Resumen de 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.
Eduardo González Partida, Antoni Camprubí, Francisco González Sanchez, Carles Canet i Miquel
- español
En el presente trabajo se utilizan datos procedentes del analisis de inclusiones fl uidas a fi n de caracterizar el papel de salmueras de cuenca en las cuencas de Sabinas y del Sureste en Mexico, en las que dichas salmueras fueron responsables de la formacion de depositos tipo Mississippi Valley (MVT) y asociados, asi como de la migracion y acumulacion de petroleo, respectivamente. Los depositos MVT de Pb-Zn presentan salinidades entre 7 y 22 wt.% NaCl equiv. y Th entre 75�� y 150��C. Los mantos de barita asociados al tipo MVT presentan salmueras cloruradas sodicas y calcicas con predominio de esta ultima, en un rango de 1 a 2 wt.% NaCl y de 8 a 24 wt.% CaCl2, con Th entre 50�� y 190��C. Los mantos de celestina asociados al tipo MVT presentan fl uidos en inclusiones con salinidades entre 1 y 12 wt.% NaCl equiv. y Th entre 70�� y 160��C. Los mantos y brechas de fl uorita asociados al tipo MVT presentan fl uidos acuosos en inclusiones con salinidades entre 6 y 14 wt.% NaCl equiv. y Th entre 50�� y 170��C, e inclusiones de hidrocarburos con Th entre 45�� y 90��C. Dichos hidrocarburos son generalmente de dos tipos: (1) con poca concentracion de metano (>20% mol) y bajas Th (45�� a 60��C), y (2) con concentraciones proximas al 30-40% mol de metano, con Th mas altas (60�� a 90��C). Las inclusiones de hidrocarburos que presentan una fase acuosa tienen una salinidad de 14 wt.% de NaCl equiv., presentan generalmente baja concentracion de CO2 y de azufre, y relaciones CH2/CH3 elevadas, lo que corresponde a alkanos de la cadena C16 formados a presiones de 300 a 160 bar.
En la Cuenca del Sureste, los paleofl uidos mas precoces relacionados a las rocas generadoras de petroleo presentan Th entre 55��C y 145��C, y salinidades entre 0.5 y 1 wt.% NaCl y entre 3 a 21 wt.% CaCl2. Las inclusiones con hidrocarburos presentan Th entre 1�� y 87��C. Las salmueras acuosas presentan concentraciones altas en metano, formadas a presiones ~1200 bar, sugiriendo que las rocas estaban sobrepresurizadas en el momento de la circulacion de los paleofl uidos. En la etapa de migracion se produjeron al menos cinco generaciones de dolomita y se paso de un regimen litostatico a uno hidrostatico, a presiones entre 900 y 500 bar y temperaturas de 130�� a 150��C, con salinidades entre 1.6 y 12 wt.% NaCl equiv. El llenado de los reservorios se produjo a temperaturas analogas por salmueras acuosas con salinidades entre 2 y 8 wt.% NaCl equiv., mientras que las inclusiones de hidrocarburos presentan una Th entre 40�� y 100��C y representan el regreso a condiciones de presion hidrostatica, entre 400 y 600 bar.
La geoquimica de halogenos, tanto en yacimientos de hidrocarburos como en yacimientos tipo MVT, sugiere la intervencion de aguas derivadas de evaporacion de agua marina, que sobrepasaron el punto de precipitacion de la halita, en equilibrio con procesos de dolomitizacion.
Los valores de �Â13C y �Â18O en carbonatos de la Cuenca del Sureste presentan valores entre -5 y 2.8�ñ, y entre -10 y 1.9�ñ, respectivamente. Tales variaciones en el comportamiento isotopico de los carbonatos se deben principalmente a (1) la introduccion de carbono organico en el fl uido a partir del que se formo la dolomita, derivado de la oxidacion del metano, y (2) a un bajo proceso de interaccion agua/roca, teniendo la temperatura un papel muy secundario. Para los yacimientos tipo MVT, los valores de �Â13C y �Â18O se presentan en rangos entre -8 y 2.8�ñ, y entre -15 y -0.1�ñ, respectivamente. Estas composiciones se interpretan como debidas a mezclas entre aguas meteoricas y salmueras de cuenca calientes que provocaron la maduracion de materia organica de la roca encajonante al momento del deposito de las menas.
- English
Fluid inclusion data are used in this paper to explain the role of basinal 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 fl uid 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 fl uid 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 fl uid inclusions with salinities that range from 1 to 12 wt.% NaCl equiv. and Th that range from 70° to 160°C. The fl uorite mantos and breccias associated with the MVT type have aqueous fl uid inclusions with salinities that range from 6 to 14 wt.% NaCl equiv. and Th that range from 50° to 170°C, and hydrocarbon inclusion fl uids with Th that range from 45° to 90°C. Such hydrocarbon-bearing inclusions can generally be classifi ed 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 CH4) 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 paleofl uids associated with Tithonian-Kimmeridgian rocks (petroleum generators) are represented by fl uid 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 paleofl uids and their interaction with country rocks. During the migration of such fl uids no less than fi ve 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 fl uids range from 1.6 to 12 wt.% NaCl equiv. The fi lling of reservoirs occurred at similar temperatures, with associated aqueous fl uids with salinities ranging from 2 and 8 wt.% NaCl equiv., whereas the aqueous fl uids 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 oversaturation in halite, in equilibrium with dolomitization processes. Such water mixed with mainly meteoric water during the fi lling of petroleum reservoirs and during the formation of some ores in MVT and associated deposits.
The ä13C and ä18O 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 fl uids 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 ä13C and ä18O 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.
