MODELING OF CHALK DIAGENESIS (ELDFISK FIELD, NORWEGIAN NORTH-SEA) USING WHOLE ROCK AND LASER ABLATION STABLE ISOTOPIC DATA

Laser ablation techniques permit the determination of the stable isotopic ratios of finely crystalline calcite cements in chalks for the first time. Modelling of fluid-rock interaction using whole rock and laser ablation stable isotopic data indicates that carbonate mineral diagenesis in the Eldfisk Field consisted largely of the dissolution and reprecipitation of calcite with little associated loss of porosity. Cementation by calcite derived from stylolites apparently occurred throughout the Eldfisk Field chalk, but had only a subsidiary effect on whole rock isotopic ratios. Oxygen isotopic data indicates a pore water temperature of 50-80-degrees-C during the bulk of chalk recrystallization. Increases in whole rock delta-C-13 values with depth are likely the result of bacterial methanogenesis during chalk recrystallization.