The Seismic Imaging and Interpretation of Faults: A Case Study from the Snøhvit Field, Barents Sea by Jennifer Elizabeth Cunningham MSc Thesis submitted in fulfilment of the requirements for degree of PHILOSOPHIAE DOCTOR (PhD) FACULTY OF SCIENCE AND TECHNOLOGY DEPARTMENT OF ENERGY RESOURCES 2020 University of Stavanger N-4036 Stavanger NORWAY www.uis.no ©2020 Jennifer Elizabeth Cunningham MSc ISBN: 978-82-7644-981-5 ISSN: 1890-1387 PhD. Thesis no. 568 Preface This thesis is submitted in fulfilment of the requirements for the degree of Philosophiae Doctor (PhD) at the university of Stavanger, Norway. This work was completed between March 2016 and October 2020 while I was enrolled in the PhD program in the Department of Energy Resources, Faculty of Science and Technology at UiS. This PhD was funded by the Research Council of Norway . My main supervisor is Professor Nestor Cardozo (UiS) and my co-supervisor is Dr Chris Townsend (Associate Professor at UiS). Dr Wiktor Weibull (UiS), Dr David Iacopini (formerly University of Aberdeen and currently University of Naples), Dr Richard Callow (Equinor) and Gard Ole Wærum (Equinor) were significant collaborators throughout this research. During my PhD I acted as a PhD Contact Person for PhD candidates in the Departments of Energy Resources and Energy and Petroleum Technology. This role included planning lunch seminars, department-wide sporting events, social events, conducting surveys to improve PhD studies, being the main communication hub for PhD candidates and being a friendly face for PhD candidates if they needed help or advice. This thesis contains one published article and two manuscripts which were submitted for publication before the submission of this dissertation. Several aspects of the work have also been presented at industrial and academic conferences. The first part of this thesis introduces the general problems, motivation for the work, the data being used, methodology, results, discussion, conclusions and recommendations for future work. The second part is a compilation of the three papers which form the main body of the thesis. The appendices provide supplementary material. _____________________________ Jennifer E. Cunningham Stavanger, October 6th, 2020 i Abstract An accurate understanding of faults in the subsurface is critical to successful petroleum exploration and production. Faults define sediment distribution pathways during deposition, act as barriers or conduits to fluid flow, and can define or compromise hydrocarbon traps. Faults are commonly analysed in the petroleum industry using reflection seismic data, and more rarely core or borehole image data. Outcrop analogues are used to provide a more complete understanding of fault rock morphologies and properties. In seismic data however, faults are often imaged as discontinuities where geological horizons are vertically and horizontally displaced. Due to the limited resolution of seismic data (typically tens of metres) comparatively little can be concluded about fault morphology, internal structure or the detailed interaction of faults with seismic waves. The first paper in this thesis focuses on bridging the knowledge gap in the seismic imaging of faults by analysing how the seismic processing methods of incidence angle stacking and azimuthal separation influence fault imaging. This first paper also studies the relationship between fault orientation and seismic acquisition direction, and the impact on the seismic imaging of faults in the subsurface. The paper also demonstrates that the internal complexity of faults can influence the dominant seismic frequencies observed in and adjacent to complex fault zones. The second paper in this dissertation integrates established and newly developed analyses in fault interpretation and seismic imaging. Analyses of throw, dip distortion, seismic attributes, unsupervised fault facies and seismic amplitudes in seismic fault analysis were applied. These attributes were compared to establish the geological significance of the seismic signature in and around faults. The paper proposed a linkage between unsupervised seismic fault facies, throw and dip separation gradient which are inherently linked to the mechanical stratigraphy ii which controls fault propagation and growth. Systematic variations in observed seismic amplitudes were found to be related to fault propagation folding and the formation of synclines and anticlines. The analyses applied in this paper can provide a more concrete methodology for analysing and understanding fault systems and when compared to conventional seismic interpretation methods, it can yield far more information on fault morphologies and interactions. Faults in seismic data are commonly interpreted on a combination of reflection seismic data and attributes derived from the same reflection data. Fault sticks are typically interpreted on vertical sections through the data volumes and then these sticks are gridded into fault surfaces. This methodology has been in place for decades, but an investigation into the impact of fault and horizon interpretation methodologies on the geological accuracy of the resulting fault surfaces has not been published. The third paper in this dissertation tests five interpretation experiments with the aim of uncovering the impact of interpretation strategy on fault and horizon morphology, fault length, and vertical displacement. The most and least successful interpretation methods were further examined to demonstrate the impact of interpretation method on geological modelling workflows and the calculation of hydrocarbon volumes. This paper proves the importance of a denser fault interpretation methodology on the resulting structural analysis, geomodelling and the estimation of petroleum reserves. It also suggests a best practice on how to approach the interpretation and modelling workflow. Having the best possible understanding of the imaging, seismic signature and interpretation of faults in seismic data is crucial for geoscientists working in hydrocarbon exploration and production. This thesis aims to strengthen the geoscientific understanding of faults and provides clear observations, conclusions and recommendations that can improve the use of existing seismic datasets and optimise the acquisition and processing of future seismic surveys. iii Acknowledgements First and foremost, I would like to express my deepest gratitude to my supervisor Professor Nestor Cardozo. Your guidance during my time at the University has been crucial to the completion of this thesis. I greatly appreciate you always taking the time for questions, discussions and review. Your help during the writing process has been very constructive, incredibly fast and is greatly appreciated. Thank you so much! I am also very thankful to my co-supervisor, Chris Townsend, who holds a Professor II position at UiS. Our collaboration over the past four years has helped me a lot. Thank you for motivating me and for the great advice. I would also like to thank my co-authors Wiktor Weibull (UiS), David Iacopini (University of Naples), Richard Callow (Equinor) and Gard Ole Wærum (Equinor) for your constant support. I could not have done this without your scientific contributions and guidance. A huge thank you to Geoteric™, Badleys Geoscience (T7™) and Schlumberger (Petrel™) for providing university licences for their softwares and for the support throughout this entire piece of work. A huge thanks to Equinor and their partners in the Snøhvit Field for providing the data used as a case study in this thesis. To my research fellows at the department, thank you for the many laughs, coffees, amazing lunches and ping-pong tournaments. A special thanks to Sanne Lorentzen and Tijana Voake – my two confidants at the University. An enormous thank you to Richard Callow for putting up with long hours and proof reading everything. Finally, to my family (Mom, Dad, Jessica, Adam and Jake) for your unwavering support during my studies. Jen Cunningham, Stavanger –October 2020 iv List of Papers Paper I Investigating the Seismic Imaging of Faults Using PS Data from the Snøhvit Field, Barents Sea and Forward Seismic Modelling Cunningham, Jennifer; Weibull, Wiktor; Cardozo, Nestor; Iacopini, David. In review at Petroleum Geoscience. Paper II Fault Deformation, Seismic Amplitude and Unsupervised Fault Facies Analysis: Snøhvit Field, Barents Sea Cunningham, Jennifer; Cardozo, Nestor; Townsend, Chris; Iacopini, David; Wærum, Gard Ole. Published in the Journal of Structural Geology v. 118, 165-180. Paper III The Impact of Seismic Interpretation Methods on the Analysis of Faults: A Case Study from the Snøhvit Field, Barents Sea Cunningham, Jennifer; Cardozo, Nestor; Townsend, Chris; Callow, Richard H. T. In review at Solid Earth. v Conference presentations and posters Cunningham, Jennifer; Cardozo, Nestor; Weibull, Wiktor; Iacopini, David. (2019). How do seismic survey parameters and artefacts affect the imaging of normal faults? A case study from the Snøhvit Field, Barents Sea. Tectonic Studies Group AGM; 2019/14/01 – 2019/16/01. Oral Presentation. Cunningham, Jennifer; Cardozo, Nestor; Townsend, Chris; Iacopini, David, Wærum, Gard Ole. (2018). Deformation, amplitude and unsupervised fault facies analysis of normal faults from the Snøhvit Field, Barents Sea. FORCE Challenges Related to Fault Modelling; 2018/20/06. Oral presentation. Cunningham, Jennifer; Cardozo, Nestor; Iacopini, David, Townsend, Chris; Wærum, Gard Ole. (2018). Deformation analysis and unsupervised fault facies classification