C. Taberner

Secondary porosity development during late burial in carbonate reservoirs as a result of mixing and/or cooling of brines

Abstract Carbonate reservoirs from different geologic provinces record an important event of porosity enhancement by corrosion (dissolution) in diagenetically late, deep burial conditions. Fluid mixing can be detected in successive cement stages preceding major corrosion by means of coupled fluid inclusion microthermometry and stable isotopic composition ( δ 13 C, δ 18 O) of carbonate cements. Major corrosion is preceded by recurrent minor corrosion between successive cement stages. The transport of dissolved carbonate away from the system, as well as arrested diagenesis related to hydrocarbon migration appears essential for porosity preservation. Rock observations demonstrate that cooling postdates main corrosion, and appears to be coeval with oil migration. Relatively low-temperature (90–100 °C) fluorite with oil inclusions characterises the stage of cooling. Fluorite (and minor pyrite -marcasite, barite- celestite, dickite and quartz) is found in minor amounts filling porosity formed during major corrosion or replacing carbonate cements and matrix.

Basin infill architecture and evolution from magnetostratigraphic cross-basin correlations in the southeastern Pyrenean foreland basin

Magnetostratigraphy, numerical dating, and facies mapping have been combined to provide the first cross-basin correlation of the Eocene southeastern Pyrenean foreland basin in northeastern Spain. This has enabled (1) depositional systems on the northern and southern margins of the basin to be dated and correlated across the basin and (2) changes in the rates of sediment accumulation in time and space to be evaluated. By sampling correlative intervals at several locations in a transect across the basin, magnetozones were detected that otherwise may have been misinterpreted due to recent magnetic overprinting, or missed by wide sampling intervals in previous magnetostratigraphic surveys. Results of this study indicate that marginal marine strata of the southeastern part of the foreland near the Vic area of Spain are 5 m.y. older than previously thought, the base of the interval (46 Ma) being middle Lutetian in age. By proposing an age of at least 35.68 Ma for the overlying Cardona evaporite unit, the age span of the marine interval in the studied part of the foreland is ~10 m.y. Contrary to earlier interpretations, no major southward migration of the southeastern Pyrenean foreland basin depocenter is recorded by the basin stratigraphy during the initial stages of middle Eocene marine sedimentation in the southern margin of the basin. However, marked basin asymmetry is observed later in its evolution as the top of the marine deposits records northward increase in the thickness of chron C17n.1n (37.47‐36.61 Ma). This asymmetry is also proved by the presence of a larger number of magnetozones within marine strata at the top of the marine interval in the northern sector. These results imply that previous age calibrations and the estimated rates of tectonic processes and sedimentation within the foreland need to be reassessed. The implications for basin infill architecture evolution through time and space, subsidence analysis, and depocenter evolution through time are addressed.

Modeling the sulfur and oxygen isotopic composition of sulfates through a halite-potash sequence: Implications for the hydrological evolution of the upper eocene Southpyrenean basin

The isotopic composition of sulfates sampled throughout a complete evaporite sequence has contributed to the understanding of the evolution of the Southpyrenean Upper Eocene basin. The δ34S and δ18O values appear to be constant in thick segments of the sequence. This pattern is clearly different from the continuous decrease in δ values predicted by the evaporation processes in a closed system. The observed trend of the δ34S and δ18O values can be successfully modeled by an evaporation process in an open system, where steady state conditions are reached, separated by periods of significant changes in the hydrological regime of the basin. Moreover, in the segments of the sequence where they are constant, the δ34S and δ18O values help to constrain the hydrological parameters of the model. n nA first marine stage of the evolution of the basin corresponds to the formation of the Basal Anhydrite and Lower Halite Units. The mineral sequence corresponds to evaporation in a basin with a progressive degree of restriction and evaporation compensated by equal inflow of major marine origin. The sharp decrease in the δ34S and particularly the δ18O values, recorded at the end of the Basal Anhydrite Unit, illustrates a limited reservoir effect produced by the increase of restriction leading to the halite precipitation. The modeling of the constant isotopic values through the Lower Halite Unit confirms the isotopic composition of Eocene marine dissolved sulfate (δ34S = +20.0%0; δ18O = +8.7%0) deduced from the average values of marginal gypsum. n nThe second continental stage of basin evolution corresponds to the formation of the Potash Unit and the Upper Halite Unit. The mineral sequence and lateral extension indicate that the basin evolved with a decreasing brine volume after disconnection from the sea. The dilution of the residual brine stopped the precipitation of camallite, and the basin then evolved as a perennial endorreic lake with continental recharge replacing evaporation. The sharp decrease in the δ34S and δ18O values in the sylvite member is consistent with the massive precipitation of sulfate occurring during the decrease in the volume of water. The δ values of the sulfates intercalated in the camallite and the Upper Halite Unit indicate the influence of the recycling of both the already deposited Upper Eocene sulfates and the Upper Triassic sulfates carried by continental recharge. The constant δ34S and δ18O values from the uppermost half of the Upper Halite Unit is used to constrain the sulfate source as 80% recycled Upper Triassic and 20% recycled Eocene gypsum.

The use of environmental markers to distinguish marine vs. continental deposition and to quantify the significance of recycling in evaporite basins

Abstract Environmental markers, namely the bromine content of halite samples, the electrolyte content of primary fluid inclusions in halite and the isotopic composition of sulphates from two Tertiary evaporite sequences, provide complementary information on the depositional environment (marine vs. continental). The use of these markers, together with lithofacies and thicknesses of precipitated evaporites, enable the detection and quantification of evaporite recycling within evaporite basins. The information provided by the isolated use of each of these geochemical markers should be used with caution, as this could lead to erroneous interpretations of the depositional environment or may not detect significant evaporite recycling processes. The complementary information provided by geochemical markers enable quantification of solute input from recycling of previous precipitates within the basins themselves or from older evaporites. The input of recycled evaporites increases progressively, together with evaporite basin restriction to the open ocean.

Solute inputs in the Salar de Atacama (N. Chile)

Abstract The provenance of saline inputs to the Salar de Atacama has been studied by means of the isotopic composition of sulphates dispersed in drilling core samples and the analysis of the water of surface inflows. The pattern shown by the ratios of 87Sr/86Sr and the isotopic composition of sulphur and oxygen in sulphates, suggests a mixing of two water inputs: one coming from the north-east of the basin, higher in 87Sr/86Sr, δ34S and δ18O and the other from the south-west, lower in 87Sr/86Sr, δ34S and δ18O. Analysis of the recent inflows to the Salar have confirmed the existence of two solute sources: northern and eastern inputs show an enrichment in those elements related to hydrothermalism and volcanism, in relation to water present in the west of the basin.

The geochemical evolution of the Catalan potash subbasin, South Pyrenean foreland basin (Spain)

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Brine-mineral reactions in evaporite basins: Implications for the composition of ancient oceans

The chemical evolution of several European Mesozoic and Tertiary evaporite basins was reconstructed by using mineral associations, primary fluid-inclusion analyses, and numerical simulations of evaporation scenarios. The solute proportion recorded in the fluid inclusions can be explained by the evaporation of present-day seawater as a major recharge. The sulfate depletion in the brines is responsible for the type of potash deposit formed, potassium-magnessium sulfates or sylvite. This sulfate depletion can be due either to dolomitization or to the addition of a CaCl 2 -rich solution to the basin. The sulfate depletion occurred in varying intensity in basins of the same age, as well as throughout the evolution of the same basin. Therefore, changes in potash mineralogy and sulfate depletion in fluid inclusions are not conclusive arguments in favor of secular variations in the composition of the ocean, as recently proposed by several authors.

Coral reef diagenesis records pore-fluid evolution and paleohydrology of a siliciclastic basin margin succession (Eocene South Pyrenean foreland basin, northeastern Spain)

An integrated field, petrographic, and geochemical study has determined the fluid-rock interaction history in part of an Eocene reef complex on the margins of the siliciclastic-dominated Catalan sector of the South Pyrenean foreland basin. Results show that at least four distinct fluid systems influenced the basin margin, and demonstrate the sensitivity of reef rocks as paleohydrological archives in siliciclastic environments.nnThe earliest calcite cements precipitated from meteoric waters at shallow burial depths, and mineralogical stabilization of reef carbonate was mostly completed during this episode. Textural and isotope trends typical of paleo-exposure surfaces are lacking, and trace element results suggest that meteoric waters were transmitted laterally and/or upward into the reef via delta-lobe sandstone paleoaquifers.nnThe distribution of late cements is highly variable and fracture-fed, demonstrating the importance of deformation in controlling fluid flow subsequent to reef lithification. Isotopic and elemental compositions of burial calcites, plus their paragenetic association with barite and dolomite, show that fluids of marine parentage were expelled from compacting basinal marls into the basin margin paleoaquifers. The Calders reef also records an episode of extrabasinal fluid circulation, probably associated with faulting of the basin margin. Ferroan saddle dolomite contains fluid inclusions with elevated homogenization temperatures, indicating thermal disequilibrium with the surrounding strata. Hydrothermal activity was possibly related to Neogene extensional tectonics. The final cement stage in the reef records the influence of telogenetic meteoric water, possibly reflecting the present-day basin margin hydrology.