Vertical Fracture Zone Research Articles

Application of Different Electrode Array in Delineating Cassiterite Bearing Layer for A Two-Dimensional Tomography Survey Within Pingel Village of Bauchi State

Electrical resistivity method consist of various arrays that had been successfully applied in mineral explorations. Three important electrode arrays in 2D imaging surveys were tested in this study by creating a tomography of the subsurface to determine which array is the most successful in defining the cassiterite bearing layer. These arrays are Wenner, Schlumberger and Dipole - dipole. The numerical modeling was created through vertical fracture zone and correlated with a representative lithology of the area produced by using tape to measure depths to different lithology at an exposed mining pit. The results showed that the Schlumberger is the most suitable electrode array that can resolve both vertical and horizontal structures present in the subsurface. It was also obvious that the Schlumberger is a better array that images the vertical and horizontal contacts and represented these geological structures more accurately than the Wenner and Dipole-dipole arrays.

Contribution of bedrock structures to the bedrock surface topography and groundwater flow systems within deep glaciofluvial aquifers in Kurikka, Western Finland

AbstractOne of the key challenges related to glaciofluvial aquifers is understanding how the underlying bedrock structures and the associated bedrock surface topography contributed to the deposition of the glaciofluvial sediments and the generation of groundwater flow pathways. In this study from Western Finland, we present a new digital elevation model of the Precambrian crystalline bedrock surface buried up to 100 m beneath the glaciofluvial sediments along the Kurikka depression. We link bedrock topography to structural anisotropy using additional data from rock outcrops, boreholes and gravity data. Brittle deformation zones are seen in the bedrock‐digital elevation model (DEM) as sharp breaks in the rock surface. These vertical fracture zones contributed to the development of interlinked elongate bedrock depressions and delimit rock blocks with different elevations. A narrow, zigzagging trench, with a stepped floor, follows WNW‐ESE and NE‐SW oriented fracture zones and forms a major hydraulic connection between two major parts of the aquifer. Furthermore, the conductive fracture zones are conduits which connect the shallow glacifluvial aquifer system to deep groundwater in the bedrock. Understanding bedrock structures and buried topography is critical for successful groundwater modelling in crystalline, fractured bedrock.

Silicification, flow pathways, and deep-seated hypogene dissolution controlled by structural and stratigraphic variability in a carbonate-siliciclastic sequence (Brazil)

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO2>80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs.

How Important Are Those Fracture Zones? Scale Dependent Characteristics Revealed Through Field Studies and an Integrated Hydrological Model of a Mountain Headwater Catchment

Modeling groundwater flow in bedrock can be particularly challenging due to heterogeneities associated with fracture zones. However, fracture zones can be difficult to map, particularly in forested areas where tree cover obscures land surface features. This study presents the evidence of fracture zones in a small, snowmelt-dominated mountain headwater catchment and explores the significance of these fracture zones on groundwater flow in the catchment. A newly acquired bare earth image acquired using LiDAR identifies a previously undetected linear erosion zone that passes near a deep bedrock well at low elevation in the catchment. Borehole geophysical logs indicate more intense fracturing in this well compared to two wells at higher elevation. The well also exhibited a linear flow response during a pumping test, which is interpreted to reflect the influence of a nearby vertical fracture zone. The major ion chemistry and stable isotope composition reveal a slightly different chemical composition and a more depleted isotopic signature for this well compared to other groundwaters and surface waters sampled throughout the catchment. With this evidence of fracturing at the well scale, an integrated land surface – subsurface hydrologic model is used to explore four different model structures at the catchment scale. The model is refined in steps, beginning with a single homogeneous bedrock layer, and progressively adding 1) a network of large-scale fracture zones within the bedrock, 2) a weathered bedrock zone, and 3) an updated LiDAR-derived digital elevation model, to gain insight into how increasing subsurface geological complexity and land surface topography influence model fit to observed data and the various water balance components. Ultimately, all of the models are considered plausible, with similar overall fit to observed data (snow, streamflow, pressure heads in piezometers, and groundwater levels) and water balance results. However, the models with fracture zones and a weathered zone had better fits for the low elevation well. These models contributed slightly more baseflow (~14% of streamflow) compared to models without a weathered zone (~1%). Thus, in the watershed scale model, including a weathered bedrock zone appears to more strongly influence the hydrology than only including fracture zones.

Heat control mechanism and productivity optimization of artificial fracture zone structure of dry hot rock in Gonghe Basin

Hydraulic fracturing is necessary to obtain geothermal energy from the hot dry rocks. The structure of the artificial fractures plays an important role in controlling the water and heat transport during the heat production. In this study, we analyzed the influence of the permeability and width of artificial fracture zone on the coupled heat and flow processes in the Gonghe Basin, China. The results showed that in the fractured reservoir with permeability lower than 5 D, the outflow temperature increases with the width of the fracture zones. This is because the injected cold water can sufficiently merge with the thermal water in the reservoir, and has a weak influence on the water temperature close to the extraction well. A special situation occurs in the horizontal reservoir with permeability higher than 10 D, where the outflow temperature decreases with the increase of fracture zone width, because the overall low temperature zone is increased due to the rapid diffusion of injected cold water. In the reservoir with horizontal fracture zone, outflow temperature decreases with the increase of the permeability, because the injected cold water easily arrives the production well. In contrast, in the vertical or tilted fracture reservoir, the outflow temperature increases with the permeability, because the free convection occurs strongly in the high-permeable reservoirs. A synthetic comparison suggested that under the same well distance, heat production is higher in the reservoirs with low-permeable horizontal fracture zone and with high-permeable vertical fracture zone, among other fracture zones.

Delineation of Potential Groundwater Zones Using Shear Wave Velocity in Eastern Deccan Volcanic Province, India

A comprehensive study based on Multichannel Analysis of Surface Waves (MASW), a near-surface geophysical technique, was carried out for the first time in the Mandla Lobe of the Eastern Deccan Volcanic Province. The study was conducted to demarcate fractures, which are essential for finding potential groundwater zones in the hard rock region and for the delineation of shallow subsurface geological features in terms of shear wave velocity. The MASW surveys were carried out at different selected locations in the study area to delineate fractures in the basaltic terrain covered with thin weathering and alluvium cover. These basalts are mainly comprised of massive, vesicular and amygdaloidal varieties and interpreted as different basaltic flows of Mandla, Dhuma and Pipardahi formations. The shear wave velocity sections divulge the layered nature of the basalt sequences. In some of the massive flows, there are low-velocity vertical zones that can be deduced to be either vertical columnar joints or fracture zones, and the low-velocity weathered flows can be the good groundwater reservoirs. The shear wave velocities Vs obtained are in the range from 300 to 2500 m/s at different locations, and the thicknesses of flows are varied in the MASW sections. The obtained shear wave data are compared with the borehole data, and the results are well correlated. The study clearly differentiates the variations within basaltic formations, which are interpreted as potential groundwater zones.

The effect of thermo-elastic stress re-distribution on geothermal production from a vertical fracture zone

Injection of cold water into fracture zones in Enhanced Geothermal Systems (EGS) induces contractive thermo-elastic strains in the host rock, locally altering the stress state of the reservoir. Building on previous studies with horizontal fractures we examined how such thermo-elastic effects act during injection and production from a doublet intersecting a vertically extensive fracture zone embedded in impermeable host rock with a vertical geothermal gradient and with vertical aperture variations in response to stress increase with depth. As in horizontal fracture zones, contractive strains propagate outward from point of injection of cold water and are surrounded, due to stress re-distribution, by a ring zone with increased effective normal stress and, therefore, reduced aperture. The reduced aperture ring affects production temperature evolution to different degrees: for fracture zones with initially homogeneous transmissivity the effect on temperature decline is most pronounced for injection in the deeper well while it is small to negligible for injection in the shallower well. Fractures zones with initially heterogeneous transmissivity distribution experience highly channelized flow and the enhanced normal stresses develop preferentially at the sides of the channels, having little effect on flow along the channel and temperature decline. The effects show systematic variation with the rock's Young's modulus as does the tendency to induce shear failure by thermo-elastic effects. The simulated temperature evolutions deviate significantly from previous models that assessed the economics of EGS with sub-horizontal wells intersecting vertically extensive fractures in which the geothermal gradient and thermo-mechanical were not included.

Heat and Fluid Transport Induced by Convective Fluid Circulation Within a Fracture or Fault

AbstractNatural water convection in subvertical fractures, fracture zones, or faults can perturb the temperature field around the fracture and enhance and focus vertical heat flow within. We investigate, by means of numerical simulation, the effects of convection in a deeply buried vertical fracture zone. Fracture zone transmissivity, defined as permeability times thickness of the permeable region, is found to be the primary control on convection style rather than fracture zone thickness or permeability alone. In an impermeable host rock, the convection‐induced thermal anomaly propagates solely via conduction, diminishing away from the fracture. Convective heat flow increases with fracture transmissivity up to ~10−8 m3, when a plateau in convective heat flow is reached, constrained by fracture size and the host rock's thermal conductivity. Permeable host rocks modify these results significantly. In a moderately permeable host rock (10−14 m2), convection in the fracture induces non‐Rayleigh fluid convection, while thermal effects and fluid exchange between host rock and fracture remain moderate. If the host rock is sufficiently permeable to allow porous medium Rayleigh convection to occur (10−13 m2), the convection patterns within the fracture are overprinted by the host's convective patterns. Fluid exchange between the fracture and the rock will be significant. Our findings provide insight into how thermal anomalies in the uppermost crust may relate to locally enhanced heat flow from convection in nonoutcropping fractures below. Furthermore, the results for permeable host rocks provide evidence for previously inferred hydrologic scenarios for the formation of certain hydrothermal, vein‐type mineral deposits.

A zoning model for coal mining - induced strata movement based on microseismic monitoring

A microseismic monitoring technique is adopted to investigate the distribution regularity of microseismic events released by the coal mining - induced roof rock mass along the vertical and horizontal directions in combination with mine geological data. On the basis of the quantity and energy distributions of the microseismic events recorded using the microseismic monitoring technique, a zoning method is first established for the roof strata movement in the vertical and horizontal directions. The vertically zoning method is then applied to analyze the microseismic monitoring results obtained in the Dongjiahe Coal Mine, which divides rock mass into six zones along the vertical direction, i.e. the caved zone, the block zone, the vertical fracture through-going zone, the vertical fracture zone, the separation zone and the continuous zone. The horizontally zoning model divides the roof strata movement into three zones, i.e. the calm zone, the generation zone impacted by working face and the historical generation zone. After that, based on the horizontally zoning model, a method is developed to determine the displacement angle using the microseismic monitoring data, which has been verified in the Dongjiahe Coal Mine.

원격탐사, 전기탐사, 전자기탐사 및 시추공영상의 융합적 분석을 통한 서산지역 방조제 누수구역 판별

본 연구에서는 원격탐사, 전기비저항탐사, 전기전도도탐사(전자기탐사), 시추공 영상촬영법을 융합하여 효과적인 제방의 안정성 및 누수 평가 방법을 서산 A지구 방조제에 적용하여 제시하였다. Landsat ETM+열적외선 밴드를 사용하여 방조제 부근에서 다른 온도의 해수와 담수가 혼합되는 양상을 통해 누수의 존재, 규모 및 기간을 유추하였다. 전기비저항탐사를 방조제를 따라 실시하여 평균비저항 값 83.197 <TEX>${\Omega}$</TEX>-m이하를 보이는 이상대의 수평적 수직적 분포를 확인하였다. 간조와 만조 시의 전기전도도 값의 분석을 통해 배경치의 영향을 최소화한 수평적 이상대 분포를 확인하였다. 시추공영상촬영을 통해 파쇄대의 수직적 분포양상과 기존 탐사법의 정확도를 확인하였다. 그 결과 전체 제방의 연장 중 41.7%에서 세 수평적 이상대 탐사법의 중첩구간이 확인되었으며, 시추공에서 확인된 수직적 파쇄양상을 고려할 때 누수구간이 제방단면의 중심부를 따라 상당부분 수평적으로 연장이 되어 방대한 양의 누수가 존재할 가능성이 높을 것으로 사료된다. 네 탐사법의 융합적 접근은 각 탐사방법이 가지고 있는 장점을 부각하고 단점을 보완하여 수평적 및 수직적인 누수구간과 기간의 확인 및 판별이 가능하고, 시추공에서 확인된 수직적 파쇄양상의 수평적 연장성을 예측하는데 높은 신뢰도를 보인다 할 수 있다. This study introduces integrated approach on detection of a leakage in a sea wall based on remote sensing, electric resistivity survey, electromagnetic survey, and borehole survey for the Seosan A-Region sea wall. The satellite temperature distribution from Landsat ETM+ data identifies water leakage distribution and period by analyzing temperature mixing patterns between sea water and fresh water. Electric resistivity survey provides both horizontal and vertical anomaly distributions over the sea wall showing below average electric resistivity. Electromagnetic survey(electrical conductivity survey) reveals the potential possible leakage areas with minimal background impact by comparing electrical conductivity values between high and low tides. Borehole image processing system confirmed the locations of anomalies identified from the other survey methods and distributions of vertical fracture zones. The integrated approach identified 41.7% of the sea wall being the most probable area vulnerable to water leakage and effectively approximated both horizontal and vertical distribution of water leakage. The integrated analysis of remote sensing, electric resistivity survey, electromagnetic survey and borehole survey is considered to be an optimal method in identifying water leakage distribution, period, and extent of fractures knowledged from the boreholes.

Modeling of deep fracture zone opening and transient ground surface uplift at KB-502 CO2 injection well, In Salah, Algeria

The Krechba gas field at In Salah (Algeria), the site of the first industrial scale on-shore CO2 storage demonstration project, is also known for satellite-based ground-deformation monitoring data of remarkable quality. In this work, we focus on the In Salah injection well KB-502, where a double-lobe uplift pattern has been observed in the ground-deformation data. On the basis of previous numerical results, semi-analytical inverse deformation solutions, and seismic analyses, we explain this pattern of uplift as resulting from injection-induced deformation in a deep vertical fracture zone. In this study, we simulate a fracture zone characterized by high permeability and low mechanical stiffness, which activates after a few months of injection, causing irreversible changes in permeability. We study the transient evolution of uplift using the observed injection rate and compare it to the field Interferometric Synthetic Aperture Radar (InSAR) data using the displacement in the satellite line-of-sight. We also carry out a sensitivity study, analyzing the extent of the fracture zone, particularly its height from the reservoir depth. Our analysis supports the notion that the fracture zone is confined within the caprock and does not penetrate into the overlying aquifer.

Response of underground pipeline through fault fracture zone to random ground motion

It is assumed that a pipeline is laid through a vertical fault fracture zone, and is excited by seismic ground motion modelled as stationary stochastic process. For horizontal incidence of waves, the cross-PSD (Power Spectral Density) function is developed using wave propagation theory, while for vertical incidence of waves the cross-PSD function is composed by auto-PSD model, coherence model and site response model. As the seismic input, the cross-PSD function is used to calculate the the axial and lateral seismic responses of underground pipeline through the fracture zone. The results show that the incident directions of seismic waves, width and soil property of the fracture zone have great influence on underground pipeline. It is suggested that the flexible joints with appropriate stiffness should be added into the pipeline near the interfaces between the fracture zone and the surrounded media.

Coupled non-isothermal, multiphase fluid flow, and geomechanical modeling of ground surface deformations and potential for induced micro-seismicity at the In Salah CO2 storage operation

Abstract We present recent results of coupled non-isothermal, fluid flow and geomechanical modeling associated with the In Salah CO2 storage operation at the Krechba gas field, Algeria. Much recent modeling efforts have been dedicated to analyze satellite based measurements of ground surface uplift at one of the injection wells where a double-lobe uplift pattern has been observed. Both semi-analytical inverse deformation analyses and coupled numerical modeling have indicated that the observed double-lobe uplift pattern can be explained by injection-induced deformation in a deep vertical fracture zone or fault intersecting the injection well and extending a few hundred meters above the injection zone (up to a depth below 1600 m). Recently, a 3D seismic survey indicated that such a fault or fracture zone may indeed intersect the well with the orientation originally predicted by the semi-analytical inverse deformation analysis. A coupled numerical analysis indicates that observed progressive uplift during active CO2 injection and relatively slower subsidence rate during a subsequent shut-in period could be modeled as an elastic response, i.e. indicating elastic deformation of an existing geological feature rather than the creation of a new hydraulic fracture. Finally, we analyzed the simulation results in terms of reservoir stress evolution and the potential for injection-induced micro-seismicity at Krechba. Our analysis shows that the highest potential for injection-induced micro-seismicity occurs along the horizontal injection wells caused by the combined effects of injection-induced cooling and pressure. However, for the best-estimated present-day strike-slip stress regime at Krechba, our analysis indicates a relatively low potential for injection-induced micro-seismicity.

Tectonic architecture through Landsat-7 ETM+/SRTM DEM-derived lineaments and relationship to the hydrogeologic setting in Siwa region, NW Egypt

Fracture zones on the Earth’s surface are important elements in the understanding of plate motion forces, the dynamics of the subsurface fluid flow, and earthquake distributions. However, good exposures of these features are always lacking in arid regions, characterized by flat topography and where sand dunes extensively cover the terrain. During field surveys these conditions, in many cases, hinder the proper characterization of such features. Therefore, an approach that identifies the regional fractures as lineaments on remotely-sensed images or shaded digital terrain models, with its large scale synoptic coverage, could be promising. In the present work, a segment tracing algorithm (STA), for lineament detection from Landsat-7 Enhanced Thematic Mapper Plus (ETM+) imagery, and the data from the Shuttle Radar Topographic Mission (SRTM) 30 m digital elevation model (DEM), has been applied in the Siwa region, located in the northwest of the Western Desert of Egypt. The objectives are to analyze the spatial variation in orientation of the detected linear features and its relation to the hydrogeologic setting in the area and the underlying geology, and to evaluate the performance of the algorithm applied to the ETM+ and the DEM data. Detailed structural analysis and better understanding of the tectonic evolution of the area could provide useful tools for hydrologists for reliable groundwater management and development planning. The results obtained have been evaluated by the structural analysis of the area and field observations. Four major vertical fracture zones were detected corresponding to two conjugate sets of strike-slip faults that governed the surface, and subsurface environments of the lakes in the region, and these correlate well with the regional tectonics.

Geophysical investigations for the characterization of fractured rock aquifers in Itu, SE Brazil

This paper presents the results of integrated geophysical investigations to characterize aquifers in fractured rocks in the Granite complex of Itu, SE Brasil, to help locate sites for tubular wells for groundwater exploration. Ground penetrating radar (GPR) profiles, dipole–dipole electric survey, gamma spectrometry, and radon gas emanometry were applied on a same line for a comparative study of these methodologies. The results allowed us to characterize structural discontinuities up to 30 m in depth, such as, dipped, or vertical fractures. The dipped fractures appear as strong GPR reflectors, probably due to the presence of water. Besides, two anomalous regions were identified, one at 50 m and other at 75 m, both showing high attenuation of the GPR signals, and they correspond to the vertical fracture zones. 2D modeling of the dipolar electric survey shows that the two anomalous areas correspond to high electric conductivity zones, and that in the position of 75 m, a vertical fracture zone is clearly identified. In the gamma rays profiles, the values of K and Th concentrations observed in the position of 50 m indicate the presence of vertical fractures filled with clay minerals. On the other hand, in the position of 70 m of the Rn gas emanometry profile, the peak of Rn emanation is five times higher than the regional level, indicating the presence of a vertical fracture zone, clearly filled with fresh water in circulation. The geophysical characterization of the vertical fracture zones with fresh water subsidized the drilling of a well for groundwater exploration in fractured aquifers, which was an important contribution for the local community because the main sources of surface water are contaminated.

The Neogene to Recent Rallier-du-Baty nested ring complex, Kerguelen Archipelago (TAAF, Indian Ocean): stratigraphy revisited, implications for cauldron subsidence mechanisms

Abstract The Kerguelen Archipelago is made up of a stack of thick piles of Tertiary flood basalts intruded by transitional to alkaline igneous centres at various times since 30 Ma ago. In the SW, the Rallier-du-Baty Peninsula is mostly occupied by two silicic ring complexes, each with an average diameter of 15 km, comprising dissected calderas cross-cut by subvolcanic cupolas. Previous radiometric determinations yield ages ranging from 15.4 to 7.4 Ma in the southern centre, and 6.2 to 4.9 Ma in the northern one. The felsic ring dykes were injected by coeval mafic magmas, forming, successively, swarms of early mafic enclaves, disrupted synplutonic cone sheets, and late cone-sheets. After the emplacement and subsequent unroofing of the plutonic ring complexes, abundant and thick trachytic pyroclastic flows and falls were emitted from the younger caldera volcanoes, while hawaiite and mugearite lava flows were erupted from marginal maars and cones. Huge trachyte ignimbritic flows filled the glacial valleys in the central Peninsula, and capped lacustrine deposits and older lava flows, while related pumice falls are widespread throughout the archipelago. This powerful plinian eruption took place after the network of glacial valleys was established, but before the Little Ice Age that occurred during the last centuries. In the south of the peninsula, even younger trachytic formations are exposed, and fumarolic vents are still active. The growth mechanisms of a caldera-related ring complex can be explained as a repetitive sequence of two eruptive episodes. The first episode of hydrofracturing, induced by volatile exsolution within the evolving magma chamber, creates a vertical circular fracture zone, along which highly vesiculated magmas are emitted during explosive eruptive events occurring at the surface in a caldera volcano. It is followed by a second episode of cauldron-subsidence of a crustal block down to the degassed magma chamber, induced by pressure release. Downward movement of the crustal block favours the emplacement at shallow depths within the older caldera-filling formations, of discrete magmatic sheets characterized by a 16-km mean diameter and a 1-km mean thickness, corresponding to an average unit volume of 200 km 3 . Actually, the estimated volumes of the different igneous episodes within the Rallier-du-Baty nested ring complex vary from 60 to 900 km 3 , and correspond to the production during 15 Ma of about 2800 ± 850 km 3 of new materials and a net crustal growth of about 100 ± 30 × 10 3 m 3 per year.

Stochastic ground water flow simulation with a fracture zone continuum model.

A method is presented for incorporating the hydraulic effects of vertical fracture zones into two-dimensional cell-based continuum models of ground water flow and particle tracking. High hydraulic conductivity features are used in the model to represent fracture zones. For fracture zones that are not coincident with model rows or columns, an adjustment is required for the hydraulic conductivity value entered into the model cells to compensate for the longer flowpath through the model grid. A similar adjustment is also required for simulated travel times through model cells. A travel time error of less than 8% can occur for particles moving through fractures with certain orientations. The fracture zone continuum model uses stochastically generated fracture zone networks and Monte Carlo analysis to quantify uncertainties with simulated advective travel times. An approach is also presented for converting an equivalent continuum model into a fracture zone continuum model by establishing the contribution of matrix block transmissivity to the bulk transmissivity of the aquifer. The methods are used for a case study in west-central Florida to quantify advective travel times from a potential wetland rehydration site to a municipal supply wellfield. Uncertainties in advective travel times are assumed to result from the presence of vertical fracture zones, commonly observed on aerial photographs as photolineaments.

Capturing Reservoir Behavior by Simulating Vertical Fracture and Super-K Zones in the Ghawar Field

Summary A dynamic simulation model containing several million cells shows geological details used to simulate the vertical fractures and high stratiform permeability of the Ghawar field. Observed performance data have shown that well intersections in the Ghawar field exhibit thin intervals of extremely high productivity. These can be related to high-permeability layers (stratiform features) or to subvertical faults and fractures. To capture observed field behavior in a dynamic simulation model, these two features must be incorporated in the simulation model. This paper discusses how vertical fractures and high stratiform permeability were simulated in the world's largest oil reservoir.

PARENTAL MAGMA AND EMPLACEMENT OF THE STRATIFORM UITKOMST COMPLEX, SOUTH AFRICA

The Uitkomst Complex, a Ni–Cu–PGE–Cr-mineralized layered basic intrusion situated approximately 200 km due east of Pretoria, South Africa, hosts commercially viable magmatic base-metal sulfide deposits. The Complex has a concordant 207Pb/206Pb zircon age of 2044 ± 8 Ma, suggesting that it is coeval with the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Chemical modeling, using summation and least-squares optimization techniques, provides evidence that the boninitic Bushveld B1 magma is parental to both the lower ultrabasic and upper basic layered series of the Uitkomst Complex. The layered series crystallized in two stages, i.e., a lower conduit and an upper closed-system stage. The tubular shape of the Uitkomst Complex is the result of the intersection of a near-horizontal bedding plane fault with an existing vertical fracture zone under tensional conditions. During the conduit stage, a combination of magma mixing, contamination and flow dynamics may have facilitated sulfide formation and segregation. The identification of Bushveld B1 magma as the major parental magma of the Uitkomst Complex has significance in the exploration for similarly mineralized sub-RLS intrusive bodies.

Hydrogeological potential of major fractures in Eritrea

Neogene rifting and associated uplift on the western flank of the Red Sea relate to a variety of steep to vertical faults and fracture zones. These include reactivation of north-northwest—south-southeast Precambrian shear zones, normal faults roughly parallel to those earlier structures and prominent east-southeast—west-northwest fractures. Where exposed, some of the last features prove to be products of large-scale dilatation and contain fills up to 25 m wide of clastic sediments and highly altered basaltic and felsic dykes. These dilatations may have considerable potential for groundwater production, especially as their trend takes them from areas of high elevation with regionally the highest precipitation to lowland semi-arid areas. The lowlands of interior Eritrea have abundant fertile soils that were derived by erosion of Tertiary sediments and volcanic rocks from the uplifting Eritrean Highlands. Geophysical profiling across several of these structures in both lowland and highland terrains reveals conductive features believed to relate to saturated zones in these fractures. Drilling of some has proved sustainably high well yields. Although dilatational fractures are potentially useful in supplying water, zones of deep weathering in their highland catchment may add soluble compounds that reduce the quality of supplies.