Influence of Early Diagenesis on Reservoir Quality of Rotliegende Sandstones, Northern Netherlands

Abstract

Reservoir quality of Rotliegende sandstones (Lower Permian) in the northern Netherlands was significantly affected by early diagenetic processes that are related to sedimentary facies in marginal desert/playa lake environments. In the past, burial-related products and processes have received more attention than early diagenetic processes, neglecting the effects that early diagenesis can have on reservoir quality and well productivity of Rotliegende sandstones. Shallow groundwater-related diagenesis is a major factor affecting Rotliegende reservoir sandstones. The precipitation of the main porosity-occluding cements (dolomite, anhydrite, and quartz) occurred early in the diagenetic history. Oxygen isotopes of dolomite (d18O = 18.4 to 23.4‰ SMOW) suggest that meteoric water has played a major role throughout the diagenetic history, and that precipitation of these cements occurred at low temperatures (<80°C). The strontium isotopes of dolomite cements (87Sr/86Sr = 0.70935-0.71387) and the sulfur isotopes of anhydrite cements (d34S = 6.7 to 9.7‰ CDT) rule out Permian or younger seawater as a source for these cements, and are consistent with formation from meteoric water within a continental basin. An early diagenetic origin of the cements is also suggested by the fact that the majority of sandstone samples have lost more porosity due to cementation than through compaction. Most samples with high (<20%) amounts of porosity loss via cementation were affected by early dolomite and anhydrite. The relative abundance of dolomite and anhydrite cements can be correlated with depositional environments. Wet depositional environments (e.g., interdune and fluvial sandstones) with water tables close to the surface show the highest amount of dolomite and anhydrite cements, whereas dry environments with relatively deep water tables (e.g., dune sandstones) are characterized by low amounts of cements. The precipitation of early diagenetic cements has strongly influenced the present-day porosity patterns, with depositional environments interpreted as drier showing less cements and significantly altering the porosity patterns formed during early diagenesis. Knowledge of the influence of sedimentology and paleogeography on the diagenetic patterns is a key element for improved understanding and prediction of reservoir quality in the Rotliegende sandstones of the northern Netherlands.