Characteristics Of Fault Zones Research Articles

Influence of Structural Symmetry of Fault Zones on Fluid-Induced Fault Slips and Earthquakes

Subsurface fluid injection and extraction can reactivate faults and induce earthquakes. In current research, faults are typically described as symmetrical structures and the presence of asymmetric structures is often overlooked. The reality is that numerous asymmetric faults exist within the Earth’s crust. The architectural and permeability characteristics of fault zones differ significantly between symmetrical and asymmetrical faults. These differences may have a great influence on fault stability during fluid injection or extraction. In this study, the impact of fault zone structures on fluid-induced slips and seismic activity were investigated through numerical analysis. The findings indicated that symmetrical faults were more likely to induce larger slips and earthquakes during various subsurface fluid operations. For asymmetric faults, larger induced slips occurred when fluid was operated in a hanging wall reservoir than in a footwall reservoir. In symmetrical faults, the opposite was true. When evaluating the stability of a fault in subsurface fluid engineering, the fault structure and fluid pattern and their combined effects must be considered comprehensively.

Diagenesis and lateral sealing types of fault zones in the Lishu Depression, Songliao Basin, NE China

The Lishu Depression in the Songliao Basin is characterized by well-developed faults, and the accumulation of oil and gas is closely related to fault sealing. However, there remains a dearth of comprehensive investigation into the diagenesis of fault zones and their influence on lateral sealing. This study employed electron probe analysis, cathodoluminescence, and carbon and oxygen isotope techniques to investigate the diagenetic processes in fault zones. This research also reconstructed the evolution of porosity and displacement pressure of fault rocks, and identified the lateral sealing characteristics of fault zones and their impact on oil and gas accumulation. The majority of faults in the study region formed earlier, and the fault zones experienced intricate diagenesis, characterized by compaction, cementation, replacement, and dissolution. The displacement pressure of fault rocks is higher during fault intermittent and static periods compared to active periods, and it is influenced by diagenesis. Late-stage calcite and pore-filling chlorite were deposited subsequent to the period of fault activity, filling dissolution pores. This led to relatively higher displacement pressure and improved lateral sealing of fault zone. The kaolinite cement exhibits a relatively elevated potassium concentration, due to its connection with the dissolution of feldspar, resulting in lower displacement pressure and poor sealing of fault zone. Based on the evolutionary characteristics of displacement pressure in fault rock and reservoir during various transportation stages, this study identified six types of lateral sealing of fault zones. Notably, the sealing types characterized by "fault core and damage zone sealing during static periods" and "fault core and damage zone sealing during intermittent and static periods" exhibit superior lateral sealing properties, effectively controlling the accumulation of oil and gas in the sand bodies on either side of the fault zone.

Trakya Havzasının Güneybatısında Fay Parametrelerinin Gravite Gradyent Verilerinden Kestirimi

Characterization of fault zones plays an important role for earthquake studies, geothermal and mineral resource exploration. This problem can be achieved by optimizing parameters of a defined synthetic model iteratively using gravity, gravity gradients or magnetic data. Curvature gradients indicate deviations of an equipotential surface from a spherical surface, therefore, reflecting deviations of mass distributions from a source point. In this study, differential curvature gradient is used as it identifies linear structures such as dip-slip faults and geological contacts. Curvature gradients are computed from gravity gradient observations which are derived from the high-resolution Earth Gravitational Model 2008. Estimating the fault parameters from gravity gradients is a complex nonlinear geophysical inverse problem that requires finding the minimum of a multi-variable cost function. A global optimization technique called simulated annealing (SA) is used to estimate location, dip angle and some depth parameters of the faults located in southwest side of the Thrace basin in Türkiye. In the optimization technique, a straight solution is inevitably applied to calculate the theoretical anomaly to be compared with the observed anomaly in each search step. In this case, the problem requires constructing a mathematical or geophysical interpretation model. Such a model is designed as a dip-slip fault model. The results show that dip angle estimates indicate high-angle faults which are found to be consistent with the seismic studies.

Extensional Faulting Around Kolumbo Volcano, Aegean Sea—Relationships Between Local Stress Fields, Fault Relay Ramps, and Volcanism

AbstractThe Christiana‐Santorini‐Kolumbo (CSK) volcanic field has hosted more than 100 explosive eruptions in the past 250,000 years, including the 1650 CE eruption of Kolumbo Volcano. Previous studies have established a link between regional tectonics and volcanism in the CSK volcanic field. While 2D seismic reflection data have given valuable insight into regional faulting, detailed fault zone characterization has been precluded by the sparsely spaced profiles. Using 3D seismic reflection data around Kolumbo Volcano, we provide the first 3D characterization of fault zones in the CSK volcanic field. Beneath the volcano's northwestern flank, and farther to the northwest, normal faults are predominantly NE‐SW trending, with mean fault trends between 044° and 049°. Normal faults beneath the southeastern flank are slightly more north‐oriented, with mean fault trends between 028° and 038°. Our detailed fault zone analysis reveals clear NW‐SE directed extension around the volcano, consistent with published focal mechanisms from microseismicity. The Kolumbo Fault Zone, ∼6 km northwest of Kolumbo Volcano, is characterized by distinct relay ramps between major overstepping normal faults. Regional 2D seismic profiles reveal a previously undocumented volcanic cone directly above the fault zone. Magma ascent to this cone has likely exploited enhanced vertical permeability associated with distributed deformation within a relay ramp. We suggest that fault relay structures may play an important role, over a range of spatial scales, in focusing magma ascent within the CSK volcanic field.

Water-blocking characteristics of fault zones with different structural combination of cementation degree

Recently, deep coal mining has posed a significant challenge owing to the threat of water inrush from highly pressured aquifers in the floor strata. Water inrush in the floor strata is often associated with the structural characteristics of the underlying fault zones. The permeability of these fault zones is a critical factor in determining their potential as conduits for water flow. The variation in permeability is attributed to the varying cementation degree across different locations within the fault zones. Hence, this study is based on the key stratum theory and introduces the concept of a water-blocking key stratum within fault zones. This research focuses on the engineering context of the 16th coal seam above the lower coal seam in the 10603 working face of the Yangcun Coal Mine's tenth mining area. The study investigates the activation characteristics of the 10605F5 reverse fault zone, which cuts through the working face. Various methods, including test of simulation of similar materials and numerical simulations, are employed to analyse the hydraulic pressure distribution and the impact of mining activities on the water-blocking key stratum within the 10605F5 reverse fault zone. The research findings will provide valuable insights for preventing fault activation and water inrush.

Characteristics and Seismic Identification Mode of Ultra-Deep Carbonate Fault-Controlled Reservoir in Northwest China

Ordovician carbonate reservoirs in the Shunbei area of the Tarim Basin are characterized by large burial depth, small vertical fault displacement of the strike-slip fault zone, small karst development scale, strong reservoir heterogeneity, complex and diverse seismic reflection types, which lead to unclear seismic response characteristics of fault zone and reservoir, and further restrict the fine description of ultra-deep (>7300 m) fault controlled reservoirs. On the basis of high-precision 3D seismic data analysis, combined with drilling, logging, and comprehensive geological interpretation data in the Shunbei area, seismic geological models of different types of reservoirs and fault zones are established. The changes in seismic response characteristics of fault zones and reservoirs are simulated and analyzed by changing reservoir size, scale, fault and fracture cavity combination modes, as well as other factors. On this basis, two seismic identification models of fault-controlled reservoirs in the Shunbei area are summarized and established. The seismic identification mode of seismic reflection marker wave of fault-controlled reservoir is mainly “fault seismic response characteristics & weak reflection” and red trough anomaly under T74. The internal seismic identification modes are mainly “fault seismic response characteristics & random reflection”, “fault seismic response characteristics & beading”, “fault seismic response characteristics & linear weak reflection”, and “fault seismic response characteristics & random reflection”. Among them, except for “fault seismic response characteristics & random reflection”, the coincidence rate of reservoir prediction of other three types of seismic response characteristics is more than 70%. The coincidence rate of the other three types of seismic response characteristics of a reservoir is more than 70%. The research results provide a reference for the prediction and description of ultra-deep carbonate fault-controlled reservoirs.

Role of Atmospheric Temperature and Seismic Activity in Spring Water Hydrogeochemistry in Urumqi, China.

Springs offer insight into the sources and mechanisms of groundwater recharge and can be used to characterize fluid migration during earthquakes. However, few reports provide sufficient annual hydrochemical and isotopic data to compare the variation characteristics and mechanisms with both atmospheric temperature and seismic effects. In this study, we used continuous δ2H, δ18O, and major ion data from four springs over 1 year to understand the groundwater origin, recharge sources, circulation characteristics, and coupling relationships with atmospheric temperature and earthquakes. We found that (1) atmospheric temperatures above and below 0 °C can cause significant changes in ion concentrations and water circulation depth, resulting in the mixing of fresh and old water in the aquifer, but it cannot cause changes in δ2H and δ18O. (2) Earthquakes of magnitude ≥ 4.8 within a 66 km epicentral distance can alter fault zone characteristics (e.g., permeability) and aggravate water–rock reactions, resulting in significant changes in δ2H, δ18O, and hydrochemical ion concentrations. (3) Hydrogen and oxygen isotopes are the most sensitive precursory seismic indicators. The results of this study offer a reference for the establishment of long-term hydrochemical and isotopic monitoring, with the potential for use in earthquake forecasting.

Resolving Differences in the Rupture Properties of M5 Earthquakes in California Using Bayesian Source Spectral Analysis

AbstractThe spectra of earthquake waveforms can provide important insight into rupture processes, but the analysis and interpretation of these spectra is rarely straightforward. Here we develop a Bayesian framework that embraces the inherent data and modeling uncertainties of spectral analysis to infer key source properties. The method uses a spectral ratio approach to correct the observed S‐wave spectra of nearby earthquakes for path and site attenuation. The objective then is to solve for a joint posterior probability distribution of three source parameters—seismic moment, corner frequency, and high‐frequency falloff rate—for each earthquake in the sequence, as well as a measure of rupture directivity for select target events with good azimuthal station coverage. While computationally intensive, this technique provides a quantitative understanding of parameter tradeoffs and uncertainties and allows one to impose physical constraints through prior distributions on all source parameters, which guide the inversion when data is limited. We demonstrate the method by analyzing in detail the source properties of 14 different target events of magnitude M5 in southern California that span a wide range of tectonic regimes and fault systems. These prominent earthquakes, while comparable in size, exhibit marked diversity in their source properties and directivity, with clear spatial patterns, depth‐dependent trends, and a preference for unilateral directivity. These coherent spatial variations source properties suggest that regional differences in tectonic setting, hypocentral depth or fault zone characteristics may drive variability in rupture processes, with important implications for our understanding of earthquake physics and its relation to hazard.

Outcrop-scale fracture analysis and seismic modelling of a basin-bounding normal fault in platform carbonates, central Italy

Faults are characterized by a complex internal architecture. In carbonates, the geometry, attitude, and distribution of fault-related fractures and subsidiary faults can largely affect the petrophysical properties and hydraulic behavior of the fault zone. This work investigates the footwall damage zone of a seismic-scale normal fault (throw ∼ 300 m) from a structural, petrophysical and seismic point of view. The studied Venere Fault (VF) bounds the intra-mountain Fucino Basin (central Italy) and crosscuts Lower Cretaceous platform carbonates. A significant portion of the footwall VF damage zone (VF-DZ) is well exposed in the 400 × 200 m Santilli Quarry. There, we assess the amount of outcrop-scale fracture porosity and permeability by in-situ fracture analyses and permeability measurements. The results show a composite power-law decay of fracture intensity away from the main slip surfaces, strongly influenced by subsidiary faults. An outcrop-based, digital 2D model of the VF-DZ is constructed and populated with acoustic properties (Vp, Vs and density) derived from both the matrix and fracture porosities. This model is enlarged five times and used for seismic modelling to investigate the seismic signature of the VF-DZ under different but realistic geological and geophysical conditions. Seismic modelling suggests that within the modelled damage zone and for wave frequencies of 20–40 Hz, seismic impedance contrasts associated with subsidiary faults may be imaged, depending on the degree of fracture porosity, fracture aperture, and the illumination angle (a measure of the maximum dip that can be imaged), the last two parameters being controlled by overburden depth. These results have implications for the seismic interpretation and characterization of fault zones in carbonates, and hence for the evaluation of fluid migration through these structures.

Sensitivity Analysis of FWI Applied to OVSP Synthetic Data for Fault Detection and Characterization in Crystalline Rocks

We have performed several sensitivity studies to assess the ability of the Full Wave Inversion method to detect, delineate and characterize faults in a crystalline geothermal reservoir from OVSP data. The distant goal is to apply the method to the Soultz-sous-Forêts site (France). Our approach consists of performing synthetic Full Wave 2D Inversion experiments using offset vertical seismic and comparing the estimated fields provided by the inversion, i.e., the estimated underground images, to the initial reference model including the fault target. We first tuned the inversion algorithmic parameters in order to adapt the FWI software, originally dedicated to a sedimentary context, to a crystalline context. In a second step, we studied the sensitivity of the FWI fault imaging results as a function of the acquisition geometry parameters, namely, the number of shots, the intershot distance, the maximum offset and also the antenna length and well deviation. From this study, we suggest rules to design the acquisition geometry in order to improve the fault detection, delineation and characterization. In a third step, we studied the sensitivity of the FWI fault imaging results as a function of the fault or the fault zone characteristics, namely, the fault dip, thickness and the contrast of physical parameters between the fault materials and the surrounding fresh rocks. We have shown that a fault with high dip, between 60 and 90° as thin as 10 m (i.e. lower than a tenth of the seismic wavelength of 120 m for Vp and 70 m for Vs) can be imaged by FWI, even in the presence of additive gaussian noise. In summary, for a crystalline geological context, and dealing with acceptable S/N ratio data, the FWI show a high potential for accurately detecting, delineating and characterizing the fault zones.

Detecting fault zone characteristics and paleovalley incision using electrical resistivity: Loma Blanca Fault, New Mexico

We have combined electrical resistivity tomography (ERT), geologic information from boreholes and outcrops, and hydrogeologic data to investigate field-scale fault-zone cementation of the Loma Blanca Fault in the Rio Grande Rift. We have collected electrical resistivity data from 16 transects and geologic samples from 29 boreholes (completed as groundwater wells to 30 m depth) across and around the fault. The 2D ERT profiles, whose interpretations are constrained by geologic data, indicate (1) a high resistivity zone in cemented portions of the fault below the water table and (2) in the unsaturated zone, a low-resistivity feature along the cemented portions of the fault. The high-resistivity zone below the water table is consistent with a 10% reduction in porosity due to the fault zone cementation. With the same porosity in the unsaturated zone, the low-resistivity feature in the cemented fault zone is consistent with saturation >0.7, in contrast to saturation 0.2–0.7 for sediment outside of the cemented fault zone. In addition, subsurface samples and ERT profiles delineate a buttress unconformity (i.e., steeply dipping erosional contact) corresponding to a paleovalley margin. This unconformity truncates the cemented fault zone and separates Pliocene axial-fluvial sand (deposited by an ancestral Rio Grande) from late Quaternary sand and gravel (deposited by the Rio Salado, a Rio Grande tributary). The cemented fault zone in the southern portion of the study area is a hydrogeologic barrier; north of the buttress unconformity, where the cemented fault zone has been removed by erosion, the fault is not a hydrogeologic barrier. The integration of geologic, geophysical, and hydrogeologic observations is key to developing our understanding of this complex system, and it allows us to demonstrate the utility of ERT in detecting subsurface fault-zone cementation.

Mechanism of Water Inrush Through Fault Zones Using a Coupled Fluid–solid Numerical Model: A Case Study in the Beiyangzhuang Coal Mine, Northern China

To better understand the mechanism of water inrush from fault zones, we investigated a typical inrush case from a coal mine in northern China, identified the water source and the role of fault zones, and analyzed the mechanical characteristics of the rock mass. We established a numerical model of fluid–solid coupling using finite difference methods based ideal elastic–plastic and Biot’s consolidation theories. The model considers softening effects caused by mine dewatering as well as the dynamic influence of mean principal stress and pore water pressure on fault zone permeability. It also simulates fault zone deformation, failure, and seepage under natural and disturbed-state conditions. The mechanism of water inrush through fault zones in a disturbed area was identified by comparing the deformation, failure, and seepage characteristics of fault zones with and without disturbance. The results show that deformation was concentrated at the bottom of the fault zone because of its naturally low strength, which created favorable conditions for water inrush. Softening of the media by mine dewatering also facilitated water inrush. Feedback effects between the fault zone and groundwater-enhanced permeability then led to groundwater bursting into the mine through the fault zone. This study provides an important framework for groundwater hazard prevention in similar mining areas.

Determination of local active tectonics regime in central and northern Greece, using primary geodetic data

Numerous active fault zones are detected into the central and northern part of Greece, while some of them are associated with destructive seismic events. Geometrically, the fault zone characteristics differentiate, showing both extensional and compressional tectonic features. The geodetic data used, recorded by permanent GPS/GNSS stations, provides a precise analysis the tectonic setting. In particular, 58 permanently installed GPS/GNSS stations are located into the study area, having collected primary data for 7 consecutive year time-period. The calculation of the East and North velocity components and their errors, respectively, derived from each GPS/GNSS station, was carried out implementing the triangulation methodology, based on the combination of three different GPS/GNSS stations each time. The dense GPS/GNSS stations network led to the construction of 1092 different triangles, while the centroid of each triangle was extracted. For each centroid, the following parameters were determined: (1) maximum horizontal extension, (2) minimum horizontal extension, (3) total velocity, (4) maximum shear strain, (5) area strain and (6) rotation. The extracted values were geostatistically processed (interpolation—grid pattern), showing low to medium values, especially north of the extension of the North Aegean Fault System, while the same parameters are high at the southern part of Greece. Therefore, the geological and seismological data are confirmed showing that Central–Northern Greece is tectonically less active than Southern Greece. However, it is mentioned that the neotectonic fault length can be related to the production of significant earthquakes, albeit in longer recurrence intervals.

Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

This study attempts to reveal the fault zone characteristics of the locked Ganos Fault based on electrical resistivity studies including audio-frequency (AMT: 10,400–1 Hz) and wide-band (MT: 360–0.000538 Hz) magnetotellurics near the epicenter of the last major event, that is, the 1912 Mürefte Earthquake (Mw 7.4). The AMT data were collected at twelve stations, closely spaced from north to south, to resolve the shallow resistivity structure to 1 km depth. Subsequently, 13 wide-band MT stations were arranged to form a grid enclosing the AMT profile to decipher the deeper structure. Three-dimensional inverse modeling indicates highly conductive anomalies representing fault zone conductors along the Ganos Fault. Subsidiary faults around the Ganos Fault, which are conductive structures with individual mechanically weak features, merge into a greater damage zone, creating a wide fluid-bearing environment. This damage zone is located on the southern side of the fault and defines an asymmetry around the main fault strand, which demonstrates distributed conduit behavior of fluid flow. Ophiolitic basement occurs as low-conductivity block beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Resistive structures on both sides of the fault beneath this transition suggest that the lack of seismicity might be related to the absence of fluid pathways in the seismogenic zone.Graphical .

Characteristics of fault zones and their control on remaining oil distribution at the fault edge: a case study from the northern Xingshugang Anticline in the Daqing Oilfield, China

Most major oil zones in the Daqing Oilfield have reached a later, high water cut stage, but oil recovery is still only approximately 35 %, and 50 % of reserves remain to be recovered. The remaining oil is primarily distributed at the edge of faults, in poor sand bodies, and in insufficiently injected and produced areas. Therefore, the edge of faults is a major target for remaining oil enrichment and potential tapping. Based on the dynamic change of production from development wells determined by the injection–recovery relationship at the edge of faults, we analyzed the control of structural features of faults on remaining oil enrichment at the edge. Our results show that the macroscopic structural features and their geometric relationship with sand bodies controlled remaining oil enrichment zones like the edges of NNE-striking faults, the footwalls of antithetic faults, the hard linkage segments (two faults had linked together with each other to form a bigger through-going fault), the tips of faults, and the oblique anticlines of soft linkages. Fault edges formed two types of forward micro-amplitude structures: (1) the tilted uplift of footwalls controlled by inverse fault sections and (2) the hanging-wall horizontal anticlines controlled by synthetic fault points. The remaining oil distribution was controlled by micro-amplitude structures. Consequently, such zones as the tilted uplift of the footwall of the NNW-striking antithetic faults with a fault throw larger than 40 m, the hard linkage segments, the tips of faults, and the oblique anticlines of soft linkage were favorable for tapping the remaining oil potential. Multi-target directional drilling was used for remaining oil development at fault edges. Reasonable fault spacing was determined on the basis of fault combinations and width of the shattered zone. Well core and log data revealed that the width of the shattered zone on the side of the fault core was less than 15 m in general; therefore, the distance from a fault to the development target should be larger than 15 m. Vertically segmented growth faults should take the separation of the lateral overlap of faults into account. Therefore, the safe distance of remaining oil well deployment at the fault edge should be larger than the sum of the width of shattered zone in faults and the separation of growth faults by vertical segmentation.

Fault zone characteristics and basin complexity in the southern Salton Trough, California

Ongoing oblique slip at the Pacific–North America plate boundary in the Salton Trough produced the Imperial Valley (California, USA), a seismically active area with deformation distributed across a complex network of exposed and buried faults. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project to construct a three-dimensional P-wave velocity model down to 8 km depth and a velocity profile to 15 km depth, both at 1 km grid spacing. A VP = 5.65–5.85 km/s layer of possibly metamorphosed sediments within, and crystalline basement outside, the valley is locally as thick as 5 km, but is thickest and deepest in fault zones and near seismicity lineaments, suggesting a causative relationship between the low velocities and faulting. Both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge-shaped basin, where two deep subbasins are located. We estimate basement depths, and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas.

Permeability distribution in the Lahendong geothermal field: A blind fault captured by thermal–hydraulic simulation

Subsurface fluid flow of reservoirs in active tectonic regions is mainly controlled by permeability of fault zones. Therefore, the characterization of fault zones is an important step toward performance assessment of a reservoir. The fluid flow is controlled also by pressure and temperature conditions. In this context, we simulated pressure and temperature fields to elaborate on the influence of permeability on subsurface fluid flow in the Lahendong geothermal reservoir. Thermal–hydraulic simulation is performed using a finite element approach. Adjusting the permeability through 370 different cases, modeling results converged to the observed data within a misfit range of 0–7 %. The best fitting models identified a deep-seated fault that has previously not been traced at the surface. Simulated temperature distribution suggests a prominent convective heat flow, driven by an upward migrating and SW–NE oriented fluid flow. This hydraulic gradient causes a pressure drop along the reservoir. High-pressure patterns are used to constrain recharge areas, in addition to infiltration measurements. Discharge flow occurs from SW to NE migrating also upward toward the hot springs. In that frame, thermal–hydraulic simulations identified previously unresolved subsurface faults, which now allow a better understanding of the subsurface permeability and its influence on fluid flow.

Characterization of seismic reflections from faults in a crystalline environment, Schneeberg, Germany

ABSTRACTWe present an approach for analysing seismic reflections from faults in a crystalline hard rock environment. We analysed 3D seismic reflection data for geothermal reservoir characterization acquired in the Erzgebirge Region, Germany. The seismic image derived from this data set revealed two main features: a less pronounced reflector corresponding to a steeply dipping major fault zone Roter Kamm and a group of pronounced reflectors attributed to the existence of conjugate mineralized faults. We analysed these reflections in the pre‐stack data to characterize the nature and origin of reflectivity. This was done by extracting the corresponding waveforms from the raw data and carefully pre‐processing them, including amplitude correction for geometrical spreading and signal‐to‐noise enhancement. Reflection coefficients were derived from the pre‐processed shot gathers by comparing the amplitudes of the reflected and direct waves. Synthetic waveform modelling using the reflectivity method has been performed for several model families consisting of one‐dimensional velocity–depth functions with varying velocities, densities, and thicknesses of the layers. A comparison of the modelled and observed waveforms revealed that a reflection coefficient of 0.18 for the conjugate mineralized faults can be explained by single layers with high impedance contrast and a thickness between 30 m and 40 m, whereas the reflection from the Roter Kamm fault zone with a reflection coefficient of −0.23 requires a model consisting of several low‐velocity layers with a total thickness of up to 100 m embedded in a high‐velocity background model. These results are in accordance with the geological interpretation of these reflectors. However, the characteristics of these reflections vary significantly within the investigation area, both in terms of the reflection coefficient and the waveform, which is also in agreement with the general lateral variation of fault zone characteristics known from tectonic investigations such as geological mapping of outcrops and fabric analysis.

Local stress anomaly in the Bavarian Molasse Basin

The characterization of fault zones in the Bavarian Molasse Basin plays a major role for further geothermal reservoir development. Hence, their identification, geological origin, and hydraulic characterization are discussed extensively. Stress indicators and fractures are interpreted from image and caliper logs of three highly deviated wells at the Sauerlach site. We transform the identified stress field into the borehole coordinate system and compare the observed orientation to the modeled stress field which assumes a homogeneous borehole surrounding. High breakout occurrence, cross-cutting fractures, and a fracture orientation from N-S to NNE-SSW are observed in Sauerlach Th1. In Sauerlach Th2 and Th3, fractures strike primarily ENE-WSW and N-S to NNE-SSW. Drilling-enhanced natural fractures and drilling-induced tensile fractures are observed in all three wells and indicate the orientation of tensile stress at the borehole wall. In Sauerlach Th2 and Th3, stress transformation indicates a SH-dir. ~ N 10°E in a strike-slip stress regime. The modeled stress orientations match the observed orientations within the well Sauerlach Th1 if either SH-dir. is N 320°E in a strike-slip regime or SH-dir. is N 10°E in a normal faulting regime. This approach improves the detection of the local stress field especially for non-vertical wells, which has, in combination with the facture pattern, a major impact on the hydraulic system of the geothermal reservoir.

Fault zone characteristics, fracture systems and permeability implications of Middle Triassic Muschelkalk in Southwest Germany

Fault zone structure and lithology affect permeability of Triassic Muschelkalk limestone-marl-alternations in Southwest Germany, a region characterized by a complex tectonic history. Field studies of eight fault zones provide insights into fracture system parameters (orientation, density, aperture, connectivity, vertical extension) within fault zone units (fault core, damage zone). Results show decreasing fracture lengths with distances to the fault cores in well-developed damage zones. Fracture connectivity at fracture tips is enhanced in proximity to the slip surfaces, particularly caused by shorter fractures. Different mechanical properties of limestone and marl layers obviously affect fracture propagation and thus fracture system connectivity and permeability. Fracture apertures are largest parallel and subparallel to fault zones and prominent regional structures (e.g., Upper Rhine Graben) leading to enhanced fracture-induced permeabilities. Mineralized fractures and mineralizations in fault cores indicate past fluid flow. Permeability is increased by the development of hydraulically active pathways across several beds (non-stratabound fractures) to a higher degree than by the formation of fractures interconnected at fracture tips. We conclude that there is an increase of interconnected fractures and fracture densities in proximity to the fault cores. This is particularly clear in more homogenous rocks. The results help to better understand permeability in Muschelkalk rocks.