Fracture systems in normal fault zones crosscutting sedimentary rocks,Northwest German Basin

Field studies of fracture systems associated with 58 normal fault zones crosscutting sedimentary rocks were performed in the Northwest German Basin. Fracture orientations, densities, apertures and lengths, as well as fault zone structural indices, were analysed separately for fault damage zones and host rocks. The results show a pronounced difference between carbonate and clastic rocks: mainly in carbonate rocks we found presence of clear damage zones, characterized by higher fracture densities than in the host rocks. While the maximum aperture is similar for both units, the percentage of fractures with large apertures is much higher in the damage zones than in the host rocks.Based on laboratory measurements of Young's moduli and field measurements of fracture densities, we calculate effective stiffnesses E_e, that is the Young's moduli of the in situ rock masses, within the normal fault zones. Compared with carbonate rocks, E_e computed for clastic-rock damage zones decreases significantly less due to lower fracture densities. We conclude that normal fault zones in carbonate rocks have more profound effects on enhancing permeability in fluid reservoirs than those in clastic rocks. The results are of great importance for modelling the hydromechanical behaviour of normal fault zones in subsurface fluid reservoirs.     

Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway

Systematic field mapping of fracture lineaments observed on aerial photographs shows that almost all of these structures are positively correlated with zones of high macroscopic and mesoscopic fracture frequencies compared with the surroundings. The lineaments are subdivided into zones with different characteristics: (1) a central zone with fault rocks, high fracture frequency and connectivity but commonly with mineral sealed fractures, and (2) a damage zone divided into a proximal zone with a high fracture frequency of lineament parallel, non-mineralized and interconnected fractures, grading into a distal zone with lower fracture frequencies and which is transitional to the surrounding areas with general background fracturing. To examine the possible relations between lineament architecture and in-situ rock stress on groundwater flow, the geological fieldwork was followed up by in-situ stress measurements and test boreholes at selected sites. Geophysical well logging added valuable information about fracture distribution and fracture flow at depths. Based on the studies of in-situ stresses as well as the lineaments and associated fracture systems presented above, two working hypotheses for groundwater flow were formulated: (i) In areas with a general background fracturing and in the distal zone of lineaments, groundwater flow will mainly occur along fractures parallel with the largest in-situ rock stress, unless fractures are critically loaded or reactivated as shear fractures at angles around 30° to σ_H; (ii) In the influence area of lineaments, the largest potential for groundwater abstraction is in the proximal zone, where there is a high fracture frequency and connectivity with negligible fracture fillings. The testing of the two hypotheses does not give a clear and unequivocal answer in support of the two assumptions about groundwater flow in the study area. But most of the observed data are in agreement with the predictions from the models, and can be explained by the action of the present stress field on pre-existing fractures.     

碳酸盐岩断裂破碎带结构、分布与发育机制

断裂带通常包括狭窄的断层核与宽阔的破碎带,并对岩石物理及其水力学、流体与渗透性具有重要作用。根据断裂破碎带结构的差异,将碳酸盐岩断层核划分为颗粒支撑、基质支撑与胶结支撑等3种类型,将碳酸盐岩破碎带划分碎裂带与裂缝带。不同类型碳酸盐岩断层核与破碎带形成多种类型组合,并造成断裂破碎带多种复杂的渗流结构。通过井一震资料的结合,可以判识沉积盆地内部较宽的碳酸盐岩断裂破碎带,其宽度一位移关系可以指示断裂破碎带的不同生长方式与机制。复杂内部结构、多期构造成岩作用与non-Andersonian破裂机制可能导致碳酸盐岩断裂破碎带的异常生长,也是碳酸盐岩断裂破碎带机理研究的重要方向。     

碳酸盐岩断裂破碎带结构、分布与发育机制

断裂带通常包括狭窄的断层核与宽阔的破碎带,并对岩石物理及其水力学、流体与渗透性具有重要作用。根据断裂破碎带结构的差异,将碳酸盐岩断层核划分为颗粒支撑、基质支撑与胶结支撑等3种类型,将碳酸盐岩破碎带划分碎裂带与裂缝带。不同类型碳酸盐岩断层核与破碎带形成多种类型组合,并造成断裂破碎带多种复杂的渗流结构。通过井—震资料的结合,可以判识沉积盆地内部较宽的碳酸盐岩断裂破碎带,其宽度—位移关系可以指示断裂破碎带的不同生长方式与机制。复杂内部结构、多期构造成岩作用与non-Andersonian破裂机制可能导致碳酸盐岩断裂破碎带的异常生长,也是碳酸盐岩断裂破碎带机理研究的重要方向。     

碳酸盐岩断层破碎带构造成岩作用——以塔中Ⅰ号断裂带为例

多旋回叠合盆地断层具有异常复杂的构造变形与成岩演变,跨学科结合的构造成岩作用研究为断层破碎带演变机制与流体-岩石作用分析提供了新思路。结合塔中Ⅰ号断裂带构造解析和地化资料分析,探讨古老碳酸盐岩断层破碎带构造成岩作用的特殊性。结果表明,塔中Ⅰ号断裂带上奥陶统台缘带碳酸盐岩断层破碎带发育多类、多期构造成岩作用,明显不同于围岩。断层破碎带宽度高达2~4km,裂缝类型多样、纵横向变化大、开启程度较高。沿断裂带压实作用较弱且有差异,发现有碳酸盐岩压实变形带发育,并有后期裂缝与溶蚀作用形成的局部高渗透带。断层破碎带观测到多期压溶低角度缝合线与高角度构造缝合线,部分具有渗透性。多达4~5期不同类型胶结作用沿裂缝带发育,大气淡水胶结较多,胶结程度相对较弱。准同生期大气淡水溶蚀与局部风化壳岩溶主要沿断层破碎带发育,而且埋藏溶蚀作用较强,发育大型缝洞体与溶蚀孔洞。构造成岩作用揭示塔中Ⅰ号断裂带东段上奥陶统碳酸盐岩断层破碎带经历断裂萌芽期-形成期-定型期-复活扩张期-局部开启期等5期演变,不同演化期构造成岩作用有明显差异。构造成岩作用的多期差异活动是造成古老碳酸盐岩断层破碎带复杂性的重要因素。     

碳酸盐岩断裂相分类特征——以新疆塔里木盆地柯坪露头为例

断裂相的概念为断裂带的内部结构研究提供了新的思路与建模方法,通过塔里木盆地柯坪露头断裂带的分析,碳酸盐岩断裂相特征有别于碎屑岩.柯坪露头碳酸盐岩断裂带不连续构造以滑动面、裂缝带和变形带发育为特征.根据形态识别出平直截切型、弯曲起伏型、渐变条带型三种类型滑动面.破碎带中裂缝带发育,裂缝充填少,是良好输导通道;断层核部存在...     

Modelling of fractured carbonate reservoirs: outline of a novel technique via a case study from the Molasse Basin, southern Bavaria, Germany

Fluid flow in low-permeable carbonate rocks depends on the density of fractures, their interconnectivity and on the formation of fault damage zones. The present-day stress field influences the aperture hence the transmissivity of fractures whereas paleostress fields are responsible for the formation of faults and fractures. In low-permeable reservoir rocks, fault zones belong to the major targets. Before drilling, an estimate for reservoir productivity of wells drilled into the damage zone of faults is therefore required. Due to limitations in available data, a characterization of such reservoirs usually relies on the use of numerical techniques. The requirements of these mathematical models encompass a full integration of the actual fault geometry, comprising the dimension of the fault damage zone and of the fault core, and the individual population with properties of fault zones in the hanging and foot wall and the host rock. The paper presents both the technical approach to develop such a model and the property definition of heterogeneous fault zones and host rock with respect to the current stress field. The case study describes a deep geothermal reservoir in the western central Molasse Basin in southern Bavaria, Germany. Results from numerical simulations indicate that the well productivity can be enhanced along compressional fault zones if the interconnectivity of fractures is lateral caused by crossing synthetic and antithetic fractures. The model allows a deeper understanding of production tests and reservoir properties of faulted rocks.     

Fluid infiltration associated with seismic faulting: Examining chemical and mineralogical compositions of fault rocks from the active Chelungpu fault

In order to understand the fault zone architecture and mechanisms that caused the Chi-Chi earthquake, the Chelungpu drilling project was conducted during April 2000 through a collaborative project between Japan and Taiwan. In this study, chemical and mineralogical variations within the overall Chelungpu fault zone, including variations between less damaged host rocks, damaged zones, and fault cores caused by the Chi-Chi earthquake were examined. Slopes of TiO₂ immobile isocons were consistently > 1 for analyses comparing host rocks with rocks from damaged zones or with gouges from fault cores, indicating that volume loss occurred in damaged zones and the fault cores. These results strongly imply that pervasive fluid infiltration occurred within the fault zone. Volume loss within the damaged zone and fault core is interpreted to result from a two-stage process involving: (i) coseismic mechanical wearing and/or dissolution in the fault core, and (ii) fluid infiltration within the fault zone during postseismic and interseismic periods along cracks caused by seismic failure. Semi-quantitative XRD analysis indicates that the kaolinite content consistently increases from the less damaged host rocks to the damaged zone and gouges in each fault core. Mineralogic changes indicate that pervasive acidic fluid infiltration occurred within the fault zones and reacted with the feldspars or muscovite to form kaolinite. Enrichment of kaolinite and illite found in the fault zones of southern drilling site could play some role on the slipping behavior of the southern part of the Chelungpu fault. Greater volume loss in the fault core may have resulted from moderate permeability, combined with the very fine grain nature of pulverized material in the fault core, which enhanced chemical reactions including transformation of feldspars and muscovite to clay minerals. The study results indicate that pervasive fluid infiltration occurred and changed the mineralogical and chemical architecture of fault zones caused by the cyclic earthquakes.     

Embedded discontinuity finite element modeling of fluid flow in fractured porous media

In many geomaterials, particularly rocks and clays, permeability is greatly enhanced by the presence of fractures. Fracture sets create an overall permeability that is anisotropic, enhanced in the directions of the fractures. In modeling the fractures via a finite element method, for example, meshing around these fractures can become quite difficult and result in computationally intensive systems. In this article, we develop a relatively simple method for including the fractures within the elements. Flow through the bulk medium is assumed to be governed by Darcy’s law, and the flow on the fracture by a generalization of the law. This model is embedded in a finite element framework, with the fractures passing through the elements. In this formulation, elements with fractures are given an enhanced permeability in the direction of the fractures. With these enhancements, the material essentially becomes anisotropically more permeable in the direction of fracture sets.     

Hydromechanical reactivation of natural discontinuities: mesoscale experimental observations and DEM modeling

Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field.

     

Structure and Fluid Transportation Performance of Faults in the Changxing-Feixianguan Formation,Xuanhan County,Northeastern Sichuan Basin

On the basis of field observations, microscopic thin-sections and laboratory data analysis of ten faults in Xuanhan County area, northeastern Sichuan Basin, central China, the internal and megascopic structures and tectonite development characteristics are mainly controlled by the geomechanical quality in brittle formation of the Changxing-Feixianguan Formation. The fluid transportation performance difference between the faults formed by different geomechanics or different structural parts of the same fault are controlled by the mcgascopic structure and tectonite development characteristics. For instance, the extension fault structure consists of a tectonite breccia zone and an extension fracture zone. Good fluid transportation performance zones are the extension fracture zone adjacent to the tectonite breccia zone and the breccia zone formed at the early evolutionary stage. The typical compression fault structure consists of a boulder-clay zone or zones of grinding gravel rock, compression foliation, tectonite lens, and dense fracture development. The dense fracture development zone is the best fluid transporting area at a certain scale of the compression fault, and then the lens, grinding gravel rock zone and compression foliation zones are the worst areas for hydrocarbon migration. The typical tensor-shear fault with a certain scale can be divided into boulder-clay or grinding gravel rock zones of the fault, as well as a pinnate fractures zone and a derivative fractures zone. The grinding gravel rock zone is the worst one for fluid transportation. Because of the fracture mesh connectivity and better penetration ability, the pinnate fractures zone provides the dominant pathway for hydrocarbon vertical migration along the tensor-shear fault.     

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.     

塔里木盆地走滑带碳酸盐岩断裂相特征及其与油气关系

通过露头与井下资料的综合分析,塔里木盆地奥陶系碳酸盐岩走滑断裂带断裂相具有多样性,根据内部构造发育程度可以分为断层核发育、断层核欠发育两类。露头走滑带断层核部以裂缝带、透镜体、滑动面等断裂相发育为特征,断裂边缘的破碎带发育裂缝带、变形带。裂缝带主要分布在断层核附近50m的破碎带内,裂缝多开启,渗流性好。断裂核部透镜体发育,在破碎带也有分布,破碎角砾组合的透镜体多致密。滑动面具有平直截切型、渐变条带型等两种类型,多为开启的半充填活动面。变形带多为方解石与碎裂岩充填,破碎带局部部位裂缝与溶蚀作用较发育。利用地震剖面、构造图、相干图等资料可以判识塔里木盆地内部奥陶系碳酸盐岩走滑断裂相的特征及其发育程度,沿走滑断裂带走向上断裂相具有分段性与差异性,根据渗流性可以定性区分高渗透相、致密相区。沿断裂带高渗透相区是碳酸盐岩缝洞体储层发育的有利部位。断裂相的横向变化造成油气分布的区段性,形成高渗透相输导模式、致密相遮挡模式等两类成藏模式。走滑断裂带碳酸盐岩断裂相的特征及其控藏作用对油气勘探开发储层建模具有重要意义。     

碳酸盐岩内断裂带结构及其与油气成藏

以野外观察描述为手段,系统研究了碳酸盐岩断裂变形机制的影响因素及断裂带结构演化过程,剖析了碳酸盐岩地层中断裂带结构与流体运移的关系。研究表明,影响碳酸盐岩内断裂变形机制的因素包括岩性、孔隙度、变形深度、温度、胶结作用、先存裂缝等,控制断裂带结构形成的因素包括滑动位移和破裂模式等。低孔隙度碳酸盐岩以裂缝发育为主,高孔隙度碳酸盐岩变形早期产生变形带,带内裂缝联接逐渐发育成断层带。随着埋藏深度的增加,断裂带结构不同：埋藏深度小于3 km,断层核主要发育无内聚力的断层角砾岩和断层泥;埋藏深度大于3 km,断层核普遍发育有内聚力的断层角砾岩和碎裂岩,破碎带发育多种成因的裂缝。随着位移的增加,破裂模式从早期的破裂作用变为后期的碎裂作用,最终形成碎裂流。断裂带演化是一个四维过程,断层核和破碎带发育情况直接影响断层对油气的运移和封闭的作用。断裂变形机制、断裂带内部结构以及与流体运移关系的研究,都可为封闭性提供重要的理论依据。     

塔中地区奥陶系碳酸盐岩储层裂缝发育特征及主控因素

主要依据塔中地区40多口钻井的奥陶系岩心裂缝观察、深浅侧向测井和FM I成像测井的响应特征等综合识别裂缝和获得裂缝特征参数。研究认为塔中地区奥陶系碳酸盐岩储层构造裂缝大量发育,以高角度缝和微裂隙最为发育,大、中缝较少,有近一半裂缝为未被充填的有效缝;平面上沿断裂带由西向东裂缝线密度变大,有效性增强;纵向上发育上奥陶统裂缝段、中—下奥陶统顶部100 m内裂缝段和深部裂缝段,中—下奥陶统顶部100 m内为裂缝集中发育段;裂缝发育程度与断裂构造和岩性关系密切,在断层发育密集区或多组断裂交汇处裂缝最发育,距离主断层或次级断层越近裂缝越发育,白云岩裂缝发育程度好于灰岩;好的裂缝发育区位于塔中地区东部多组断裂带交汇的部位。     

渤中凹陷北部中生界顶面断层破碎带量化研究

渤中凹陷是渤海海域最大的生烃凹陷,环渤中凹陷北部潜山带是目前重要的油气勘探区带。渤中凹陷北部中生界以火成岩为主,裂缝对火成岩储层的形成具有重要影响。利用地震方差属性值量化断层破碎带,建立断层破碎带宽度与断层斜距之间的数量关系,构建方差属性值等值线图的方式,结合前人研究的图版和薄片资料,对渤中凹陷北部中生界顶面断层破碎带进行量化、验证和分析。结果显示:研究区内的断层均有多个断层核,断层系统复杂,断层破碎带宽度由几百米至几千米不等,断层破碎带对中生界火山岩影响作用显著;断层破碎带的影响程度与距断层的距离呈负相关,即距离断层越远,影响程度越弱,方差属性值越小;利用地震方差属性值量化断层破碎带的结果与相关研究的图版规律、薄片资料的显示结果契合度高,对断层破碎带的量化研究具有较高的可行性。     

Fault/fracture density and mineralization: a contouring method for targeting in gold exploration

A widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore-forming process. In Archaean greenstone-hosted deposits, high-density fracturing was accompanied by enhanced fluid flow during fault/fracture network development, producing regional-scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas. Fracture density is both increased and decreased during faulting and fault healing, and fracture density accumulates over time, in zones of high palaeo-fluid flow. Localised zones where the density of fracturing is increased by deformation, become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation. The Ora Banda mining centre in Western Australia contains significant gold deposits that appear to demonstrate a close association between high-density fracturing and gold precipitation. Fracture density in the Ora Banda mines was enhanced by fault–fault intersections, fault–contact intersections and changes in fault geometry. The mine-scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales, hence these relationships may be used in targeting for gold exploration. Contouring the density of fracturing in a region provides a semi-quantitative way to rank areas for exploration and uses data from mapping, drilling and high-quality geophysical data as a basis for analyses. Fracture density contouring is complementary to other prospectivity-analysis methods.     

First results on the LPO-derived seismic properties of iron ores from the Quadrilátero Ferrífero region, southeastern Brazil

The role of hydrothermal fluids in assisting the activity of strike-slip faults is investigated using a range of new geological, geophysical, and geochemical data obtained on the Argentat fault, Massif Central, France. This fault zone, 180-km-long and 6 to 8 km-width, has experienced coeval intense channeling of hydrothermal fluids and brittle deformation during a short time span (300–295 Ma). According to seismic data, the fault core is a 4-km-wide, vertical zone of high fracture density that rooted in the middle crust (~ 13 km) and that involved fluids in its deeper parts (9–13 km depth). If stress analyses in the fault core and strain analyses in the damage zone both support a left-lateral movement along the fault zone, it is inferred that hydrothermal fluids have strongly influenced fault development, and the resulting fault has influenced fluid flow. Fluid pressure made easier fracturing and faulting in zones of competent rocks units and along rheological boundaries. Repeated cycles of increase of fault-fracture permeability then overpressure of hydrothermal fluids at fault extremity favored strong and fast development of the crustal-scale strike-slip fault. The high permeability obtained along the fault zone permitted a decrease of coupling across the weak fault core. Connections between shallower and lower crustal fluids reservoirs precipitate the decrease of fault activity by quartz precipitation and sulfides deposition. The zones of intense hydrothermal alteration at shallows crustal levels and the zones of fluid overpressure at the base of the upper crust both controlled the final geometry of the crustal-scale fault zone.