Sub-surface geology and velocity structure of the Krafla high temperature geothermal field, Iceland: Integrated ditch cuttings, wireline and zero offset vertical seismic profile analysis

John M. Millett*, Sverre Planke, Felix Kästner, Anett Blischke, Gylfi Páll Hersir, Sæunn Halldórsdóttir, Ólafur G. Flóvenz, Sigurveig Árnadóttir, Helga M. Helgadóttir, Sergey Vakulenko, Sergey Buryak, Ögmundur Erlendsson, Rüdiger Giese, Jehanne P. Cavailhes, Dougal A. Jerram, Ásgrímur Guðmundsson, Egill Júlíusson

*Corresponding author for this work

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Abstract

The Krafla geothermal area in northern Iceland comprises one of the best studied examples of a high temperature geothermal field associated with an active volcanic rift zone. Of key importance to improved resource exploration and development in volcanic areas such as Krafla, is the interpretation and understanding of the subsurface geology. Within this study we present results from an integrated analysis of the downhole volcanic stratigraphy from the K-18 borehole within the Krafla caldera based on combined wireline, ditch cuttings, and zero-offset VSP (vertical seismic profile) analyses. This study presents the first published sonic log velocity data from a high temperature geothermal borehole in Iceland and clearly demonstrates the importance of borehole velocity data for improving volcanic facies interpretations. The shallow subsurface geology of the K-18 site from c. 0–380 m comprises an inter-layered sequence of sub-aerial basaltic lavas, hyaloclastites and tuffaceous lithologies of both felsic and basic composition, which are progressively replaced by basaltic sheet intrusions with increasing depth. An interval of variably basic to more evolved mixed tuffaceous facies is identified based on cuttings analysis, gamma and sonic velocities between c. 790–1120 m depth. Discrete high sonic Vp units cut the lower c. 100 m of this interval and are interpreted as either sheet intrusions or lava interiors. At c. 1800 m, a sharp increase in P-wave velocity from c. 4.5 to c. 5.5 km/s, represents the transition from a mixed lava and sheet intrusion dominated sequence, into a dense basic intrusion forming the local basement that persists to the bottom of the borehole at c. 2215 m. Reduced travel time analysis of processed zero-offset VSP results reveal good correspondence with the major stratigraphic boundaries in the borehole, supporting the potential for VSP studies to robustly characterize complex volcanic stratigraphy in high temperature geothermal fields. Finally, the application of percentage-based ditch cuttings analyses methods for volcanic facies analysis in geothermal boreholes is tested and assessed to have future potential.

Original languageEnglish
Article number106342
Number of pages18
JournalJournal of Volcanology and Geothermal Research
Volume391
Early online date30 Mar 2018
DOIs
Publication statusPublished - 1 Feb 2020

Keywords

  • Geothermal
  • Iceland
  • IMAGE
  • Krafla
  • Sonic velocity
  • VSP
  • FLOOD-BASALT PROVINCE
  • SEQUENCES
  • IGNEOUS PROVINCE
  • FAROE-ISLANDS
  • STRATIGRAPHY
  • DEEP DRILLING PROJECT
  • BOREHOLE
  • ATTENUATION
  • EXPLORATION
  • LITHOFACIES

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    Millett, J. M., Planke, S., Kästner, F., Blischke, A., Hersir, G. P., Halldórsdóttir, S., Flóvenz, Ó. G., Árnadóttir, S., Helgadóttir, H. M., Vakulenko, S., Buryak, S., Erlendsson, Ö., Giese, R., Cavailhes, J. P., Jerram, D. A., Guðmundsson, Á., & Júlíusson, E. (2020). Sub-surface geology and velocity structure of the Krafla high temperature geothermal field, Iceland: Integrated ditch cuttings, wireline and zero offset vertical seismic profile analysis. Journal of Volcanology and Geothermal Research, 391, [106342]. https://doi.org/10.1016/j.jvolgeores.2018.03.024