Methods

The Appendix summarises the INGV geochemical surveys during the Weyburn IEA-EC Project, from August 2000 to September 2004, for strontium isotope, major, minor and trace element and dissolved gas analyses. The laboratory analytical methods, applied to the liquid phase, were used after sampling the oil waters at the oil or gas well-head within collapsible, approx. 1-gal LDPE containers and allowing them to sit for a 1-2 minutes to achieve gravity separation of water and hydrocarbons.

• 87Sr/86Sr isotopic ratios were analysed from Sr dissolved in oil-water using standard mass spectrometry methods coupled with the following sample preparation technique (modified after Jones et al., 1994; Quattrocchi et al., 2003): 100 mL of sample is added to 50 mg of Na2CO3 to transform the sulphates (SrSO4, BaSO4) into carbonates. The final product of this reaction is filtered and washed to completely eliminate the sulphate ions. The residual on the filter was dissolved with 2.5 N HCl and the obtained solution was evaporated. The dry residue was dissolved with 2.5 N HCl and the strontium was separated from the other elements using a cation exchange resin (DOWEX resin type). This element is placed (as Sr(NO3)2) on a tantalium filament and the 87Sr/86Sr ratio is measured by a VG 54E mass spectrometer (University of Rome). The 87Sr/86Sr values were then normalised to the standard ratio of 0.1194. The analytical measurements are affected by an error expressed as 2 times the standard deviation (± 0,00002).

• The dissolved gas analytical method is based upon the partitioning of gases between the liquid and gas phase in the head-space of the sampler (Capasso & Inguaggiato, 1998). A Chrompack™ CP2002 portable gaschromatograph is used in the laboratory to analyse CO2, CH4, H2S, H2, N2, O2, and light hydrocarbons (C2-C4), while He is also analysed with an ALCATEL mass spectrometer. As the dissolved gases were not sampled at borehole P,T,[X] conditions, it is necessary to correct the analytical data to the reservoir pressure of 16 MPa (1300-1500 m), considering also kinetic effects. The degassing during the rise of fluids from 16.5 MPa (reservoir pressure) to 0.1 MPa (atmospheric pressure) can be calculated following certain principles (Chapoy et al., 2004; Tian et al., 2004; Renè Perez, personnel communication, work in progress). This can be done for each dissolved species, assuming a constant pump rate, sampling time and stripping effects before the immediate sampling of the oil waters at surface. At present we will only discuss the data gathered by sampling at 0.1 MPa pressure, as the "gross composition" of the reservoir evolution.

• Alkalinity is measured (U of C partner) by an ORION 960 titrometer (with 0.16M H2SO4) soon after the oil water sampling, considering HCO3 + CO3 and oily alkali.

• trace elements (partially discussed in this paper) were analysed by multi-component ELAN™ 6000 ICP for the following elements:, Be, B, Al, Si, Fe, Ni, Pb, Mn, Rb, Sr, Ag, Ba, Br, Cd, Cs, Cr, Cu, As, and Zn. Measurements are affected by an error expressed as 2 times the standard deviation.

• Major elements (partially discussed in this paper) are analysed by DIONEX HPLC; a charge imbalance of less than ± 5-10 % is considered acceptable.

• Geochemical Modeling, partially discussed in this paper (Cantucci B, PhD thesis in progress, INGV-University of Florence Italy 2004-2006), is accomplished by PHREEQC version 2.10 (Parkhurst and Appelo, 1999). The oil water composition is imposed both during kinetic evolution and in final equilibrium with the Vuggy and Marly rock compositions.

• Contour maps are created with the kriging technique within the program Surfer, while the "gross composition" of dissolved gases are discussed using normal probability plots (Sinclair, 1991).

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