Influence of Early Diagenesis on Reservoir Quality of Rotliegend Sandstones, Dutch On- and Offshore

Joachim E. Amthor, Jos Okkerman

Reservoir quality in Rotliegend reservoirs (Lower Permian) is significantly affected by early-diagenetic processes which are related to sedimentary facies in marginal desert-sabkha environments. In the past, burial-related products and processes have received more attention than early-diagenetic processes which has resulted in an under-appreciation of the effects that early diagenesis can have on reservoir quality and well productivity.

Groundwater-related cementation is one of the major processes affecting Rotliegend reservoir rocks in the northern Netherlands. The precipitation of the main porosity occluding cements dolomite, anhydrite and quartz occurred very early in the diagenetic history. Oxygen isotopes of dolomite (^dgr¹⁸O = 20.4 to 28.4 ^pmil SMOW) and quartz cements (^dgr¹⁸O = 18.4 to 23.4 ^pmil SMOW) suggest that meteoric water has played a major role throughout the diagenetic history and that precipitation of these cements occurred at low temperatures (20-60° C. The strontium isotopes of dolomite cements (⁸⁷Sr/⁸⁶Sr = 0.70935 to 0.71387) and the sulphur isotopes of anhydrite cements (^dgr³⁴S = 6.7 to 9.7 ^pmil CDT) rule-out Permian or younger seaw ter 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 very close to the surface show the highest amount of dolomite and anhydrite cements, whereas dry environments (e.g., dune sandstones) with relatively deep water tables are characterised by low abundances.

The presence of these early-diagenetic cements strongly influences the porosity patterns with dry environments having the least amounts of early cements and better reservoir properties. Subsequently, burial-related processes have modified but not significantly altered these early-diagenetic patterns. The implication for exploration and production is that the effects of early diagenesis on reservoir properties and well productivity may be predictable by mapping sedimentary facies units.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995