Jökull


Jökull - 01.12.1987, Side 48

Jökull - 01.12.1987, Side 48
The permeability in the upflow is provided by near vertical tectonic fractures. It may be this control of permeability that forces the water and steam phases to ascend together, thus causing extensive boiling of the reservoir water. The low N2/Ar ratios (<37) in some of the steam samples from the Sveifluháls area can be explained by boiling of the reservoir water in two steps. The first step involves limited boiling at high temperatures (300°C), steam formation amounting to up to 0.5% by weight. The second step involves extensive boiling generating the fumarole steam. Boiling of the reservoir water in two steps also explains the relatively low H, concentrations in the fumaroles in the Sveifluháls area. The interpreta- tion for the N2/Ar ratios and the H2 concentrations imply that there is no equilibrium steam present in the reser- voir. The conceptual model of the evolution of volcanic systems in Iceland (Steinthórsson et al., 1986) indicates that the Krísuvík field is a youthful system, probably devoid of large high level intrusions except for feeder dykes. The temperature reversals observed in wells in the field (Arnórsson et al., 1975a) may be due to the shape of the intrusive heat source and the permeable fractures; hot water and steam rise along these fractures and at a shallow level the boiled water spreads laterally. REFERENCES Árnason, B. 1976: Groundwater Systems in Iceland Traced by Deuterium. Societas Scientarum Islandica, Reykjavík, publ. XLII, 236 pp. Arnórsson, S, 1978: Precipitation of calcite from flashed ge- othermal waters in Iceland. Contrib. Mineral. Petrol. 66: 21-28. Arnórsson, S. 1986: The chemistry of gases associated with geothermal activity and volcanism in Iceland: A review. J. Geophys. Res. 91: 12,261-12,268. Arnórsson, S. 1971: Fumarole discharge. In: G. Gudmundsson (editor) Geothermal exploration of the Krísuvík field. Na- tional Energy Authority, Prof. Paper (In Icelandic). Arnórsson, S., A. Björnsson, G. Gíslason and G. Gudmunds- son 1975a: Systematic exploration of the Krísuvík high-tem- perature area, Reykjanes Peninsula, Iceland. Proc. 2nd Unit- ed Nations Symposium on the Development and Use of Ge- othermal Resources 2: 853-864. Arnórsson, S., G. Gudmundsson, S. G. Sigurmundsson, A. Björnsson, E. Gunnlaugsson, G. Gíslason, J. Jónsson, P. Einarsson and S. Björnsson 1975b: The Krísuvík geothermal field: Final report on exploration. National Energy Author- ity, Prof. Paper OSJHD 7554 (In Icelandic). Arnórsson, S. and E. Gunnlaugsson 1983: Gas chemistry in geothermal systems. Proc. 9th Stanford Geothermal Reservoir Engineering Workshop, 231-237. Arnórsson, S. and E. Gunnlaugsson 1985: New gas geother- mometers for geothermal exploration — Calibration and ap- plication. Geochim. Cosmochim. Acta 49: 1307-1325. Arnórsson, S., E. Gunnlaugsson and H. Svavarsson 1983a: The chemistry of geothermal waters in Iceland III. Chemical ge- othermometry in geothermal investigations. Geochim. Cos- mochim. Acta 47: 567-577. Arnórsson, S., E. Gunnlaugsson and H. Svavarsson 1983b: The chemistry of geothermal waters in Iceland II. Mineral equilib- ria and independent variables controlling water composi- tions. Geochim. Cosmochim. Acta 47: 547-566. Bunsen, R. 1847: Uber den inneren Zusammenhang der pseudo- vulkanischen Erscheinungen Islands. Liebigs Ann. Chem. u. Pharm. 62: 1-59. DAmore, F. and R. Celati 1983: Methodology for calculating steam quality in geothermal reservoirs. Geothermics 12:129- 140. D’Amore, F., R. Celati and C. Calore 1983a: Hypotheses on possible equilibria between N2 and other gases at Larderello and Cerro Prieto. Proc. 9th Stanford Geothermal Reservoir Engineering Workshop, 247-252. D'Amore, F., R. Celati, C. Calore and R. Bertrami 1983b: Effects of natural recharge on gas composition in the Larde- rello-Castelnuovo area. Proc. 9th Stanford Geothermal Res- ervoir Engineering Workshop, 221-229. D’Amore, F. and S. Nuti 1977: Notes on the geochemistry of geothermal gases. Geothermics 6: 39^15. D'Amore, F. and C. Panichi 1980: Evaluation of deep temper- atures of hydrothermal systems by a new gas geothermom- eter. Geochim. Cosmochim. Acta 44: 549-556. D’Amore, F. and A. H. Truesdell 1979: Models for the steam chemistry at Larderello and the Geysers. Proc. 5th Stanford Geothermal Reservoir Engineering Workshop, 283-297. D’Amore, F. and A. H. Truesdell 1980: Gas thermometry for drillhole fluids from vapor-dominated and hot water geother- mal fields. Proc. 6th Stanford Geothermal Reservoir Engi- neering Workshop, 351-360. D’Amore, F. andA. H. Truesdell 1985: Calculation of geother- mal reservoir temperatures and steam fractions from gas compositions. Trans. Geothermal Resources Council 9: 305- 310. Elísson, G. 1967: Analyses of geothermal gases 1962-68. Un- publ. report, Science Institute, Reykjavík, 15 pp. Ellis, A. J. and W.A.J. Mahon 1977: Chemistry and geothermal systems. Academic Press, New York. Giggenbach, W. F. 1980: Geothermal gas equilibria. Geochim. Cosmochim. Acta 44: 2021-2032. Gunnlaugsson, E. 1975: Chemistry of postglacial lavas in the Krísuvík area, SW-Iceland. Societas Scientarum Islandica, Reykjavík, publ. 23: 160-166. Imsland, P. 1973: The geology of Sveifluháls. Unpubl. B.Sc. thesis, Univ. Iceland, 86 pp. (In Icelandic). Jónsson, J. 1978: Geological map of the Reykjanes Peninsula. National Energy Authority Prof. Paper OSJHD 7831 (In Icelandic). Líndal, B. 1951: Chemical analysis of hot spring discharges. Unpubl. report of the State Drilling Contractors, 33 pp. (In Icelandic). 46
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