Abstract
Fracture-filling calcite and quartz from the Lopra-1/1A well (at 2380 m and 3543 m depth) contains both aqueous low-salinity fluid inclusions and hydrocarbon-dominated fluid inclusions. Microthermometry indicates that the aqueous fluids contain 0.2 to 1.4 equivalent weight% NaCl and occasionally contain traces of hydrocarbons. Homogenisation to liquid occurred between 90°C and 150°C. Modelling based on these fluid inclusion observations indicates that during burial the basaltic section was subjected to temperatures of 160°C and 170°C, occasional pressures of 600–700 bars and the simultaneous percolation of aqueous and hydrocarbon fluids. These fluid conditions may also be relevant to the formation of zeolite observed in the Lopra-1/1A well.
Highlights
Types and setting of fluid inclusions in selected samplesThe samples were selected by examining about 40 thin sections taken between 2204 m and 3543 m depth in the Lopra-1/1A well
Fracture-filling calcite and quartz from the Lopra-1/1A well contains both aqueous low-salinity fluid inclusions and hydrocarbon-dominated fluid inclusions
A reconnaissance study was undertaken of fluid inclusions in vug- and fracture-filling quartz and calcite from samples taken from the basalts penetrated by Lopra1/1A
Summary
The samples were selected by examining about 40 thin sections taken between 2204 m and 3543 m depth in the Lopra-1/1A well. From 2380 m and m depth, were found to contain fracture-filling quartz and calcite with fluid inclusions suitable for further study. Sample 2380 m (Lopra-1, core 1) is a sparsely plagioclase-glomerophyric olivine-clinopyroxene basalt with almost complete alteration of plagioclase and olivine. The quartz and calcite studied occur in mm-wide veins. The veins are rimmed by chlorite, calcite and quartz that appear to have been precipitated contemporaneously. Sample 3543 m (Lopra-1A, sidewall core 1) is a nearaphyric lapilli-tuff with extensively altered plagioclase, olivine and clinopyroxene phenocrysts in a cryptocrystalline groundmass. The irregular veins contain laumontite, prehnite, calcite and rare quartz. The fluid inclusions are characterised by immature morphologies and occur in irregular groupings or in curved internal planar arrangements, suggesting periods for their entrapment which do not markedly postdate the growth of the host mineral. All inclusions indicative of having been influenced by necking down were avoided during the heating and freezing stage work
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