Chemical and Isotopic Approach to Identify Hyperthermal Groundwater Origin in The Parigi Fracture Limestone Aquifer, Mt. Kromong Area, West Java – Indonesia
Deny Juanda Puradimaja, Hendri Silaen, Dasapta Erwin Irawan
Applied Geology Research Group, Faculty of Earth Sciences and Technology Mineral
Institut Teknologi Bandung, Indonesia – Jl. Ganesa 10, Bandung, West Java, Indonesia, Tel/Fax: +62 22 251 0802 – email@example.com
Submitted t: Journal of Hydrology
Abstract Parigi Limestone Aquifer is exposed mainly at Mount (Mt) Kromong areas. The formation exposure lies at Palimanan, 20 km to the west of Cirebon, West Java, Indonesia. There are 13 hot springs emerge from fracture aquifer in limestone and intrusion rock. Three (3) hot springs of limestone aquifer are containing traces of oil seeps. Ten (10) groundwater samples have been tested to measure the concentration of major elements and stable isotope (Deuterium and Oxygen-18).
Hot spring samples from limestone aquifer system of Mt. Picung and Kali Asin area are characterized by Cl/HCO3 and Cl/SO4-type water. Based on Piper plot, hot spring samples are analogous with hot spring samples from Mt. Ciremai, but separated from normal temperature springs.
Moreover, the stable isotope compositions in the deuterium and oxygen-18 chart show the similarity of hot spring sample with meteoric water composition. The condition indicates the origin of hot spring water from meteoric water. The chart also show the shifting of isotopic line to formation water area. The shifting indicates mixing process of meteoric water with formation water from deep-seated aquifer. The interaction confirms the multiphase flow between groundwater and traces of oil seeps in Parigi Limestone Aquifer.
Keywords : aquifer system, groundwater chemistry, multiphase flow, isotope hydrogeology
Mount (Mt) Kromong is located at Palimanan area, 20 km to the west of Cirebon, West Java, Indonesia (Figure 1). This mountain is sited 15 km to the north from Mt. Ciremai (3078 masl). Geothermal features found in the northern part of Mt. Kromong comprise 13 hot springs and a mud pool, with temperature ranges between 32,8 to 58 oC. The hot springs located in four different areas, namely Mt. Picung area (8 hot springs), Kali Asin area (3 hot springs), Kedondong area (1 hot spring), and Bobos area (1 hot spring). Three hot springs in Mt. Picung area are mixed with oil seeps.
Previous studies on regional geology, tectonics, and stratigraphy in Palimanan area had been carried out by Pringgoprawiro (1976), Adnan (1991), Situmorang (1995), and Djuri (1996). Kartokusumo (1982) also reported the result of preliminary study on hot springs hydrochemistry. However, there is no data available on the utilization of stable isotope for hot spring study in this area.
The combination of oxygen isotope, deuterium isotope, and water chemistry had been used in many studies on hot water origin. White (1957), Craig (1963), Clayton et.al. (1966) examined the variations of chemical and isotopic composition of hot water, which provide an important tool for determining different types of water. The hot water could be generated from meteoric water, formation water, magmatic type, or the mixing of different types. The Craig and Clayton models has been used in an attempt to understand the characteristics of hot (hyperthermal) water aquifer based on chemical composition of hot water. The flow system is then analyzed based on geologic, topographic, and hydrologic conditions.
2. HYDROGEOLOGICAL SETTING
Studies carried out by Pringgoprawiro (1976), Situmorang (1995), and Djuri (1996) had shown that the stratigraphy of the area consists of five lithological units as seen in Figure 2.
1. Cibulakan Formation, composed of Middle Miocene shale with few sandstone and limestone intercalations. At Jatibarang area, approx. 20 km to the north of study area, oil and gas are produced from those sandstones and limestone layers.
2. Parigi Formation, composed of Late Miocene – Early Pliocene limestone with the maximum thickness of 150 meters. Oil is also produced from reef limestone of this unit.
3. Cisubuh Formation, composed of Late Pliocene shale/clay. No hydrocarbon was found.
4. Plio-Pleistocene Intrusive Rocks, composed of andesite, hornblende andesite, and hyperstene andesite. The intrusion took place during Plio-Pleistocene time and intruded the limestone and shale layer.
5. Quaternary Volcanic Rock, composed of breccias, lavas, and tuff. These deposits derived from Ciremai volcano which is located 15 km to the north of the area. Kromong breccias contain fragments from older sediment layers.
The geological structure of mount Kromong is associated with the southeast – northwest and southwest – northeast trending fault systems, which characterize the regional structure of West Java (Adnan, 1991; Djuri, 1996). The thermal discharges in Kali Asin area are located near to the southeast – northwest normal fault, while the thermal discharges in Mt. Picung area are located near to the southwest – northeast normal fault, east – west strike slip fault, and southeast – northwest normal fault (see Figure 3).
Based on the geological condition and result of spring’s observation, the aquifer system in this area can be divided into three types:
1. Volcanic aquifer, composed of fracture aquifer in lavas layer of Quaternary Volcanic Rocks. Several normal springs are found in this aquifer with low temperature (21,4 – 24 oC) and high discharge (70 – 159 l/sec; IWACO, 1989).
2. Carbonates aquifer, composed of fracture aquifer in limestone layer. Eleven hot springs are found in this aquifer with high temperature (33,8 – 58 oC) and low discharge (up to 0,4 l/sec; fluctuated by rainfall effect).
3. Intrusive rock aquifer, composed of fracture aquifer in intrusion rock and porous aquifer in weathering intrusion rock. Two hot springs with high temperature (36,5 – 37,2 oC) and 3 springs with low temperature (26,8 – 28,3 oC) are found in this aquifer. Spring discharges are about 0,2 to 0,8 l/sec.
The study area has a high annual rainfall (1500 – 3200 mm/year). Maximum rainfall occurs during the month of November to March, while minimum rainfall in May to September (see Figure 4).
Six of eight hot springs in Mt. Picung area appear only during the rainy season (March – April 2002), while in August 2001 (dry season) there are only 2 hot springs remained. The total discharge rate of each hot spring also fluctuated from 0,1 l/sec in dry season to 2 l/sec in rainy season.
3. WATER TYPES
Sixteen water chemistry analyses have been carried out in the laboratory, using samples collected from 7 normal springs, 8 hot springs (5 in dry season and 3 in rainy season), and 1 mud pool. The parameter of analysis comprises major elements (Na, K, Ca, Mg, HCO3, Cl, and SO4) and minor elements (B and F) (see Table 1).
In limestone aquifer system of Mt. Picung and Kali Asin area (PC1, PC2a/b, PC6, PC7, KA1, and KA2), hot water is characterized by Cl/HCO3 and Cl/SO4-type water (see Figure 5 for Mt. Picung area) with neutral pH (7-8) and high chloride content (2438 – 5095 mg/l), high bicarbonate (1803 – 1405 mg/l), and low sulphate (except KA1:4755 mg/l). Low B/Cl and F/Cl ratios indicate the mixing process of meteoric water with water from deeper aquifer (White, 1957). Meanwhile, the high bicarbonate indicates the occurrence of magmatic process.
Hot water in intrusive rock aquifer of Kedondong 1 and Bobos hot springs is characterized by HCO3-type water with neutral pH and high bicarbonate content (452,3 – 844,7 mg/l). Relative high B/Cl ratios indicate the magmatic water influence.
Hot water from mud pool of Kedondong area (MP) shows SO4-type water with high sulphate content (4755 – 6128 mg/l) and acid condition (pH=2). All water from normal springs is bicarbonate type with neutral pH.
4. HYPERTHERMAL WATER ORIGIN
Oxygen-18 and Deuterium are utilized as indicators for hot water origin (Craig, 1963) and degree of water-rock interaction at high temperature (Clayton et.al., 1966). Five isotopic analyses using deuterium (D) and oxygen-18 (18O) have been carried out in the laboratory on samples collected from 1 normal spring of intrusive rocks aquifer, 3 hot springs of limestone aquifer, and 1 hot spring of intrusive rocks aquifer. The stable isotope compositions of these samples are listed in Table 1.
The isotopic compositions of water from Kedondong 1 and Kedondong 2 springs are close to meteoric water line, dD = 8 d18O + 10 for the global meteoric water line (see Figure 6). Undoubtedly, the water is derived from meteoric water. The comparation of isotope composition of Kedondong 1 and Kedondong 2 indicates an 18O enrichment as a result of water – rock interaction at high temperature.
The values of dD and d18O for Mt. Picung 2, 6, and 7 hot springs water are higher than Kedondong springs (see Figure 7). The sequence of dD and d18O increases, starts from PC6 (-42;5;-5,37), PC2 (-41.2;-4,7), to PC7 (-41.0;-3,18). Such increase reflects the isotope enrichment process due to steam loss of hot water. However, the shifting of isotopic compositions of hot water also indicates the mixing of water from deeper aquifer with shallow groundwater (meteoric origin). The deep water could be originated from formation water or brine.
The main source of water in geothermal system is meteoric water that comes from rainfall or shallow groundwater (Craig, 1963). In limestone aquifer of Mt. Picung area, the fluctuation of hot spring discharge is related to the rainfall, marked by the significant increase of discharge and number of springs in rainy season. Fracture and solution opening are the characteristic features of the limestone aquifer. These features enable high infiltration rate in limestone aquifer.
The result of chemical and isotopic analyses also indicated that the meteoric water has a significant influence on the hot spring system, and that Mt. Picung hot water is resulted from the mixing of shallow groundwater (meteoric origin) with deeper aquifer water.
The Cl – 18O diagram (see Figure 8) shows that the 18O increase is corresponding with the chloride increase. The chloride type water often occurs as formation water or regional flows groundwater. In this case, the decrease of chloride is controlled by the amount of meteoric water mixed with deep water. The more meteoric water mixed with deep water, the lower 18O and chloride concentrations in hot water.
The mixing process of meteoric with deep water for PC6 springs is greater than other springs, due to higher volume of meteoric water available. The recharge system of PC6 hot spring is better than PC2 and PC7 hot springs. The PC6 spring is located near the west – east fault (see Figure 3), presumably the main channel of meteoric water flows to the aquifer beneath. On the other hand, the southwest – northeast and southeast – northwest normal faults are main conduits for formation water to emerge to the surface (see Figure 9).
As a matter of fact, the number and the discharge rate of springs in intrusive rock aquifer are relatively stabile. Their hot water isotopic composition and chloride concentration strongly indicate its meteoric origin. Such condition is made possible by relatively poor flow system, due to the existence of less fracture forms.
5. RELATION OF HOT WATER TO OIL SEEPAGE
Hydrocarbon occurs in Parigi limestone layer, which is known as hydrocarbon bearing layer in Jatibarang oil field, north of Palimanan area. The existence of hydrocarbon in hot water aquifer of Mt. Picung area is detectable from the presence of oil droplets in hot water of PC4, PC6, and PC8. Since oil seeping only occurs during rainy season, we can conclude that it is caused by hot water movements.
Observation on limestone outcrop – near rock quarry – shows the occurrence of dome-like shape opening filled with asphalt. In this case, the fracture and solution opening act as oil trap; while in other parts they also act as medium for hot water to emerge to the surface as hot springs and oil seeping.
However, oil seeping is not spreading to the whole area of Mt. Picung. It only occurs near the west – east fault, the same location where the meteoric water infiltrates. Hence, hot water flow containing hydrocarbon is presumably generated by the meteoric water heating process near this fault.
Using chemical tracer and isotopic ratio, it can be inferred that hot water in limestone aquifer system of Mt. Picung and Kali Asin areas is resulted from the mixing of shallow groundwater and deep aquifer water, while hot water in intrusive rock aquifer of Kedondong 1 and Bobos hot springs belongs to meteoric origin.
The values of dD and d18O in water from Mt. Picung 2, 6, and 7 hot springs are higher than Kedondong spring, indicating that the isotope enrichment process occurred due to mixing of shallow groundwater of meteoric origin with formation water from deeper aquifer. Such inference is also justifiable from the occurrence of high composition of chloride.
In Mt. Picung area, the limestone aquifer properties such as fracture and opening solution significantly determine the occurrence of hot springs and oil seeping. On the other hand, the west-east fault, southwest – northeast and southeast – northwest normal faults act as the medium for hot water to flow from heat source to the surface.
The authors recognize many inputs of potentiometric map from Lambok Hutasoit, Ph.D and undergraduate students whom have helped the data acquisition.
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LIST OF FIGURES AND TABLES (in separate files)
Figure 1. Location map.
Figure 2. Geological map of the study area (TH =Telaga Herang spring, Bj=Bojong spring, TR=Telaga Remis, TN=TelagaNilam, Kdd=Kedondong, GJ=Mt. Jaya, PC=Mt. Picung, KA=Kali Asin, Bbs=Bobos, MP = Mud Pool).
Figure 3. Detailed geological map of Mt. Picung area, Palimanan.
Figure 4. Rainfall fluctuations observed from three rain gauge station during July 2001 – May 2002.
Figure 5. Comparison of chemical and isotopic composition of hot spring waters of Mt. Picung area, Palimanan.
Figure 6. Diagram Cl. SO4, and HCO3 of spring and hot spring waters of Palimanan area.
Figure 7. Relation of δD and δ 18O of hot spring and spring water.
Figure 8. Relation of chloride and δ 18O composition.
Figure 9. Schematic model of hot (hyperthermal) water of Mt. Picung area, Palimanan.
Table 1. Chemical and isotopic composition of water from springs at Palimanan area.