitate the upward migration of thermogenic fluids from underlying sediments. Fluids can be temporarily trapped below the gas hydrate stability zone, but fluid advection may cause gas hydrate dissociation and affect the thickness of gas hydrate zone. The fluid accumulation leads to the generation of excess pore fluids that release along faults, forming pockmarks and mud volcanoes on the seafloor. These features are indicators of fluid flow in a tectonically‐quiescent sequence, Qiongdongnan Basin. Geofluids (2010) 10 , 351–368     

Geothermometry and geobarometry of overpressured environments in Qiongdongnan Basin,South China Sea*

We demonstrate the use of PVT fluid inclusion modelling in the calculation of palaeofluid formation pressures, using samples from the YC21‐1‐1 and YC21‐1‐4 wells in the YC21‐1 structural closure, Qiongdongnan Basin, South China Sea. Homogenisation temperatures and gas/liquid ratios were measured in aqueous fluid inclusions, and associated light hydrocarbon/CO₂‐bearing inclusions, and their compositions were determined using a crushing technique. The vtflinc software was used to construct P–T phase diagrams that enabled derivation of the minimum trapping pressure for each order of fluid inclusion. Through the projection of average homogenisation temperatures (155, 185.5 and 204.5°C) for three orders of fluid inclusion on the thermal‐burial history diagram of the Oligocene Yacheng and Lingshui formations, their trapping times were constrained at 4.3, 2.1 and 1.8 Ma, respectively. The formation pressure coefficient, the ratio of fluid pressure/hydrostatic pressure established by PVT modelling coupled with DST data, demonstrates that one and a half cycles of pressure increase–discharge developed in the Yacheng and Lingshui formations for about 4.3 Ma. In comparison, the residual formation pressure determined by 2D numerical modelling in the centre of LeDong depression shows two and a half pressure increase–discharge cycles for about 28 Ma. The two different methods suggest that a high fluid potential in the Oligocene reservoir of the YC21‐1 structure developed at two critical stages for regional oil and natural gas migration and accumulation (5.8 and 2.0 Ma, respectively). Natural gas exploration in this area is therefore not advisable.     

Quantifying secondary migration efficiencies

Exploration success relies on properly risking the hydrocarbon system relevant for each prospect. Accurate risking of secondary migration efficiencies has been difficult due to lack of simple procedures that relate rock properties such as permeability and entry pressures to migration velocities, oil stringer heights and saturations. In order to achieve improved estimates of charge probabilities, equations for the secondary migration process are formulated based upon the Darcy flow and buoyancy conditions. An analytical solution of the formulated equations is shown, making it possible to construct charts for efficiently assessing the column height of secondary migration hydrocarbon stringers. The average oil (hydrocarbon) saturation of the migrating stringer can be computed, making it easy to compute the permeability related, secondary migration losses. Inputs to the chart are hydrocarbon flow‐rates and flow‐path width, hydrocarbon viscosity and density, carrier bed dip, permeability and entry pressures. Outputs are stringer heights, hydrocarbon saturation, relative permeability, migration velocities and migration losses. A procedure for including the new equations into existing basin scale fluid flow simulators is outlined and a Java applet for calculating the properties is described. The Java applet is useful for sensitivity studies, and can also be used to test results from basin simulators with the new migration efficiency equations. The analytical solution suggests that many published methods for calculating hydrocarbon migration in fluid flow simulators will over‐estimate hydrocarbon saturations and therefore losses. Calculated migration velocities will also be too low.     

Seismic constraints on the effects of gas hydrate on sediment physical properties and fluid flow: a review

The formation of gas hydrates in marine sediments changes their physical properties and hence influences fluid flow. Here, we review seismic indicators of gas hydrates and relate these indicators to gas hydrate formation and fluid migration. Analyses of seismic data from sediments containing gas and gas hydrates in a variety of locations have shown that the characteristic bottom‐simulating reflector (BSR), which commonly marks the hydrate phase boundary is caused mainly by the presence of gas beneath the gas hydrate stability zone (GHSZ). The amplitude of the BSR is also dependent on the hydrate concentration and on the porosity of the sediment. The presence of gas hydrate alters the elastic properties of sediments, particularly if it cements sediment grains. However, multifrequency studies in various geological provinces show that any loss of reflectivity or blanking observed within the GHSZ is dependent on both the nature of the sediments and concentration of hydrate present. Gas beneath the BSR may cause amplitude anomalies and may result in bright spots and enhanced reflections. The presence of gas beneath the BSR is the primary cause of observed amplitude versus offset (AVO) anomalies, but the amplitude of these anomalies is also dependent on the amount of cementation brought by the gas hydrates within the GHSZ. Fluid migration appears to play an important role in the formation and dissociation of gas hydrates in both active and passive margin settings. Fluid migration in accretionary prisms influences hydrate accumulation and may therefore control the spatial distribution of BSRs. Fluid migration may influence also the type of hydrate formed by bringing thermogenic gas containing higher order hydrocarbons to the GHSZ from below. Fluid advection may cause local dissociation of gas hydrates by bringing heat from below, thus shifting the gas hydrate phase boundary. Fluid flow within the GHSZ is limited by the formation of hydrate in the pore space, which reduces the permeability of the sediment. Features such as pockmarks, acoustic masking and acoustic turbidity are indirect indicators of fluid flow and identification of these features in seismic sections within and beneath the GHSZ may also suggest the formation of gas hydrate.     

Direct inversion of Young's and Poisson impedances for fluid discrimination

Seismic inversion with prestack seismic data such as amplitude variation with offsets (AVO) inversion is an important tool in the estimation of elastic parameters for predicting lithology and discriminating fluid in conventional or unconventional hydrocarbon reservoirs. The product of Young's modulus and density (Young's impedance, YI) and the product of Poisson ratio and density (Poisson ratio impedance, PI) show great potential in lithology prediction and fluid discrimination of unconventional resources such as shale gas or oil. The high quality requirements for prestack data in density inversion render the estimation of YI and PI arduous and inaccurate with a conventional prestack inversion approach. In this study, a direct AVO inversion approach is proposed to estimate YI, PI, and density directly from P‐wave seismic data. The linearized P‐wave reflectivity approximate equation in terms of YI, PI, and density is initially derived. Five models, including four typical AVO classes, are utilized to verify the accuracy of the derived linearized P‐wave reflectivity equation in comparison with the exact P‐wave reflectivity equation and the frequently used linearized reflectivity approximate equation involving P‐ and S‐wave velocities and density. Parameter sensitivity analysis illustrates that YI and PI can reasonably be estimated from P‐wave reflectivity if a decorrelation scheme is utilized in the inversion algorithm. In addition, a pragmatic AVO inversion using a Bayesian scheme is suggested for the direct inversion of YI and PI from prestack seismic data. Synthetic and field data examples demonstrate the feasibility of the proposed inversion approach in the estimation of YI and PI and show the potential of this approach in fluid discrimination.     

Fluid mapping in deeply buried Ordovician paleokarst reservoirs in the Tarim Basin,western China

The storage spaces within deeply buried Ordovician paleokarst reservoirs in the Tarim Basin are mostly secondary and characterized by strong heterogeneity and some degree of anisotropy. The types of fluids that fill the spaces within these reservoirs are of great importance for hydrocarbon exploration and exploitation. However, fluid identification from seismic data is often controversial in this area because the seismic velocity for this particular reservoir could be significantly influenced by many factors, including pore shapes, porosity, fluid types, and mineral contents. In this study, we employ the differential effective medium‐Gassmann rock physics model to interpret and discuss the characteristics of conventional karstic carbonate reservoirs in the Tarim Basin that are filled with different fluids (oil, gas, and water) using logging data and thus objectively build corresponding fluid identification criteria. These criteria are subsequently evaluated by amplitude versus offset (AVO) forward analysis based on typical logging data and further applied to ascertain the reservoir fluid types in two different areas in the Tarim Basin based on prestack inversion results. For conventional carbonate reservoirs, gas can be distinguished from heavy oil and water, but heavy oil and water are broadly similar on seismic data. For condensate carbonate reservoirs, water can be differentiated from light oil (i.e., condensates) and gas, but light oil and gas demonstrate substantial similarities in terms of their seismic responses. The predicted fluid results are in good agreement with the results of drilling and oil testing. In particular, modeling the seismically resolvable reservoirs in the carbonate strata in the Tarim Basin, which have needle‐ and sphere‐shaped storage spaces (pore aspect ratio > 0.3) and clay content that is lower than 5%, indicates that fluid properties could be properly evaluated if the porosity is larger than 5% for conventional carbonate reservoirs and >7% for condensate carbonate reservoirs.     

GRAVITY AND SPATIAL STRUCTURE: THE CASE OF INTERSTATE MIGRATION IN MEXICO*

ABSTRACT The estimation of gravity models of internal (aggregate) place‐to‐place migration is plagued with endogeneity (omitted‐variable) biases if the unobserved effects of spatial structure are not accounted for. To address this econometric problem, this paper presents a more general specification of the gravity model, which allows for (bilateral) parameter heterogeneity across individual migration paths—along with (unilateral) origin‐ and destination‐specific effects. The resultant “three‐way fixed‐effects” (3FE) model is applied for an analysis of interstate migration in Mexico based on cross‐sectional data. To overcome parameter‐dimensionality problems (due to limited or incomplete information), the 3FE model is estimated using the Generalized Maximum Entropy (GME) estimator. The empirical implications of this new modeling strategy are illustrated by contrasting the 3FE‐GME estimates with those for the traditional and two‐way fixed‐effects (2FE) models. The former are far more plausible and intuitively interpretable than their traditional and 2FE counterparts, with parameter estimates changing in expected directions. The (average) effect of the migrant stock is markedly smaller than usually estimated, providing a more realistic measure of network‐induced migration. Migration outflows from centrally located origins have significantly steeper distance decay. Path‐specific distance effects exhibit directional asymmetries and spatial similarities.     

Pliocene sand injectites from a submarine lobe fringe during hydrocarbon migration and salt diapirism: a seismic example from the Lower Congo Basin

Large‐scale conical and saucer‐shaped sand injectites have been identified in the Upper Miocene sediments of the Lower Congo Basin. These structures are evidenced on the 3D high‐resolution seismic data at about 600 ms TWT (two‐way traveltime) beneath the seabed. The conical and saucer‐shaped anomalies range from 20 to 80 m in height, 50 to 300 m in diameter, and 10 to 20 ms TWT in thickness. They are located within a sedimentary interval of about 100 m in thickness and are aligned over 20 km in dip direction (NE‐SW), above the NW margin of an underlying Upper Miocene submarine fan. We have interpreted the conical and saucer‐shaped anomalies as upward‐emplaced sand injectites sourced from the Upper Miocene fan because of their discordant character, the postsedimentary uplifting of the sediments overlying the cones and saucer‐shaped bodies, the alignment with the lateral fringe of the Upper Miocene submarine fan, and the geological context. Sand injection dates from the Miocene–Pliocene transition (approximately 5.3 Ma). The prerequisite overpressure to the sand injection process may be due to the buoyancy effect of hydrocarbons accumulated in the margins of the fan. Additionally, overpressure could have been enhanced by the lateral transfer of fluids operating in the inclined margins of the lobe. The short duration of sand injection and the presence of many sandstone intrusions suggested that the process of injection was triggered by an event, likely due to a nearby fault displacement related to diapiric movements. This is the first time that sand injectites of seismic scale have been described from the Lower Congo Basin. The localized nature of these injectites has led to a change in the migration path of fluids through the sedimentary cover. Consequently, the sand intrusions are both evidence and vectors of fluid migration within the basin fill.     

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.     

Evolution of deformation and fault‐related fluid flow within an ancient methane seep system (Eocene,Varna, Bulgaria)

Faults are often important in fuelling methane seep systems; however, little is known on how different components in fault zones control subsurface fluid circulation paths and how they evolve through time. This study provides insight into fault‐related fluid flow systems that operated in the shallow subsurface of an ancient methane seep system. The Pobiti Kamani area (NE Bulgaria) encloses a well‐exposed, fault‐related seep system in unconsolidated Lower Eocene sandy deposits of the Dikilitash Formation. The Beloslav quarry and Beloslav N faults displace the Dikilitash Formation and are typified by broad, up to 80 m wide, preferentially lithified hanging wall damage zones, crosscut by deformation bands and deformation band zones, smaller slip planes and fault‐related joints. The formation of a shallow plumbing system and chimney‐like concretions in the Dikilitash Formation was followed by at least two phases of fault‐related methane fluid migration. Widespread fluid circulation through the Dikilitash sands caused massive cementation of the entire damage zones in the fault hanging walls. During this phase, paths of ascending methane fluids were locally obstructed by decimetre‐thick, continuous deformation band zones that developed in the partly lithified sands upon the onset of deformation. Once the entire damage zone was pervasively cemented, deformation proceeded through the formation of slip planes and joints. This created a new network of more localized conduits in close vicinity to the main fault plane and around through‐going slip planes. ¹³C‐depleted crustiform calcite cements in several joints record the last phase of focused methane fluid ascent. Their formation predated Neogene uplift and later meteoric water infiltration along the joint network. This illustrates how fault‐related fluid pathways evolved, over time, from ‘plumes’ in unconsolidated sediments above damage zones, leading to chimney fields, over widespread fluid paths, deflected by early deformation structures, to localized paths along fracture networks near the main fault.     

‘We’re moving out’: Youth Out‐Migration Intentions in Coastal Non‐Metropolitan New South Wales

This article discusses youth out‐migration on the non‐metropolitan New South Wales Eastern Seaboard. High levels of in‐migration and counter‐urbanisation, typical of many coastal non‐metropolitan towns in NSW, mask the out‐migration of youth. There are relatively few 15–24 year olds in the coastal communities of non‐metropolitan New South Wales, because many youths out‐migrate to larger centres, for a range of reasons. Out‐migration also demarcates a life transition away from school life, adolescence and the parental home. This paper draws from research with senior high school students in one coastal town – Coffs Harbour – where such trends have been particularly apparent. It examines the propensity for youth out‐migration and discusses how young people articulate their migration intentions. Young people's perceptions of their current and future prospects feature prominently in their discourses about intended migration, although this research also demonstrates that the life courses of regional youth are unorthodox and diverse in nature.     

Spatial Cluster Detection in Spatial Flow Data

As a typical form of geographical phenomena, spatial flow events have been widely studied in contexts like migration, daily commuting, and information exchange through telecommunication. Studying the spatial pattern of flow data serves to reveal essential information about the underlying process generating the phenomena. Most methods of global clustering pattern detection and local clusters detection analysis are focused on single‐location spatial events or fail to preserve the integrity of spatial flow events. In this research we introduce a new spatial statistical approach of detecting clustering (clusters) of flow data that extends the classical local K‐function, while maintaining the integrity of flow data. Through the appropriate measurement of spatial proximity relationships between entire flows, the new method successfully upgrades the classical hot spot detection method to the stage of “hot flow” detection. Several specific aspects of the method are discussed to provide evidence of its robustness and expandability, such as the multiscale issue and relative importance control, using a real data set of vehicle theft and recovery location pairs in Charlotte, NC.     

Overpressure in the Malay Basin and prediction methods

Predrill overpressure prediction is important for well planning and migration modeling for prospect evaluation. The Eaton (Journal of Petroleum Technology, 24 , 1972, 929) and Bowers (SPE Drilling & Completion, 10 , 1995, 89) methods are used worldwide for postdrill overpressure prediction using sonic log and predrill overpressure prediction using seismic interval velocity. In this research, these two methods were used for overpressure prediction using 3D anisotropic prestack depth‐migrated seismic interval velocity in a field of the Malay Basin. In the shallow overpressured zone, where the mechanism of overpressure is undercompaction, the onset of overpressure was predicted reasonably well using the Eaton and Bowers methods with their standard parameters (i.e., Eaton exponent 3 and Bowers loading curve) for seismic velocity. However, in the deep overpressured zone, where fluid expansion is the cause of overpressure generation, these methods underpredicted the high overpressure. In the deep overpressured zone, the overpressures were better predicted by applying a correction to the Eaton method. On the other hand, the Bowers unloading parameters for the fluid expansion mechanisms did not show any significant effect on overpressure prediction. Hence, in the study area, the Bowers method is not effective for 3D overpressure prediction using seismic velocity, whereas the Eaton method is more robust and can be used for 3D overpressure prediction from seismic velocity.     

Role of compressive tectonics on gas charging into the Ordovician sandstone reservoirs in the Sbaa Basin,Algeria: constrained by fluid inclusions and mineralogical data

Structure‐ and tectonic‐related gas migration into Ordovician sandstone reservoirs and its impact on diagenesis history were reconstructed in two gas fields in the Sbaa Basin, in SW Algeria. This was accomplished by petrographical observations, fluid inclusion microthermometry and stable isotope geochemistry on quartz, dickite and carbonate cements and veins. Two successive phases of quartz cementation (CQ1 and CQ2) occurred in the reservoirs. Two phase aqueous inclusions show an increase in temperatures and salinities from the first CQ1 diagenetic phase toward CQ2 in both fields. Microthermometric data on gas inclusions in quartz veins reveal the presence of an average of 92 ± 5 mole% of CH₄ considering a CH₄‐CO₂ system, which is similar to the present‐day gas composition in the reservoirs. The presence of primary methane inclusions in early quartz overgrowths and in quartz and calcite veins suggests that hydrocarbon migration into the reservoir occurred synchronically with early quartz cementation in the sandstones located near the contact with the Silurian gas source rock at 100–140°C during the Late Carboniferous period and the late Hercynian episode fracturing at temperatures between 117 and 185°C, which increased in the NW‐direction of the basin. During the fracture filling, three main types of fluids were identified with different salinities and formation temperatures. A supplementary phase of higher fluid temperature (up to 226°C) recorded in late quartz, and calcite veins is related to a Jurassic thermal event. The occurrence of dickite cements close to the Silurian base near the main fault areas in both fields is mainly correlated with the sandstones where the early gas was charged. It implies that dickite precipitation is related to acidic influx. Late carbonate cements and veins (calcite – siderite – ankerite and strontianite) occurred at the same depths resulting from the same groundwater precipitation. The absence of methane inclusions in calcite cements result from methane flushing by saline waters.     

Walking the Land: Aboriginal Trails,Cultural Landscapes,and Archaeological Studies for Impact Assessment

Aboriginal people have occupied northern Alberta since the end of the last ice age. For most of that time they travelled across the land by foot, producing complex networks of trails, many of which may have great antiquity. Aboriginal people also modified the landscape extensively by the use of controlled burning. Lastly, they are immersed in and “read” the land as places with multiple cultural meanings, which in turn helped shape their cultures and identities. Together, these elements indicate the existence of a series of overlapping cultural landscapes for which the cross-country trails and waterways provide the grid. This article addresses the importance of traditional trails for identifying the cultural landscapes of northeastern Alberta and points to the rapid disappearance today of knowledge about such trails. It considers how archaeological investigations done in Alberta for Impact Assessment purposes fail to consider either trails or cultural landscapes in their surveys or to consult with Aboriginal people. As a result, government Review Panels making recommendations for whether or not an industrial project should be approved are basing their findings on incomplete information about Aboriginal land uses and meanings.     

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.