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Hydrothermal bitumen generated from sedimentary organic matter of rift lakes – Lake Chapala, Citala Rift, western Mexico
Applied Geochemistry Applied Geochemistry 20 (2005) 2343–2350 www.elsevier.com/locate/apgeochem
Hydrothermal bitumen generated from sedimentary organic matter of rift lakes – Lake Chapala, Citala Rift, western Mexico Pedro F. Za´rate-del Valle a, Bernd R.T. Simoneit a b
b,*
Departamento de Quı´mica, Universidad de Guadalajara – CUCEI, Ap. Postal 4-021, Guadalajara, Jalisco CP 44410, Mexico Environmental and Petroleum Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Building 104, Corvallis, OR 97331-5503, USA Received 20 January 2005; accepted 7 September 2005 Editorial handling by R. Fuge Available online 25 October 2005
Abstract Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleoand active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appear as small islands composed of solid, viscous and black bitumen. Aliquots of tar were analyzed by GC-MS and the mixtures are comprised of geologically mature biomarkers and an UCM. PAH and n-alkanes are not detectable. The biomarkers consist mainly of hopanes, gammacerane, tricyclic terpanes, carotane and its cracking products, steranes, and drimanes. The biomarker composition and bulk C isotope composition (d13C = 21.4%) indicate an organic matter source from bacteria and algae, typical of lacustrine ecosystems. The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala exceeding 40 ka (14C) in age and then forced to the lakebed by tectonic activity. The absence of alkanes and the presence of an UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered in future energy resource exploration in such regions. 2005 Elsevier Ltd. All rights reserved.
1. Introduction Lake Chapala, in western Mexico, is located in the Citala Rift (E–W) which, in association with the Tepic-Zacoalco (NW–SE) and Colima (N–S) Rifts, forms the well-known neotectonic Jalisco con*
Corresponding author. Tel.: +1 541 737 2155; fax: +1 541 737 2064. E-mail address: [email protected] (B.R.T. Simoneit).
tinental triple junction (JTJ; 20.14N; 103.48W) (Barrier et al., 1990). The JTJ is characterized by active volcanism (Ceboruco and Volcan de Fuego) and tectonics (1995, earthquake M = 8). The JTJ neotectonics are due to the motion of the Cocos Plate relative to the North America Plate (Fig. 1; Allan et al., 1991; Barrier et al., 1990; Michaud et al., 1994). The Citala Rift is characterized by evidence for both paleo- and active hydrothermal activity (Fig. 2) as follows: (a) 3 types of thermal springs:
0883-2927/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2005.09.001
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Fig. 1. Rift structures and associated volcanism from the Pliocene to Recent in western Mexico (modified from Allan et al., 1991). Caption: Normal faults are generalized and shown by lines with hachures on the down-thrown side, lineaments are shown as solid lines. Open squares refer to cities. Closed triangles refer to volcanoes or volcanic fields: SJ = San Juan, N = Navajas, S = Sanganguey, Tp = Tepeltitic, SP = San Pedro, Cb = Ceboruco, Tq = Tequila, P = Primavera, CN = Ca´ntaro, NC = Nevado de Colima, VC = Volca´n Colima, M = Mascota volcanic field, LV = Los Volcanes volcanic field, Pa = Paricutı´n, and J = Jorullo. The asterisks show the locations of the Villa Corona and Ixtla´n de los Hervores inland hydrothermal fields, and the Los Negritos mud volcano.
inland (Fig. 1; Villa Corona and Ixtla´n de los Hervores), nearshore (Fig. 2; San Juan Cosala´), and offshore (Fig. 2; Los Gorgos, sublacustrine) hydrothermal fields; (b) fossil sinter deposits (Fig. 2; La Calera); (c) hydrothermal alteration halos and incipient XIX century mining activity (Fig. 2; San Juan Cosala´); (d) geyser activities (Figs. 1 and 2; San Juan Cosala´ and Ixtla´n de los Hervores); (e) gas seeps at Los Gorgos and El Fuerte vents; (f) hydrothermal bitumen at Columba tar islands near Los Gorgos (Figs. 2 and 3); and (g) Los Negritos mud volcano eruption in 1900 (Figs. 1 and 2; Moreno Garcı´a and Funes, 1991). Inland fumarole activity is not observed at JTJ. In 1868 the Mexican local press reported that manifestations of bitumen were appearing in the lake at the mid-southern shore off Cape Columba near Emiliano Zapata village. Because of these hydrocarbon manifestations, the national company Petro´leos Mexicanos drilled
an oil exploration well in that vicinity to a depth of 2348 m in 1961 (Lo´pez-Ramos, 1979). No reservoired oil was found. Hydrothermal activity at JTJ has been studied principally for its geothermal energy potential since the 1980s by the Comisio´n Federal de Electricidad (Venegas et al., 1985). 2. Geological setting The JTJ is located at the western end of the Transmexican Volcanic Belt geological province (GP) and was generated by the activity of a Pleistocene continental arc (Ortega-Gutie´rrez et al., 1991). The Tepic-Zacoalco and Colima continental rifts of the JTJ are the boundaries of a western tectonostratigraphic assemblage (tsa) named the Jalisco Block (Fig. 1; Ferrari et al., 1999), consisting of granodioritic Meso-cenozoic batholiths and stocks (Jalisco Batholith GP) that have partially intruded
P.F. Za´rate-del Valle, B.R.T. Simoneit / Applied Geochemistry 20 (2005) 2343–2350
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Fig. 2. Geology (modified from Rosas-Elguera and Urrutia-Fucugauchi, 1998) and geothermal features of the Lake Chapala region.
Fig. 3. Bathymetry of the ‘‘Los Gorgos’’ sector in Lake Chapala.
a Cretaceous back-arc volcano-sedimentary sequence (Guerrero-Colima Orogenic Complex GP; Ortega-Gutie´rrez et al., 1991). In the Jalisco Block these sequences are partly covered by Pleistocene volcanic rocks (i.e., andesites of the Chapala Group;
Rosas-Elguera et al., 1997). The Citala and Colima continental rifts delineate an eastern-southwestern tsa called the Michoaca´n Block (Fig. 1), which abuts the south shore of Lake Chapala. Both north and south shores of Lake Chapala (Fig. 2) consist
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mainly of calc-alkaline and alkaline andesitic lava (500–1020 m; Chapala Group; 6.2–3.5 Ma; Ferrari et al., 1997), with subordinate basalt flows (3.4–2.2 Ma) and monogenetic basaltic volcanoes (<2.0 Ma; Rosas-Elguera et al., 1997). A lacustrine volcano-sedimentary (continental) sequence named the Chapala Formation (Downs, 1958) is in the JTJ region, with an average thickness between 300 and 900 m, and forms partially the north shore of the lake (Fig. 2). The Chapala Formation, tilted NE, is made of alternating lacustrine medium-coarse sediments and pyroclastic ash, pumice units and diatomite horizons (Rosas-Elguera and Urrutia-Fucugauchi, 1998). The sedimentation rates throughout Lake Chapala are not uniform. The rate from west to east varies from <2 to <4 mm a1 (based on 210Pb, 137 Cs and 240Pu; Fernex et al., 2001). Terrigenous minerals of a feldspathic affinity (Si/Al ratio 2–3; Jones and Bowser, 1978) dominate the infilling at Lake Chapala, with a granulometric media ranging from 10 to 50 lm in diameter (Ramı´rez-Sa´nchez, 2001). The actual thickness of the Lake Chapala infill is estimated to be >500 m in places (Za´rate-del Valle et al., 2002). The mean lake level is 1526 m above sea level with a mean depth of only about 4 m (Sandoval, 1994). The deepest zone of the lake was 26.8 m as
measured in 2003 (Anaya Maldonado et al., 2004). The bathymetry of the Los Gorgos sublacustrine thermal spring system (GSTS) is based on a survey by the Secretariat of Hydraulic Resources (SRH, 1981, Fig. 3). The major depression of the GSTS is Thermal Spring I (Fig. 3), a quasi-elliptic funnel, 700 m E–W · 400 m N–S, and about 8 m deeper than the lake bottom. The GSTS has a second sublacustrine thermal spring (II, 200 · 250 m) located N 108 and 1700 m from Thermal Spring I, and the tar islands located N270 and 500 m from Thermal Spring I (Fig. 3). 3. Sample acquisition and analysis There are two tar emanations near the Los Gorgos sub-lacustrine thermal spring close to the south shore of Lake Chapala (20.19N; 102.96W). They appear as small islands (<3–4 m2), named in this paper Columba tar islands (Figs. 2 and 3), composed of solid, viscous and black bitumen (Fig. 4). The tar islands are sometimes covered by water but when the water level decreases sufficiently (i.e., drought) it is possible to access them. In June 2000, the tar islands were exposed due to severe drought, which permitted the sampling of tar. The samples were taken with a stainless steel corer (1 cm diameter) and a hammer. The weathered crust
Fig. 4. View of a Columba tar island (composed of solid, viscous, and black bitumen).
P.F. Za´rate-del Valle, B.R.T. Simoneit / Applied Geochemistry 20 (2005) 2343–2350
was avoided in sampling. Samples were put into glass vials (5 ml) with Teflon lined caps. A tar sample from each island was 14C dated by the combustion method at a commercial laboratory (Cambridge Isotope Laboratory). Aliquots of tar were dissolved in dichloromethane and analyzed directly by gas chromatography-mass spectrometry (GC-MS). The GC-MS was a Hewlett-Packard 6890 gas chromatograph coupled to a 5973 Mass Selective Detector, with a DB-5MS (Agilent) fused silica capillary column (30 m · 0.25 mm i.d., 0.25 lm film thickness) and He as carrier gas. The GC was temperature programmed from 65 C (2 min initial time) to 300 C at 6 C min1 (isothermal for 20 min final time). The MS was operated in the electron impact mode at 70 eV ion source energy.
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Mass spectrometric data were acquired and processed using the GC-MS ChemStation data system. Compounds were identified by GC retention index and MS comparison with authentic standards, literature and library data, and characterized mixtures. 4. Results and discussion The GC-MS data for the tar samples from both islands are the same (Fig. 5(a) vs. (b)). The mixtures are comprised of geologically mature biomarkers and an UCM (unresolved complex mixture of branched and cyclic hydrocarbons). Polycyclic aromatic hydrocarbons (PAH) and n-alkanes are not detectable. The biomarkers consist mainly of 17a(H),21b(H)-hopanes ranging from C27 to C34
Fig. 5. GC-MS total ion current (TIC) and key ion traces for the total tar bitumens: (a) Sample 1.1 (island 1) TIC, (b) sample 2.1 (island 2) TIC, (c) m/z 191, key ion for hopanes (aH) and tricyclic terpanes (T), and (d) m/z 217 key ion for steranes [numbers refer to carbon skeleton, a, b, R, S refer to configuration, 18 = 2-(3 0 -methyloctyl)-1,3,3-trimethylcyclohexane, 19 = 2-(3 0 ,7 0 -dimethyloctyl)-1,3,3trimethylcyclohexane].
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Fig. 5 (continued)
(no C28), gammacerane, tricyclic terpanes (C20–C26, no C22) (Fig. 5(c)), carotane and its cracking products, C28 steranes and dinosterane, and drimanes (C14–C16) (Fig. 5(d)). The biomarker composition indicates an organic matter source from bacteria, algae and diatoms, typical of lacustrine ecosystems (Seifert and Moldowan, 1978; Jiang and Fowler, 1986; Kawka and Simoneit, 1987; Zumberge, 1993). The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala (Simoneit et al., 2000). The absence of PAHs supports the interpretation that the tars are lower temperature (<200 C) products from rapid thermal alteration of sedimentary organic matter (Kawka and Simoneit, 1990; Simoneit and Fetzer, 1996; Simoneit et al., 2004). Higher molecular weight PAHs form in hydrothermal systems at temperatures above 200 C and readily from 300 to 500 C by dehydrogenation of other organic
compounds. The paucity of n-alkanes and isoprenoids, and the presence of the UCM, mature hopanes, steranes, and carotenoid biomarkers are also consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 C, intermediate compared to hydrothermal systems in marine sediments (Simoneit, 1985). This temperature window has been confirmed by laboratory simulations (Leif et al., 1991; Za´rate-del Valle et al., 2005). Biodegradation of conventional petroleum is an alternate interpretation of the molecular composition of these tars, because that was the conclusion presented for oils with similar compositions from NW China (Jiang and Fowler, 1986; Jiang et al., 1988; Zhang et al., 1988). The authors favor the first interpretation that these tars are lower temperature hydrothermal products and not biodegradation residues. This is based on laboratory simulation experiments producing no PAHs, but generating the biomarkers.
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Furthermore, lacustrine or marine organic matter is converted to alkane-rich petroleum with PAHs in hydrous pyrolysis simulation at temperatures >250 C (Leif et al., 1991). Biodegradation of such a petroleum formed rapidly at high temperatures would leave some isoprenoid hydrocarbons and the full suite of PAHs. The 14C dating of samples from the two tar islands yielded ages exceeding 40 ka. This indicates that geologically old C formed bitumen in deeper sedimentary horizons (300–500 m) which then migrated by hydrothermal/tectonic processes and remobilization to the lake bottom (Peter et al., 1991; Simoneit and Kvenvolden, 1994). These islands appear to be elongations of bitumen veins hosted E–W in inshore faults reaching the lake bed from the bottom. The bulk C of the tars has a mean d 13CPDB of 21.4% which is typical of lacustrine organic matter (Degens, 1969; Meyers and Ishiwatari, 1993). Based on the source sediments, locale, geology and tectonics it can be concluded that the bitumen compositions of these tars are not conventional biodegraded petroleum residues as described for NW China (Zhang et al., 1988), but they are similar to hydrothermal petroleum from Lake Tanganyika in the East African Rift (Simoneit et al., 2000). This tar/bitumen was generated rapidly from lacustrine organic matter at temperatures below that for hydrothermal cracking of alkanes from kerogen (<250 C). It was forced to the lakebed by tectonic activity from a depth where the 14C age is >40 ka. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be included as a target in exploration for future energy resources in such regions. 5. Conclusions Evidence for hydrothermal activity is found in the Lake Chapala region as thermal springs, sinter deposits, alteration halos, mud volcano and hydrothermal bitumen. Based on the bitumen compositions, source sediments, locale, geology and tectonics, it can be concluded that these tars are not biodegraded conventional petroleum residues, but are similar to hydrothermal petroleum from the East African Rift. The tars contain no n-alkanes, no PAH or significant other aromatics, but consist of a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine microbiota. Thus, this bitumen was generated rapidly from lacustrine organic matter at
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temperatures below that for hydrothermal cracking of alkanes from kerogen (<250 C). It was forced to the lakebed by tectonic activity from a depth where the 14C age is >40 ka. Acknowledgements We thank the PARECyL-1 and Apoyo a Ponentes (2002) programs of the Universidad de Guadalajara and the ECOS M95-U01 project for facility and logistic support for field work and F.A. Romero and A.I. Rushdi for technical assistance. We are grateful to R. Fuge and K.A. Kvenvolden for their review comments which improved this paper. References Allan, J.F., Nelson, S.A., Luhr, J.F., Carmichael, I.S.E., Wopat, M., Wallace, P.J., 1991. Pliocene–Holocene rifting and associated volcanism in southwest Mexico: An exotic terrane in the making. In: Dauphin, J.P., Simoneit, B.R.T. (Eds.), The Gulf and Peninsular Province of the Californias. Am. Assoc. Petrol. Geolog. Memoir, vol. 47, pp. 425–445. Anaya Maldonado, J.L., Badillo Camacho, J., Ochoa Are´valo, A.E., Quezada Magan˜a, C., 2004. Balance hidroquı´mico de afluentes y efluentes del sistema hidrolo´gico local del Lago de Chapala. Tesis profesional, Licenciatura en Quı´mica, Universidad de Guadalajara. Inedit. Barrier, E., Bourgois, J., Michaud, F., 1990. Le syste`me de rifts actifs du point triple de Jalisco: vers un proto-golfe de Jalisco. Compt. Rend. Acad. Sci. Paris 310 (se´rie II), 1513–1520. Degens, E.T., 1969. Biogeochemistry of stable carbon isotopes. In: Eglinton, G., Murphy, M.T.J. (Eds.), Organic Geochemistry – Methods and Results. Springer Verlag, New York, pp. 304–329. Downs, T. 1958. Fossil vertebrates from Lago de Chapala, Jalisco, Mexico. In: Proceedings of the XX International Geological Congress, Mexico, pp. 73–77. Fernex, F., Za´rate-del Valle, P.F., Ramı´rez-Sa´nchez, H., Michaud, F., Parron, C., Dalmasso, J., Barci-Funel, G., Guzma´n Arroyo, M., 2001. Sedimentation rates in Lake Chapala (western Mexico): Possible active tectonic control. Chem. Geol. 177, 213–228. Ferrari, L., Nelson, S.A., Rosas-Elguera, J., Aguirre-Dı´az, G.J., Venegas-Salgado, S., 1997. Tectonic and volcanism of the western Mexican Volcanic Belt. In: Aguirre-Dı´az, G.J., Aranda-Go´mez, J.J., Carrasco-Nu´n˜ez, G., Ferrari, L. (Eds.) Magmatism and Tectonics in the Central and Northwestern Mexico – a selection of the 1997 IAVCEI Assembly Excursions: Mexico, DF, UNAM-Geological Institute, Excursion 12, pp. 85–129. Ferrari, L., Pasquare`, G., Venegas-Salgado, S., Romero-Rı´os, F., 1999. Geology of western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block. Geol. Soc. Am., Spec. Pap. 334, 65–83. Jiang, Z.S., Fowler, M.G., 1986. Carotenoid-derived alkanes in oils from northwestern China. Org. Geochem. 10, 831–839. Jiang, Z.S., Philp, R.P., Lewis, C.A., 1988. Fractionation of biological markers in crude oils during migration and the
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Sandoval, F.P., 1994. Pasado y futuro del Lago de Chapala. Unidad Editorial Gobierno del Estado de Jalisco, Me´xico. Seifert, W., Moldowan, M., 1978. Application of steranes, terpanes and monoaromatics to the maturation, migration, and source of crude oils. Geochim. Cosmochim. Acta 42, 77– 95. Simoneit, B.R.T., 1985. Hydrothermal petroleum: Genesis, migration and deposition in Guaymas Basin, Gulf of California. Can. J. Earth Sci. 22, 1919–1929. Simoneit, B.R.T., Fetzer, J.C., 1996. High molecular weight polycyclic aromatic hydrocarbons in hydrothermal petroleums from the Gulf of California and Northeast Pacific Ocean. Org. Geochem. 24, 1065–1077. Simoneit, B.R.T., Kvenvolden, K.A., 1994. Comparison of 14C ages of hydrothermal petroleums. Org. Geochem. 21, 525– 529. Simoneit, B.R.T., Aboul-Kassim, T.A.T., Tiercelin, J.J., 2000. Hydrothermal petroleum from lacustrine sedimentary organic matter in the East African Rift. Appl. Geochem. 15, 355–368. Simoneit, B.R.T., Peters, K.E., Rohrback, B.G., Brenner, S., Kaplan, I.R., 2004. Thermal alteration of Cretaceous black shale from the Eastern Atlantic. III: Laboratory simulations. In: Hill, R.J., Leventhal, J., Aizenshtat, Z., Baedecker, M.J., Claypool, G., Eganhouse, R., Goldhaber, M., Peters, K. (Eds.), Geochemical Investigations in Earth and Space Sciences: A Tribute to Isaac R. Kaplan, The Geochemical Society Special Publication, 9. Elsevier, Amsterdam, pp. 321–340. SRH (Secretarı´a de Recursos Hidrau´licos), 1981. Levantamiento Hidrogra´fico Lago de Chapala, Jalisco y Michoaca´n. Esc. 1:20,000; equidistancia entre curvas de nivel 0.5 m; hoja 7/11. Venegas, S.S., Herrera, F.J.J., Maciel, F.R., 1985. Algunas caracterı´sticas de la faja volca´nica mexicana y sus recursos geote´rmicos. Geofı´s. Int. 24, 47–82. Za´rate-del Valle, P.F., Michaud, F., Parron, C., Solana-Espinoza, G., Israde-Alca´ntara, I., Ramı´rez-Sa´nchez, H.U., Fernex, F., 2002. Geology, sediments and soil. In: Hansen, A.M., van Afferden, M. (Eds.), The Lerma-Chapala Watershed. Evaluation and Management. Kluwer Academic/Plenum Publishers, pp. 31–57. Za´rate-del Valle, P.F., Rushdi, A.I., Simoneit, B.R.T., 2005. Hydrothermal petroleum of Lake Chapala, Citala Rift, Western Mexico:- Bitumen compositions from source sediments and hydrous pyrolysis. Appl. Geochem., in press. Zhang, D.-J., Huang, D.-F., Li, J.-C., 1988. Biodegraded sequence of Karamay oils and semi-quantitative estimation of their biodegraded degrees in Junggar Basin, China. Org. Geochem. 13, 295–302. Zumberge, J.E., 1993. Organic geochemistry of Estancia Viega oils, Rio Negro Norte block. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 461–471.
Hydrothermal bitumen generated from sedimentary organic matter of rift lakes – Lake Chapala, Citala Rift, western Mexico Pedro F. Za´rate-del Valle a, Bernd R.T. Simoneit a b
b,*
Departamento de Quı´mica, Universidad de Guadalajara – CUCEI, Ap. Postal 4-021, Guadalajara, Jalisco CP 44410, Mexico Environmental and Petroleum Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Building 104, Corvallis, OR 97331-5503, USA Received 20 January 2005; accepted 7 September 2005 Editorial handling by R. Fuge Available online 25 October 2005
Abstract Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleoand active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appear as small islands composed of solid, viscous and black bitumen. Aliquots of tar were analyzed by GC-MS and the mixtures are comprised of geologically mature biomarkers and an UCM. PAH and n-alkanes are not detectable. The biomarkers consist mainly of hopanes, gammacerane, tricyclic terpanes, carotane and its cracking products, steranes, and drimanes. The biomarker composition and bulk C isotope composition (d13C = 21.4%) indicate an organic matter source from bacteria and algae, typical of lacustrine ecosystems. The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala exceeding 40 ka (14C) in age and then forced to the lakebed by tectonic activity. The absence of alkanes and the presence of an UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered in future energy resource exploration in such regions. 2005 Elsevier Ltd. All rights reserved.
1. Introduction Lake Chapala, in western Mexico, is located in the Citala Rift (E–W) which, in association with the Tepic-Zacoalco (NW–SE) and Colima (N–S) Rifts, forms the well-known neotectonic Jalisco con*
Corresponding author. Tel.: +1 541 737 2155; fax: +1 541 737 2064. E-mail address: [email protected] (B.R.T. Simoneit).
tinental triple junction (JTJ; 20.14N; 103.48W) (Barrier et al., 1990). The JTJ is characterized by active volcanism (Ceboruco and Volcan de Fuego) and tectonics (1995, earthquake M = 8). The JTJ neotectonics are due to the motion of the Cocos Plate relative to the North America Plate (Fig. 1; Allan et al., 1991; Barrier et al., 1990; Michaud et al., 1994). The Citala Rift is characterized by evidence for both paleo- and active hydrothermal activity (Fig. 2) as follows: (a) 3 types of thermal springs:
0883-2927/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2005.09.001
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Fig. 1. Rift structures and associated volcanism from the Pliocene to Recent in western Mexico (modified from Allan et al., 1991). Caption: Normal faults are generalized and shown by lines with hachures on the down-thrown side, lineaments are shown as solid lines. Open squares refer to cities. Closed triangles refer to volcanoes or volcanic fields: SJ = San Juan, N = Navajas, S = Sanganguey, Tp = Tepeltitic, SP = San Pedro, Cb = Ceboruco, Tq = Tequila, P = Primavera, CN = Ca´ntaro, NC = Nevado de Colima, VC = Volca´n Colima, M = Mascota volcanic field, LV = Los Volcanes volcanic field, Pa = Paricutı´n, and J = Jorullo. The asterisks show the locations of the Villa Corona and Ixtla´n de los Hervores inland hydrothermal fields, and the Los Negritos mud volcano.
inland (Fig. 1; Villa Corona and Ixtla´n de los Hervores), nearshore (Fig. 2; San Juan Cosala´), and offshore (Fig. 2; Los Gorgos, sublacustrine) hydrothermal fields; (b) fossil sinter deposits (Fig. 2; La Calera); (c) hydrothermal alteration halos and incipient XIX century mining activity (Fig. 2; San Juan Cosala´); (d) geyser activities (Figs. 1 and 2; San Juan Cosala´ and Ixtla´n de los Hervores); (e) gas seeps at Los Gorgos and El Fuerte vents; (f) hydrothermal bitumen at Columba tar islands near Los Gorgos (Figs. 2 and 3); and (g) Los Negritos mud volcano eruption in 1900 (Figs. 1 and 2; Moreno Garcı´a and Funes, 1991). Inland fumarole activity is not observed at JTJ. In 1868 the Mexican local press reported that manifestations of bitumen were appearing in the lake at the mid-southern shore off Cape Columba near Emiliano Zapata village. Because of these hydrocarbon manifestations, the national company Petro´leos Mexicanos drilled
an oil exploration well in that vicinity to a depth of 2348 m in 1961 (Lo´pez-Ramos, 1979). No reservoired oil was found. Hydrothermal activity at JTJ has been studied principally for its geothermal energy potential since the 1980s by the Comisio´n Federal de Electricidad (Venegas et al., 1985). 2. Geological setting The JTJ is located at the western end of the Transmexican Volcanic Belt geological province (GP) and was generated by the activity of a Pleistocene continental arc (Ortega-Gutie´rrez et al., 1991). The Tepic-Zacoalco and Colima continental rifts of the JTJ are the boundaries of a western tectonostratigraphic assemblage (tsa) named the Jalisco Block (Fig. 1; Ferrari et al., 1999), consisting of granodioritic Meso-cenozoic batholiths and stocks (Jalisco Batholith GP) that have partially intruded
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Fig. 2. Geology (modified from Rosas-Elguera and Urrutia-Fucugauchi, 1998) and geothermal features of the Lake Chapala region.
Fig. 3. Bathymetry of the ‘‘Los Gorgos’’ sector in Lake Chapala.
a Cretaceous back-arc volcano-sedimentary sequence (Guerrero-Colima Orogenic Complex GP; Ortega-Gutie´rrez et al., 1991). In the Jalisco Block these sequences are partly covered by Pleistocene volcanic rocks (i.e., andesites of the Chapala Group;
Rosas-Elguera et al., 1997). The Citala and Colima continental rifts delineate an eastern-southwestern tsa called the Michoaca´n Block (Fig. 1), which abuts the south shore of Lake Chapala. Both north and south shores of Lake Chapala (Fig. 2) consist
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mainly of calc-alkaline and alkaline andesitic lava (500–1020 m; Chapala Group; 6.2–3.5 Ma; Ferrari et al., 1997), with subordinate basalt flows (3.4–2.2 Ma) and monogenetic basaltic volcanoes (<2.0 Ma; Rosas-Elguera et al., 1997). A lacustrine volcano-sedimentary (continental) sequence named the Chapala Formation (Downs, 1958) is in the JTJ region, with an average thickness between 300 and 900 m, and forms partially the north shore of the lake (Fig. 2). The Chapala Formation, tilted NE, is made of alternating lacustrine medium-coarse sediments and pyroclastic ash, pumice units and diatomite horizons (Rosas-Elguera and Urrutia-Fucugauchi, 1998). The sedimentation rates throughout Lake Chapala are not uniform. The rate from west to east varies from <2 to <4 mm a1 (based on 210Pb, 137 Cs and 240Pu; Fernex et al., 2001). Terrigenous minerals of a feldspathic affinity (Si/Al ratio 2–3; Jones and Bowser, 1978) dominate the infilling at Lake Chapala, with a granulometric media ranging from 10 to 50 lm in diameter (Ramı´rez-Sa´nchez, 2001). The actual thickness of the Lake Chapala infill is estimated to be >500 m in places (Za´rate-del Valle et al., 2002). The mean lake level is 1526 m above sea level with a mean depth of only about 4 m (Sandoval, 1994). The deepest zone of the lake was 26.8 m as
measured in 2003 (Anaya Maldonado et al., 2004). The bathymetry of the Los Gorgos sublacustrine thermal spring system (GSTS) is based on a survey by the Secretariat of Hydraulic Resources (SRH, 1981, Fig. 3). The major depression of the GSTS is Thermal Spring I (Fig. 3), a quasi-elliptic funnel, 700 m E–W · 400 m N–S, and about 8 m deeper than the lake bottom. The GSTS has a second sublacustrine thermal spring (II, 200 · 250 m) located N 108 and 1700 m from Thermal Spring I, and the tar islands located N270 and 500 m from Thermal Spring I (Fig. 3). 3. Sample acquisition and analysis There are two tar emanations near the Los Gorgos sub-lacustrine thermal spring close to the south shore of Lake Chapala (20.19N; 102.96W). They appear as small islands (<3–4 m2), named in this paper Columba tar islands (Figs. 2 and 3), composed of solid, viscous and black bitumen (Fig. 4). The tar islands are sometimes covered by water but when the water level decreases sufficiently (i.e., drought) it is possible to access them. In June 2000, the tar islands were exposed due to severe drought, which permitted the sampling of tar. The samples were taken with a stainless steel corer (1 cm diameter) and a hammer. The weathered crust
Fig. 4. View of a Columba tar island (composed of solid, viscous, and black bitumen).
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was avoided in sampling. Samples were put into glass vials (5 ml) with Teflon lined caps. A tar sample from each island was 14C dated by the combustion method at a commercial laboratory (Cambridge Isotope Laboratory). Aliquots of tar were dissolved in dichloromethane and analyzed directly by gas chromatography-mass spectrometry (GC-MS). The GC-MS was a Hewlett-Packard 6890 gas chromatograph coupled to a 5973 Mass Selective Detector, with a DB-5MS (Agilent) fused silica capillary column (30 m · 0.25 mm i.d., 0.25 lm film thickness) and He as carrier gas. The GC was temperature programmed from 65 C (2 min initial time) to 300 C at 6 C min1 (isothermal for 20 min final time). The MS was operated in the electron impact mode at 70 eV ion source energy.
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Mass spectrometric data were acquired and processed using the GC-MS ChemStation data system. Compounds were identified by GC retention index and MS comparison with authentic standards, literature and library data, and characterized mixtures. 4. Results and discussion The GC-MS data for the tar samples from both islands are the same (Fig. 5(a) vs. (b)). The mixtures are comprised of geologically mature biomarkers and an UCM (unresolved complex mixture of branched and cyclic hydrocarbons). Polycyclic aromatic hydrocarbons (PAH) and n-alkanes are not detectable. The biomarkers consist mainly of 17a(H),21b(H)-hopanes ranging from C27 to C34
Fig. 5. GC-MS total ion current (TIC) and key ion traces for the total tar bitumens: (a) Sample 1.1 (island 1) TIC, (b) sample 2.1 (island 2) TIC, (c) m/z 191, key ion for hopanes (aH) and tricyclic terpanes (T), and (d) m/z 217 key ion for steranes [numbers refer to carbon skeleton, a, b, R, S refer to configuration, 18 = 2-(3 0 -methyloctyl)-1,3,3-trimethylcyclohexane, 19 = 2-(3 0 ,7 0 -dimethyloctyl)-1,3,3trimethylcyclohexane].
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Fig. 5 (continued)
(no C28), gammacerane, tricyclic terpanes (C20–C26, no C22) (Fig. 5(c)), carotane and its cracking products, C28 steranes and dinosterane, and drimanes (C14–C16) (Fig. 5(d)). The biomarker composition indicates an organic matter source from bacteria, algae and diatoms, typical of lacustrine ecosystems (Seifert and Moldowan, 1978; Jiang and Fowler, 1986; Kawka and Simoneit, 1987; Zumberge, 1993). The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala (Simoneit et al., 2000). The absence of PAHs supports the interpretation that the tars are lower temperature (<200 C) products from rapid thermal alteration of sedimentary organic matter (Kawka and Simoneit, 1990; Simoneit and Fetzer, 1996; Simoneit et al., 2004). Higher molecular weight PAHs form in hydrothermal systems at temperatures above 200 C and readily from 300 to 500 C by dehydrogenation of other organic
compounds. The paucity of n-alkanes and isoprenoids, and the presence of the UCM, mature hopanes, steranes, and carotenoid biomarkers are also consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 C, intermediate compared to hydrothermal systems in marine sediments (Simoneit, 1985). This temperature window has been confirmed by laboratory simulations (Leif et al., 1991; Za´rate-del Valle et al., 2005). Biodegradation of conventional petroleum is an alternate interpretation of the molecular composition of these tars, because that was the conclusion presented for oils with similar compositions from NW China (Jiang and Fowler, 1986; Jiang et al., 1988; Zhang et al., 1988). The authors favor the first interpretation that these tars are lower temperature hydrothermal products and not biodegradation residues. This is based on laboratory simulation experiments producing no PAHs, but generating the biomarkers.
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Furthermore, lacustrine or marine organic matter is converted to alkane-rich petroleum with PAHs in hydrous pyrolysis simulation at temperatures >250 C (Leif et al., 1991). Biodegradation of such a petroleum formed rapidly at high temperatures would leave some isoprenoid hydrocarbons and the full suite of PAHs. The 14C dating of samples from the two tar islands yielded ages exceeding 40 ka. This indicates that geologically old C formed bitumen in deeper sedimentary horizons (300–500 m) which then migrated by hydrothermal/tectonic processes and remobilization to the lake bottom (Peter et al., 1991; Simoneit and Kvenvolden, 1994). These islands appear to be elongations of bitumen veins hosted E–W in inshore faults reaching the lake bed from the bottom. The bulk C of the tars has a mean d 13CPDB of 21.4% which is typical of lacustrine organic matter (Degens, 1969; Meyers and Ishiwatari, 1993). Based on the source sediments, locale, geology and tectonics it can be concluded that the bitumen compositions of these tars are not conventional biodegraded petroleum residues as described for NW China (Zhang et al., 1988), but they are similar to hydrothermal petroleum from Lake Tanganyika in the East African Rift (Simoneit et al., 2000). This tar/bitumen was generated rapidly from lacustrine organic matter at temperatures below that for hydrothermal cracking of alkanes from kerogen (<250 C). It was forced to the lakebed by tectonic activity from a depth where the 14C age is >40 ka. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be included as a target in exploration for future energy resources in such regions. 5. Conclusions Evidence for hydrothermal activity is found in the Lake Chapala region as thermal springs, sinter deposits, alteration halos, mud volcano and hydrothermal bitumen. Based on the bitumen compositions, source sediments, locale, geology and tectonics, it can be concluded that these tars are not biodegraded conventional petroleum residues, but are similar to hydrothermal petroleum from the East African Rift. The tars contain no n-alkanes, no PAH or significant other aromatics, but consist of a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine microbiota. Thus, this bitumen was generated rapidly from lacustrine organic matter at
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temperatures below that for hydrothermal cracking of alkanes from kerogen (<250 C). It was forced to the lakebed by tectonic activity from a depth where the 14C age is >40 ka. Acknowledgements We thank the PARECyL-1 and Apoyo a Ponentes (2002) programs of the Universidad de Guadalajara and the ECOS M95-U01 project for facility and logistic support for field work and F.A. Romero and A.I. Rushdi for technical assistance. We are grateful to R. Fuge and K.A. Kvenvolden for their review comments which improved this paper. References Allan, J.F., Nelson, S.A., Luhr, J.F., Carmichael, I.S.E., Wopat, M., Wallace, P.J., 1991. Pliocene–Holocene rifting and associated volcanism in southwest Mexico: An exotic terrane in the making. In: Dauphin, J.P., Simoneit, B.R.T. (Eds.), The Gulf and Peninsular Province of the Californias. Am. Assoc. Petrol. Geolog. Memoir, vol. 47, pp. 425–445. Anaya Maldonado, J.L., Badillo Camacho, J., Ochoa Are´valo, A.E., Quezada Magan˜a, C., 2004. Balance hidroquı´mico de afluentes y efluentes del sistema hidrolo´gico local del Lago de Chapala. Tesis profesional, Licenciatura en Quı´mica, Universidad de Guadalajara. Inedit. Barrier, E., Bourgois, J., Michaud, F., 1990. Le syste`me de rifts actifs du point triple de Jalisco: vers un proto-golfe de Jalisco. Compt. Rend. Acad. Sci. Paris 310 (se´rie II), 1513–1520. Degens, E.T., 1969. Biogeochemistry of stable carbon isotopes. In: Eglinton, G., Murphy, M.T.J. (Eds.), Organic Geochemistry – Methods and Results. Springer Verlag, New York, pp. 304–329. Downs, T. 1958. Fossil vertebrates from Lago de Chapala, Jalisco, Mexico. In: Proceedings of the XX International Geological Congress, Mexico, pp. 73–77. Fernex, F., Za´rate-del Valle, P.F., Ramı´rez-Sa´nchez, H., Michaud, F., Parron, C., Dalmasso, J., Barci-Funel, G., Guzma´n Arroyo, M., 2001. Sedimentation rates in Lake Chapala (western Mexico): Possible active tectonic control. Chem. Geol. 177, 213–228. Ferrari, L., Nelson, S.A., Rosas-Elguera, J., Aguirre-Dı´az, G.J., Venegas-Salgado, S., 1997. Tectonic and volcanism of the western Mexican Volcanic Belt. In: Aguirre-Dı´az, G.J., Aranda-Go´mez, J.J., Carrasco-Nu´n˜ez, G., Ferrari, L. (Eds.) Magmatism and Tectonics in the Central and Northwestern Mexico – a selection of the 1997 IAVCEI Assembly Excursions: Mexico, DF, UNAM-Geological Institute, Excursion 12, pp. 85–129. Ferrari, L., Pasquare`, G., Venegas-Salgado, S., Romero-Rı´os, F., 1999. Geology of western Mexican Volcanic Belt and adjacent Sierra Madre Occidental and Jalisco Block. Geol. Soc. Am., Spec. Pap. 334, 65–83. Jiang, Z.S., Fowler, M.G., 1986. Carotenoid-derived alkanes in oils from northwestern China. Org. Geochem. 10, 831–839. Jiang, Z.S., Philp, R.P., Lewis, C.A., 1988. Fractionation of biological markers in crude oils during migration and the
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