Failure Modes in Normal Faults - an investigation into structure and permeability evolution in physical models and outcrop studies
| العنوان: | Failure Modes in Normal Faults - an investigation into structure and permeability evolution in physical models and outcrop studies |
|---|---|
| المؤلفون: | Kettermann, Michael |
| المساهمون: | Urai, Janos, Reicherter, Klaus, Vrolijk, Peter |
| المصدر: | Aachen 1 Online-Ressource (IV, 249 Seiten) : Illustrationen, Diagramme (2016). doi:10.18154/RWTH-2017-06716 = Dissertation, RWTH Aachen University, 2016 |
| سنة النشر: | 2016 |
| مصطلحات موضوعية: | clay smear, joints, flow simulation, ddc:620, structural geology, permeability, normal faults, analogue modeling |
| الوصف: | RWTH Aachen University, Diss., 2016; Aachen, 1 Online-Ressource (IV, 249 Seiten) : Illustrationen, Diagramme(2017). = RWTH Aachen University, Diss., 2016 This thesis deals with structures and processes related to failure mode transition in normal faults inmechanically layered sediments. Dilatant, hybrid and shear mode faults are common in the uppercrust and associated with different deformation processes, fault and fracture geometries and transportproperties. Fault systems are of great importance for applications such as hydrocarbon explorationand production, groundwater flow, hydro-thermal systems, CO2 sequestration and natural hazardassessment. However, our knowledge of faults still has room for improvement and here I addresssome issues more or less directly related to different failure modes and mechanical stratigraphy.To this end a variety of methods, including analogue modeling, field work and numerical flowsimulations are used to determine a general structural trend from dilatant to shear failure modeand later on to study details of fault zones in some of these settings. This failure mode transition isthe central theme of my thesis.While some experimental studies have been done on failure mode transition in experimentallydeformed samples and 2-D discrete element models, we lack a good understanding of the effect offailure mode transition on 3-D structures on normal faults. This is addressed in Chapter 3 of thisthesis. Here I present a series of scaled analogue models of normal faulting using cohesive powderembedded in cohesionless sand. By varying the overburden thickness, the failure mode changes fromtensile to hybrid and finally to shear. 3-D investigation of the hardened and excavated cohesivelayers shows three structural domains: (1) The tensile domain is characterized by steep open fissuresand sharp changes in strike at segment boundaries and branch points; (2) the shear domain showsshallower fault dips, fault planes develop striations and map-view fault traces undulate with smallerchanges in strike at branches; (3) the hybrid domain shows characteristic features of dilatant andshear domain, alternating both laterally and vertically. These attributes can be recognized inmap-view, which is a useful tool in understanding and interpreting further work in this thesis.I study dilatant faults in the graben system of Canyonlands National Park, Utah, USA (Chap. 4).Here, I present results of ground-penetrating radar (GPR) surveys in combination with fieldobservations and analysis of high-resolution airborne imagery. This study shows that Quaternarysediments at the flat graben floors are intensely faulted, implying a more complex fault structurethan visible at the surface. The presence of sinkholes gives insights in local dilatancy and showswhere water and sediments are transported underground. These field observations are later comparedwith a series of scaled analogue models using cohesive powder in a half-graben setting (Chap. 5).Pre-formed vertical, cohesionless joints in the upper 5 cm of the model vary in strike directionwith respect to a predefined basement fault, to cover a range of joint-fault angles observed inCanyonlands NP. The surface trace of faults follows the joints geometry to a certain extent andtherefore, fault geometry and amount of secondary fractures differ strongly from experiments withoutjoints. Map-view analyses show that with increasing angle between joint-set and basement-faultstrike the number of secondary fractures and the number of connected joints increases, while thearea fraction of massively dilatant fractures shows only a minor increase.The transition from dilatant to hybrid mode faulting in this thesis is explored by a ’proof-ofconcept’study on salt intrusion into underlying normal faults (Chap. 6). The motivation of thisproject is the deformation-rate depending lubricating effect of the rock salt on the fault movementand potential effects on induced seismic events in the Groningen gas field. We combine cohesivepowder as analog for brittle carbonates with different viscous salt analogs in two experiments.Using resin as salt analog shows the resulting geometry of salt in the faults which form in dilatantmode as result of low overburden stress. Using transparent silicone oil as salt analogue allowsdynamic observation of the developing fault geometry, where faults form in hybrid mode due tolarger overburden stress.I present a study on hybrid failure in clay smears with 3-D structural investigation and flowmeasurements (Chap. 7). Despite numerous published outcrop and analogue studies, this structuraldomain is underrepresented in current research. Water saturated sandbox experiments with largeclay smear surfaces (500 cm²) are coupled with across-fault flow measurements to determine thedynamic permeability evolution. This is combined with structural analyses of excavated clay smears.We observe an initial hybrid failure of the clay with early breaching of the clay layer and increasedcross-fault flux, followed by subsequent phases of fault back-stepping, shearing and reworking. Wefind diagnostic relationships between fault structures and flow responses that allow a predictionof fluxes from observed structures or vice versa. In addition to simple single-layer experimentswe present experiments with two source clay layers, which show a different characteristic fluxresponse with a generally lower permeability at the same total clay volume in the faulted sequence.Experimental observations are then compared with numerical flow simulations using finite elementmethod (FEM) in 2-D and 3-D, showing a good fit between the measurements and flow simulations.The thesis concludes with an outcrop study on clay smears forming in shear mode in unlithifiedclay-sand sequences in lignite mines of the Lower Rhine Embayment (Chap. 8). This work focuseson 3-D outcrops in freshly cut normal faults with shale gouge ratios of 0.1 - 0.3. Vertical profilesshow that fault zones are often layered, with sheared sand, sheared clay and tectonically mixedsand–clay gouge. 3-D thickness data derived from sequential slicing in excavated fault zones areapproximately log-normal and show heterogeneous thickness distributions. The thinnest parts ofclay smears are often located close to source layer cut-offs rather than in the center of the smear asproposed in earlier studies. Profile and 3-D data show, that clay smears are strongly affected by R- andR’-shears, which mostly form at the footwall side. These shears can locally cross and offsetclay smears, which we identify as prominent process in the formation of holes. On micro-scale, claysmear samples show that grain-scale mixing can lead to thickening of the low permeability smearsand intense mixing and amalgamation of parallel smears may lead to resealing of holes. Published by Aachen |
| اللغة: | English |
| DOI: | 10.18154/RWTH-2017-06716 |
| URL الوصول: | https://explore.openaire.eu/search/publication?articleId=doi_dedup___::0f414870fa184eaefb77699771ac443c |
| Rights: | OPEN |
| رقم الانضمام: | edsair.doi.dedup.....0f414870fa184eaefb77699771ac443c |
| قاعدة البيانات: | OpenAIRE |
| DOI: | 10.18154/RWTH-2017-06716 |
|---|