Inversion of reservoir quality by early diagenesis: an example from the Triassic Buntsandstein, offshore the Netherlands

Abstract

Sandstones of the Triassic Main Buntsandstein form a major gas reservoir in the Netherlands offshore. The sequence is dominated by siliciclastics deposited in a semi-arid continental setting, and includes dune, interdune, sheetsand and fluvial sandstones. Reduction in reservoir quality is caused primarily by dolomite, halite and anhydrite cementation, with minor authigenic illite and chlorite. Integration of petrographie and isotopic data has allowed the origins and relative timing of the different cements to be constrained. The carbon and oxygen isotopic composition of dolomite (δ13C = −3 to +2.9‰ PDB, δ180 = −3.7 to −9.3‰ PDB) combined with strontium isotopic data (0.7091 to 0.7109 86Sr/87Sr) suggests that it precipitated from meteoric groundwater. Halite and anhydrite formed from a mixture of meteoric water and saline fluids expelled from underlying evaporites and claystones. Sulphur isotopic data (δ34 S = +4.2 to +12.1‰ CDT) support this interpretation for the origin of the anhydrite. Precipitation of the major authigenic minerals occurred during early diagenesis, prior to burial depths of 500 m. Cementation and groundwater flow preferentially followed the zones of highest permeability and caused an inversion of reservoir quality. Sandstones with the highest depositional porosity and permeability (i.e. dune sandstones) are the most cemented, and have poorer reservoir quality than the fluvial and interdune sandstones which originally had lower depositional porosity and permeability. Formation of authigenic illite and chlorite occurred during burial and has significantly reduced permeability further.