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Numerical Studies of the Deformation of Salt Bodies with Embedded Carbonate Stringers

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Numerical Studies of the Deformation of Salt Bodies with Embedded Carbonate Stringers Numerical Studies of the Deformation of Salt Bodies with embedded Carbonate Stringers Shiyuan Li - Doctoral Thesis "Numerical Studies of the Deformation of Salt Bodies with embedded Carbonate Stringers" From the Faculty of Georesources and Materials Engineering of the RWTH Aachen University in respect of the academic degree of Doctor of Natural Sciences approved thesis submitted by M.Sc. Shiyuan Li from Shanghai, China Advisors: Univ.-Prof. Dr. Janos Urai Advisors: Univ.-Prof. Peter Kukla, Ph.D. Date of the oral examination: 28. November 2012 This thesis is available in electronic format on the university library’s website III IV "Numerical Studies of the Deformation of Salt Bodies with embedded Carbonate Stringers" Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von M.Sc. Shiyuan Li aus Shanghai, China Berichter: Univ.-Prof. Dr. Janos Urai Berichter: Univ.-Prof. Peter Kukla, Ph.D. Tag der mündlichen Prüfung: 28. November 2012 Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar V VI Acknowledgements Janos L. Urai is sincerely thanked for supervising my thesis. His wisdom, patience and creativity encourage me to do my research step by step. With his guidance I can deepen my understanding of geomechanics, especially how to make a good combination between numerical model and structural geology. Meanwhile he used his experience to evaluate the effectiveness and correctness of geomechanical models. Peter A. Kukla is the other supervisor of my thesis. He is greatly acknowledged because he provides me many chances such as workshop, seminar or discussion to learn much information of the geological background. Steffen Abe is the coordinator of my PhD work here. Since he is an outstanding expert of numerical modeling, many discussions have been done with him so that I can have confidence to make a series of models which are compatible to the geological situation. I thank the Ministry of Oil and Gas of the Sultanate of Oman and Petroleum Development Oman LLC (PDO) for granting permission to publish the results of this study. I also thank PDO for providing the seismic data and Gideon Lopez Cardozo for useful discussion. The secretaries, Bettina Dulle and Evelyn Bützler, are also thanked for their big helpfulness in my daily work and daily life since I am a foreign student in Germany. I am very grateful that I can work in good team and work with my colleagues, Guillaume Desbois, Joyce Schmatz, Maartje Houben, Andrea Komoroczi, Zoltan Komoroczi, Jop Klaver, Sohrab Noorsalehi-Garakani, Susanne Hemes, Simon Virgo, Max Arndt, Ben Laurich and Michael Kettermann. Furthermore, I have a very good cooperation with Heijn van Gent in our institute, Lars Reuning, Frank Strozyk and Stephan Becker in geological institute. Many geomechanical models I have done are related to their seismic interpretations and diagenetic analysis. Through deep discussions I have learned quite a lot of geological background and methods. Johannes Schoenherr is also thanked for his help at the beginning stage of my PhD study VII Summary Many salt bodies contain large carbonate or anhydrite inclusions (so-called rafts, floaters or stringers) and these respond to the movements of the salt in a variety of ways, including displacement, folding and fracturing. The process of the movement and deformation of salt bodies and embedded carbonate or anhydrite inclusions has not been fully understood. In particular, no numerical research has been done to simulate the brittle deformation of individual stringers in the initial phases of salt tectonics. Through collecting rocksalt rheology data from laboratory experiments from published literatures and combining with the analysis from microstructure of naturally deformed rocksalt at low stress, we simplify the rock salt rheology to Power law creep (n=5) and Newtonian creep (n=1) with the appropriate material parameters in the salt tectonics modeling. Meanwhile, geomechanical modeling on the gravitational sinking of stringers are provided as one other effective approach to investigate the relation between velocity and displacement of stringers and long-term deformed salt rheology. This method also contributes to the consideration of the appropriate rheology in the salt tectonics modeling on a case study from South Oman Salt Basin. In addition, we also propose a new method to evaluate long-term deformation salt rheology by inverting the gravitational sinking of anhydrite inclusions (“stringers”) using a case study from Zechstein salt which was tectonically inactive in the Tertiary. The results of geomechanical model are compared to the seismic observation of stringers and then we conclude that the rheology of the Zechstein salt masses in the Tertiary follows power law creep. Since the sinking displacement and velocity of stringer fragments are very sensible to the salt rheology, we can invert the sinking process with different salt rheology and compare the numerical result with the observations in the seismic data to investigate the possible salt rheology. We can conclude that the modeling of gravitational sinking can be an effective approach to measure salt rheology. VIII We presented numerical models of deformation of salt body with embedded stringers using the case study from the South Oman Salt Basin. We used Abaqus package (finite element models) to explore how the salt flow and the associated deformation of stringers (including both brittle and viscous material properties) is influenced by passive tectonic process. In our research, the main work was to use and develop techniques in Abaqus to make models of rheological behaviour of salt tectonics and brittle or ductile behaviour of carbonate stringers embedded in salt. A series of techniques were used in order to make successful models. We simplified the geometry obtained from seismic data, define carbonate or anhydrite and rocksalt material, model passive tectonic process through applying boundary conditions, model large displacement through adaptive remeshing technique, model consequent brittle fractures through an iterative scheme which includes stress monitoring, stringer breaking, material replacing and calculation restarting by adaptive remeshing techniques. The simplified model offers a practical method to investigate complex stringer motion and deformation. The brittle and ductile deformations of embedded stringers in both compressive and tensile environments have been investigated in our research. Models suggest that brittle stringers can break very soon after the onset of salt tectonics. The extension can make brittle stringer to fracturing and boudinage while the compression can make brittle stringer bending, fracturing and thrusting. Models suggest that viscous stringers have folding and extension deformation. Results also show that the internal structure of salt body and deformation patterns of stringers are strongly dominated by the various geometrical or material properties of models. IX Zusammenfassung Viele Salzkörper enthalten große Carbonat- oder Anhydrit-Einschlüsse (so genannte Rafts, Floater oder Stringer) welche durch Versatz, Faltung und Fracturing auf die Bewegungen des Salzes reagieren. Das Verständnis der Bewegung und Deformation von Salzkörpern und darin eingebetteter Anhydrit- oder Karbonat-Einschlüsse ist immer noch sehr unvollständig. Insbesondere fehlen bisher numerische Studien zur spröden Verformung einzelner Stringers in der Initialphase der Salztektonik. Unter Berücksichtigung bisher veröffentlichter Daten zur Rheologie von Steinsalz, ermittelt in Laborexperimenten, und in Kombination mit der Mikrostrukturanalyse von in- situ unter niedrigen Spannungen deformierten Steinsalzproben, vereinfachen wir die Verformung von Steinsalzunter der Annahme, dass diese durch Power-law-Creep(n=5) und Newtonian-Creep(n=1) beschrieben werden kann. Weiterhin wird eine Methode zur geomechanische Modellierung desgravitativen Absinkens von Stringern präsentiert, um die Beziehung zwischen Geschwindigkeit und Versatz von Stringern und der Rheologie langsam verformenderSalzkörper zu erfassen. Diese Methode leistet einen Beitrag zum Verständnis der Rheologie von Steinsalz im Rahmen einer Fallstudie zur tektonischen Modellierungdes South Oman Salt Basin. Zudem wird eine neue Methode zur Evaluierung der Rheologie der langsamen Deformation von Steinsalz vorgestellt, welche auf der Inversion des gravitativen Absinkens der Anhydrit-Stringer basiert. Im Rahmen einer Fallstudie werden geomechanische Modelle mit seismischen Beobachtungen des in Zeiten des Tertiärs inaktiven Zechsteinsalzesverglichen, woraus wir schließen, dass die Rheologie des Zechsteinsalzes im Tertiär den Gesetzen des Power-Law-Creep folgt. Da sowohl der vertikaleVersatz als auch die Versatzgeschwindigkeit der Stringer-Fragmente stark von der Rheologie des Salzes abhängig sind, stellt die Methode des gravitativen Absinkenseine effektive Technik zur Bestimmung der Rheologie des Salzesdar. Wir präsentieren numerische Modelle der Deformation vonSalzkörpern mit eingebetteten Stringern im Rahmeneiner Fallstudieim South Oman Salt Basin. Wir verwendendie finite X Elemente Methode (Abaqus Suite) um Salz-Flusszu modellieren und zu untersuchen, wie die damit einhergehende, sowohl spröde als auch duktile Deformation der Stringer durch passive tektonische Prozesse beeinflusst wird. Der wichtigste Teil unserer Forschung
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