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.
Bibliographical noteThe research leading to these results has received funding from the European Community's Seventh Framework Programme under grant agreement No. 608553 (Project IMAGE). The VMAPP project run by VBPR, DougalEARTH Ltd. and TGS also contributed funding to the borehole characterization of the K-18 borehole. Landsvirkun is acknowledged for their effort and assistance in this work and in particular for allowing the use of the data from well K-18. We further acknowledge the support from the Research Council of Norway through its Centres of Excellence funding scheme, project 22372 (SP and DAJ).
- Sonic velocity
- FLOOD-BASALT PROVINCE
- IGNEOUS PROVINCE
- DEEP DRILLING PROJECT