Hydrothermal bitumen generated from sedimentary organic matter of rift lakes - Lake Chapala, Citala Rift, western Mexico

摘要

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 paleo-and active hydrothermal activity. At the South shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appeal, 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 (delta(13)C = -21.4%) indicate all 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 (C-14) in age and then forced to the lakebed by tectonic activity. Tile absence of alkalies and the presence of all UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200-250 degrees C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered ill future energy resource exploration ill Such regions. (c) 2005 Elsevier Ltd. All rights reserved.