Diagenetic influence on reservoir quality evolution, examples from Triassic conglomerates/arenites in the Edvard Grieg field, Norwegian North Sea

Abstract

Abstract The Utsira High (Southern Norwegian North Sea) has been a petroleum exploration target since the late 1960s. The southwestern part of the Utsira High complex (Haugaland High) attracted new interest when in 2007 Lundin Norway AS and partners Revus Energy ASA and RWE Dea Norge AS discovered commercial hydrocarbon reservoirs in Triassic to Early Cretaceous formations (Edvard Grieg field). In the present study, four siliciclastic cores from the Edvard Grieg field have been investigated sedimentologically, including core logging, optical mineralogy, X-ray powder diffraction (XRD) and scanning electron microscope (SEM). Quantification and characterization of the different mineralogical phases in thin section have been executed to better understand porosity evolution through time. Intergranular volume (IGV) and cement volume were also compared in order to estimate the most important factors effecting porosity loss within the studied samples. The results show that through time the various diagenetic events and phases, such as dissolution of unstable grains and precipitation of clay minerals and cements, strongly influenced reservoir quality in the Edvard Grieg field. Two main lithology groups are identified within the cored intervals, conglomerates and sandstones. These two groups can further be sub-divided into four units: 1) sandy conglomerate, 2) silty conglomerate, 3) interbedded sandstone, 4) large scale cross-bedded sandstone. Most of the reservoir quality reduction in the conglomerate matrix can be related to poor sorting, carbonate cement and pore-filling/pore-lining authigenic clay minerals. Better reservoir quality characterizes the sandstone-dominated alluvial fan intervals, which generally are associated with a greater degree of sorting that led to higher intergranular porosities. Large scale cross-stratified sandstone units, interpreted to be of aeolian origin, have the highest reservoir quality. These deposits are moderately to well sorted with only minor occurrences of patchy carbonate cement and generally low proportions of authigenic minerals. However, locally abundant kaolinite has been observed, filling the pore space and reducing the porosity, predominantly by blocking interconnected primary pores.