Petrological, Mineralogical and Geochemical Constraints on Hydrocarbon Bearing North Sea Reservoir Chalk

Abstract

Summary A study of the geochemistry and petrology characterizing the North Sea reservoir chalk is central in the efforts of refining or developing new Increased Oil Recovery (IOR) methods, as it provides an insight in the chemical composition, mineral structures and textures of the reservoir rock and the grounds for a pilot test within the National IOR Centre of Norway. The study is based on cores collected over a decimeter-scale, under different flooding status, from unflooded to waterflooded at lower or higher temperatures, swept and unswept regions from the Tor and Ekofisk formations directly sampled from the Ekofisk field. Optical petrography shows a very fine, micritic carbonate matrix, with various microfossils such as calcispheres, foraminifers, or sponge spicules. SEM micrographs reveal post-depositional calcite precipitation inside the calcispheres, sometimes entirely cementing their cavities. The amount of clay minerals observed with SEM varies and there is a clear decrease in porosity stratigraphically downwards, along with more cementation and compaction. X-ray diffraction confirms calcite as most abundant in the whole-rock composition, with quartz and few other non-carbonate minerals like smectite, illite and kaolinite present. The silica content varies highly from <2 wt% in the shallower cores to 6 – 8 wt% in areas close to tight zones and up to 11 wt% in the deeper cores. δ13C and δ18O are lower than the secular global isotopic values for this period. Since similar disturbed stable isotope values are seen in other hydrocarbon-rich samples unexposed to any fluid for IOR purposes, the disturbance is assigned to a post-depositional diagenetic overprint, or to the influence of a secondary fluid of unknown origin, rather than the effect of the cores’ flooding status. Given the compositional variety of the Ekofisk reservoir rocks, selecting a single on-shore exposure as a standard equivalent for the Ekofisk chalk would be problematic. The complexity of the reservoir chalk and consideration of many other IOR influencing parameters, compel caution when transferring results from the onshore chalk modeling to the reservoir chalk (e.g. Hjuler and Fabricius, 2009 ). Beside the mineralogical composition of chalk strongly influencing compaction, the palaeo-environmental conditions at the time of deposition, the diagenetic history, calcite recrystallization and fossil preservation may affect the strength of the rock. Hence, a further thorough geological study on the reservoir chalk is necessary to verify the prospect of comparisons based on geological grounds.