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Fracture Systems (fracture + system)
Selected AbstractsHydraulic pathways in the crystalline rock of the KTBGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Günter Zimmermann Fracture systems and fluid pathways must be analysed in order to understand the dynamical processes in the upper crust. Various deterministic as well as stochastic fracture networks in the depth section of the Franconian Lineament (6900 to 7140 m), which appears as a brittle ductile shear zone and prominent seismic reflector, were modelled to simulate the hydraulic situation at the two boreholes of the Continental Deep Drilling Program (KTB). They led to estimations of the hydraulic permeability in crystalline rock. The geometrical parameters of the fractures, such as fracture locations and orientations, were determined from structural borehole measurements, which create an image of the borehole wall. The selection of potentially open fractures was decided according to the stress field. Only fractures with the dip direction (azimuth) of the fracture plane perpendicular to the maximum horizontal stress field were assumed to be open. The motivation for this assumption is the fact that the maximum horizontal stress is higher than the vertical stress from the formation, indicating that the state of stress is a strike-slip faulting. Therefore, the probability of open fractures due to this particular stress field at the KTB sites is enhanced. Length scales for fracture apertures and extensions were stochastically varied and calibrated by hydraulic experiments. The mean fracture aperture was estimated to be 25 ,m, assuming an exponential distribution, with corresponding permeability in the range of 10,16 m2. Similar results were also obtained for log-normal and normal distributions, with a variation of permeability of the order of a factor of 2. The influence of the fracture length on permeability of the stochastic networks was also studied. Decreasing the fracture length beyond a specific threshold of 10 m led to networks with vanishing connectivity and hence vanishing permeability. Therefore, we assume a mean fracture length exceeding the threshold of 10 m as a necessary assumption for a macroscopic hydraulically active fracture system at the KTB site. The calculated porosity due to the fracture network is of the order of 10,3 per cent, which at first sight contradicts the estimated matrix porosity of 1 to 2 per cent from borehole measurements and core measurements. It can be concluded from these results, however, that if the fluid transport is due to a macroscopic fracture system, only very low porosity is needed for hydraulic flow with permeabilities up to several 10,16 m2, and hence the contribution of matrix porosity to the hydraulic transport is of a subordinate nature. [source] Modelling and analysis of attenuation anisotropy in multi-azimuth VSP data from the Clair fieldGEOPHYSICAL PROSPECTING, Issue 5 2007Sonja Maultzsch ABSTRACT Anisotropic variations in attenuation are of interest since they can give information on the fracture system and may be more amenable to measurement than absolute attenuation values. We examine methods for detecting changes in relative attenuation with azimuth from VSP data, and validate the techniques on synthetic data. Analysis of a multi-azimuth walkaway VSP data set from a fractured hydrocarbon reservoir indicates that such azimuthal variations in P-wave attenuation are observable. The effects are localized in the reservoir, and analysis allows the prediction of a fracture strike direction, which agrees with geological information. The observed effects can be modelled under reasonable assumptions, which suggests the validity of the link between the anisotropic attenuation and the fracturing. [source] Carbonatitic Volcanic Genesis of Hetaoqing Fe-Cu Deposit in Central Yunnan, ChinaRESOURCE GEOLOGY, Issue 4 2003Yongbei Zhang Abstract. The Kunyang rift on western margin of Yangtze Platform is a continental rift, and also a rare Precambrian Fe-Cu mineralization zone in China. The Wuding-Lufeng mineralization area in the middle section of the rift is an important part of the zone, and an elliptic-shaped volcanic collapsed basin, controlled by a ring fracture system with carbonatitic volcanic rocks mainly occurring along the northwestern edge of the basin. The Hetaoqing Fe-Cu ore deposit at the western side of the basin is hosted in carbonatitic volcanic rocks and pyroclastic sedimentary rocks. The original ore bodies occur as layers, bands and lenses conformable to the host carbonatitic rocks. The ores usually appear as massive, impregnated and granular in carbonatitic rocks, and as brecciform and sandy in pyroclastic sedimentary rocks. Ore-forming minerals are magnetite, hematite, chalcopy-rite, bornite, pyrite, carrollite, molybdenite, cobaltite and skinnerite, and secondary minerals limonite, chalcocite, azurite, malachite and tenorite. Gangue minerals are calcite, dolomite, ankerite, common hornblende, arfvedsonite, augite, aegirine-augite, albite, phlogopite, biotite, chlorite and apatite. Evidences of mineral chemistry, trace elements and isotopic ratios of ores, as well as geological features, suggest that the original ores are igneous in origin. Chemical features of magnetites in the deposit belong to carbonatite type, and are similar to those from the Bayan Obo carbonatites. The ores are rich in iron, titanium, rare earth elements, niobium, tantalum, gold, silver, phosphor and sulfur. These features indicate that the Fe-Cu deposit associated with volcanic activity in the Wuding-Lufeng basin is alkali-carbonatite volcanic type. [source] Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well, Lower Cretaceous, Valencia Trough, Spain)GEOFLUIDS (ELECTRONIC), Issue 3 2010E. PLAYÀ Abstract Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre-Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene-early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host-rock, suggesting a high degree of fluid-rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher 87Sr/86Sr ratios than the host-limestones. These CS3 karstic infillings recrystallized early within evolved-meteoric waters having very little interaction with the host-rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic 87Sr/86Sr values, indicating low fluid-rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm). Geofluids (2010) 10, 314,333 [source] Fluid evolution in base-metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern IrelandGEOLOGICAL JOURNAL, Issue 1 2005Martin Baron Abstract The Dalradian and Ordovician,Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base-metal sulphide-bearing vein deposits associated with kilometre-scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H2O,CO2,salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base-metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9,kbar. Caledonian mineralization in Ordovician,Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9,kbar. A later, probably Carboniferous, stage of mineralization was associated with base-metal sulphide deposition and involved a low to moderate temperature (Th 70 to 240°C), low to moderate salinity (0 to 20,wt% NaCl eq.), H2O,salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long-term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley & Sons, Ltd. [source] The Geysers geothermal field: results from shear-wave splitting analysis in a fractured reservoirGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2005Maya Elkibbi SUMMARY Clear shear-wave splitting (SWS) is observed in 1757 high signal-to-noise ratio microearthquake seismograms recorded by two high density seismic arrays in the NW and the SE Geysers geothermal fields in California. The Geysers reservoir rocks within the study area are largely composed of lithic, low-grade metamorphism, well-fractured metagraywackes which commonly lack schistosity, warranting the general assumption that shear-wave splitting here is induced solely by stress-aligned fracturing in an otherwise isotropic medium. The high quality of observed shear-wave splitting parameters (fast shear-wave polarization directions and time delays) and the generally good data spatial coverage provide an unprecedented opportunity to demonstrate the applicability and limitations of the shear-wave splitting approach to successfully detect fracture systems in the shallow crust based on SWS field observations from a geothermal reservoir. Results from borehole stations in the NW Geysers indicate that polarization orientations range between N and N60E; while in the SE Geysers, ground surface stations show polarization directions that are generally N5E, N35E-to-N60E, N75E-to-N85E, and N20W-to-N55W. Crack orientations obtained from observed polarization orientations are in good agreement with independent field evidence, such as cracks in geological core data, tracer tests, locally mapped fractures, and the regional tectonic setting. Time delays range typically between 8 and 40 ms km,1, indicating crack densities well within the norm of fractured reservoirs. The sizeable collection of high resolution shear-wave splitting parameters shows evidence of prevalent vertical to nearly vertical fracture patterns in The Geysers field. At some locations, however, strong variations of SWS parameters with ray azimuth and incident angle within the shear-wave window of seismic stations indicate the presence of more complex fracture patterns in the subsurface. [source] The modelling of multi-fracturing solids and particulate mediaINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 1 2004D. R. J. Owen Abstract Computational strategies in the context of combined discrete/finite element methods for effective modelling of large-scale practical problems involving multiple fracture and discrete phenomena are reviewed in the present work. The issues considered include: (1) Fracture criteria and propagation mechanisms within both the finite and discrete elements, together with mesh adaptivity procedures for discretization and introduction of fracture systems; (2) Detection procedures for monitoring contact between large numbers of discrete elements; (3) Interaction laws governing the response of contact pairs; (4) Parallel implementation; (5) Other issues, such as element methodology for near incompressible behaviour and generation of random packing of discrete objects. The applicability of the methodology developed is illustrated through selected practical examples. Copyright © 2004 John Wiley & Sons, Ltd. [source] Geneses of High Chlorine and Silver,Lead,Zinc,Mineralized Granitoids in Tsushima, JapanRESOURCE GEOLOGY, Issue 3 2000Shunso ISHIHARA 7037. (5) They are high in Cl and S, which occur in fluid inclusions and as pyrrhotite>pyrite, respectively. Two genetic models are considered for the source of the unique granitoid magmas: the continental crust or the upper mantle fertilized with Si, K and 18O. The latter may be the case for the Tsushima granitoids, because of the low initial Sr ratio. The age of the granitoids (16 Ma) indicates the magmatism related to the opening of the Sea of Japan. It is suggested that both basaltic and granitic magmas were generated in the continental lithosphere under an extensional tectonic setting; the two magmas could have been partly mingled. The mingled magma was originally an oxidized type, but reduced during the emplacement by repeated inflow of S and C-bearing gases from the pelitic wall rocks. Because of the reduction, SO3 sulfur is almost nil in the rock-forming apatite, and most of sulfur remained in fluid phase of the magma as reduced species. Cl content was high in the original magma and concentrated in the fluid phase of the residual system which dissolved silver, lead and zinc metals. Such a fluid migrated into the Taishu fracture systems, as the magma crystallized, and formed the silver,lead,zinc deposits. [source] Fluid dynamics and subsurface sediment mobilization processes: an overview from Southeast CaribbeanBASIN RESEARCH, Issue 4 2010Éric Deville ABSTRACT This paper discusses the origin and the dynamics of subsurface sediment mobilization processes in tectonically mobile regions and shale-rich environment. This is illustrated by the example of Trinidad and the south of the Barbados prism. In this area of the southeast Caribbean, geophysical acquisitions have spectacularly shown the widespread development of sediment mobilization features in the interference area between the southern part of the Barbados prism and the active turbidite system of the Orinoco. Numerous mud volcanoes are especially developed along ramp anticline crests through hydraulic fracture systems. The area also exhibits trends of structures that correspond to massive uplifts of well-preserved turbidite and hemipelagic sediments that cut up the surrounding sediments. Some of these structures are complicated by the development of collapse structures, calderas and superimposed mud volcanoes. The mobilized sediments expelled by the mud volcanoes are not only liquefied argillaceous but also fine sandy material from deep horizons, and various shallower formations pierced by the mud conduits. Both in the Barbados prism and in Trinidad, the expelled mud is rich in thin, angular and mechanically damaged quartz grains, which are probably cataclastic flows issued from sheared and collapsed deep sandy reservoirs. The exotic clasts and breccias result mostly from hydraulic fracturing. In Trinidad, the gas phase is mainly deep thermogenic methane associated with hydrocarbon generation at depth. Subsurface sediment mobilization notably differs from salt mobilization by the role taken by the fluid dynamics that control overpressured shale mobilization and induce sediment liquefaction. A reaction chain of several deformation processes develops around the conduits. Massive sedimentary uplift corresponds to large movements of stratified solid levels, possibly due to the tectonic inversion of pre-existing mud volcano systems. All these phenomena are controlled by the development of overpressure at depth. No evidence for piercing shale diapirs has been observed in the area studied. [source] Gas seeps linked to salt structures in the Central Adriatic SeaBASIN RESEARCH, Issue 4 2008Riccardo Geletti ABSTRACT The analyses of about 800 km of Chirp sub-bottom profilers and 600 km2 of Multibeam data acquired during the 2005 and 2007 surveys of the R/V OGS Explora, and their correlation with one new, and several public, multichannel seismic profiles, allow us to propose a relation between the distribution of gas seepages, fracture systems and deep salt features present in the Central Adriatic Sea. Gas seepage is evident from pockmarks on the seabed and in the shallow sub-bottom, where acoustic chimneys and bright spots have been highlighted and analyzed. The Mid-Adriatic Depression (MAD) is a distinct morphological feature in the Central Adriatic Sea elongated in a NE,SW direction. The area is affected by salt doming of Triassic evaporites which cause the two main alignments of the Mid-Adriatic Ridge as far as the Palagruza High and the Jabuka Ridge. These salt tectonics have existed since, at least, Paleogene times and are still active: they characterize sectors with less resistance to deformation produced by successive regional compressive regimes that have affected the area differently during the different geodynamic phases. Gas-seep features are distributed preferentially above and along the fracture systems produced above and around the salt mounds. [source] | ||||||||||