STABLE ISOTOPE CHEMISTRY OF WELL FLUIDS

For a few wells, both steam and gas samples were taken for ¹⁸O and ²H analysis through a side tapping located at a distance of approximately ten times the pipe diameter downstream of the throttling plates of the discharge pipe using a webre separator at a measured pressure (Table IV). The isotopic analysis of the separate samples were then combined to obtain the total discharge composition, e.g. ô²^ for the case of deuterium, according to

= (1 - y^s)o²H, + y^so²H^v (3)

where the subscripts 1 and v refer to the liquid and vapor fractions respectively (Table V).

Analytical results for water samples only (i.e., the steam fraction was not analyzed) were corrected to total discharge composition assuming isotopic equilibration at the sampling condition by the relation

62Ht = 02H, - ys!03lna2H (4)

where a^2H is the fractionation constant of ²H between liquid water and steam. Analogous relations were also used for ¹⁸O.

Figure 10 shows a plot of 5²H versus 5^I8O for all the wells sampled together with the LMWL and the measured values for rain and groundwaters. The cold lake samples (Rangas,

PAL-40 PAL-130 PAL-90 pAL.,6D

-180

PAL-140

•PAL-4D PAL-90 CN- 3D MO-3

-10 - 4 - 2 . . 0 . 2

DELTA -180

LEGEND:

RAINWATER WELLS:

O PAL D OP O CN

CI Spring SEAWATER MO

T———————I——

8 10 12

FIG. 10. A plot of 618O versus 82H for well waters at Bacon-Manito geothermal field. Line drawn through the points is a regression line. The square labelled "andesitic water" is from Giggenbach (1992). The line drawn through the rain samples (represented by asterisks,*) is the local meteoric water line (LMWL) obtained by regression.

203

LRAN and Osiao, LOS) were excluded since these are subject to evaporation and variable inputs due to runoff. To discriminate the deep subsurface from near surface processes, all of the acid-sulfate and bicarbonate thermal springs have been omitted. The chloride springs, while useful for comparing them with the well waters, were excluded since these are subject to evaporation (NAG) as shown earlier, dilution with different types of water and condensation (e.g., INA, BUG). The PAW spring was included to represent a natural thermal feature containing the deep chloride geothermal fluid diluted with meteoric water.

A regression line of well data including PAW spring gives

02H = 0.3918O - 25.9 (5)

which is almost horizontal. The intersection of this line with the LMWL gives a tentative isotopic signature of the recharge meteoric water which is 5²H = -28%o and 6¹⁸O = -5.3%o.

Several groups of springs located southwest and southeast in relation to the main production sector, Palayang Bayan, approximates this isotopic composition.

The regression line has the appearance of the "classical" oxygen isotopic shift due to water-rock interaction [18]. The waters tapped by well OP-6D appear to be most shifted, and should represent closely the deep, hot temperature reservoir fluids. The Cl-enthalpy plot presented earlier showed that this is not the case. Wells OP-3D and OP-4D are the most likely representative fluid, although their oxygen isotopic content is less shifted.

Other explanations have to be invoked. OP-6D, as shown earlier in the Cl-enthalpy plot, is fed by a relatively shallow and gassy horizon. Its fluids could have undergone high

1000 2000 3000 4000 5000 6000 7000 aooo 9000

CHLORIDE TD,mg/kg

FIG. 11. A plot of chloride-818O (in total discharge) for Bacon-Manito wells and springs. Big hatched square represents parent water. The size indicates the level of uncertainty. The parent fluid, P, has 8000 ± 500 mg/kg Cl and a possible SISO ranging from 0 to + 1.5%o.

204

Albay Gulf

MO-2

MO-3 MO-1'

'.|>EEP RECHARGE

+ * V WATERS

* 4- *

SHALLOW METEORIC

«^»RECHARGE WATERS

°0

SECO RESE

\\V CI RESER

• \ 200 V ^50 280

m.AMSL

Sorsogon Bay

LEGEND:

BOILING

^J, TEMPERATURE

Not drawn to scale

FIG. 12. A conceptual geochemical model of the field. Big arrows pointing upwards represent upflowing geothermal waters. Temperature contours below 350 °C are based on well data, while higher values are purely hypothetical. Downward pointing smaller arrows represent meteoric waters.

temperature, ¹⁸O-shift with CaCO³ rather than with H²O and subsequent steam addition upon entering the wellbore. Pending additional data, it is well OP-3D which is likely to be tapping deep reservoir fluids which have encountered similar high temperatures and undergone pro-longed interaction with both CaCO³ in the GSF and silicates in the CIC and PV. This could explain its higher Cl, gas and ¹⁸O shift relative to OP-4D.

An alternative explanation is mixing with magmatic water. Recently, Giggenbach [19]

has shown that waters from geothermal and volcanic systems along convergent plate boundaries, of which Bacon-Manito is an example, are mostly mixtures of the local meteoric water and waters associated with andesitic magmatism. All Philippine volcanic [14, 19, 20]

and geothermal systems [5, 6, 19, 21] studied so far have the above characteristics which makes the Bacon-Manito field unlikely to be an exception.

Extending the regression line to more positive values would, within errors, intersect the postulated location of the "andesitic" water of Giggenbach [19]. OP-3D and OP-4D, could then represent possible mixing combinations between these two end-members. With the slightly greater 18O-shift from OP-4D to OP-3D, the possible magmatic water contribution to the deep reservoir fluids increases from about 38% to 50%. Based on the chloride values of the isotope samples there is a corresponding increase in Cl from 6500 to about 9000 mg/kg.

Based on these values, the end-member magmatic water has a Cl concentration ranging from 15000 to 18000 mg/kg. These values are close to the magmatic Cl value of 15000 mg/kg postulated for the andesitic White Island volcano and Mt. Ruapehu in New Zealand [12].

To verify the actual processes involved, chloride is plotted against 518O (Figure 11).

The trends are similar to that of the Cl-enthalpy plot, thus, confirming that OP-3D and OP-4D are nearest to the upflowing reservoir fluids. Thus, the deep reservoir fluid is estimated to have Cl concentration of 7500-8500 mg/kg Cl.