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Fluid flow record from fracture-fill calcite and its relationship to oil shows (Upper Jurassic limestones, Maestrat Basin, Iberian Range)
Journal of Geochemical Exploration 101 (2009) 17
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Fluid flow record from fracture-fill calcite and its relationship to oil shows (Upper Jurassic limestones, Maestrat Basin, Iberian Range) M.A. Caja a,⁎, A. Permanyer b, R. Salas b, R. Marfil a a b
Departamento de Petrología y Geoquímica, Facultad C.C. Geológicas, Universidad Complutense de Madrid, Avda. Jose Antonio Novais s/n, 28040-Madrid, Spain Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona. Martí i Franquès, s/n. 08028-Barcelona, Spain
The presence of oil shows associated with partially cemented fractures allowed the study of the relationships between fluid flow and oil migration through fractures. The carbonate-cemented tectonic fractures and oil shows occur in the Upper Jurassic limestones of the Maestrat Basin (Iberian Range, Spain). The studied samples correspond to an onshore oil exploration well (i.e. Maestrazgo-1), located in the Salzedella sub-basin which represents the main sedimentary depocenter and contains the source rock (Ascla Fm) of the Mediterranean Amposta oil field. Fractures were partially cemented by Mg-calcite cement (up to 3.6% mol MgCO3). Primary aqueous fluid inclusions show low to moderate homogenisation temperatures (45 to 117 °C) and low salinity (4.5 to 6% wt. eq. NaCl). Two-phase (vapor and liquid) oil fluid inclusions, showing yellowish fluorescence colors under ultraviolet light (365 nm), were identified in the fracture filling calcite cement. Fluid inclusions occur as long trails, parallel to the fracture margins which suggest a primary origin. However, in the same fractures yellow fluorescence trails of oil fluid inclusions perpendicular to the fracture margins were observed, which may indicate a secondary origin after calcite precipitation. Stable isotope analyses revealed δ18O ratios from −9.2 to −4.6‰ VPDB, positive δ13C ratios from +1.5 to +2.0‰ VPDB. Minor amounts of fluorite cement/replacement
after Mg-calcite were observed. Oil shows are occupying the remaining porosity in the fracture and are postdating both cements. GC-MS of the saturated hydrocarbon fraction of oil shows associated to the partially cemented fractures shows an n-alkanes distribution dominated by medium weight n-alkanes in the n-C16 to n-C18 range, with n-C17 predominant over n-C16 and n-C18. Moreover, an even predominance is systematically observed in the n-C20 to n-C28 range. The pristane/ phytane ratio is commonly ≤ 1 indicating a reducing carbonate environment of the source rocks. Thus, the fluids involved in the formation of carbonate cements were slightly Mg-rich with low salinity, low to moderate temperature, isotopically lighter than the host rock and related to marine-derived waters and compactional fluids. The precipitation of fluorite suggests major changes in fluid chemistry and/or temperature after carbonate precipitation. For this reason, the precipitation of fluorite is commonly considered a good marker for oil migration. Thus, the studied fracture filling calcite cements recorded the migration of oil from the lowermost organic rich shale levels toward the uppermost carbonates of the Ascla Fm. The oil migration distance seems to be relatively short, approximately 50– 100 m. Oil migration was focused through fractures coevally with precipitation of calcite cement and continued after cementation.
⁎ Corresponding author. E-mail addresses: [email protected] (M.A. Caja), [email protected] (A. Permanyer), [email protected] (R. Salas), marfi[email protected] (R. Marfil). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.005
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Fluid flow record from fracture-fill calcite and its relationship to oil shows (Upper Jurassic limestones, Maestrat Basin, Iberian Range) M.A. Caja a,⁎, A. Permanyer b, R. Salas b, R. Marfil a a b
Departamento de Petrología y Geoquímica, Facultad C.C. Geológicas, Universidad Complutense de Madrid, Avda. Jose Antonio Novais s/n, 28040-Madrid, Spain Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona. Martí i Franquès, s/n. 08028-Barcelona, Spain
The presence of oil shows associated with partially cemented fractures allowed the study of the relationships between fluid flow and oil migration through fractures. The carbonate-cemented tectonic fractures and oil shows occur in the Upper Jurassic limestones of the Maestrat Basin (Iberian Range, Spain). The studied samples correspond to an onshore oil exploration well (i.e. Maestrazgo-1), located in the Salzedella sub-basin which represents the main sedimentary depocenter and contains the source rock (Ascla Fm) of the Mediterranean Amposta oil field. Fractures were partially cemented by Mg-calcite cement (up to 3.6% mol MgCO3). Primary aqueous fluid inclusions show low to moderate homogenisation temperatures (45 to 117 °C) and low salinity (4.5 to 6% wt. eq. NaCl). Two-phase (vapor and liquid) oil fluid inclusions, showing yellowish fluorescence colors under ultraviolet light (365 nm), were identified in the fracture filling calcite cement. Fluid inclusions occur as long trails, parallel to the fracture margins which suggest a primary origin. However, in the same fractures yellow fluorescence trails of oil fluid inclusions perpendicular to the fracture margins were observed, which may indicate a secondary origin after calcite precipitation. Stable isotope analyses revealed δ18O ratios from −9.2 to −4.6‰ VPDB, positive δ13C ratios from +1.5 to +2.0‰ VPDB. Minor amounts of fluorite cement/replacement
after Mg-calcite were observed. Oil shows are occupying the remaining porosity in the fracture and are postdating both cements. GC-MS of the saturated hydrocarbon fraction of oil shows associated to the partially cemented fractures shows an n-alkanes distribution dominated by medium weight n-alkanes in the n-C16 to n-C18 range, with n-C17 predominant over n-C16 and n-C18. Moreover, an even predominance is systematically observed in the n-C20 to n-C28 range. The pristane/ phytane ratio is commonly ≤ 1 indicating a reducing carbonate environment of the source rocks. Thus, the fluids involved in the formation of carbonate cements were slightly Mg-rich with low salinity, low to moderate temperature, isotopically lighter than the host rock and related to marine-derived waters and compactional fluids. The precipitation of fluorite suggests major changes in fluid chemistry and/or temperature after carbonate precipitation. For this reason, the precipitation of fluorite is commonly considered a good marker for oil migration. Thus, the studied fracture filling calcite cements recorded the migration of oil from the lowermost organic rich shale levels toward the uppermost carbonates of the Ascla Fm. The oil migration distance seems to be relatively short, approximately 50– 100 m. Oil migration was focused through fractures coevally with precipitation of calcite cement and continued after cementation.
⁎ Corresponding author. E-mail addresses: [email protected] (M.A. Caja), [email protected] (A. Permanyer), [email protected] (R. Salas), marfi[email protected] (R. Marfil). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.005