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Durham E-Theses Structural evolution of the Nam Con Son Basin: quantitative fault analysis applied to a 3-simensional seismic dataset Pugh, Adam How to cite: Pugh, Adam (2007) Structural evolution of the Nam Con Son Basin: quantitative fault analysis applied to a 3-simensional seismic dataset, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/2497/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 STRUCTURAL EVOLUTION OF THE NAM CON SON BASIN: QUANTITATIVE FAULT ANALYSIS APPLIED TO A 3-DIMENSIONAL SEISMIC DATASET by Adam Pugh The copyright of this thesis rests with the author or the university to which it was submitted. No quotation from it, or information derived from it may be published without the prior written consent of the author or university, and any information derived from it should be acknowledged. A thesis submitted for the degree of Doctor of Philosophy at the Department of Earth Sciences, University of Durham. 2007 0 6 OCT 2008 COPYRIGHT The copyright of this thesis rests with the author. No quotation from it should be published without prior written consent and information derived from it should be acknowledged. No part of this thesis has been submitted for a degree at this university or any other university. The work described in this thesis is entirely that of the author, except where reference is made to previous published or unpublished work. © 2007 Adam Philip Pugh 11 ABSTRACT The Nam Con Son Basin is one of several Tertiary rift basins located on the continental shelf, offshore Indochina. It has received relatively little attention until recently and remains a poorly understood sedimentary basin. The discovery and production of significant volumes of hydrocarbons from within fault-bounded structural highs has spurred a drive to better understand the basin structure. Previous interpretations of regional 2-dimensional seismic data suggested a complex structure, with E-W trending Eocene-Oligocene faults, overprinted by a rhomboidal pattern of mainly N-S and NE SW trending Miocene faults and the possible involvement of compressional and/or strike-slip tectonics. The driving mechanism for extension on the continental shelf is not fully understood, but the Cenozoic structural evolution was undoubtedly influenced by a heterogeneous crust cut by a number of pre-existing Palaeozoic and Mesozoic structures. Crucially, the western extent of South China Sea rifting in the region may have been controlled by the ~N-S trending edge of the Indochina craton, a major rheological boundary that likely underlies the Nam Con Son Basin. In addition to its economic importance, the proximity of the basin to the tip of the South China Sea, the possible involvement of wrench tectonics and the potential influence of pre-existing fabrics, makes it an ideal target for academic study. A 3-dimensional seismic dataset from the centre of the Basin has been used to build a fault/horizon model of the Early-Middle Miocene syn-rift sequence. The faults have a wide range of orientations and, in all cases, the hangingwall is down-thrown consistent with apparent normal offsets; there is no evidence for strike-slip or reverse faulting. By accurately modelling the fault surfaces and the fa4lt/horizon intersections, the dip, strike, dip-azimuth and offset of 225 faults have been calculated and used to quantitatively analyse the fault sample. The timing of fault activity has been constrained for each fault and shows that all the faults are broadly contemporary but that the number of active faults increased through time. This is partly attributed to continuing subsidence on graben bounding faults, which focussed faulting and extension within the graben. There is no correlation between fault strike and fault age and the faults show no significant rotation about vertical axes with progressive deformation. The range of fault orientations likely reflects the conflicting influences of the ~NW -SE regional extension direction and the dominant ~N-S trending basement fabric. A systematic variation in maximum dip-slip fault offset (defined as the distance between the footwall and hangingwall cutoffs measured parallel to the fault surface) with fault strike has revealed an 'ideal' fault orientation, which can be used to infer the direction of maximum horizontal extension. The relationship between this extension direction, the dominant pre-existing basement fabric and the regional displacement direction is consistent with a model of moderately extension-dominated dextral transtension for the Nam Con Son Basin synchronous with the opening of the South China Sea. At a local scale, the influence of the underlying Eocene-Oligocene structures has produced offset depocentres, along strike fault polarity flips, and complex accommodation zones. lll ACKNOWLEDGEMENTS The project has been made possible by NERC (Natural Environment Research Council) who have provided funding in the form of an Industrial Case Studentship and BP (Industrial Case partner) who have provided additional funding and the seismic data that I have used for this project. I wish to thank the people at BP for their help and hospitality on those occasions when I have visited the company, in particular Steve Matthews at BP Sunbury and James Stewart and the Asset Team at BP Ho Chi Minh City. A big thanks to my supervisors for their guidance, support and advice, Bob Holdsworth, Ken McCaffrey and Jonny Imber at Durham University and Steve Matthews at BP Sunbury. Jonny deserves additional thanks for loading the 2D and 3D seismic data that I have used and for working to overcome a number of software and data issues that have been encountered along the way. I would also like to thank the other academic staff and support staff within the Earth Science department who have helped me during my time at Durham. On a personal note, I have made some great friends whilst at Durham who have made the whole process much more enjoyable. So a very big thanks to all of them. This thesis marks the end of over 7 years at university in full time education. Throughout the whole time my family have been a great support, both financially when it was needed and by taking such an interest in what I have been doing and encouraging me all the way. I am lucky to have such a caring family. For the last four years I have had my girlfriend Kerry with me. To say that she has improved my time in Durham would be a gross understatement. I am incredibly grateful that she moved up to Durham to be with me. Together we have really enjoyed our time here. IV LIST OF TABLES AND FIGURES CHAPTER 1 Figure 1.1 1 CHAPTER2 Figure 2.1 6 Figure 2.2 7 Figure 2.3 10 Figure 2.4 11 Figure 2.5 12 Figure 2.6 13 Figure 2.7 14 Figure 2.8 16 Figure 2.9 18 Figure 2.10 19 Figure 2.11 21 Figure 2.12 24 Figure 2.13 27 CHAPTER3 Figure 3.1 31 Figure 3.2 33 Figure 3.3 35 Figure 3.4 37 Figure 3.5 38 Figure 3.6 40 Figure 3.7 43 Figure 3.8 45 Figure 3.9 47 Figure 3.10 49 Figure 3.11 50 Figure 3.12 52 Figure 3.13 54 Figure 3.14 55 Figure 3.15 56 Figure 3.16 58-59 Figure 3.17 60-61 Figure 3.18 62-63 Figure 3.19 64-65 Figure 3.20 66-67 Figure 3.21 68-69 Figure 3.22 70-71 Figure 3.23 72-73 Figure 3.24 74-75 Figure 3.25 76-77 Figure 3.26 78-79 V Figure 3.27 81 Figure 3.28 82 Figure 3.29 84 Figure 3.30 87-89 Figure 3.31 90 Figure 3.32 92 Figure 3.33 95 Figure 3.34 98 Figure 3.35 99 Figure 3.36 101 Figure 3.37 102 Figure 3.38 104 Figure 3.39 106 Figure 3.40 107 Figure 3.41 108 Figure 3.42 109 Figure 3.43 114 CHAPTER4 Figure 4.1 119 Figure 4.2 122 Figure 4.3 123 Figure 4.4 124 Figure 4.5 128 Figure 4.6 129 Figure 4.7 130 Figure 4.8 131 Figure 4.9 132 Figure 4.10 133 Figure 4.11 135 Figure 4.12 138 CHAPTERS Figure 5.1 143 Figure 5.2 145 Figure 5.3 149 Figure 5.4 151 Table 5.1 153 Figure 5.5 154 Figure 5.6 155 Figure 5.7 156 Figure 5.8 157 Figure 5.9 160 Table 5.2 161 Figure 5.10 162 Figure 5.11 163 Figure 5.12 166 VI CHAPTER6 Figure 6.1 170 Figure 6.2 171 Figure 6.3 172 Figure 6.4 174 Figure 6.5 178 Figure 6.6 179 Figure 6.7 180 Figure 6.8 185 Vll CONTENTS TITLE COPYRIGHT 11 ABSTRACT 111 ACKNOWLEDGEMENTS IV LIST OF TABLES AND FIGURES V 1 INTRODUCTION 1 1.1 AIMS AND CONTENTS 4 2 STRUCTURAL GEOLOGICAL BACKGROUND 6 2.1 STRAIN 6 2.1.1 3-dimensional strain 9 2.1.2 Faulting during 3-D strain 12 2.1.3 Analogue modelling of 3-D strain 13 2.1.4 Folding during transtension 17 2.2 FAULTING 19 2.2.1 Post-sedimentary normal faults 19 2.2.2 Syn-sedimentary normal faults 21 2.2.3 Fault growth, linkage and fault population evolution 23 2.3 REACTIVATION 26 3 STRUCTURE AND EVOLUTION OF THE NAM CON SON BASIN 29 3.1 JINTRODUCTION 29 3.2 REGIONAL GEOLOGICAL SETTING 30 3.3 METHOD AND DATASET 42 3.3.1 Sampling faults 48 3.3.2 Depth conversion 49 3.3 .3 Constraining the timing of fault movement 51 3.4 STRUCTURAL MODEL DERIVED FROM THE 3-D SURVEY 56 3.4.1 Cross sections 56 3.4.2

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