Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland

Abstract

a b s t r a c tThe wholerock major and trace element composition of drill cutting samples are compared to drill core samples from adjacent depths in the seawater recharged Reykjanes geothermal system in Iceland. The first appearance of alteration minerals and lithologies in drill cutting samples is a useful tool for interpreting broad subsurface characteristics. However, use of drill cutting samples for determining igneous affinity and elemental exchanges during hydrothermal alteration is problematic. Samples recovered from immediately above and below the cored intervals in wells RN-17B and RN-30 demonstrate that drillcutting samples are biased towards preservation of least altered primary igneous minerals and more resistant alteration minerals, including albite, quartz, and epidote, with preferential loss of finer-grained and less resistant minerals including chlorite and actinolite. This selective recovery obscures elemental exchanges resulting from hydrothermal alteration processes. For some elements, compositional variations (enrichments and depletions) measured from 9.5 m of core exceeds that observed in ∼3000 m of cutting analyses. Concentration ratios of hydrothermally immobile elements including Zr, Nb, V, Y, HREE, Hf, Ta and Th in deep (>2245 m) spot drill core samples record bimodal, trace element-enriched and trace element-depleted precursor compositions similar to subaerial Reykjanes Peninsula basalts. The same elements in nearly 3000 m of drill cutting samples from well RN-17 overwhelmingly reflect the more common trace element-enriched igneous precursor, demonstrating that mixing of drill cutting samples obscures details of their igneous affinity. A new and different drill rig was used to deepen well RN-17 below 2266 m in a sidetrack hole (RN-17ST), which resulted in a change in drilling conditions, accompanied with an increased well deviation angle from ∼0• to ∼4• . Wholerock geochemical results for drill cutting samples from RN-17ST are homogenous for virtually every element; suggesting the change in drilling conditions resulted in extreme mixing of the drill cuttings. Anomalously high concentrations of Cu, Ni, Cr and Ta in some drill cutting samples likely reflects contamination of drill cutting samples by metal alloys used in drill bits and drill collars or more resistant spinel and sulfide phases.

Elemental changes and alteration recorded by basaltic drill core samples recovered from in situ temperatures up to 345°C in the active, seawater‐recharged Reykjanes geothermal system, Iceland

“…The alteration history recorded in the RN‐17B core, a sidetrack of hole RN‐17, provides a very different picture of alteration compared to investigation of cuttings from RN‐17 and RN‐17B [ Marks et al ., ; Fowler and Zierenberg , ]. Geochemical variations in RN‐17 and RN‐17B drill cuttings analyzed at 50 m depth intervals from downhole depths ranging between 350 and 3050 m [ Marks et al ., ] are relatively minor compared to the variations seen in the RN‐17B core, particularly for depths below 2000 m where intrusive rocks predominate [ Fowler and Zierenberg , ].…”

“…The alteration history recorded in the RN‐17B core, a sidetrack of hole RN‐17, provides a very different picture of alteration compared to investigation of cuttings from RN‐17 and RN‐17B [ Marks et al ., ; Fowler and Zierenberg , ]. Geochemical variations in RN‐17 and RN‐17B drill cuttings analyzed at 50 m depth intervals from downhole depths ranging between 350 and 3050 m [ Marks et al ., ] are relatively minor compared to the variations seen in the RN‐17B core, particularly for depths below 2000 m where intrusive rocks predominate [ Fowler and Zierenberg , ]. Based on the relatively unaltered nature of the cuttings and oxygen and strontium isotope profiles showing little downhole variation below 2000 m, the impression is that the Reykjanes system is rock dominated with abundant intervals of relatively unaltered rock, and fluid flow is mostly controlled by infrequent fractures [ Marks et al ., ].…”

“…Based on the relatively unaltered nature of the cuttings and oxygen and strontium isotope profiles showing little downhole variation below 2000 m, the impression is that the Reykjanes system is rock dominated with abundant intervals of relatively unaltered rock, and fluid flow is mostly controlled by infrequent fractures [ Marks et al ., ]. In stark contrast, core RN‐17B shows pervasive alteration and much larger variations in chemical composition, including zones of Na addition and adjacent zones of Na depletion [ Fowler and Zierenberg , ]. The coarse‐grained cuttings that are routinely analyzed in order to infer downhole lithology and alteration are clearly biased by selective recovery of relatively unaltered rock and resistant alteration minerals, especially quartz and epidote, and caution should be used when inferring chemical exchange based on drill cuttings [ Fowler and Zierenberg , ].…”

“…In stark contrast, core RN‐17B shows pervasive alteration and much larger variations in chemical composition, including zones of Na addition and adjacent zones of Na depletion [ Fowler and Zierenberg , ]. The coarse‐grained cuttings that are routinely analyzed in order to infer downhole lithology and alteration are clearly biased by selective recovery of relatively unaltered rock and resistant alteration minerals, especially quartz and epidote, and caution should be used when inferring chemical exchange based on drill cuttings [ Fowler and Zierenberg , ].…”

“…Lithology, alteration mineralogy, and temperature characteristics of the Reykjanes geothermal system are well characterized from the more than 30 wells that have been drilled for the production of geothermal energy, some to depths exceeding 3000 m. The majority of subsurface samples from these wells are returned as drill cuttings, which are of limited use for determining seawater‐basalt element exchanges due to mixing of different lithologies and contamination of samples by drilling equipment [ Fowler and Zierenberg , ]. Recently, the Iceland Deep Drilling Project (IDDP) recovered spot drill cores to test newly designed drilling equipment during the drilling of three new geothermal wells (RN‐17B, RN‐19, and RN‐30) in the Reykjanes geothermal system.…”