Doubly terminated quartz crystals usually named âHerkimer Diamondsâ occur in fractures in. Late Jurassic-Early Cretaceous dolomitized limestones from the ...
P14
European Current Research on Fluid Inclusions (ECROFI-XIX) University of Bern, Switzerland, 17–20 July, 2007. Abstract Volume, p. 160
PVTx modelling of fluid inclusions in diamond quartz crystals from the Sierra Madre Oriental, Southeast Mexico Ramos-Rosique, Aldo*, Aldo Levresse, Gilles*, Tritlla, Jordi* and Jiménez Sandoval, Sergio** *Centro de Geociencias, UNAM, Campus Juriquilla, 76230 Juriquilla, Queretaro, Mexico **CINVESTAV, Libramiento Norponiente No. 2000, 76230 Juriquilla, Queretaro, Mexico
Doubly terminated quartz crystals usually named “Herkimer Diamonds” occur in fractures in Late Jurassic-Early Cretaceous dolomitized limestones from the petroliferous province of Cuenca del Sureste in southern Mexico´s Sierra Madre Oriental. The Sierra is a mountain belt formed during the Laramide Orogeny from Late Cretaceous – Early Tertiary. The main source rocks for organic matter in the oil fields are Tithonian shales and mudstones (Muir, 1938) Thermodynamic modelling of hydrocarbon inclusions was performed using the homogenization temperature (Th) measured by microthermometry, and the degree of gas bubble filling (gas vol. %) measured by confocal scanning laser microscopy (CSLM). For aqueous inclusions the method uses the homogenization and last melting temperature (Tm) in addition to the molar fraction of methane quantified by Raman microspectrometry (Pironon et al., 2002). The procedure reconstruct the isopleth and the isochore for both aqueous (Peng & Robinson, 1976) and hydrocarbon fluid systems (Thiery et al., 2000), and the intersection of the two isochores in a P-T diagram represent the best approach to the trapping conditions. Aqueous inclusions. Only one primary inclusion was observed present in petrographic association with hydrocarbon inclusions. Th = 135º C, Tm = 1.8º C, or 3.03 wt% eq. NaCl. Raman analyses indicate the presence of CH4, in a concentration of 0.3 molal. No CO2 has been detected. No aqueous inclusions are observed in fractures. Hydrocarbon inclusions. The Th of primary intra-crystalline inclusions shows a distribution ranging from 109 to 132º C with a maximum at 130º C. In fractures crossing the whole crystal, inclusions show a Th homogeneous distribution ranging from 116 to 120º C. CSLM volumetric reconstructions of both primary and secondary
160
hydrocarbon-bearing inclusions indicate a gas vol. % ranging from 10 to 20% and 8 to 15% respectively. There are numerous one-phase methane inclusions present in fractures, as indicated by the strong picks in Raman spectra that reveal the presence of this gas under high pressures. All these data has been used to model the PVTx conditions for quartz precipitation using the PIT software (Thiery et al., 2000). The PT minimum conditions for the hydrocarbon primary inclusions are 120º C and 180 bar. The minimum trapping conditions calculated for the aqueous inclusion is 135º C and 825 bar. The isochores of both systems will never intersect. So we can assume that they are not in equilibrium. The PT trapping conditions of the inclusions in fractures could not be determined due to the lack of aqueous inclusions. The minimum PT conditions are calculated at 120º C and 210 bar, which are very similar than those calculated for the intra-crystalline hydrocarbon inclusions, and confirm as the high methane density inclusions present in fractures, the post trapping re-equilibrium of the intra-crystalline hydrocarbon inclusions. Therefore we propose to consider the minimum crystalization PT conditions to be at least comparable with the aqueous inclusion PT minimum trapping conditions (135º C and 825 bar). REFERENCES Muir, J. M. (1938) The Science of Petroleum, vol. 1, Oxford Univ. Press Peng, D.Y. & Robinson, D.B., (1976) Industrial and Engineering Chemistry, 15, 59-64. Pironon, J., Grimmer, J.O.W., Teinturier, S., Guillaume, D., Dubessy, J., (2002). PACROFI VIII, Halifax, Canada, Abstract. Thiéry, R., Pironon J., Walgenwitz F, Montel F, (2000) Jour. Geochem. Exp. 69-70, 701-704.
