Origin of palaeo-waters in the Ordovician carbonates in Tahe oilfield, Tarim Basin: constraints from fluid inclusions and Sr, C and O isotopes

摘要

Petrographic features, isotopes, and trace elements were determined, and fluid inclusions were analyzed on frac-nture-filling, karst-filling and interparticle calcite cement from the Ordovician carbonates in Tahe oilfield, Tarimnbasin, NW China. The aim was to assess the origin and evolution of palaeo-waters in the carbonates. The initialnwater was seawater diluted by meteoric water, as indicated by bright cathodoluminescence (CL) in low-tempera-nture calcite. The palaeoseawater was further buried to temperatures from 57 to 110u0002C, nonluminescent calcitenprecipitated during the Silurian to middle Devonian. Infiltration of meteoric water of late Devonian age into thencarbonate rocks was recorded in the first generation of fracture- and karst-filling dull red CL calcite with tempera-ntures from <50u0002Cto83u0002C, low salinities (<9.0 wt%), high Mn contents and high 86nSr ⁄n87nSr ratios from 0.7090 ton0.7099. During the early Permian,n87nSr-rich hydrothermal water may have entered the carbonate rocks, fromnwhich precipitated a second generation of fracture-filling and interparticle calcite and barite cements with salini-nties greater than 22.4 wt%, and temperatures from 120u0002C to 180u0002C. The hydrothermal water may have collectednisotopically light CO2 (possibly of TSR-origin) during upward migration, resulting in hydrothermal calcite and thenpresent-day oilfield water having d13nC values from )4.3 to )13.8& and showing negative relationships ofn87nSr ⁄n86nSr ratios to d13nC and d18nO values. However, higher temperatures (up to 187u0002C) and much lower salinitiesn(down to 0.5 wt%) measured from some karst-filling, giant, nonluminescent calcite crystals may suggest thatnhydrothermal water was deeply recycled, reduced (Fe-bearing) meteoric water heated in deeper strata, or waterngenerated from TSR during hydrothermal water activity. Mixing of hydrothermal and local basinal water (or dia-ngenetically altered connate water) with meteoric waters of late Permian age and ⁄ or later may have resulted innlarge variations in salinity of the present oilfield waters with the lowest salinity formation waters in the palaeo-nhighs.