Diagenesis and reservoir quality of Triassic sandstones in the Central Graben, Norwegian sector : A mineralogical, petrographical and petrophysical approach

Abstract

This study examines the diagenesis and reservoir quality of Triassic sandstones in the Central Graben, Norwegian sector. Cored reservoir intervals from four wells (6/3-1, 7/8-4, 7/11-8, 7/11-9) have been examined by petrophysical- (wire line logs, cross plots), petrographical- (SEM-BSE, SEM-SEI) and mineralogical analysis (XRD, modal analysis). The sandstones are fine grained moderately sorted subrounded to subangular with an average subarkosic composition (Q85F11R4). A wide range of diagenetic minerals has been identified: quartz, chlorite, smectite, kaolin, illite, dolomite, calcite, ankerite and pyrite. Intergranular volume – quartz cement relationships have been used to predict the effect of grain size, grain shape, compaction (mechanical and chemical) and grain coating on compaction. An increase of 7% average quartz cement in sandstones between well 6/3-1 (2,9 – 3.1 km) and 7/8-4 (4,1 km) is caused by the temperature difference of about 40ºC. Kaolin is found in the shallow well (6/3-1). Kaolin could also have been present in the deeper wells (7/8-4, 7/11-9, 7/11-8) but may have been dissolved and replaced by illite. More quartz cement is found in very fine grained sandstones than in medium grained sandstones, which is related to higher surface area for quartz cementation. Early carbonate cement common and may be an important fluid flow barrier. Mapping the distribution of carbonate cement was done by comparing XRD data together with petrophysical data and interpreting log responses. The distribution of carbonate cement is facies dependant within floodplain associated facies with most likely pedogenic origin. Authigenic iron rich chlorite occurs as both pore-filling and grain coats. High porosities are found in sandstone with minor amount of chlorite typical of grain coating chlorite, thus grain coating chlorite has most likely prevented precipitation of quartz overgrowths. Authigenicg illite reduces the permeability by pore-bridging. Morphological evidence suggests smectite to the precursor of both chlorite and illite. Authigenic kaolin in the Upper Triassic channel assicated facies indicates leaching due to meteoric water flow in permeable rocks. Presence of detrital kaolin in combination with authigenic kaolin suggests a more humid climate towards the end of Triassic and into the Jurassic.