Geological constraints on Cretaceous reservoir rocks from the North Sea

KALLESTEN E.; GOMEZ I.; MORALEDA, L. R; ZIMMERMANN, U., ; MADLAND, M.V.; BERTOLINO, S.R.A.

Stavanger

Congreso; 31st Geological Winter Meeting, NGF,; 2015

Geological Society of Norway

'Reservoir chalk' is chalk, which has been deposited in the North Sea and is related to hydrocarbon reservoirs. This chalk (porosity around 40 %) is associated with chalk of lower porosities, which is devoid of massive hydrocarbon reservoirs. To study enhanced oil recovery (EOR), we test on-shore chalk in rock mechanical experiments and try to understand the chemomineralogical changes, which affect rock mechanical parameters. Hence, the ideal on-shore analogue to 'reservoir chalk' needs to be identified, a work which is still in progress. This implies that 'reservoir chalk' is defined as a specific rock type, which is yet not.Therefore we developed this study to investigate in detail geological characteristics of 'reservoir chalk' and associated off-shore chalks to test if on-shore chalk can be a valid substitute at all. We will study the reservoir rocks with thin sections, SEM methodology, X-ray diffraction on chalk and the non-carbonate phases in the chalk, whole-rock geochemistry, MLA, C-0-Sr isotopes, to characterize the material at best for comparison. The same amount of data is available for the on-shore chalk deposits we use for EOR experiments. Samples for this study had been taken from the Ekofisk area (Tor and Ekofisk formations; Maastrichtian to Paleocene) containing hydrocarbons in different amounts, from the Hod Formation (midTurnnian to Campanian) and the confonnably overlying Tor Formation (mainly Maastrichtian) at Ekofisk and in other areas. The Hod Formation consists of grey, partly pink to red, fine-grained argillaceous chalky limestones and varies in its thickness between 200 and 700 m. Most of the chalk is laminated and burrowed, but grainstone turbidites are common. Porosity and penneability vary and do not correlate always to depth of burial. First geochemical results show for major elements elevated concentrations in Cao (< 50%), while silica concentrations in the Tor and Hod formations vary between I and 6 % without a systematic distribution. The trace element geochemistry does not point to significant elastic input(> 5 %). 'Reservoir chalk' does contain primary o13C isotope composition. In contrary, 0180 are not reflecting the global marine composition during the Late Cretaceous. Hence, reservoir chalk had been affected by fluid flow events not visible in onshore chalk or suffered a higher diagenetic overprint comparable to on-shore chalk from Kansas. Alternatively, or additionally, the depositional environment of chalk from the central North Sea is different from some of the on-shore chalk deposits, which might indicate a different geological history.