Sedimentology, petrology, and diagenesis of core 16/1-13 from the Edvard Grieg Field, Utsira High, Norwegian North Sea

Abstract

Sedimentary successions of Permo-Triassic to Early Cretaceous age have been studied in core 16/1-13 from the Edvard Grieg Field, located on the Utsira High, in the North Sea. Sedimentary field data combined with petrographical and mineralogical studies (XRD, thin sections, SEM and EMP) have been used to interpret the depositional environment and to gain knowledge about the diagenetic history of the sediments. The results obtained from this master thesis, will provide information about the properties of the sediments, and can be used in later work on the porosity and permeability history in the Edvard Grieg Field. It is important to point out that the results obtained in this master thesis, only derive from the study of one core, and that the results are derived from the data obtained during this half-year thesis. Further studies, from the same core and adjacent cores, should be conducted for a quality check of the results found during this study. The studied section, core 16/1-13 consists of sediments from the Permo-Triassic and the Early Cretaceous, displaying depositional environments within different alluvial and marine regimes. An alluvial fan and an aeolian depositional environment represent the Permo-Triassic sedimentary succession, while marine carbonates represent the sediments deriving from the Early Cretaceous. Emphasis has been on the sediments of Permo-Triassic age, where the diagenetic history contains several phases of authigenic clay mineral formation, Fe-oxide precipitation, authigenic overgrowths and the precipitation of carbonate cement. The sediments from the alluvial fan and the aeolian regimes are found to carry similar diagenetic histories. The alluvial fan section contains debris flow successions, represented by conglomerates, displaying eroded basement clasts and matrixes originating from reworked paleo-weathering profiles located on the Utsira High. The study reveals that the basement clasts have experienced both sub-aerial weathering and alteration connected to diagenesis. The clay minerals present in the conglomeratic matrixes, are all found to have a diagenetic origin, deriving from the alteration and degradation of feldspars and micas during burial (neoformation), or from recrystallization of detrital clay (regeneration). Multiple layers of clay coating and pore filling clays are found. Differences in degree of weathering and original mineralogy of basement clasts, together with varying alteration of feldspars, points to at least two provenance areas for the alluvial fan sediments. The sediments from the alluvial fan, are in thin section, found to display relatively low porosity values. The aeolian sediments display higher porosities, compared to the alluvial fan sediments, but the aeolian sandstones are found to have a higher content of authigenic kaolinite and sparitic carbonate cement. These authigenic minerals are found to reduce the porosities, especially in the medium grained sandstones. The study reveals that the sediments of core 16/1-13, have been affected by early diagenetic processes related to the sedimentary facies in alluvial environments. The diagenetic influence has to some extent reduced the porosity, and may have influenced the present day reservoir quality. The sediments from the aeolian regime are found to have the highest porosity values, giving these sandstones good reservoir quality.