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BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA http://dx.doi.org/10.18268/BSGM2008v60n1a3 |
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Fisicoquímica de salmueras e hidrocarburos en cuencas petroleras y en depósitos minerales tipo Mississippi Valley y asociados. Parte II: ejemplos de la Cuenca de Sabinas y la Cuenca del Sureste, México
Physicochemical characteristics of brines and hydrocarbons in petroliferous basins and Mississippi Valley type and associated ore deposits. Part II: examples in the Sabinas and Southeast basins, Mexico
Eduardo González–Partida1,*, Antoni Camprubí2, Carles Canet3 y Francisco González–Sánchez1,2
1 Programa de Geofluidos, Centro de Geociencias, Universidad Nacional Autónoma de México; Campus Junquilla, Carretera 57 km. 15.5, 76230 Santiago de Querétaro, Qro.
2 Departamento de Geoquímica, Instituto de Geología, Universidad Nacional Autónoma de México; Ciudad Universitaria, 04510 México, D.F.
3 Departamento de Recursos Naturales, Instituto de Geofísica, Universidad Nacional Autónoma de México; Ciudad Universitaria, 04510 México, D.F.
* This email address is being protected from spambots. You need JavaScript enabled to view it..
Abstract
Fluid inclusion data are used in this paper to explain the role ofbasinal brines for both the Sabinas and Southeast basins in Mexico, in –which such brines –were responsible for the formation of MVT and associated deposits, and for the migration and accumulation of petroleum, respectively. Salinities and temperatures of homogenization (Th) of fluid inclusions of the Pb–Zn MVT deposits of the Sabinas Basin range from 7 to 22 wt.% NaCl equiv. and Th range from 75° to 150°C. The barite mantos have aqueous fluid inclusions with sodium and calcium chloride brines, –with dominant CaCl2, that range from 1 to 2 wt.% NaCl and from 8 to 24 wt.% CaCl2, and Th that range from 50° to 190°C. The celestine mantos associated to the MVT type have aqueous fluid inclusions with salinities that range from 1 to 12 wt.% NaCl equiv. and Th that range from 70° to 160°C. Thefluorite mantos and breccias associated with the MVT type have aqueous fluid inclusions with salinities that range from 6 to 14wt.% NaCl equiv. and Th that range from 50° to 170°C, and hydrocarbon inclusion fluids with Th that range from 45° to 90°C. Such hydrocarbon–bearing inclusions can generally be classified as (1) of low CH4 concentration (>20% mol) and low Th (45° a 60°C) inclusions, or (2) inclusions with higher methane concentrations (about 30–40% mol CH) and higher Th (60° to 90°C). Hydrocarbon–bearing inclusions that show an aqueous phase have salinities of about 14 wt.% NaCl equiv. and generally low CO2 and sulfur concentrations, and high CH2/CH3 ratios, which correspond to C16–chain alkanes that formed at pressures between 160 to 300 bar.
In the Southeast Basin the earliest paleofluids associated with Tithonian–Kimmeridgian rocks (petroleum generators) are represented by fluid inclusions that show calcic brines with Th that range from 55 °C to 145 °C and salinities that range from 0.5 to 1 wt.% NaCl and from 3 to 21 wt.% CaCl2. The hydrocarbon–bearing inclusions have Th that range from 1° to 87°C. The aqueous brines may have high methane concentrations, and formed at ~1200 bar, suggesting that the rocks were overpressured during the circulation of paleofluids and their interaction with country rocks. During the migration of such fluids no less than five generations of dolomitization occurred, and the pressure regime shifted from lithostatic to hydrostatic, thus forming hydraulic breccias that were cemented by dolomite and late calcite. Such shift occurred at pressures that ranged from 900 to 500 bar and temperatures from 130° to 150°C, and the salinities of associated fluids range from 1.6 to 12 wt.% NaCl equiv. The filling of reservoirs occurred at similar temperatures, with associated aqueous fluids with salinities ranging from 2 and 8 wt.% NaCl equiv, whereas the aqueous fluids in hydrocarbon–bearing inclusions have Th that range from 40° to 100°C. Such inclusions were trapped during a shift back to the hydrostatic pressure regime, between 400 and 600 bar.
In both petroleum and MVT deposits the geochemistry of halogens suggests the occurrence of water derived from the evaporation of seawater that reached the over saturation in halite, in equilibrium with dolomitization processes. Such water mixed with mainly meteoric water during the filling of petroleum reservoirs and during the formation of some ores in MVT and associated deposits.
The δ13C and δ18O values in carbonates of the Southeast Basin range from –5 and 2.8‰, and from –10 to 1.9‰, respectively. Such variation in isotopic compositions may be due to (1) the introduction of organic carbon in the fluids that produced dolomitization, after the oxidation of methane, and (2) a process of low water/rock interaction in which the role of temperature would have been marginal. The δ13C and δ18O values in carbonates of MVT and associated deposits in the Sabinas Basin range from –8 to 2.8‰, and from –15 to –0.1‰, respectively. Such variation in isotopic compositions is interpreted as due to the mixing between meteoric water and hot basinal brines that induced the maturation of organic matter within country rocks during the formation of ores.
Key words: Sabinas Basin, Southeast Basin, Mexico, petroleum, MVT deposits, fluid inclusions, halogens.
