Diagenesis and its controls on reservoir quality of the Tambar oil field, Norwegian North Sea

A study of the diagenetic evolution of the Late Jurassic sandstones of the shallow marine facies in the Tambar oil field, Norwegian North Sea was carried out to understand its controls on reservoir quality. Core samples and a set of wireline logs obtained from three wells were used to carry out petrographic studies including light optics and scanning electron microscopy, well log analysis, fluid inclusions, X-ray diffraction (XRD) for whole rock samples, clay mineral extracts and stable isotopes. The Tambar reservoir sandstones range mainly from siltstone to fine-grained sandstones, exclusively arkoses cemented mainly by microcrystalline quartz, euhedral quartz overgrowth, dolomite cement, illite and chlorite. Early diagenetic/eodigenetic minerals include pyrite, calcite, and microcrystalline quartz and late diagenetic/mesodiagenetic minerals include quartz overgrowth, dolomite, illite and chlorite. The δ18O values of dolomite cement in the Tambar sandstone reservoir range from −11.77‰ to −3.57‰ PDB, while the δ13C values of the dolomite cement are from −5.07‰ to −1.12‰ PDB. Homogenization temperature for fluid inclusions trapped in authigenic quartz fall between 130 °C and 169 °C. The reservoir quality is controlled mainly by early formed grain-coating microcrystalline quartz that precipitated from the dissolution of sponge spicules which significantly inhibits quartz overgrowth precipitation. Oil emplacement do not show significant control on reservoir quality because considerable quartz cementation has taken place before the onset of major oil charge. Only the coarse-grained sands show a correlation between quartz cement volume and water saturation. However, the interpreted temperature for dolomite cementation is significantly higher for water leg-suggested effect of oil emplacement on the late dolomite. Understanding these controls on the reservoir quality will enhance the exploration strategy for the Tambar oil field. [Display omitted] •Dominant detrital minerals are monocrystalline quartz, feldspar and polycrystalline quartz.•Microcrystalline quartz inhibits quartz cementation.•Overgrowth of microcrystalline and syntaxial, euhedral quartz on authigenic phase. .•Reservoir quality is mainly controlled by quartz overgrowth and microcrystalline quartz.