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Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

There are several organic matter-rich shale geologic formations in Mexico that could be considered non-conventional potential oil reservoirs; one of those formations is the Eagle Ford Formation (Late Cenomanian - Turonian), aim of this study. The Eagle Ford Formation is located within the Sabinas paleobasin and along the Burros-Peyote paleoplatform, in the Coahuila State. This formation has been previously described as a transgression system deposited in a shallow marine environment with a total organic carbon between 0.5 to 8% with type II organic matter. For these reasons, it could be considered a potential source for non-conventional hydrocarbons. Therefore, it is considered that a reservoir of shale gas must have certain requirements, such as: 1) high total organic carbon (> 2% of COT), 2) presence of types II and III kerogen, 3) vitrinite reflectance values greater than 1.2% Ro (0.7% for shale oil), 4) thickness greater than 30 m and large extension, 5) fracturability (<40% clays or absence of expandable clays) or the presence of a microcracking system, 6) overpressure, 7) adequate depth, 8) heterogeneity, and others. A systematic sampling was carried on outcrops of Coahuila State with the goal to determine the thermal maturity degree of the Eagle Ford Formation using a detailed stratigraphic analysis of three columns (CSI, CSII and CSV) and micro-thermometric of fluid inclusions. Our results indicate that the Eagle Ford Formation is a sedimentary sequence rich in organic matter, locally composed of carbonaceous carbonate shale in laminar stratification alternating with shaly limestone (mudstone- wackestone) and clastic o bioclastic limestone (packstone-grainstone). Regarding the diagenetic alteration, primary porosity is affected by compaction and cementation processes. However, it was also observed a secondary micro-porosity development due to neomorphism, dolomitization, fracturing and stylolitization processes. Measured homogenization temperatures (Th) in fluid inclusions are between 65 °C and 125 °C, reaching the temperature window to generate oil and humid gas. The thermal evolution through its facies changes in the basin is not homogeneous. Our results indicate that the Eagle Ford Formation is a sedimentary sequence rich in organic matter, locally composed of carbonaceous carbonate shale in laminar stratification alternating with shaly limestone (mudstone- wackestone) and clastic o bioclastic limestone (packstone-grainstone). [ABSTRACT FROM AUTHOR]

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In the tract of the Chihuahua and Sabina intracratonic basins, stratabound copper deposits are systematically hosted by siliciclastic rocks of the Las Vigas, San Marcos and Huizachal formations. The deposits in the Las Vigas area, Chihuahua, occur in the upper part of the Valanginian-Hauterivian Las Vigas Formation, which is conformably overlain by gypsum and limestones of the Barremian La Virgen Formation. Other local evaporites are those found in the Jurassic La Casita Formation. The Las Vigas stratabound copper deposit was mined with grades between 2 and 4 % Cu and occasional grades of 1 g/t Au and 80 to 100 g/t Ag in four 1 to 4 m-thick mantos. The hypogene metallic minerals in this deposit are chalcopyrite, bornite, pyrite, sphalerite and galena, along with quartz and calcite, mostly as disseminations. Notably, part of these ores occurs as replacements of fossil tree logs (m-sized) that contain the same minerals as mantos. The close examination of fossil tree logs emphasizes the epigenetic and stratabound character of copper ores. Representative fluid inclusion associations were studied in calcite and quartz from mantos, lenses and fossil logs. Fluid inclusions contain halite, calcite and chalcopyrite daughter crystals, as determined by means of Raman microspectroscopy. The calculated salinity of mineralizing fluids ranges between 16.6 and 32.0 wt. % NaCl equiv., temperatures of homogenization range between 104º and 205 ºC, and relevant N2, CO, CO2 and CH4 contents were determined. δ34SVCDT values in chalcopyrite range between -12.0 and 13.7 ‰. These values are compatible with sedimentary or metasedimentary sources for sulfur, significant amounts of sulfates that were leached from evaporites, besides the occurrence of bacteriogenic sulfate reduction processes orthe interaction of mineralizing fluids with organic matter. δ13CVPDB and δ18OVSMOW values in hydrothermal calcite range between -7.96 and -0.71 ‰ and between 22.38 and 26.39 ‰, respectively; these are compatible with a trend in diagenesis due to burial of marine sediments, much alike MVT deposits in Coahuila and Texas. The above data indicate that mineralizing fluids in Las Vigas are basinal brines with minor meteoric inputs, which underwent convection within the siliciclastic series by means of differences in density as they dissolved evaporites of the overlying La Virgen Formation. Abundant organic matter in the Las Vigas Formation would have enabled mineral precipitation by means of sulfate reduction -such interpretation is supported by the occurrence of Cu ores along with silicification in fossil tree logs. As a consequence, the Las Vigas deposit is generically ascribed to the group of epigenetic copper deposits hosted by red-bed series that rim marine basins, or deposits of the Kupferschiefer type. [ABSTRACT FROM AUTHOR]

The petroleum systems of the Southeast basin (Tabasco, Mexico) are characterized by having source rocks mainly from the Upper Jurassic, Cretaceous storage rocks, and Upper Cretaceous to Cenozoic seal rocks. In general, most of these reservoirs reflect a complex diagenetic history that depends on the prevailing porosity and permeability. A new approach using tools such as scanning electron microscopy (SEM) and X-ray 3D microtomography contribute to the understanding of how hydrocarbons accumulate and flow even at low permeabilities. The rock specimens observed by means of SEM are strongly altered by diagenetic processes of corrosion-dissolution-precipitation, which generated fractures, dissolution, stylolites, vug lining, decarbonatization, and dedolomitization. The paragenetic succession of such diagenetic processes can be schematized thus: dolomite1 → calcite1 → quartz1 → pyrite → dolomite2 → calcite2 → quartz2 → calcite3 → quartz3 → chlorite → zeolites → clay minerals → celestine → calcite4 with hydrocarbons. It is worth noting the widespread presence of quartz and the partial replacement of some fossils by framboidal pyrite. The latter, along with clays and zeolites usually occupy spaces left after the corrosion of dolomite crystals, and also occur within vugs and fractures, as an intermediate to late stage in the paragenetic sequence. The latter is characterized by the following stages: (1) cementation of carbonates, (2) thermobaric alteration, associated with the incursion of fluids that were characterized by means of microthermometry of fluid inclusions: a. in dolomite, with salinities that range between 12 and 19 wt. % NaCl equiv., and temperatures of homogenization (Th) between 99 and 141 °C, b. in diagenetic quartz in vugs and fractures, with salinities that range between 12 and 15 wt. % NaCl equiv., and Th between 112 and 131 °C, c. in the earliest calcite, with salinities that range between 12 and 14 wt. % NaCl equiv., and Th between 110 and 149 °C, (3) a late stage of corrosion. New information obtained by means of X-ray microtomography reveals that most of the vugs are not interconnected, and that the samples have abundant fossils randomly distributed in the micritic matrix and oriented according to the stratification. Microfractures and nano-pores in the matrix are partially responsible for the development of the vugs where cavities are formed after corrosive solutions that initially generated micro-breccias, followed by a partial lining by quartz and calcite. [ABSTRACT FROM AUTHOR]

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Boletín de la Sociedad Geológica Mexicana is the property of Sociedad Geologica Mexicana and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper is a review on the principles for the interpretation of microthermometric data from brine- and hydrocarbon-bearing fluid inclusions. Furthermore, a state-of-the-art analytical methodology is described, that allows to determine the composition of dissolved gas in brines, as well as to determine the type of trapped hydrocarbon, its density and, from such data, to reconstruct its isochore and isoplete. These data cannot be easily obtained to date but allow the crossing of isochores calculated for two insoluble liquids (brine and oil), and to estimate with precision the initial pressure and temperature during the trapping of fluid inclusions. Such results are necessary (1) to precisely reconstruct the conditions for hydrocarbon migration, (2) to determine the responsible mechanisms for porosity changes in a certain rock type and for the filling of reservoirs, (3) to understand the hydrodynamics of paleofluids in basins with associate formation of stratabound deposits, and (4) to better understand diagenetic processes. [ABSTRACT FROM AUTHOR]

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 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. The fluorite mantos and breccias associated with the MVT type have aqueous fluid inclusions with salinities that range from 6 to 14 wt.% 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 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 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 oversaturation 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.… [ABSTRACT FROM AUTHOR]