https://orcid.org/0000-0002-5194-4349
Abstract
Upper Carboniferous sandstones in NW-Germany consist of thick successions of cyclic deposits and are major tight gas reservoirs. This study presents the heterogeneity exposed in a large quarry near Osnabrück, Germany, which contains faulted and jointed fourth-order coarse- to fine-grained tight sandstone cycles separated by anthracite coal seams. First, we characterize the rocks and the lateral variation of rock properties such as porosity, diagenesis and structural inventory. Than we test whether the quarry may act as a tight gas reservoir analogue to better constrain input data for reservoir modeling. The tight sandstones are intensely compacted and cemented with quartz and generally characterized by low matrix porosities < 8 % (He-pycnometry on plugs and cuttings) and very low permeabilities (< 0.01 mD). Porosity is generally secondary intraparticle, formed by detrital and authigenic carbonate dissolution and dissolution of feldspars. Matrix porosity significantly increases up to 25% in corridors around faults. Main rock types can be distinguished by spectral gamma ray in the quarry. Quartz veins and fault mineralizations indicate fluid flow within and around faults. Normal faults show bands of clay smear and gouge, forming compartments. Fractures were analyzed in a 50 x 50 m section of the quarry wall using LIDAR laser scanning. This digitized quarry face also allows the characterization of the lithology and quantitative measurement of bedding, fracture and fault orientation data in inaccessible areas. Our high-resolution field analogue enables a better understanding of tight sand reservoir properties and reservoir quality at a sub-seismic scale, considering both the change of porosity during diagenesis and the formation of structures. Results may be used to develop data-driven exploration strategies and improved development options for similar subsurface tight gas reservoirs, especially in Northern Germany.
