Indonesian Journal on Geoscience Vol. 4 No. 2 August 2017: 61-70 INDONESIAN JOURNAL ON GEOSCIENCE Geological Agency Ministry of Energy and Mineral Resources Journal homepage: h p://ijog.geologi.esdm.go.id ISSN 2355-9314, e-ISSN 2355-9306 Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java Hendra Amijaya and Pracoyo Adi Pameco Department of Geological Engineering, Gadjah Mada University Jln. Grafika No. 2 Bulaksumur, Yogyakarta 55281, Indonesia 1 G Corresponding author: pameco.adi.p@gmail.com Manuscript received: May 21, 2016; revised: November 15, 2016; approved: February 15, 2017; available online: March 20, 2017 IJ O Abstract - Three seepage gas samples collected from Boto Area, Bancak, Semarang, Central Java, were studied to determine their chemical characteristics using GC and GC-IRMS methods. They are composed 53 - 85% of methane predominantly. However, gas seep Site 3 sample has the highest N2 compound and the lesser extent to the samples Site 2 and Site 1 respectively. The two hydrocarbon gas seeps (Site 1, 2, and Site 3 samples) that are characterized by δ13C methane of -35.61‰ and -27.97‰, and values of δD methane of -112‰ and -109‰ respectively, are each isotopically distinct from all others suggesting, at least, they are derived from different maturity level. The Site 3 gas sample is suggested to be more mature than the others. Keywords: isotope, gas seepage, methane, Boto, Bancak © IJOG - 2017. All right reserved How to cite this article: Amijaya, H. and Pameco, P.A., 2017. Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java. Indonesian Journal on Geoscience, 4 (2), p.61-70. DOI: 10.17014/ijog.4.2.61-70 Introduction Background Natural gas seepages found in Boto Area, Bancak, Semarang, Central Java (see Figure 1) have become a topic of studies related to the petroleum system in this area which have been done. Amijaya and Winardi (2006) examined the composition of seepage gases and concluded that the gas was composed of methane, ethane, carbon dioxide, and nitrogen. Pramono (2008) who studied about oil seep in this area concluded that the oil derived from mixed organic matter source rock. Wiloso (2008) and Wiloso et al. (2008) also showed mixed kerogen in Galeh oil seep analysis. However, the genetic of the gas is still unknown, whether it is of biogenic or ther- mogenic nature. Geochemical characterization of the gas, especially in this study which used stable isotope analysis, was conducted to find out the genetic type of the gas. Natural gas is a gas phase of petroleum which generally contains 70 - 100% methane, 1 - 10% ethane, a lower percentage of wetter gases, and low up to high percentage of non-hydrocarbon gases (hydrogen sulfide, carbon dioxide, and nitrogen) (Hunt, 1996). Based on their formation, natural gas can be divided into two kinds: biogenic gas (a gas formed at low temperature as a result of decomposition of organic material by microorganism activity) and thermogenic gas (a gas formed by destruction of the source rock structure or oil at high pressure and temperature) (Rice, 1993). Accredited - LIPI, valid August 2016- -Acredited August 2021 IJOG/JGI by: (Jurnal Geologi Indonesia) by LIPI No. 547/AU2/P2MI-LIPI/06/2013. valid 21 June 2013 - 21 June 2016 - RISTEKDIKTI, valid May 2016 - May 2021 61 Indonesian Journal on Geoscience, Vol. 4 No. 2 August 2017: 61-70 110 20'0" E o 110 40'0" E 7o20'0" S 7o20'0" S G 7o0'10" S 7o0'10" S 7o0'0" S 7o0'0" S o IJ O N 0 2.5 5 10 km Study area 110o20'0" E 110o40'0" E Figure 1. Locality map of the studied area (modified BNPB, 2010). The composition and stable isotope of natural gas have an important role in the investigation of gas. Gas composition can indicate the genetic of natural gas. Biogenic gas typically contains less than 0.2% ethane (Schoell, 1983). Thermogenic gas may be either wet (C2+ > 5%) or dry depending on the level of its thermal maturity. Dry thermogenic gases are associated with higher levels of thermal stress (Katz, 2002). However, it should be emphasized that the C2+ content of a gas is sensitive to many secondary processes like the present-day temperature and pressure (Schoell, 1983; Katz, 2002). Stable isotope concentration indicates the type and origin of natural gas (Schoell, 1983). The comparison of stable isotope 13C/12C directly shows heavy concentration of this isotope, usually reported in one per mil (‰) relative to the standard. 62 δ= (R sample - R standard) x 1000 (ppt) R standard Stable isotopes of carbon and hydrogen from methane are fractionated by biological processes, so it is possible to distinguish methane formed biologically from methane formed thermally during catagenesis. When bacteria form methane from carbon dioxide reduction, they tend to eat lighter carbon from 12CO2 than the heavier (13CO2) and make the value of comparison 13C/12C becomes more negative. That is around -109‰ relative to Peedee belemnite (Hunt, 1996). Carbon isotope values of thermal gases can also be distinguished. Carbon isotope values of thermal gases are thought to increase with increasing maturity of their precursors. The δ13C values of wet thermogenic methane range between -60‰ to -30‰, whereas dry thermogenic methane range Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java (H. Amijaya and P.A. Pameco) between -40‰ to -15‰ (Hunt, 1996). Berner and Faber (1996) also provided empirical relationships between the value of carbon isotopic of methane, ethane, propane, and vitrinite maturity level for different organic matter types. Methanogenic bacteria also utilize lighter hydrogen than the heavier (deuterium) one, so methane formed by the bacteria had isotope deuterium values (δD) more negative than -250%, relative to standard mean ocean water (SMOW) (Hunt, 1996). Geological Setting Late Miocene to Middle Pliocene The Banyak Beds mainly consist of andesitic material deposited in a marine environment: tuffs, coarse tuff-sandstones, calcareous tuffaceous sandstones, and volcanic breccia alternating with marls bearing planktonic foraminifera. G The stratigraphy and structural geology of the Kendeng Zone have been described by numerous authors (e.g. van Bemmelen, 1949; de Genevraye and Samuel, 1972; Sukardi and Budhitrisna, 1992; Smith et al., 2005). The stratigraphy of the area is summarized below using a lithostratigraphy nomenclature. ages appear through this formation. In the westernmost part of the Kendeng Zone, the formation has been divided into two parts: Merawu Member and Penyatan Member. The Merawu Member, correlated with the Lower Kerek Formation, is a volcano-clastic sequence consisting of conglomeratic layers with pebbles of quartz, basalt, andesitic tuffs, and limestones. The Penyatan Member is essentially clastic and tuffaceous corresponded to the upper part of the Kerek Formation. IJ O Late Pliocene The Damar Formation lies conformably on the argillaceous Kalibiuk Member. The Damar Formation consists of andesitic tuff-sandstones and conglomerates beds bearing the first vertebrate fossils. In the west and central Kendeng Zone, Pucangan Formation unconformably overlies the Upper Kalibeng carbonate and consists of continental coarse pyroclastic sediments, those are volcanic conglomerates and sandstones. Stratigraphy The studied area is located in Kendeng Zone (van Bemmelen, 1949). Kendeng Zone is a west - east trending anticlinorium which extends from the Gunung Ungaran Volcano in the west to the Brantas River in the east. de Genevraye and Samuel (1972) distinguished Kendeng Zone into three parts. The studied area is included in the western part (Figure 2) which has a high content of volcanic material and major structural complications. The stratigraphy of the western part of Kendeng Zone according to de Genevraye and Samuel (1972) is: Late Oligocene to Early Miocene The Pelang Formation is the oldest sedimentary exposure in Kendeng Zone. It consists of marls and argillaceous marls. The Pelang Formation is very rich in planktonic foraminifera and was deposited in an open marine environment. Middle to Late Miocene The Kerek Formation consists of massive argillaceous and calcareous sequences where volcano-clastic material is very abundant. Seep- Early Pleistocene The continental Notopuro Formation rests unconformably either on the Kabuh Formation or on the Pucangan Formation. The formation consists of coarse to very coarse volcanic conglomerates, tuffaceous sandstones, tuffs, and lahar deposits. Structural Geology In Kendeng Zone, there are many structural complications. The major structures are west east trending folds, west - east trending reverse faults, and wrench faults which transversely cut the Kendeng Zone (de Genevraye and Samuel, 1972). The folds are tightly folded with steep, vertical, or reverse flanks. Faults are extremely frequent in this area. The intensity of folding and faulting appears to decrease from west to east of the Kendeng Zone. 63 t Tun Volcanic facies Putjangan Fm Lower Kalibeng Fm with Kapung limestone limy facies argillaceous facies Fault Putjangan Fm Upper Kalibeng Fm Assumed Flexure Boundary of Formation Kabuh Fm marly facies Pleistocene N19 N20 N21 N22 N23 Tf Tgh P18 P19 P20 (N1) P21 (N2) P22 (N3) N4 N5 N6 N7 N8 N9 N10 N11 N12 N13 N14 N15 N16 N17 N18 Tcd Te G Djuwan Young volcanic products K. n ana k ang Upper Kalibeng Fm o 110 40'0" E Gubug Notopuro Fm Suluk 10 km Damar Fm o 110 30'0" E Salatiga 5 IJ O 0 Godong Blow Zonation (1969-1970) o ? Merawu Kalibiuk Kapung limestone Banyak Penyatan Damar Notopuro/volc Ungaran area ? Pelang Northern flank Kerek Lower Kalibeng Notopuro/ Puncangan Southern flank Western Kendeng Zone Figure 2. Geological map and general stratigraphy of western Kendeng Zone (de Genevraye and Samuel, 1972). The gas in the studied area leaks through Kerek Formation. Pelang Fm Kerek Fm Banjak Fm Study Area Ambarawa G. Surolojo G. Ungaran G. Turun N Age Pliocene Miocene 110o40'0" E 7o10'0" S 7o20'0" S 7 10'0" S o Larte Middle Early Oligocene 7 20'0" S Letter Stages Upper Lower Upper 64 Lower 110o30'0" E Indonesian Journal on Geoscience, Vol. 4 No. 2 August 2017: 61-70 Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java (H. Amijaya and P.A. Pameco) by the emergence of gas bubbles in the rice fields (see Figure 3b). One of the seepages, the Site 3 (see Figure 3d), had been cemented and the gas is streamed and utilized by local residents. Flame characteristics of gas when burned in daylight condition are partially reddish yellow colour and some are invisible (see Figures 3c and d for example). Sampling and Analytical Methods A field observation includes determination of location and visual observation of the seepage condition has been done. Samplings of gases were using some traditional equipments, such as vacuum pump, gas sampling bag, hose, and funnels. In this study, the seepage gases were analyzed for molecular as well as C1 and C2 stable carbon isotope compositions. Samples were collected in sample bag following an in-house sampling protocol for seepage gas sample collection. The determination of the gas composition was conducted at the Laboratory of Gas Technology PPPTMGB LEMIGAS, using a Gas Chromatography instrument from Agilent Technologies 7890A series. Laboratory GHGeochem Ltd. analyzed the isotope compositions of methane using gas chromatography–isotope ratio mass spectrometry (GC-IRMS) systems with a precision of ±0.3‰ - 0.4‰ for δ13C and ±5‰ for δD values. Samples were prepared and analyzed according to internal standard procedures by the laboratory. After sample preparation, both the 13 C/12C and 2H/1H analyses are performed on an Isoprime. Table 2 shows the results geochemical data for the δ13C and δD isotopic methane and gas composition of gas seepage samples. The geochemical data were evaluated to constrain gas origin and thermal maturity. G Beside the longitudinal reverse faults that spread along the anticlinorium, the latter is transversely cut by numerous wrench faults. Wrench faults between the Tuntang River and Salatiga, are structures which occur in the studied area (see Figure 2), and are believed to be deep-seated faults originating from the basement. The most conspicuous examples of the relative displacement transversely to the Kendeng Zone trending west - east, occur in its westernmost and easternmost parts. Westernmost block, which is the studied area, is situated between the Gunung Ungaran faulted area and the Tuntang River/Salatiga network of deep-seated faults. It has been displaced towards the north as shown by the northwards shift of the anticlinorium axis in this area. Material and Methods IJ O Geochemical analysis of a gas includes both molecular composition (relative abundance in the gas sample of C1 to C3 hydrocarbons and nonhydrocarbon gases such as H2, CO2, and N2) and isotopic composition. The isotopic composition of a gas includes characteristics such as the stable carbon and hydrogen isotope ratios of the C1-C5 hydrocarbon gas species, stable isotope composition of non-hydrocarbon gases, radioactive carbon, and hydrogen isotopes of C1. Data for molecular and isotopic characteristics of natural gas can be used to constrain the origin of a gas (bacterial and/ or thermogenic) as well as the type and maturity of the organic matter that generated the thermogenic hydrocarbons in a sample. These data can also identify any secondary processes, such as gas oxidation, that may have affected a sample (Schoell, 1983; Whiticar and Faber, 1986; Jenden et al., 1993; Laughrey and Baldassare, 1998). Locations of Seepage Samples Three seepage gas samples were found in Boto Area (see Figure 3a). Table 1 shows the coordinates for the sample locations. Site 1 is located in Dusun Gunung and the others are located in Dusun Galeh, about 1.5 km to the southwest of the Site 1. The presence of seepages was known Results and Discussion Gas Composition The major gas compositions and stable isotope data of the seepage gas samples collected are given in Table 2. GC results in the table show 65 Indonesian Journal on Geoscience, Vol. 4 No. 2 August 2017: 61-70 a 456000 457000 9201000 9201000 Panggung 13 1 Gunung 7.5 5 137. 13 7.5 137.5 137 Godean Krasak 5 5 5 12 100 100 87.5 112.5 175 112.5 125 87.5 100 9199000 12 .5 .5 2.5 11 75 100 9199000 112. Bancak 112 2 11 .5 162.5 Bancak 125 5 112. 100 Galeh 125 137 75 75 100 2 .5 5 137. 3 87.5 2.5 125 Boto 16 Bantal 5 125 87.5 137. 87.5 87.5 87.5 Salaran 11 2 .5 125 125 454000 455000 456000 125 N .5 150 0 250 500 1 50 m 2.5 16 250 150 2.5 26 453000 Contour Oil seepage location River 112 .5 11 2 7.5 200 Road 175 .5 23 250 Gas seepage location 2.5 16 2 11 5 137.5 22 Legend 175 5 212. 9198000 187.5 275 150 9200000 87.5 125 137.5 9200000 455000 137.5 454000 137.5 9198000 b Seepages Location Map Boto Area, Bancak, Semarang, Central Java 453000 457000 c IJ O G d Figure 3. Locations of the three natural gas seepage samples found in Boto Area (a) showing flame tips indicating high hydrocarbon content on Site 1 (b) in Dusun Gunung; Site 2 (c) and Site 3(d) both are in Dusun Galeh, around 1.5 km to the southwest of the Site 1. Table 1. Location of Gas Seepages in the Studied Area No 1 Location Dusun Gunung Dusun Gunung Dusun Galeh Dusun Galeh 2 3 Coordinates 49S 454579 9200797 (+ 10 m to west from location 1) 49S 453639 9199601 49S 453879 9199753 Sample ID 1.1 1.2 2 3 Table 2. δ13C and δD Isotopic Methane and Gas Composition of Gas Seepages in the Studied Area Sample ID Isotopic Composition Gas Composition (%) δ13C CH4a δD CH4b H 2S N2 CO2 CH4 C2H6 C3H8 O2 1.1 - - N/A 9.40 0.75 85.00 0.16 0.00 4.70 1.2 -35.61 -112 N/A 10.18 0.44 83.37 0.03 0.00 5.99 2 - - N/A 19.91 0.08 68.93 0.00 0.00 11.08 3 -27.97 -109 N/A 34.80 0.17 53.75 0.33 0.11 10.86 N/A: not analyzed, need another measurement technique a ‰ relative to Peedee belemnite (PDB) b ‰ relative to Standard Mean Ocean Water (SMOW) 66 Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java (H. Amijaya and P.A. Pameco) that all the seepage gas samples are generally have the similar hydrocarbon compositions. Gases in the studied area as expected are dominated by methane which normalized hydrocarbons to reach more than 99% (not shown in the table). Other than that, other hydrocarbon gases, i.e. ethane and propane, are found but in a small percentage. With exception to the N2 for the seepage gas sample from the Site 3, in general, nonhydrocarbon gases found are relatively small percentage of CO2, N2, and O2. Relatively high O2 compound in the samples are probably as atmospheric contaminant due to improperly samples collecting tool. Isotopic Composition Sources of CH4 include natural gas reservoirs, gas associated with oil and coal, landfills and the products of anaerobic degradation of organically contaminated groundwater. Although, there are many ways to characterize the methane source, a combination of C and H isotope analysis of natural gases is a powerful tool to discriminate different origins of gases. A plot of δ 13C vs. δD (see Figure 4a) not only reveals a distinction of biogenic and thermogenic gases from different environments, but it also allows delineating mixtures between the different types. For example, plots of the stable isotope ratios for C vs. H (e.g. Schoell, 1983) can be used to distinguish petroleum groups. Identifying the gas origin may be very much more complicated if there is more than one source of methane exists, and where compositional evidence alone is used. It is important to have a well-defined database for the likely composition of possible source gases and to evaluate whether changes in composition have occurred during gas migration. Stable isotope ratios of carbon and hydrogen (i.e. 13C/12C, 2H/1H) potentially provide additional means by which methane can be characterized, but such data are scant for sources in Indonesia. Different gases δ13C values of given hydrocarbon generally increase with the increasing thermal maturity. Migrating methane could be enriched in 12C or 13C as 1 - 3‰ depending on the amount of organic matter on shales, the mechanism of IJ O G Gas seepage composition of Site 1 From several gas seepages in the Site 1 area, only two seepage gas samples have a good pressure capacity and suitable to be sampled, i.e. Seep 1.1 and Seep 1.2 gases. These gas samples are compositionally similar (see Table 2) consisting of ± 85% CH4, < 1% C2H6, < 1% CO2, ± 10% N2, and ± 5% O2. After normalization for the gas hydrocarbons only, the composition of gas seepage samples are almost identical with methane predominant to reach more than 99%. The presence of oxygen is probably atmospheric contamination. oxygen. It is interesting to note that N2 content is very high compared to the other samples. N2 is sometimes a major constituent of natural gases, but the origin of this N2 is still enigmatic. While atmospheric air contains some 78% N2 and 21% O2 by volume along with other components and rare gases. Unless otherwise N2 in the gas seep samples come from the atmosphere during sample collection, relatively high N2 fraction may be released from sedimentary organic matter during burial, several nonsedimentary sources of N2 may also contribute to the natural gas. Gas seepage composition of Site 2 The composition of the seepage gas sample in the Site 2 (see Table 2) has a different composition with that of in the Site 1, showing lower hydrocarbon composition ± 68.93% CH4 only and relatively higher nonhydrocarbon gases in percentages of respective ± 0.08% CO2, ± 19.91% N2, and ± 11.08% O2. The smaller CH4 composition is due to higher non-hydrocarbon content (see Table 2). Gas seepage composition of Site 3 The composition of gas in the Site 3 (see Table 2) is dominated by hydrocarbon gas which comprises ± 53.74% methane. Other hydrocarbons found are ± 0.326% ethane and ± 0.11% propane. Non-hydrocarbon gases found are ± 0.17% carbon dioxide, ± 34.80% nitrogen, and ± 10.86% 67 Indonesian Journal on Geoscience, Vol. 4 No. 2 August 2017: 61-70 a b C-, H-isotope signatures of CH4 sources Sample 3 Sample 1.2 -70 -120 Sample 3 Sample 1.2 Biogenic gas -100 -60 Diagenetic? -80 d Cmethane, (ppt) Bacterial Carbonate Reduction mix and transition Bacterial Methyl--type Fermentation -60 -20 T Ms Atmospheric -40 early mature Geo ther -40 -50 13 o oo d13C-methane ( / , PDB) Bacterial “M” mal, Hyd Thermogenic roth erm al, C rysta Artificial, Bit Metamorphic llin TT (m) Md associated Mixed source humic e Mixed -30 1.2% TT (h) Migrated 2.0% 3.0% 0 -350 -250 G Abiogenic ? Mantle ? -450 -20 -150 -50 10 dD-methane ( /oo, SMOW) o “M” = mixed gas with intermediate composition Ms = compositional shift due to shallow migration Md = compositional shift due to deep migration C2+ (%) 20 30 50 Figure 4. (a) Plotting of δ13C and δD methane values on the CD diagram (Whiticar, 1968; in Whiticar, 1999) showing sample 3 is derived from humic source, but samples 1.2 is not accurately plotted on humic. (b) Plotting of carbon stable isotopes and relative concentrations on Schoell’s Diagram (Schoell, 1983). which each are isotopically distinct from all others. The figures show that both samples indicate dry thermogenic gas within the range of -40‰ to -15‰ for δ13C1 and -150‰ to -70‰ for δDC1 suggesting, at least, they are derived from different maturity level. The Site 3 gas sample is prone to be more mature than the other. All gas possibly derived from humic organic matter source rock based on the CD diagram (Whiticar, 1968; in Whiticar, 1999). Plotting carbon and deuterium isotope values can be seen in Figure 4a. Samples 1, 2, and 3 are plotted in the intersection zone between thermogenic nonassociated gas from marine (sapropelic liptinitic) source rock and thermogenic non-associated gas from coal (humic) source rock. The amount of C2+, samples 1, 2, and 3 have been derived from mixed source (Figure 4b). Pramono (2008) studied oil seep near gas seepages in the studied area. Its distance is around + 2 km from gas seepage locations. He concluded that the oil was derived from mixed organic matter source rock. Wiloso (2008) and Wiloso et al. (2008) also showed mixed kerogen in Galeh oil seep analysis. Galeh oil seep located IJ O migration, and on the properties of the medium through which the gas is moving. There are two ways to determine maturation using isotopes that are isotopic composition of CH4 and separation of carbon isotopes between hydrocarbon components. Thermogenic (methane and wet gases), source, maturity, and chemical kinetics have the primary controls on the gas composition and the proportion of gaseous and liquid hydrocarbons (Rice, 1993). With maturity, the δ13C and δD values of thermogenic gases systematically increase from C1 to C5. Increasing temperatures modify the organic matter due to various chemical reactions such as cracking and H2 disproportionation in kerogen. As these results in a 13C enrichment of the residue, more 13C - 12C bonds are broken with increasing temperatures which produce higher δ 13C values. Figure 4 shows plots of δ13C versus δD of CH4 (Figure 4a) and δ13C methane versus gas wetness (Figure 4b) for samples in this study. The two hydrocarbon gase seeps (Site 1.2 and Site 3 samples) that are characterized by δ13C methane are -35.61‰ and -27.97‰, and the values of δD methane are -112‰ and -109‰ respectively 68 Geochemistry of Natural Gas Seepages in Boto Area, Bancak, Semarang, Central Java (H. Amijaya and P.A. Pameco) in the Site 3 correlated with Ngimbang Formation rock sample from Rembang 1 well which is also mixed kerogen source rock (Wiloso, 2008; Wiloso et al., 2008). There is a linkage between gas and oil in the studied area. However, it can not be known whether it is a result of oil thermal alteration process or source rock thermal cracking process. It requires an isotope analysis of ethane and propane (Katz, 2002) which are not found in the gas seepages. bnpb.go.id/wp-content/uploads/2010/09/ indeks_peta/250K/ID-N09-250K.pdf de Genevraye, P. and Samuel, L., 1972. Geology of The Kendeng Zone (Central and East Java). Proceedings of First Annual Convention Indonesian Petroleum Association, p.17-30. Hunt, J.M., 1996. Petroleum Geochemistry and Geology. 2 nd Edition. W.H. Freeman and Company, New York. 743pp. DOI: 10.1017/ S0016756800007755 Jenden, P. D., Drazan, D. J., and Kaplan, I. R., 1993. Mixing of thermogenic natural gases in northern Appalachian Basin. The American Association of Petroleum Geologists Bulletin, 77, p.980-998. Katz, B.J., 2002. Gas geochemistry - a key to understanding formation and alteration processes. Proceedings of 28th Annual Convention Indonesian Petroleum Association, p.789-802. Laughrey, C. D. and Baldassare, F. J., 1998. Geochemistry and origin of some natural gases in the Plateau Province, central Appalachian Basin, Pennsylvania and Ohio. The American Association of Petroleum Geologists Bulletin, 82, p.317-335. Pramono, W., 2008. Studi Geokimia Rembesan Minyak Bumi di Daerah Bantal, Kecamatan Bancak, Kabupaten Semarang, Propinsi Jawa Tengah. Bachelor Thesis at Geological Engineering Department, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, 90pp. Unpublished. Rice, D.D., 1993. Composition and origins of coalbed gas. In: Law, B.E. and Rice, D.D. G Conclusion and Future Work Desa Boto, Kecamatan Bancak, Kabupaten Semarang. Laporan Penelitian, Fakultas Teknik Universitas Gadjah Mada, Yogyakarta. 28pp. Unpublished. Berner, U. and Faber, E., 1996. Empirical carbon isotope/maturity relationships for gases from algae kerogens and terrigenous organic matter, based on dry, open-system pyrolysis.Organic Geochemistry, 24, p.947-955. DOI: 10.1016/ S0146-6380(96)00090-3 BNPB, 2010. Topography map of Semarang District scale 1:250.000. http://geospasial. IJ O Natural gases in the Boto area consist of methane predominant amounting 53 - 85% and more than 99% after hydrocarbon normalization. Those seepage gas samples that are dominated by thermogenic gas are each isotopically and compositionally distinct from all others. The exception to the Site 2 sample, the gas tends to indicate being derived from mixed organic matter source rocks. The Site 2 sample has not yet to determine its source rock due to lack of evidence in this study. Further work is needed especially in term of isotope study of gas in order to disclose the gas potential within the region. Acknowledgements This research project was supported by Universitas Gadjah Mada. Special appreciation is directed to Y. Andriani for her assistance on laboratory analysis in LEMIGAS and S. H. Gunawan for the information on isotope analysis laboratory. The authors wish to thank E. A. Subroto, R. Susilawati, and I. B. Sosrowidjojo for their valuable suggestions. References Amijaya, D. H. and Winardi, S., 2006. Geokimia Rembesan Air Formasi dan Gas Alam Di 69 Indonesian Journal on Geoscience, Vol. 4 No. 2 August 2017: 61-70 Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chemical Geology, 161, p.291-314. Whiticar, M. J. and Faber, E., 1986. Methane oxidation in sediment and water column environments-Isotope evidence. Organic Geochemistry, 10, p.759-768. DOI: 10.1016/ s0146-6380(86)80013-4 Wiloso, D.A., 2008. Studi Batuan Induk Hidrokarbon di Cekungan Jawa Timur Bagian Barat. Abstract of Master Thesis at Geological Engineering Magister Study Program, Institut Teknologi Bandung, Indonesia, 1pp. Wiloso, D.A., Subroto, E.A., and Hermanto, E., 2008. Confirmation of the Paleogene Source Rocks in the Northeast Java Basin, Indonesia, Based from Petroleum Geochemistry. AAPG International Conference and Exhibition, South Africa, 6pp. IJ O G (Eds.), Hydrocarbons from Coal. The American Association of Petroleum Geologists, Tulsa, Oklahoma, p.159-184. Schoell, M., 1983. Genetic Characterization of Natural Gases: The American Association of Petroleum Geologist Bulletin, 67 (12), p.222-2238. Smith, H., Hall, R., Hamilton, J.P., and Kinny, P., 2005. East Java: Cenozoic basins, volcanoes and ancient basement. Proceedings of 30th Indonesian Petroleum Association Annual Convention, Jakarta, p.251-266. Sukardi and Budhitrisna, T., 1992. Peta Geologi Lembar Salatiga, Jawa. skala 1:100.000. Pusat Penelitian dan Pengembangan Geologi, Bandung. Van Bemmelen, R. W., 1949. The Geology of Indonesia, vol. I A, General Geology of Indonesia and Adjacent Archipelagoes, Special Edition of The Bureau of Mines In Indonesia. Department of Transport, Energy and Mining, Batavia, 732pp. 70