he generation of hydrothermal oil in sediments of the Chapala Lake and its relation with the geothermal activity in the Citala rift, Jalisco State, Mexico

Abstract

Lake Chapala, located in the Citala Rift in western Mexico, is characterized by its active and fossil geothermal activity, which includes terrestrial and sublacustrine hot springs, alteration halos, carbonated sinter deposits and mud volcanoes. Sub-lacustrine hot springs and asphalt emanations that constitute islets coexist in the lake. Oil generated in geothermic zones has an hydrothermal origin because circulating hot water generates both thermic alteration of organic matter and mass transference. For this reason, this oil is named hydrothermal petroleum (HP). Analyses by gas chromatography coupled to mass spectrometry (GC–MS) of the HP from Lake Chapala show that it consists of mature biomarkers and an unresolved complex mixture of branched and cyclic hydrocarbons (UCM). The mature biomarkers, derivated from lacustrine microbiota, consist mainly of 17α(H),21β(-H)-hopanes from C₂₇ to C₃₄ (no C₂₈); gammacerane, tricyclic terpanes (C₂₀–C₂₆, no C₂₂); carotane and its cracking products; C₂₈ and C₂₉ steranes and drimanes (C₁₄–C₁₆). The hydrothermal petroleum of the Chapala Lake does not contain polycyclic aromatic hydrocarbons (PAH) nor n-alkanes. The composition of this HP does not fit with conventional biodegraded petroleum residues. The absence of n-alkanes and isoprenoids and presence of UCM, mature hopanes, steranes and carotenoid biomarkers are consistent with rapid hydrothermal oil generation, similar to hydrothermal petroleum from the East African Rift. We propose that the HP from Lake Chapala was generated rapidly from lacustrine organic matter at temperatures below that required for hydrothermal cracking of alkanes from kerogen (250 ºC). The hydrothermal petroleum of Lake Chapala was forced by tectonic activity to the lake bed, from a depth estimated in 300–500 m, where the sediments have ¹⁴C ages >40 ka. The bulk carbon of the HP of Lake Chapala has a mean δ ¹³C_PDB of -21.4‰ which is a typical value for lacustrine organic matter. The occurrence of hydrothermal petroleum in continental rift systems is now well understood and should be included as a target in exploration for future energy resources in such regions.