Reservoir Characterization of Jurassic Sandstones of the Johan Sverdrup Field, Central North Sea Hans-Martin Kaspersen Master Thesis Geology 60 credits Department of Geosciences The Faculty of Mathematics and Natural Sciences UNIVERSITY OF OSLO 01/12 /2016 Reservoir Characterization of Jurassic Sandstones of the Johan Sverdrup Field, Central North Sea Hans-Martin Kaspersen Thesis for master degree in Geology December 2016 Supervisor: Associate Professor, Nazmul Haque Mondol © Hans-Martin Kaspersen 2016 Reservoir Characterization of Jurassic Sandstones of Johan Sverdrup Field, Central North Sea Hans-Martin Kaspersen http://www.duo.uio.no/ Printed: Reprosentralen, Universitetet i Oslo Preface This thesis is submitted to the Department of Geoscience, University of Oslo (UiO), in candidacy of the M.Sc. in Geology. The research has been performed at the Department of Geosciences, University of Oslo, and at Lundin Norway at Lysaker (Bærum, Norway) during the period of January 2016 to November 2016 under the supervison of Nazmul Haque Mondol, Associate Professor, Department of Geosciences, University of Oslo, Norway. I II Acknowledgment First of all I would like to thank my supervisor, Associate Professor, Nazmul Haque Mondol for giving me the opportunity to work on this project. His guidance and encouragement have been very helpful for me to accomplish the goals set for this study. I am also grateful to Lundin Norway for giving me the opportunity to write parts of my thesis at their office at Lysaker. The input from them have helped to understand some of the issues regarding the reservoir, and the working environment made it a nice place to write the last sections of this thesis. I would like to thank my fellow students at the department for making my time at the university the best it could be, and especially Henrik, Jørgen and Kristoffer, who have not only been my study partners, they have also become great friends over the last 5 years. Lastly, I would like to thank my family for their support throughout my studies in Oslo, and the opportunities they have given me. Finally, my partner Kristina for always being there for me when I have needed something. Hans-Martin Kaspersen 01.12.2016 III IV Abstract This study presents reservoir characterization of Middle and Upper Jurassic sandstones by using petrophysical analysis, compaction study and rock physics diagnostics with data from 26 exploration wells from the Johan Sverdrup field, Central North Sea. The primary focus is the Upper Jurassic sandstone of Intra Draupne Formation, with comparisons to the Middle Jurassic sandstones of Hugin and Sleipner Formations of Vestland Group. The studied sandstones have been deposited in different depositional environments. The sandstones of Intra Draupne Formation represent high density flows along a delta slope and the sandstones of Hugin and Sleipner Formations represent more fluvial and channelized deposits. By studying well logs potential reservoirs were identified. The analysis conducted on the Intra Draupne Formation shows that the thickness of sandstone decreases from the west towards the east from 40 meter in well 16/2-17 S before it decreases to approximately 6 meters in well 16/5- 4 in the south and 16/3-4 A in the east. The petrophysical properties of the Intra Draupne sandstone are found to be extremely good with porosities exceeding 25% and very low shale content. The Net–to–Gross values for the Intra Draupne sandstones, where the formation fulfills the reservoir cutoffs set in the analysis (Sw=0.55, PHI=0.1 and Vsh= 0.3), reaches almost 1 in 9 of the wells. The wells where the Intra Draupne sandstone is occurring below the regional Oil Water Contact, the Net–to–Gross is understandably be zero with the cutoffs used. The Middle Jurassic sandstones in the Vestland Group also show variations in thickness across the field from 2 meters in well 16/2-13 A to above 30 meters in wells 16/2-10 and 16/2-8 respectively. The porosity in this interval is as in the Intra Draupne sandstone found to be ranging from 20% to above 30%, but with higher percentage of shale in the formation. The Net–to–Gross results from this interval ranges from 0 to 1 with an average of 0.5. By analyzing the velocity and density depth trends for all the wells in the database, and calculating the geothermal gradient for each individual well the mechanical compaction and chemical compaction regimes can be separated. The geothermal gradient in the study area is calculated to be 40oC/km. The main focus for the compaction study was to investigate if the reservoir intervals were located in the chemical compaction regime as cementation can be an important factor when the reservoir quality is to be estimated. The velocities in the studied sandstone intervals show slightly higher values than the published reference curves suggested for mechanical compaction of sandstones at the depth the reservoir sandstones are located. In some thin zones the velocity greatly exceeds the expected values. These zones where further investigated by utilizing rock physics diagnostics. By cross plotting both Vp and Vs versus the porosity and using different cement models (digitized and basin specific) the sorting and cement volume of the studied sandstone intervals have been estimated. The results of the rock physics diagnostics confirm that the sandstones are in transition zone to early phase of chemical compaction reflected by small amounts of cement (average of 4.5 %) present in the formations. Further analysis proves the effect of fluid changes within the sandstones and the Vp/Vs versus AI (Acoustic Impedance), LMR and Vp versus Vs crossplots are utilized to differentiate between water saturated intervals and hydrocarbon saturated intervals. The results from this analysis show that there are clear effects of fluid change on rock properties within the reservoir sandstones. V Nomenclature AI/IP: Acoustic Impedance/P-Impedance API: American Petroleum Institute BHT: Bottom Hole Temperature CC: Chemical Compaction Frac.: Fraction Hc: Hydrocarbon IGR: Gamma Ray Index IP: P-Impedance IS: Shear Impedance/S-Impedance Km: Kilometer LMR: Lambda-Mu-Rho MC: Mechanical Compaction MD: Measured Depth N/A: Not Available N/G: Net-to-gross ratio NPD: Norwegian Petroleum Directorate PHI: Porosity PHIE: Effective Porosity PHIT: Total Porosity RKB: Relative to Kelly Bushing RPT: Rock Physics Template Shc: Hydrocarbon Saturation Sw: Water Saturation TVD: Total Vertical Depth TVDss: Total Vertical Depth subsea Vp: Compressional wave velocity Vs: Shear wave velocity Vsh: Shale Volume VI Contents Preface ......................................................................................................................................... I Acknowledgment ..................................................................................................................... III Abstract ..................................................................................................................................... V Nomenclature ........................................................................................................................... VI Contents ................................................................................................................................... VII List of Figures .......................................................................................................................... XI List of Tables ......................................................................................................................... XIX List of Appendices ................................................................................................................. XX Chapter 1 Introduction ............................................................................................................... 1 1.1 Background and Motivation ............................................................................................. 1 1.2 Research Objectives ......................................................................................................... 1 1.3 Study Area ........................................................................................................................ 2 1.4 Database ............................................................................................................................ 4 1.5 Limitations and Further Work .......................................................................................... 6 1.6 Chapter Description .......................................................................................................... 7 Chapter 2 Geology of the Johan Sverdrup Field ........................................................................ 8 2.1 Tectonic Evolution ........................................................................................................... 8 2.1.1 Pre-Permian ............................................................................................................... 8 2.1.2 Permian ...................................................................................................................... 9 2.1.3 Triassic ...................................................................................................................... 9 2.1.4 Jurassic ...................................................................................................................... 9 2.1.5 Cretaceous - Recent ................................................................................................