Inversion of Reservoir Quality: An Example from the Triassic Buntsandstein, Offshore Netherlands

Abstract

Sandstones from the Triassic Main Buntsandstein form a major gas reservoir in the Netherlands offshore. The sequence is dominated by siliciclastics deposited in an arid continental setting, and include 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 petrographic 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 ([delta][sup 13]C = -3.76 to -9.3%, [delta][sup 18]O = -3 to +2.9% PDB) combined with strontium isotopic data (0.7091 to 0.7109 [sup 86]Sr/[sup 87]Sr) suggest that is precipitated from meteoric groundwater. Halite and anhydrite formed from a mixture of meteoric water and saline fluids expelled from underlying evaporites and claystones. Sulfur isotopic data (+4.2 to +12.1 CDT) support6 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 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 poorermore » reservoir quality compared to the fluvial and interdune sandstones, which originally had lower depositional porosity/permeability. Formation of authigenic illite and chlorite occurred during burial and has significantly reduced permeability. Information on the depositional settings and paleogeography, combined with expected groundwater flow, has helped define potential exploration areas of reduced reservoir quality as a result of extensive early cementation.« less