Correlating three Upper Permian Zechstein-2-Carbonate outcrops across the Eichsfeld-Altmark Swell – Facies, reservoir properties, and outcrop analogue potential

  • Author:

    Becker I,

    Koehrer B,

    Hilgers C

  • Source:

    Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 169:517-531, doi

  • Date: 2018
  • Understanding the lateral distribution of carbonate facies is essential to understand reservoir continuity, geometry and quality, and, as a consequence, can help improving development strategies as reservoir characteristics vary between dif- ferent types of depositional environments. This study focuses on three outcrops of Zechstein-2-Carbonates (second cycle, Stassfurt, Ca2) along the SW margin of the Harz Mountains. The carbonates were deposited on the Eichsfeld-Altmark Swell, a NNE–SSW striking palaeohigh at the southern margin of the Southern Permian Basin. For each outcrop, different gross depositional environments (GDE) from slope to platform margin to inner platform deposition are interpreted and correlated over a lateral distance of 25 km. Reservoir characteristics are analysed with regard to the interpreted type of depositional environment and exemplarily compared to subsurface data of a gas-producing Ca2 field, representing one interpreted GDE type, at a distance of approximately 130 km to the NW in the Lower Saxony Basin. Porosity values of the slope carbonates in outcrop are in good accordance with the provided subsurface slope data. Both show a similar range of values and average porosities of approximately 6 % (outcrop) and 4 % (reservoir). In contrast, per- meability values of the subsurface samples are increased compared to outcrop values which may be due to micro-fracturing. Platform-related grainstone facies show best matrix porosities and permeabilities, and highest proportions of those outcrop carbonates are located in the centre of the studied Eichsfeld-Altmark Swell. Porosity-permeability relationships are derived for the interpreted gross depositional environments. In combination with the presented lateral GDE correlation lengths of 25 km, they can be incorporated into models of Ca2 reservoirs in NW Germany. Subsurface information about the depositional environment distribution potentially derived from core descriptions are the basis to choose the correct outcrop analogue for future studies of the structural inventory and lateral reservoir quality variations.