Jacques Pironon

Methane-bearing aqueous fluid inclusions: Raman analysis, thermodynamic modelling and application to petroleum basins

Abstract Calibration for the determination of the CH4/H2O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions with 0 m NaCl. Spectra of the symmetric stretching band of methane (ν1,CH4), and the bending (ν2,H2O) and stretching (νS,H2O) bands of water were obtained in the aqueous phase co-existing with the vapour phase at variable temperatures from 25° to a few degrees above the homogenisation temperature. A software program, based on the model of Duan et al. (1992a) [Duan, Z., Moller, N., Greenberg, J.H., Weare, J.H. 1992a. The prediction of methane solubility in natural waters to high ionic strength from 0°C to 250°C and from 0 to 1600 bar. Geochim. Cosmochim. Acta, 56, 1451–1460.] has been developed to calculate the composition of the aqueous liquid phase co-existing with the vapour phase in the synthetic fluid inclusions. Application to natural samples from the Alwyn South area (North Sea) shows that Raman analyses permit discrimination between inclusion populations that are not recognised using only microthermometric measurements. These results provide important constraints on the P–T conditions of inclusion formation and suggest that oil migration occurred at fluid pressures of approximately 240 bars.

PIT (Petroleum Inclusion Thermodynamic): a new modeling tool for the characterization of hydrocarbon fluid inclusions from volumetric and microthermometric measurements

A recent technique, the confocal scanning laser microscopy, allows for the accurate measuring of the bubble filling degree (Fv) of oil fluid inclusions as a function of the temperature [Pironon, J., Canals, M., Dubessy, J., Walgenwitz, F., Laplace-Builhe, C., 1998. Volumetric reconstruction of individual fluid inclusions by confocal scanning laser microscopy. Eur. J. Mineral. 10, 1143–1150]. These data, combined with measurements of homogenization temperature (Th), give us new constraints for characterizing individually hydrocarbon fluid inclusions. For this purpose, a new modeling tool, PIT (Petroleum Inclusion Thermodynamic), based on applied thermodynamics of natural oils, has been constructed for interpreting volumetric (Fv) and microthermometric (Th) measurements obtained on hydrocarbon fluid inclusions. The software allows us: (1) to reconstruct the paleo-thermobarometric trapping conditions of oils; (2) to model petroleum inclusion composition; and (3) to understand (Fv–Th) differences among a population of fluid inclusions in terms of various pre- or post-trapping processes (inclusion stretching, oil mixing, liquid/vapor de-mixing, oil leaching by a gas, etc.).

Calibration of methane analysis by Raman spectroscopy in H2O-NaCl-CH4 fluid inclusions

Abstract Calibration of the determination of CH 4 /H 2 O ratio using Raman spectroscopy is carried out using synthetic fluid inclusions at different NaCl concentration (0, 0.05, 0.66, 0.98, 1.00, 1.60, 2.25 and 3.5 m NaCl). Spectra of the stretching bands of methane and water in the aqueous phase were collected at variable temperatures up to a few degrees above the homogenisation temperature. The composition of the aqueous phase for temperatures below the homogenisation temperature was calculated with a computer program, using the model of Duan et al. [Geochim. Cosmochim. Acta 56 (1992) 1451]. Results show the dependency of the estimate of the CH 4 concentration on salinity: at constant CH 4 concentration, the CH 4 /H 2 O area ratio of Raman bands decreases with increasing salinity from 0 to 1.6 m and remains constant up to 3.5 m NaCl. The P – T projection of the isopleth of a natural fluid inclusion is deduced from the homogenisation temperature, its composition obtained from cryometry (mNaCl eq.) and Raman analysis (mCH 4 ) ratio. This methodology was applied to a sample from the Cave-in-Rock MVT deposit (fluorite–Pb–Zn district, southern Illinois, USA) presenting petroleum fluid inclusions associated with fluid inclusions of the system H 2 O–NaCl–CH 4 . Hydrocarbon isochore intersects the isopleth of the H 2 O–NaCl–CH 4 inclusions at the homogenization temperature, which validates this procedure.

Semi-quantitative FT-IR microanalysis limits: Evidence from synthetic hydrocarbon fluid inclusions in sylvite☆

Infrared microspectrometry is an essential technique for the in situ analysis of individual fluorescent hydrocarbon inclusions. In order to calibrate this technique alkane and benzene inclusions have been synthesized in sylvite crystals below 100°C at atmospheric pressure. We show that the infrared spectra obtained on pure alkane synthetic inclusions differ from equivalent composition standards. The n-alkane and cyclohexane inclusions exhibit an intense spectral deformation characterized by a broadening, a flattening, and a waving of the peak. This spectral deformation takes place in the C-H stretching and bending infrared regions and affects the CH2CH3 peak area ratio measurements. No spectral deformation occurs for benzene inclusions in the C-H stretching region. Raman analysis confirms that no change in composition occurs during trapping. When the dilution of alkanes in a solvent increases, the infrared spectral deformation observed for the inclusions disappears. This dilution effect is not observed for pure alkane standards. It is possible to approximate the alkyl chain carbon number (X) which is between the pure n-alkane standard value and the synthetic inclusion value: (AREA [∑CH2]/AREA [∑CH3] + 0.1)/0.27 < X < (AREA [∑CH2]/AREA [∑CH3] − 0.8)/0.09. These results can be applied to the natural environments taking into account the limits of the FT-IR (Fourier Transform-Infra Red) microanalysis of fluid inclusions.

CO2–H2S–COS–S8–AlO(OH)-bearing fluid inclusions in ruby from marble-hosted deposits in Luc Yen area, North Vietnam

Abstract Fluid inclusions have been studied in rubies from marble-hosted deposits in the Luc Yen mining district, North Vietnam. Raman and Infrared spectroscopy combined with microthermometry investigations on primary and secondary fluid inclusions provided evidence of CO 2 –H 2 S–COS–S 8 –AlO(OH)-bearing fluids with native sulphur and diaspore daughter minerals, without visible water. Diaspore appeared as a nonvisible film of 2–3 μm coating the wall of the fluid inclusion cavity and native sulphur was generally nucleated during Raman irradiation. The presence of diaspore and COS in the fluid inclusions indicates that water was present in the paleofluid. The mole fractions of H 2 O and CO 2 are around 10 −2 and the expected concentration of water in the fluid inclusions is in the 1–10 mol.% range. Crush-leach identified sulphates and chlorides that are assigned to the presence of anhydrite and Na–Ca–Cl salts found by SEM in the ruby crystals. The CO 2 -rich inclusions which do not coexist with an aqueous phase by immiscibility process demonstrate that ruby grew from this CO 2 -rich and water-poor fluid at equilibrium with Na–Ca–Cl salts. Thermal reduction of evaporitic sulphates based on an initial assemblage of anhydrite, calcite and graphite, originating from the metamorphism of organic matter is proposed to explain the original fluid chemistry of these marble-hosted rubies. The marbles acted as a closed system and the carbonic composition of the parent fluids in ruby indicates that aluminium can be transported in CO 2 -rich fluids at high P – T .

Characterization of smectite and illite by FTIR spectroscopy of interlayer NH+4 cations

Abstract FTIR spectroscopy has been applied to NH4+-exchanged dioctahedral clay minerals to determine the molecular environment of NH4+ and to quantify N concentration. FTIR under vapourpressure control, coupled with heating and freezing treatments has shown that NH4+ ion symmetry varies with the nature of clay minerals. NH4+ has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the Td symmetry group. The NH4+ -bending vibration is centred at 1450 and 1425 cm-1. The Si4+-Al3+ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH4+ tetrahedron which acquires a C2v symmetry. As a consequence, the Td -C2v transition can be used to characterize the smectite-illite transition. Quantification of NH4+ content per half unit cell is provided by nNH₄ = k[NH4]/[OH] where [NH4]/[OH] is the band area ratio of the NH4+ -bending vibration to the OH-stretching vibration. k = 1.1 for hydrated smectite, 0.9 for dehydrated smectite and 0.8 for illite or tobelite. The bending vibration of NH4+ is chosen for the calculation because it is not affected by superimposed contributions.

Individual characterization of petroleum fluid inclusions (composition and P-T trapping conditions) by microthermometry and confocal laser scanning microscopy : inferences from applied thermodynamics of oils

Abstract This paper proposes a new method to characterize individual oil-bearing fluid inclusions. It uses both the homogenisation temperatures measured by microthermometry, and the degree of gas bubble filling measured by confocal laser scanning microscopy, in conjunction with thermodynamic modelling for describing liquid–gas phase transitions and the volumetric behaviour of hydrocarbon mixtures. It is associated with a two-parameter (α, β) compositional model that describes the wide range of compositions of petroleums. We show that this method can give (1) useful estimations of the compositions and pressure–temperature entrapment conditions of oils in fluid inclusions, and (2) insights into the various processes that have affected these fluids either before entrapment (liquid–gas unmixing, gas leaching, mixing, etc.) or after (leakage, etc.).

Ultraviolet-fluorescence alteration of hydrocarbon fluid inclusions

Abstract Ultraviolet fluorescence emission spectra were recorded on natural and synthetic hydrocarbon fluid inclusions. Trapping and heating of oil inside the inclusions causes a red shift and a decrease of the fluorescence intensity. The thermal alteration of the fluorescence varies with the expansion of the inclusions. When the inclusions are expanded by non-isochore evolution with temperature, the fluorescence alteration is positive. The alteration of fluorescence with time induces irreversible changes in inclusions trapped in salt crystals: the intensity of the fluorescence emission and the red/green ratio increase. Analysis by FT-IR microspectroscopy was performed before and after u.v. treatment at 300°C for 4h. The preexisting conjugated ketones are totally consumed to produce aryl-ethers and carbon dioxide. However, the alteration of fluorescence is more sensitive to chemical changes than i.r. spectroscopy. There is no significant alteration for inclusions trapped in quartz crystals; no temporal alteration is detected, the thermal alteration cannot be recorded because of decrepitation, and the i.r. spectrum is masked by the spectrum of quartz. The alteration of the fluorescence of the inclusions depends both on the nature of the oil and on the nature of the host crystal. The alteration parameter can be used for the characterization of the oil inclusions and indication of oxidation state.

Fluorite deposits at Encantada–Buenavista, Mexico: products of Mississippi Valley type processes

Petroleum and aqueous fluid inclusions from the Encantada–Buenavista fluorite mineralized zone in northern Mexico were analyzed by microthermometry, UV fluorescence, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and Confocal Scanning Laser Microscopy (CSLM) to evaluate the geochemical evolution of the mineralizing fluids. Two-phase (petroleum or brine+vapor) and three-phase (petroleum+brine+vapor) inclusions are described. Aqueous and petroleum-rich inclusions commonly occur in the same plane. Vapor-decrepitated and stretched fluid inclusions are present. A low-salinity methane-saturated fluid and a high salinity-fluid with highly variable methane contents are recognized. H2S is not quantified but is always detected in close association with methane. Petroleum inclusions are of two types: a low methane petroleum fluid (20 mol%) with low Th (60 jC) and a petroleum fluid with a methane of content near 30 mol% and a Th of 90 jC. Pressure and temperature diagrams for the aqueous and petroleum inclusions show three main intersects that allow P–T–X reconstruction of fluid evolution at La Encantada–Buenavista. A CH4- and H2S-rich low-salinity brine was mixed with oil that migrated under hydrostatic conditions with a thermal gradient of 70 jC/km. The arrival and mixing of a high-salinity aqueous fluid produced overpressure to 300 bars. A return to hydrostatic conditions was accompanied by an increase in the thermal gradient. The brine related to the fluorite orebodies appears to have a genetic relationship with the brines reported from the Jurassic petroleum basins located west of the fluorite bodies and similarities with reported fluids from Mississippi Valley type deposits. It is interpreted that the fluorine-rich fluids migrated toward the platform margins during the mid-Tertiary (30 to 32 Ma) using extension zones related to Basin and Range tectonism. Mixing of two different brines was responsible for precipitation and mineralization. Heat from magmas, related to tectonic extension, caused decrepitation and changes in the shape of fluid inclusions near the contact zones. D 2003 Elsevier Science B.V. All rights reserved.

NIR FT-Raman microspectroscopy of fluid inclusions : Comparisons with VIS Raman and FT-IR microspectroscopies

Abstract The first Raman spectra of hydrocarbon inclusions using Fourier transform (FT) Raman microspectroscopy were obtained with a 1064 nm laser excitation in the near-infrared range (NIR FT-Raman). Some inclusions reveal the typical CH vibrational bands of organic compounds, but most of the inclusions that are fluorescent during visible Raman microspectroscopy (514 nm excitation) are still fluorescent in the NIR range. These Raman spectra are presented and compared to the conventional visible (VIS) Raman and FT-IR spectra. For spectra obtained on the same nonfluorescent inclusion, the signal/background ratio is lower in NIR FT-Raman than in VIS Raman. This ratio should be improved by application of more sensitive detectors. The increase of the power density (laser power/impact laser area) could be a future improvement in the limit of thermal background excitation and pyrolysis of the oils trapped in inclusions.