| Keywords |
Formation water, Mexico, geothermal reservoirs, oil field reservoirs, fluid origin, hydrochemistry, stable isotopes, radioactive isotopes |
| Abstract |
Fluids from four geothermal reservoirs in Mexico (Cerro Prieto and Las Tres VÌrgenes in NW-Mexico; Los Humeros and Los Azufres in Central Mexico) are compared with formation water from oil field reservoirs in SE-Mexico, located offshore (Pol-Chuc) and onshore (Activo Luna, Samaria-Sitio Grande). Geothermal fluids are extracted from more shallow depths (350 - 3,500 m) with maximum temperature conditions of 400oC, while oil field waters are encountered at depths between 3,000 and 6,200 m. The fast accumulation of thick sedimentary piles in the Gulf basin explains low temperature conditions from 140oC (Activo Samaria-Sitio Grande) to 170oC (Activo Luna) under extremely high pressure conditions (up to 800 bar). An increasing mineralization of groundwater with depth is observed in deep Mexican reservoirs, whereby gravity-driven separation represents the principal physical process for the descent of heavy Cl-brines. A linear 18O-isotopic gradient of 0.43 â per 100 m is proposed as a temperature and pressure dependent regional trend for the isotopic evolution of deep formation water in Mexican geothermal and oil reservoirs. Additionally, local effects cause an individual chemical and isotopic evolution of each reservoir fluid: Oil field waters from the Gulf of Mexico are characterized by a heterogeneous, saline to hypersaline, NaCl and NaCaCl composition, formed by varying mixing proportions between three components - meteoric water, seawater and/or evaporated seawater. Low Cl/Br ratios exclude the influence of halite dissolution from salt domes as a potential Cl-source. In the case of geothermal fluids, chemical and isotopic fingerprints are mainly defined by high-temperature hydrothermal reactions during convective circulation of boiling fluids. Low saline (brackish to marine), NaCl-type water reflect the average composition in Mexican geothermal reservoirs, although Los Humeros fluids can range from NaCl to NaHCO3 and NaHCO3ClSO4-types. The simultaneous enrichment in Ca++ and depletion of Mg++ in oil field groundwater is caused by the partial dolomitization of carbonate host rock. Potassium concentrations of geothermal fluids above the evaporation trajectory reflect the addition of K+ through water/rock interaction, perhaps as albitization of K-feldspar. Measured 14C-activities between 0.8 pmC and 28.2 pmC for oil field water reflect enhanced infiltration processes during late Pleistocene and/or early Holocene as a period of increased humidity in the Gulf region. Probably, the origin of meteoric and marine components in geothermal fluids from Mexican reservoirs can - although low 14C-concentrations indicate the possible dilution by dead carbon from magmatic CO2 sources - be attributed in parts to this interglacial recharge event. Elevated D-ratios with average values of -10.0â support the hypothesis of warmer climatic conditions during this period. |