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

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 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 (δ13C = -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

The generation of hydrothermal oil in sediments of the Chapala Lake, and its relation with the geothermal activity in the Citala rift, Jalisco State, Mexico [La generación de petróleo hidrotermal en sedimentos del Lago Chapala y su relación con la actividad geotérmica del rift Citala en el estado de Jalisco, México]

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 17?(H),21?(-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 °C). 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 ?13CPDB 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

The generation of hydrothermal oil in sediments of the Chapala Lake, and its relation with the geothermal activity in the Citala rift, Jalisco State, Mexico [La generación de petróleo hidrotermal en sedimentos del Lago Chapala y su relación con la actividad geotérmica del rift Citala en el estado de Jalisco, México]

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 °C). 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 δ13CPDB 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

How rude! Teaching impoliteness in the second-language classroom

Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250°C over brief geological times. " 2006 Elsevier Ltd. All rights reserved.",,,,,,"10.1016/j.apgeochem.2006.01.002",,,"http://hdl.handle.net/20.500.12104/42006","http://www.scopus.com/inward/record.url?eid=2-s2.0-33645321856&partnerID=40&md5=34036707fb18b2f5ce55d4343943c0b2",,,,,,"4",,"Applied Geochemistry",,"70

Pregnanes as molecular indicators for depositional environments of sediments and petroleum source rocks

Steroids with unconventional side chains have increasingly been applied as diagnostic markers for geological source and age assessments. However, one of the most distinctive characteristics, the abnormal abundance of pregnane and homopregnane in ancient sediments and petroleum, remains unresolved. Higher pregnane and homopregnane, as well as C23–C26 20-n-alkylpregnanes, relative to the regular steranes were observed in samples collected from different petroleum basins in China. These included Precambrian marine carbonate-derived petroleum (NW Sichuan Basin), Lower Paleozoic marine marl derived crude oils (Tarim Basin), and Eocene hypersaline lacustrine carbonate source rocks and associated petroleum (Bohai Bay Basin). However, all of the samples have many common biomarker characteristics, such as pristane/phytane ratios < 1, low amounts of diasteranes and high C29/C30 hopane (~0.6–1), C35/C34 hopane (mostly ≥ 1) and dibenzothiophene/phenanthrene (DBT/PHEN, mostly 0.5–1) ratios revealing a contribution from anoxic carbonate/marl source rocks deposited in restricted, clastic-starved settings. We suggest that 5α,l4β,l7β-pregnane and homopregnane, as well as their higher C23–C26 homologues, are geological products derived from steroids bound to the kerogen by a sulfurized side chain. Carbon or carbonate minerals are considered to be natural catalysts for this cracking reaction via preferential cleavage of the bond between C-20 and C-22. Similar distributions occur in the short chain analogues of 4-methylsterane, triaromatic steroid and methyltriaromatic steroid hydrocarbons, providing circumstantial evidence for this proposal. The ratio of pregnane and homopregnane to the total regular steranes and the ratio of C27 diasteranes to cholestanes can be sensitive indicators of sedimentary environments and facies. In general, high diasteranes and low pregnanes (with homologues) indicate an oxic water column or significant input of terrigenous organic matter in clay rich source rocks and some organic lean carbonate rocks. Low diasteranes with high pregnanes implies restricted, sulfur rich conditions, typical of anoxic carbonate source rocks. Furthermore, the two ratios may be useful to assess the variation of mineralogy and openness of source rock depositional settings

Hydrothermal petroleum of Lake Chapala, Citala Rift, western Mexico: Bitumen compositions from source sediments and application of hydrous pyrolysis

Hydrothermal simulation experiments were performed with contemporary sediments from Lake Chapala to assess the source of the lake tars. The precursor-product relationships of the organic compounds were determined for the source sediments and their hydrous pyrolysis products. The pyrolysis products contained major unresolved complex mixtures of branched and cyclic hydrocarbons, low amounts of n-alkanes, dinosterane, gammacerane, and immature and mature hopane biomarkers derived from lacustrine biomass sources. The results support the proposal that the tar manifestations in the lake are not biodegraded petroleum, but were hydrothermally generated from lacustrine organic matter at temperatures not exceeding about 250°C over brief geological times. © 2006 Elsevier Ltd. All rights reserved

Input of organic matter in Brunei Bay, East Malaysia, as indicated by sedimentary steroids and multivariate statistics

Brunei Bay is one of the most important marine environments of East Malaysia (South China Sea), covering many productive ecosystems with activities including fisheries, tourism, and main shipping lanes for petroleum transfers. Evaluation of the sources and distributions of steroids in the surface sedimentary organic matter was carried out by gas chromatography–mass spectrometry (GC–MS). The concentrations of the total identified sterols (TIS) ranged between 0.81 and 12.69 μg g−1 dry weight, and the total sterones were between 0.11 and 5.66 μg g−1 dry weight. The coprostanol level was comparatively low (<0.10 μg g−1), and the multi-biomarker proxies indicated that the region did not exhibit significant contamination from sewage effluents. Principal component analysis (PCA) revealed the coastal environment of the study area was dominated by allochthonous (mainly terrestrial) organic matter input