Geochemical methods and examples for ascertaining corrosion fluid of palaeokarst oil-gas reservoir

摘要

With development of oil & gas exploration, the mechanism of palaeokarst forming have been attached importance to in the recent years. Macro and micro appraisal symbol of palaeokarst have been posed by technique of well drilling, logging, seismic and geological geomorphology. However, series research geochemical methods to identifying palaeokarst reservoir origin have not been established. A series of geochemical methods and examples for ascertaining corrosion fluid of palaeokarst oil-gas reservoir have been established in this paper. There are three types include 10 parameters, i.e. elements(MgO/Al2O3-MnO/TiO_2、Sr-Ba、B、rare earth elements、radio elements)、isotopes（C-O，Sr ，δD-δ18O ) and fluid inclusion(fluid inclusion homogenization temperature and salinity、fluid inclusion liquid anion）. The palaeo-corrosion-fluid type and reservoir origin might be ascertained by these parameters. With a large of experiment data and diagrams, the reservoir palaeo corrosion fluid origin of Tarim basin Ordovician analyzed. As the results, in northern Tarim basin, the mineral forming fluid of cave-filling mud and calcite was mainly meteoric -brackish water and cave-filling calcite possibly formed at shallow-medium burial periods. In central Tarim basin, the mineral forming fluid of cave-filling mud and calcite was mainly brackish-saline water, and cave-filling calcite possibly formed at medium-deep burial periods. And in central Tarim basin, it was in saline water environment at early Carboniferous sedimentary period; it was in meteoric water environment at early Hercynian exposure period; it just met saline line at Silurian sedimentary period; and it was in meteoric water environment at late Caledonian exposure period.