Origin of palaeo-waters in the Ordovician carbonates in Tahe oilfield, Tarim Basin

Constraints from fluid inclusions and Sr, C and O isotopes

K. Li, C. Cai*, H. He, L. Jiang, L. Cai, L. Xiang, S. Huang, C. Zhang

*Corresponding author for this work

    Research output: Contribution to journalArticle

    38 Citations (Scopus)

    Abstract

    Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on fracture-filling, karst-filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarim basin, NW China. The aim was to assess the origin and evolution of palaeo-waters in the carbonates. The initial water was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low-temperature calcite. The palaeoseawater was further buried to temperatures from 57 to 110°C, nonluminescent calcite precipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into the carbonate rocks was recorded in the first generation of fracture- and karst-filling dull red CL calcite with temperatures from <50°C to 83°C, low salinities (<9.0wt%), high Mn contents and high 86Sr/87Sr ratios from 0.7090 to 0.7099. During the early Permian, 87Sr-rich hydrothermal water may have entered the carbonate rocks, from which precipitated a second generation of fracture-filling and interparticle calcite and barite cements with salinities greater than 22.4wt%, and temperatures from 120°C to 180°C. The hydrothermal water may have collected isotopically light CO2 (possibly of TSR-origin) during upward migration, resulting in hydrothermal calcite and the present-day oilfield water having δ13C values from -4.3 to -13.8‰ and showing negative relationships of 87Sr/86Sr ratios to δ13C and δ18O values. However, higher temperatures (up to 187°C) and much lower salinities (down to 0.5wt%) measured from some karst-filling, giant, nonluminescent calcite crystals may suggest that hydrothermal water was deeply recycled, reduced (Fe-bearing) meteoric water heated in deeper strata, or water generated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or diagenetically altered connate water) with meteoric waters of late Permian age and/or later may have resulted in large variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeohighs.

    Original languageEnglish
    Pages (from-to)71-86
    Number of pages16
    JournalGeofluids
    Volume11
    Issue number1
    DOIs
    Publication statusPublished - Feb 2011

    Keywords

    • Carbon isotopes
    • Carbonates
    • Fluid inclusion
    • Fracture-filling
    • Hydrothermal water
    • Strontium isotopes
    • Tarim basin

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