Abstract
A first and detailed study of the geochemistry and mineralogy characterizing the North Sea reservoir and non-reservoir chalk is provided in this work. The study is based on 185 cores from exploration and development wells in the North Sea. The cores related to reservoir development have different flooding status – unflooded or waterflooded at various temperatures – and are directly sampled from the Ekofisk field. Optical petrography shows a micritic carbonate matrix, with grains represented by various microfossils such as foraminifers and sponge spicules. Scanning electron microscopy (SEM) reveals post-depositional calcite precipitation and cementation. Dolomite is found only in the reservoir samples, but it is discussed as a diagenetic feature, unrelated to the hydrocarbon content or EOR exposure. The non-carbonate minerals observed with BSE-SEM and XRD include mostly quartz but also smectite, illite, kaolinite, mica, and pyrite. The abundance of clastic input varies, and there is a clear decrease in porosity stratigraphically downwards, with stronger cementation and higher compaction. δ 13 C reflects primary trends for Upper Cretaceous stages while δ 18 O in all samples is lower than the secular global isotopic values for this period. However, the δ 18 O values are not sufficiently low to imply a strong diagenetic overprint, but rather suggest the influence of a secondary fluid. This fluid cannot be a hydrocarbon-rich one, nor EOR fluids, as non-reservoir samples, as well as flooded and unflooded reservoir samples show very similar stable isotope values. • Mineralogical and geochemical characterization of broad North Sea chalk sample set. • Comparison between flooded and unflooded reservoir chalk and non-reservoir chalk. • The diagenetic overprint is low in all lithotypes. • The presence of hydrocarbons is not responsible for geochemical alterations. • The geochemical effect of EOR flooding of reservoir chalk is not obvious.
Highlights
Carbonate reservoirs hold significant amounts of the hydrocarbon reserves worldwide
There is no visible distinction between reservoir (Fig. 2, left column) and nonreservoir samples (Fig. 2, right column) nor between flooded and unflooded samples in terms of texture and fabric
Corroborated results from X-ray diffraction (XRD), BSE-Scanning electron microscopy (SEM)-energy dispersive system (EDS) and geochemistry indicate the presence of other non-carbonate min eral phases like illite, smectite, kaolinite, which confirms a significant clastic input
Summary
Carbonate reservoirs hold significant amounts of the hydrocarbon reserves worldwide. Most of the largest Norwegian carbonate reservoirs are found in the North Sea, an intracratonic basin on the Norwegian Continental Shelf (NCS) formed as the result of several major tectonic events between the Devonian and Late Jurassic. Since the discovery of the Ekofisk field in 1969, the chalk play re mains among the most prolific hydrocarbon resources in the North Sea, Ekofisk field alone accounting for approximately 10% of the produced net oil equivalents on the NCS. After the primary oil recovery, the initiation water injection program on Ekofisk in 1987 the seawater has been remarkably efficient for oil recovery, leaving water flooded zones with irreducible oil saturation of around 30%. Even with this good recovery already achieved, the amount of the resources left behind in the flooded zones is significant, due to the size of the reser voirs. This rose the motivation for studying the factors that govern this oil replacement
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