HR: 0830h
AN: V51H-0372    [PDF]
TI: Hydrothermal Petroleum in Active Continental Rift: Lake Chapala, Western Mexico, Initial Results.
AU: * Zarate-del Valle, P F
EM: pzarate@ccip.udg.mx
AF: Universidad de Guadalajara. Depto. de Quimica., Bd. M. Garcia Barragan 1451. Z5 Olimpica., Guadalajara, JAL 44430 Mexico
AU: Simoneit, B R
EM: simoneit@coas.oregonstate.edu
AF: Oregon State Univeraity, College of Oceanic and Atmospheric Sciences, Corvallis, OR 97331 United States
AU: Ramirez-Sanchez, H U
EM:
AF: Universidad de Guadalajara. Instituto de Astronomia y Metereologia, Av. Vallarta 2602, Guadalajara, JAL 44130 Mexico
AB: Lake Chapala in western Mexico is located partially in the Citala Rift, which belongs to the well-known neotectonic Jalisco continental triple junction. The region is characterized by active volcanism (Ceboruco, Volcan de Fuego), tectonic (1995 earthquake, M=8, 40-50 mm to SW) and hydrothermal (San Juan Cosala \& Villa Corona spas and La Calera sinter deposit) activities. Hydrothermal petroleum has been described in active continental rift (East African Rift) and marine spreading zones (Guaymas Basin, Gulf of California). In 1868 the Mexican local press reported that manifestations of bitumen were appearing in front of the Columba Cap on the mid south shore of Lake Chapala. This bitumen is linked to the lake bottom and when the water level decreases sufficiently it is possible to access these tar bodies as islands. Because of these manifestations the Mexican oil company (PEMEX) drilled an exploration well (2,348m) at Tizapan El Alto without success. Hydrothermal activity is evident in the tar island zone as three in-shore thermal springs (26.8 m depth, $48.5\deg$C, pH 7.8 and oriented N-S). The preliminary analyses by GC-MS of the tar from these islands indicate hydrothermal petroleum derived from lake sedimentary organic matter, generated at low temperatures ($150\deg$-$200\deg$C). The tars contain no n-alkanes, no PAH or other aromatics, but a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine biota. The biomarkers consist of mainly 17$\alpha$(H),21$\beta$(H)-hopanes ranging from C$_{27}$ to C$_{34}$ (no C$_{28}$), gammacerane, tricyclic terpanes (C$_{20}$-C$_{26}$), carotane and its cracking products, and drimanes (C$_{14}$-C$_{16}$). The biomarker composition indicates an organic matter source from bacteria and algae, typical of lacustrine ecosystems. $^{14}$C dating of samples from two tar islands yielded ages exceeding 40 kyrs, i.e., old carbon from hydrothermal/tectonic remobilization of bitumen from deeper horizons to the surface. 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.
DE: 1010 Chemical evolution
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting