Jökull - 01.12.1987, Page 41

mean annual temperature in Krísuvík) the value taken
by F is 4.03.
The values of N2 c and Zb can obtained by a common
solution of equations (1), (11) and (14) using the mea-
sured nitrogen (N2 m) and carbon dioxide concentrations
in the steam.
Results for the Krísuvík field
The amount of steam condensation in the upflow of
the Krísuvík field has been estimated by the two meth-
ods just described (Table 4, Fig. 5). The results indicate
relatively limited condensation for the majority of the
samples; less than 50% and 30% in the case of conduc-
tive heat loss and mixing with cold water, respectively.
Estimation assuming conductive heat loss always gives
higher values. This is so because the other model, in-
volving condensation in cold water, accounts for some of
the N2 in the steam sample by degassing of this cold
water.
Atmospheric contamination causes values for calcu-
lated condensation to be too high. Such is clearly the
case for samples 7, 8 and 23 in Table 4 as deduced from
the elevated N2 content together with high N2/Ar ratios.
Estimated condensation for some of the samples gives
negative values (Table 4). Apart from analytical error
which will, of course, contribute this could be due to: (1)
TABLE 4. Gas geothermometry results (°C) for fumarole samples from the
Krfsuvík field and estimated condensation in the upflow
No. C02 co2/n2 h2s h2 co2/h2 FTS DPS h2co l H2SC02 K «b
1 296 (288) 277 278 252 215 391 339 313 320 47 25
2 293 (283) 271 277 246 208 379 333 303 318 51 28
3 296 (294 290 279 259 227 406 352 324 322 22 9
4 291 (292) 295 272 259 232 404 355 322 312 -II -4
5 294 (296) 303 250 196 120 285 233 231 289 -31 -9
6 285 (279) 270 259 249 221 381 338 304 296 3/ 18
7 292 (204) 184 268 257 229 393 339 319 308 95 91
8 294 (267) 248 245 270 248 414 333 341 284 76 60
9 290 (292) 297 253 249 217 393 337 306 291 -21 -/
10 292 (292) 294 250 249 214 389 329 306 288 -4 -2
11 294 (295) 298 254 244 204 385 326 300 293 -12 -4
12 294 (281) 265 252 245 205 380 317 302 290 60 37
13 292 (283) 270 252 208 144 299 247 247 290 50 2/
14 293 (290) 286 262 245 206 378 323 301 301 23 10
15 293 (283) 270 257 244 204 377 321 299 296 53 29
16 294 (275) 255 249 246 207 369 302 302 287 70 48
17 295 (295) 296 257 238 193 366 307 291 295 -2 -1
18 297 (270) 247 231 204 130 290 220 243 269 79 61
19 290 (290) 292 257 237 196 369 321 288 295 -4 -2
20 306 (280) 256 247 233 171 338 253 287 287 80 60
21 266 (265) 261 200 193 142 271 231 217 235 10 5
22 285 (263) 242 262 270 266 270 201 232 233 71 50
23 328 (195) 179 298 248 175 " " 323 355 99 98
“Values in brackets represent CO?-temperatures corrected for condensation
according to Zk. SGeothermometer of ARNÓRSSON and GUNNLAUGSSON (1985)
which assumes Fischer-Tropsch reaction equilibrium. ÖGeothermometer of
D'AMORE and PANICHI (1980). ^The and H^S geothermometers of NEHRING
and D'AMORE (1984), respectively. YCondensation assuming conductive cool-
ing. ^Condensation assuming mixing of 100°C steam with 5°C water contain-
ing 0.71 mmoles/kg of N^.
-20 O 20 40 60 80
%zb
Fig. 5. Flistograms showing calculated amount of
steam condensation in the upflow of the Krísuvík field.
Mode of condensation: Zc conductive heat loss, Zb mix-
ing of steam at 100°C with water at 5°C. — Súlurit sem
sýnir reiknaða þéttingu gufu í uppstreymisrásum á Krísu-
víkursvœði. Háttur þéttingar: Zc varmatap með leiðni, Zb
blöndun 100°C gufu við 5°C vatn.
Presence of equilibrium steam in the reservoir fluid that
will cause C02 concentrations to be high for its temper-
ature and (2) secondary boiling of partly degassed wa-
ter. As C02 is more soluble in water than N2, primary
boiling involving partial degassing will cause the remain-
ing water to attain high C02/N2 ratio for its temperature
and, therefore, also steam formed by extensive boiling
of this water. Re-equilibration of partly degassed water
with respect to C02 would enhance the increase in the
C02/N2 ratio.
It seems unlikely that equilibrium steam in the reser-
voir accounts for the negative Z-values found for some
samples. The presence of such steam would tend to
cause H2 concentrations in the fumaroles to be high
relative to C02 as H2 is considerably less soluble than
C02. As indicated by the H2-temperatures in Table 4
such is not the case.
Fig. 6A shows how partial degassing of equilibrated
reservoir water will affect the C02/N2 temperatures for
steam formed by extensive secondary boiling of this
water and Figs. 6B and 6C show how the extent of the
primary degassing affects values for calculated conden-
sation. Degassing in excess of some 50% of the initial N2
leads to strongly negative values, both for Zc and Zb;
39