STRUCTURAL DIAGENESIS OF AN UPPER CARBONIFEROUS TIGHT GAS SANDSTONE RESERVOIR ANALOG, OSNABRUECK AREA, NW-GERMANY

Upper Carboniferous tight gas sandstone reservoirs in NW-Germany consist of thick successions of cylcothems. Our focus is a Westphalian D reservoir analog, exposed in the active Piesberg quarry which exposes tight gas rock analogs on a seismic scale. Third-order fine- to coarse-grained sandstone cycles show predominantly downstream accretion at the base, overlain by lateral accretion with heterolithic channels at the top. All fining-upward cycles end with siltstones, claystones and meta-anthracite coal seams. Three cycles are exposed, each approximately 50 m thick. They can be correlated laterally on the several 100’s of meter scale across the Piesberg quarry. Paleocurrent measurements indicate an overall NW-directed flow direction, its deposits originating from the Variscan Mountains in the south. Exposed rock types can be clearly distinguished by gamma ray spectra. Sandstone types generally show low porosities (<10 %) and very low permeabilities (<0.1 mD), mainly due to intense compaction and quartz cementation. Semivariograms will display the porosity variations within and across the massive sandstone beds and will be integrated with the diagenetic evolution inferred from petrography.

Cyclothems are truncated by synthetic south-dipping normal faults and graben structures, which were mapped throughout the quarry. Normal faults expose continuous bands of clay and coal smeared into the fault plane at minor throws of 1-2 meters, and fault gouges several meters in width at throws of more than 10 meters. Microveins accumulate around normal faults and may act as potential flow barriers. Joints show a regular pattern and are at least partly opened up along microveins. The massive grey sandstones are strongly anisotropic with a uniaxial strength of more than 100 MPa normal to bedding, and approximately 50 MPa parallel to bedding. Solid sandstone beds within and close to faults are leached by fluids, their uniaxial strength is reduced by a factor of more than two.

Data compilation result in a high-resolution 3D digital reservoir model of a tight gas sandstone reservoir analog based on field mapping and terrestrial Lidar. Results of the study allow to better understanding fluid migration pathways from seismic- to micro-scale within analogous subsurface tight gas reservoirs in the region.