Sedimentary facies reconstruction and kinematic restoration of an Upper Permian tight gas field, north-western Germany

This study of a tight gas reservoir, located at ~4200 metres depth in the Upper Rotliegend II (Upper Permian) of north-western Germany, includes a geological framework model for the sedimentary facies distribution, which is strongly influenced by the structural grain. Research focused on unravelling the structural and sedimentary complexity of Permian (Rotliegend) tight gas fields in Central Europe requires multidisciplinary studies combining tectonic-stratigraphic interpretation of three-dimensional seismic reflection, wireline log and core data, laboratory analysis, numerical modelling and field based analogue studies. Reservoir rocks in the study area are of fluvio-aeolian origin, and include dune, fluvial and wet to dry interdune deposits. Dune and minor sandflat deposits represent the most important gas reservoir rocks of the Upper Rotliegend II. Source rocks are Westphalian coals, and the top seal is provided by Zechstein evaporites. Detailed analysis of palaeo-topography, fault activity and accommodation for strata of Permian age (Rotliegend and Zechstein groups) indicate that local depocentres developed within small-scale, fault-controlled transtensional sub-basins. Synsedimentary halfgraben development and fault activity were identified during Upper Rotliegend II deposition. Many Upper Rotliegend structures were reactivated during the Triassic, Jurassic and Cretaceous, often developing enhanced offsets and lateral extent. As a consequence, much of the original Rotliegend structures and stratigraphy were significantly displaced, ultimately re-arranging the location of former Rotliegend depocentres and associated tight gas reservoir facies. This tectonic overprint hampers the prediction of potential Rotliegend reservoirs, emphasizing the necessity of palaeo-environmental restorations in areas with multi-phase tectonic histories. In addition, the sedimentary environment and involved sedimentary processes were only punctually reconstructed from core material and log data. A field analogue study in Panamint Valley, California, United States, was carried out to improve the understanding of the interactions between the sedimentary systems and tectonic processes in graben / halfgraben structures similar to those in the Upper Rotliegend of north-western Germany. The comparison shows that both study areas are characterized by (i) synsedimentary transtensional tectonics resulting in elongated graben structures with large-scale, bounding fault zones, (ii) a fault-controlled (palaeo-)topography, which is one of the key controlling mechanisms of the sediment facies distribution, comprising different types of alluvial fans, dunes, wet and damp interdune sandflats, and ephemeral dry lake deposits, and (iii) very similar sediment sources comprising clastic input from fault zone footwalls and input from volcanics. Consequently, the field analogue-based study provides a detailed model of the sedimentary facies for the tight gas reservoir in Germany prior to multi-phase tectonic overprinting, including the composition and distribution of aeolian sandstone reservoirs and their relation to fault-induced topography. For a better understanding of the multi-phase tectonic overprinting that influenced the primarily good reservoir rocks, a sequential 2D retro-deformation of the study area with focus on the temporal differentiation of salt diapirism and its relation to regional tectonic phases was carried out. Backstripping considers sedimentation, decompaction, fault-related deformation, salt movement, thermal subsidence and isostasy to gain a better understanding of the link between tectonics and reservoir diagenesis. The modeling results of this study indicate that reactive diapirism started during the Lower Buntsandstein, followed by active diapirism starting in the Lower Keuper accompanied by salt piercement of the overburden. Since the Lower Cretaceous, the salt rises by passive diapirism. From isopach maps, fluid inclusion studies, and K/Ar ages a clear linkage between the study area’s structural evolution and the timing of diagenetic processes can be observed