Abstract
The geophysical characteristics of the carbonate-dominated Ordovician succession is described using wire-line logging data from exploration wells located within the Swedish part of the Baltic Basin, both offshore and from the island of Gotland. The petrophysical properties and log-motifs are compared and correlated with the lithology of cores from the Hamra-10, Skåls-1 and Grötlingbo-1 wells on southern Gotland. The 80–125-m-thick Ordovician succession is divided into five log stratigraphic units Oa–Oe, which are correlated throughout the study area. The proposed log stratigraphy and wire-line log characteristics are evaluated and compared with the established Ordovician stratigraphy from the adjacent areas of Öland, Östergötland and South Estonia. The newly established log stratigraphy is also linked to the existing seismic stratigraphic framework for the study area and exemplified with a selection of interpreted seismic type sections from Gotland and the south Baltic Sea. The presented characterization, division and correlation provide a basis for understanding the lateral and vertical variation of the petrophysical properties, which are essential in assessing the sealing capacity of the Ordovician succession, in conjunction with storage of CO2 in the underlying Cambrian sandstone reservoir.
Highlights
Geophysical well logs have long been used for lithological characterization and sequence analysis
Bildstein et al (2010) showed, by integrated geochemical modeling, that the impact of dissolution on a homogeneous carbonate caprock is less than a few meters over a period of 10,000 years. These results suggest that dissolution is not a considerable risk factor regarding the integrity of carbonate cap rocks
Comparison with core material from Hamra-10, Skåls-1 and Grötlingbo-1 have allowed the correlation of the log-motifs on Gotland to lithofacies and detailed stratigraphic analysis performed on similar strata on Öland, in Östergötland and Estonia
Summary
Geophysical well logs have long been used for lithological characterization and sequence analysis. Perhaps the most commonly used log for this purpose is the natural gamma ray log, the response of which, often relates to the amount of potassium-bearing clay minerals in the the use of log-motifs for the identification of shifts in the depositional setting for carbonate-dominated successions is much less straightforward. This is mostly because the carbonate system differs from the siliciclastic one in that the changes in carbonate productivity must be taken into consideration. Where a sea level rise will likely result in the deposition of a higher proportion of clay material, which will correspond to relatively high gamma ray values
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