AAPG Bulletin, V. 82 (1998), No. 12 (December 1998), P. 2246-2265.

Influence of Early Diagenesis on Reservoir Quality of Rotliegende Sandstones, Northern Netherlands¹

Joachim E. Amthor² and Jos Okkerman³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received April 3, 1997; revised manuscript received February 24, 1998; final acceptance March 18, 1998.
²Petroleum Development Oman, P.O. Box 81, Muscat, PC 113, Sultanate of Oman; e-mail: [email protected]
³Norsk Hydro E&P ASA, 5020 Sandsli, Bergen, Norway; e-mail: [email protected]

This study was conducted in the course of the Nederlandse Aardolie Maatschappij (NAM) integrated Rotliegende Task Force. We are grateful for information, discussion, and comments from the team members of the Rotliegende Task Force: Manfred Epting (project supervision), Johannes Walzebuck (project management), Tom Reijers and Mieke Kosters (sedimentology), Mark Ormerod (basin modeling), Richard Huis in’t Veld and Koos Pipping (petrophysics). The reviews by Karen Love, Paul Thayer, an anonymous reviewer, and by editor Neil Hurley are greatly appreciated. We thank the Nederlandse Aardolie Maatschappij (NAM) for permission to publish this paper. 

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.