Orkustofnun, Grensasvegur 9, Reports 2016 IS-108 Reykjavik, Iceland Number 33 CHEMISTRY OF GEOTHERMAL FLUIDS IN DIENBIEN AND SONLA PROVINCES, NW-VIETNAM Pham Dieu Linh Vietnam Institute of Geosciences and Mineral Resources 67 Chien Thang Road, Ha Dong District Hanoi VIETNAM [email protected] ABSTRACT North-west Vietnam is a very special area where development of geothermal energy will be significant for socio-economic activities. The data in this project is taken from a previous research in 2004. Thirty-four samples are taken from 119 hot springs for further study, with the purpose of estimating and interpreting the chemical compositions of the water, by using the methods as triangle diagrams, geothermometers, evidence of mixing and log Q/K plot. The 34 samples were divided into 5 groups based on the location and structure of geology. The study results show that Group 1 with samples S1, S2, and S3 give a good estimation of the temperature of the reservoir from 73°C to 150°C with small deviation between methods. Group 2 with samples S8, S18, and S19 give a temperature from 76°C to 111°C. In Group 3 with samples S11 and S12, reservoir temperature is from 59°C to 110°C. Group 4 with S72 and S73 samples, the temperature of reservoir drops between 60-100°C. Group 5 with samples S6, S65, S75, S77, and S78 the reservoir temperatures estimated by silica geothermometers are quite low and they might be from a different reservoir. 1. INTRODUCTION Geothermal energy is a clean, renewable and enviromentally benign energy source which is based on the heat in the earth. Geothermal energy is number fourth of the renewable energy sources in world electricity production after hydro, biomass and wind. Vietnam is a country with an area of 331,689 km2 situated on the Indochina peninsula and more than 300 thermal springs. There have been some geothermal studies done since 1986 that involve basic surveying such as mapping, sampling and analysic of geothermal water and evaluation for the areas in north and central Vietnam, with the aim of supporting socio-economic development. Until now, geothermal utilization in Vietnam is simple, such as swimming pools, spa treatments and vegetable dehydration. The aim of this study is about working with geothermal fluid as a part of a surface exploration. During this process, sampling locations are selected, water samples are collected and analysed to find out the chemical composition of the thermal water. Geothermometers and chemical programs are applied on 693 Pham Dieu Linh 694 Report 33 the data from the analytical results. The interpretation of geochemical data aims at estimating the reservoir temperature, the properties of the geothermal fluids, fluids origin and recharge, and if possible, physical processes such as cooling, boiling or condensation, and geothermal potential of area. 2. GEOLOGICAL BACKGROUND Vietnam is located in southeast Asia and is a part of the South China plate and Indochina plate. The plate boundary is the Red River fault zone that strikes NW-SE and extends over a length of more than 1000 km between the Gulf of Tonkin and Tibet (Tapponnier et al., 1990). According to the model of Tapponnier et al. (1986), the whole of Indochina extruded to the southest during the collision of India with Asia. The relationship between Indochina and South China movement along the Red River fault zone has been estimated to be from 330 km (Lacassin et al., 1993) to 500-740 km (Tapponnier et al., 1986, 1990) of left lateral motion during the Tertiary (from 65 Ma to 1.8 Ma). According to Tran (1995) the territory of Vietnam can be divided into five units as structural blocks consisting of the Northeast (NE), Northwest (NW), Truongson (north center of Vietnam), Kontum (south center of Vietnam) and Nambo blocks (South Vietnam). The NE block includes the stratigraphy and igneous rocks ranging from the Late Proterozoic to Quaternary age, 542 Ma to 1.6 Ma. The NW and Truongson blocks involve the thickest Paleozoic strata in Vietnam which are recognized as NW-SE trending Paleozoic folded systems with some differences in tectonic development between them, especially in the late Paleozoic. The Kontum block is an uplifted massif where the oldest stratigraphy of the Archean is found, but Paleozoic rocks are almost absent. The Nambo block is defined as a continental rift filled with thick (6 km) Cenozoic deposits. The study area is located in a part of Dienbien and Sonla provinces in the NW block which have abundant active tectonic faults (Figure 1). About 34 hot springs have been discovered in this area with temperatures ranging from 35 to 78°C. The geology of this area is quite complicated with presentation of many types of rock that are structured in NW-SE direction. With regard to geology and tectonics, the region is composed of the following zones: The Dienbien zone is located southwest of study area which lies along the Vietnam-Laos border in southwest and Songma fault at northeast. Dienbien zone was filled up with volcano clastic and coal- bearing sediments in Triassic age, sandstone, siltstone, silicic, tuff basalt, andesite and limestone in Carbon – Permian age and sandstone, clay schist in Silurian – Devon age. Samples marked S1-S5, S7, S8, S14, S15, S16, S17, S18, S19, S20 and S82 are located in this zone. The Songma zone belonging to the northwest Vietnam folded system (late Caledonian time), which is located in the southwestern study area, is composed of a femic (mostly ferromagnesian minerals) basement complex. This zone was mixed with ophiolite of an old subduction zone. Samples marked S77, S79, S83, S6, S78, S80, S81, S65, S74 and S75 are located in this zone. The Songda zone was the rift of Mesozoic time which is located in the northeastern part of the study area. A superimposed depression structure overlaps the Songma zone and comprises a terrigenous- carbonate volcanic complex with a prevalence of porphyritic basalt and carbonate-terrigenous formations. Samples marked S11 and S12 are located in this zone. The Tule zone is a volcano-tectonic depression formed in Jura-Cretacious time. The bedrocks are alkaline-calcareous and volcano-platonic. The Fansipan mountain ridge which is a part of the Tule zone is a granite-granosyenite intrusion. Samples marked S67, S68, S69, S70, S71, S72 and S73 are located in this zone. Report 33 695 Pham Dieu Linh . M N u N . M V u 21.7° III IV S á n g ‡ S12 ¡ S67 n S11 Å S69 S72 i N h . C M N. uái S70 S71 S68 S73 n S1 o h K . N S5 S á n g S4 S2 ‡ S65 S3 gan a ¡ N. N M N. S78 Nõèc NÜng S80 S6 N a . M . N N õ a S14 II S án g N N º M m . ¬ P S75 h P u a S16 n S74n g S15 S17 S81 N. Ban S20 S18 S19 S8 S77 Mêng Lãi 1 S82 S79 Location S83 20.8° Lao PDR 01020 East Sea kilometers 102.8° Cambodia Legend Carbon – Pecrmi: Sandstone, siltstone, silic, tuf basalt, andesite &limestone Pleistocene: Sand, silt, clay Devon: Sandstone, quartzite, conglomerate, schist Silur-Devon: Sand stone, clay schist Upper Jura-Creta: Ortophyr, tuf, clunch, bazalt Trias: Siltstone, sandstone, clunchstone Permi: Tuf basalt, clay, silic, coal, limestone Cambri: Limestone, lime clay, quartz sandstone, quartzite, sericite schis Late Neoproterozoi: Clay schist, marble Dien Bien Phu complex: Diorite, granodiorite, granite Holocene: Sand, silt, clays Fault FIGURE 1: Geological map of the study area (modified from VIGMR, 2004), Vietnam map on the right and samples from S1 to S82 showing on the map The three main active tectonic faults are the Dienbien-Laichau fault, Songma fault and Sonla fault, where earthquakes are intensively active in this region (Figure 1). The faults and their cataclastic zones are the vents for geothermal heat discharge. Anomalous heat flow is often revealed in the form of thermal springs, which are densely located near the tectonic-zoned faults (Hoang, 1996). The Pomlot (sample S3, Table 1) hot spring which appeared after an earthquake occurred in 1935 is an illustration of this. Pham DieuLinh TABLE 1: The chemical compositions of the thirty-four thermal springs (mg/l) in the study area (VIGMR, 2004) “-“ = no analysis No. Temp. Surf (oC) pH TDS Q (l/s) + + 2+ 2+ SiO - BLiF 2- - CO + Rb Mn del 18O del D Charge balance Na K Ca Mg 2 Cl SO4 HCO3 2 NH4 S1 53,8 7,1 220 1,7 69 4,2 16 6 51,4 4,7 0,02 0,047 0,2 1 238,4 22,5 0,01 0,008 0,11 -9,8 -70,9 4% 696 S2 78 7,3 410 130,4 15,6 10 1,8 125,8 12,1 0,72 0,244 2,15 81 319,7 - 0,62 0,078 0,04 -9,1 -65,9 -4% S3 74 6,8 510 0,8 119,3 8,5 46 7,2 66,8 27,6 1,37 0,336 1,3 37,9 518,5 - 1 0,028 0,17 -8,8 -64,8 -9% S4 57 6,6 260 0,4 104,6 7,1 12 7,2 39,1 6,4 - - - 8,9 235,9 2,7 0,01 - - - - 17% S5 37 7,4 220 0,3 111,4 4,6 12 3,6 27,8 6,4 - - - 7,4 157,4 27,5 0,01 - - - - 34% S7 37 6,6 870 1 47,4 5,6 208,5 18,3 34,2 6,4 0,12 0,166 0,885 473,8 221,3 22,5 0,01 0,01 0,105 - - 2% S8 36,5 6,8 410 0,1 94,7 5,1 67,5 6,3 54,1 11,2 0,215 0,206 0,31 17,2 418,6 25,4 0,01 0,017 0,145 - - 4% S14 47 8 900 1 340,6 11,4 18 9,8 - 58,1 - - - 23 878,7 - - - - - - 1% S15 43 8 740 0,5 270 8,4 21,9 4 - 30,5 - - - 97,5 613,6 - - - - - - 2% S16 38 7,5 210 0,4 29,4 1,4 47,1 4,8 - 33 - - - 189,2 - - - - - - 0% S17 30 6 280 2 25 1,2 50,6 22,8 - 11 - - - 30,1 277,6 - - - - - - 0% S18 43 6 530 0,2 176,8 12,4 23,1 4,6 60 15,7 - - - 9,1 570,5 - - - - - - -2% S19 46 7,5 430 0,2 135 11,2 23,9 8,6 55 22,7 - - - 35,5 389,9 - - - - - - 2% S20 32 6 450 0,5 72 6,2 95,9 7 28 8,1 - - - 17,3 482 - - - - - - 1% S82 48,9 8,3 370 0,7 57,1 3,7 55 16,2 18,2 16 - - - 25,4 396,5 16,5 0,01 - - - - -6% S11 30 7,5 330 0,5 17,4 2,7 54,9 35,6 38 7,6 - - - 48 323,3 - - - - - - 0% S12 50,5 7,3