Outcrop to Well - Transferability of Zechstein Ca2 Carbonates from the Eichsfeld-Altmark Swell to the North German Subsurface
Becker I, Koehrer B, Jelinek W, Hilgers C
Natural fracture networks can control hydrocarbon migration pathways in tight carbonate reservoirs. As their distribution depends on mechanical properties which are controlled by the depositional environment, diagenetic overprints, and the tectonic history of each setting, fractured reservoir modelling appears challenging. Outcrop analogs are incorporated to improve such models by adding additional data as e.g. information about the sub-seismic structural inventory or fracture network geometries, which are not accessible from vertical wells. This study investigates three outcrops of Ca2 carbonates on the southern margin of the Harz mountains resembling slope to platform deposits on the former Eichsfeld-Altmark Swell at the southern margin of the Southern Zechstein Basin. We evaluate their suitability to act as analogs for actually operated naturally-fractured Ca2 gas reservoirs approximately 130 km to the NW by comparing different parameters.
Different facies units exposed in the three outcrops are linked over the Eichsfeld-Altmark Swell and related to their depositional environment. Diagenetic features are related to reservoir characteristics. Porosity values from surface samples can be well compared to reservoir data all showing dolomitic composition and vary between 2 and 20%, but matrix permeabilities appear slightly higher in the subsurface (0.001-100 mD) than in the outcrop (0.0001-10 mD) samples. Fracture network geometries are determined from observations on quarry floors and supplemented by scanline measurements using conventional manual methods and our digital workflow based on terrestrial laser scanning. Well-FMI data is used to evaluate the transferability to subsurface fracture pattern and the present day stress orientation controlling subsurface fracture permeability is taken into account. Overall, we show to what extent analogous data can be used as direct or indirect input to improve reservoir models and thus, development strategies.