Mineralogy, geochemistry and isotope geochemistry to reveal fluid flow processes in flooded chalk under long term test conditions for EOR purposes

BERTOLINO, S.; ZIMMERMANN U.; MADLAND, M.V; HILDEBRAND-HABEL, T.; HIORTH A.; KORSNES R.

Rio de Janeiro

Conferencia; XV International Clay Conference; 2013

AIPEA

In the search of an on-shore equivalent of reservoir rocks for hydrocarbon deposits of the North Sea we studied 5 chalks successions of similar ages (Campanian-Maastrichtian) using trace element geochemistry and C-O stable isotopes. The on-shore chalk (n=45) has been sampled in Kansas (USA), Liège and Mons (Belgium), Aalborg and Stevns Klint (Denmark) selected by comparable porosities to reservoir chalk. Earlier studies based on petrography, petrophysics and mineralogy (XRD) argued for Liège (Hjuler and Fabricius, 2009) as the most comparable chalk. Off-shore chalk was sampled from the Tor and Hod Formations at Ekofisk, Brynhild, Enoch, Sleipnir and Jotun fields. Off-shore samples show d18O values (n=42) mainly between -3.5 and -6 and are therefore more negative than secular values, while d13C are in the range of the values for primary Upper Cretaceous seawater. The latter accounts as well for sampled on-shore chalk. In contrast, all on-shore samples, but some from Kansas, show primary d18O values. Rare earth element concentrations (REE) in on-shore chalk show ranges of Y/Ho ratios between 34 and 52 or even as high as 60 (Mons). REE pattern are similar to typical seawater. The clastic input (measured by Zr and Rb concentrations) is the lowest at Mons and Stevns Klint («1%), while samples from Kansas are very variable («1-3%), and the highest in chalk from Aalborg, which may have disturbed REE pattern and decreased Y/Ho ratios. Clastic input in off-shore chalk is comparable besides very few samples. But Y/Ho ratios are generally lower in the latter with only few samples higher than 40 and REE patterns are enriched in middle REE in comparison to typical seawater, and the on-shore samples (Nozaki et al., 1997). XRD analyses of the clay minerals separated by dissolution from several kg of chalks from Kansas, Liège and Mons showed significant differences on their mineral assemblages: Kansas contains illite/smectite (R1 70%illite) ? kaolinite > illite; Liège is dominated by smectite with minor illite, opal-C, zeolites (analicime or mordenite), quartz and traces of kaolinite while Mons has poorly ordered illite/smectite (70%illite) and illite along with abundant calcite, minor quartz and scarce analcime. These assamblages suggest that the chalks were subjected to different diagenetic conditions. Therefore we can conclude that the clay minerals found in the chalk of Kansas display significant higher diagenetic grades than those in the chalk of Central European chalk. This would point to a different diagenetic history, which might have affected the O-isotope composition. Combining all results from the differnet applied methods, we can state that off-shore chalk has been affected by either higher diagenetic overprint than the European comparable chalk deposits or/and by a different secondary fluid flow, which disturbed REE concentrations and d18O values, but not by a higher clastic input. This shows that further geological studies are necessary to understand reservoir chalk when testing on-shore chalk.