Lempur, unlimited resources of Geothermal

Geothermal Info at Lempur

The Lempur geothermal field is situated on the north and east flanks of G.Kunyit, a dacitic volcano which erupted lava flows and domes, and hydroclastic flow rocks in the late Pleistocene. This is the youngest volcano in the immediate area and the associated magma chamber is the likely local heat source for the thermal system. G.Kunyit is built up on top of the products of two other Quaternary volcanoes, G.Raja to the NW and G.Tua - G.Atapijuk to the SW. These are andesitic volcanoes and poured large volumes of lava into the depression caused by down-faulting along the Great Sumatera Fault Zone. Subsidence in this zone is considered to have been coincident with the eruption of the voluminous Miocene-Pliocene silicic ignimbrites and rhyolites which were also accompanied by some basaltic andesites (J.I.C.A., 1981, 1983). All these volcanic products rest on the Mesozoic basement of limestone and minor shale in the Lempur area but elsewhere this unit also includes low grade metamorphic, granitic and some volcanic lithologies.

Fumaroles and area of steaming ground constitute the main surface thermal features on the flanks of G.Kunyit. Fumarole temperatures are mainly between 91-94oC with the hottest fumarole at Grao Rasau having a temperature of 97oC. Several zones of altered groundare delineated. However, it is not known if they are hot, warm or cold.Hot water and mud pools are also present and generally have no or only small outflows. The Abong spring is the main over-flowing feature with a temperature of 59oC. However warm springs (Tebat Buyuk, Rasau Hangat) are shown to the east of the main group of surface features (J.I.C.A., 1981, Fig. 3-2) but no information is available on their physical or chemical characteristics. All the features up on the flanks of G.Kunyit are the result of steam heating from below.

Most of the thermal features are clustered along or around the Duabelas Fault within an area of about 1.5 km2. This fault trends NNE-SSW and is parallelled by the Sikai Fault, which is further to the west cross cutting G.Kunyit, and along which is the Grao Sikai alteration zone. Faults of this trend appear to be the main conduits for the thermal fluids reaching the surface. This NNE-SSW fault direction is one of the three main fault trends in the region, the others being WNW-ESE and NW-SE. The NW-SE trending faults in the Lempur region are part of the Great Sumatera Fault zone and also partly control the flow of thermal fluids as seen by the existence of the Kering Alteration zone along the Aur Fault SE of G.Kunyit. The influence of the WNW-ESE faults on the thermal system is not clear.

GEOCHEMISTRY

The Lempur prospect lies on the northern and eastern slopes of Gunung Kunyit at an altitude of 1100 -1800 metres ASL. Surface manifestations are limited to a number of areas of fumarolic activity and alteration some of which contain hot water. These NNE-SSW along a fault (Duabelas Fault) secondary to the Great Sumatera Fault, and, to the north of these, lies another spring, Abang. To the west of Abang lies a further area of fumarolic activity, Grao Sikai, which has the highest molar percentage of gas to steam (approximately 2.8%). The other fumarolic areas have gaseous mole % content approximately 0.8%. Carbon dioxide content makes up 90-95% of this, and hydrogen sulphide contributes 1-4% by molar volume. CO2/H2 ratios imply a decreasing trend from Grao Bujang in the north to Grao Rasau in the south. The CO2/H2S ratio at Grao Sikai approximates that at Grao Bujang. Soil/air CO2 levels (J.I.C.A., 1982) follow the main NNE-SSW fracture trends in the region. Secondary fracturing running WNW-ESE has also been defined.

The highest water temperature (94oC) was increased (J.I.C.A.,1981) at Grao Sikai and closely approximates boiling point at that altitude. Water chemistries in the area imply that surface waters are steam heated and have no direct contact with the reservoir beneath. Chloride levels are low and baren, sulphate and bicarbonate concentration increase from south to north as does the pH (from 5.8 to neutral). The trend in sulphate agrees with the CO2/H2S trend which implies that the reservoir water level is closest to the surface in the south. This is in agreement with CO2 field measurements (J.I.C.A., 1981).

It can be inferred that the water to rock ratio is higher than normally found in a vapour-dominated reservoir,warm springs are known to exist to the east of the prospect at 200 m and information from these may give some indication of leakage from the system.

An exploratory well (LP-1) was sited and successfully drilled in the Grao Duabelas area based on results of CO2 soil anomaly measurements. The total mass flow rate was 21 tonnes/hour with a fluid enthalpy approximating 840 kJ/kg at a WHP of 0.6 bar g. The water was continuously monitored for pH, chloride and bicarbonate during the discharge period (24 days) and initial results indicated interference from drilling fluids returning from the formation to the surface. Chloride and bicarbonate levels eventually stabilised at 745 and 104 ppm and discharged waters had a pH of 8.8. Gas content in the fluid is very low (o.2% wt of total fluid).

GEOPHYSICS

The geophysics of the Lempur prospect is reviewed in the second Phase Survey of the 3 part report by Japan International Cooperation Agency on Lempur Geothermal Development (1982). The principal geophysical surveys were Schlumberger soundings and a gravity survey. A 1 m depth temperature survey was carried out concurrently with a soil geochemistry survey, but this survey revealed little new information about the extent of thermal ground. Outside of the obvious thermal manifestations, the ground temperature at 1 m depth was generally between 20-25oC, with most variations, and degree of exposure the weather, rather than the deep temperature variations.

The resistivity surveys were in the form of soundings to AB/2= 2000 m, centred at 300-400 m intervals, along four ENW-trending lines on the northeast flanks of G.Kunyit. A total of 75 soundings were completed. The data are presented in the form of apparent resistivity maps (AB/2= 500 m, AB/2= 1000 m, and AB/2= 1600 m), pseudosection plots on the four lines and modelled resistivity (1-D) along the lines. The lines are typically 1 km apart, and about 4 km in length. On all 4 lines there is a general trend of decreasing resistivity with depth. The main exception is in the area of the thermal features of Grao Bujang and Grao Duabelas. Here, the near-surface apparent resistivity is low (<10 2=" 1600">200 m depth along at least 3 km of line D (this line is on G.Kunyit and has the highest elevation of all the lines). However, higher resistivities (50-100 ohm-m) are modelled at 600-1000 m depth, indicating most of the sounding curves showed some turn-up at large current spacings. It is not known whether this is a real increase, and if so, howwell determined are the modelled resistivities in this deepest layer. The width of the low resistivity zone along each line decreases with decreasing elevation, and the modelled resistivity increases with decreasing elevation. This is suggestive of an outflow regime, with progressive dilution, and cooler subsurface conditions at lower elevations.

The survey area was not large enough to fully delineate the region of low resistivity at depth. The prospect appears to extend beneath the summit of G.Kunyit. The probable area in the datafile is given as 4 km2, and this is only the area delineated by the resistivity soundings. A possible area of 8 km2 assumes that the prospect could extend beneath the summit of G.Kunyit.

The gravity survey comprised 189 stations, made over a total area of 150 km2 around the Lempur prospect. Measurements were concentrated in a 40 km2 area around the thermal features, where the average station spacing is 300-600 m. A regional NW-trending gravity low coincides with the Lake Kerinci depression, which is associated with the Great Sumatera Fault Zone. The Lempur prospect lies on the SW flank of this low and a 20 mgal gravity gradient is superimposed on the prospect area. Beacuse of problems with chosing the appropriate regional filed, and the appropriate topographic density correction, the data has been reduced using a value of 2.30 Mg/m3 (based on 57 density measurements on outcrop samples). Local gravity highs and lows are present in the vicinity of the Lempur prospect, but most are difficult to interpret in terms of subsurface structure. The most prominent local feature is a WNW-trending zone of high gravity gradient. This gravity trend coincides with, and links the Sikal geothermal area with the Duabelas geothermal area. It may be an important subsurface fault zone, having a large horizontal density contrast. Its relationship with the two above-mentioned thermal areas suggests it may influence the ascent of thermal fluids to the surface.

WELL DATA

Only one well has been drilled in the Lempur Field. Well LP-1 completed in January 1983 is about 1000 m depth with a final diameter of 75 mm. Maximum temperature encountered are about 210oC at 700 m depth. A slight temperature inversion is apparent between 700 m and well bottom. Injection tests suggest poor formation permeability. A combination of low permeability, small well-bore diameter and low temperatures resut in a poor steam production characteristics. The well was discharges for one month in early 1983 and produced fluid with an enthalpy of 840 kJ/kg. Steam production was about 3.5t/hr at wellhead pressures less than 2.5 bar abs. It may be concluded that the well draws from a reservoir containing liquid water at temperatures around 200 to 210oC.

REFERENCES

J.I.C.A., 1981. Report on Lempur geothermal development, first phase survey. Unpublished Report for V.S.I. Japan International Cooperation Agency.

J.I.C.A., 1982. Report on Lempur geothermal development, second phase survey. Unpublished Report for V.S.I. Japan International Cooperation Agency.

J.I.C.A., 1983. Report on Lempur geothermal development, thrid phase survey. Unpublished Report for V.S.I. Japan International Cooperation Agency.

Van Padang, N.M., 1951. Catalogue of the active volcanoes of the worl including solfatara fields. Pt 1. Indonesia. International Volcanological Association Italy.


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