Sedimentary facies and cyclostratigraphy of a potential CO₂ storage reservoir: The Early Triassic Bunter Sandstone Formation in the Rødby area, southern Denmark

The Early Triassic (Olenekian) Bunter Sandstone Formation represents a major prospective reservoir for CO2 storage in southern Denmark especially in the four-way dip structure in the Rødby area Lolland, southern Denmark), and is expected to play an important role for reaching climate targets. The Bunter Sandstone Formation in southern Denmark formed along the northern margin of the North German Basin and is composed of terrestrial sediments and subdivided into the Volpriehausen, Detfurth and Solling members. Each member has a basal sandstone unit overlain by a mudstone unit. The sandstone units form potential reservoirs in the formation. Their thickness and composition, however, vary somewhat along the basin margin from west to east in southern Denmark. In this paper focus is on the 256 m thick Bunter Sandstone Formation in the Rødby-1 well on Lolland. Visual analysis of the gamma ray records from wireline logging of the sedimentary succession in the Rødby-1 well indicates the presence of a composite cyclicity both in sandstone and mudstone units. Spectral analysis finds identical, significant cycles both in mudstone-rich and sandstone-rich units; in mudstone-rich units cycles have the following thicknesses: 62, 16, 7.5, 5.8, 4.4, 3.6, 3.0, 2.6, and 2.3 m. Assuming that these sedimentary cycles record astronomical climate control on sedimentation and the 62 m cycle records the 405,000 years eccentricity cycle, then the remaining cycles represent the following periods: 104,500; 49,000; 38,000; 28,500; 23,500; 19,500; 17,000; and 15,000 years. Thus both a 100,000 years eccentricity as well as precession cyclicity with periods between 15,000 and 23,500 years can be identified. More surprising also a likely obliquity signal seems to be present as given by cycles with periods of 28,500, 38,000 and 49,000 years. In Rødby-1, the sandstone units vary in thickness between 23 and 29 m. The new cyclostratigraphy indicates that deposition of the sandstone units took place during the arid parts of a 405,000 years eccentricity cycle, when the central playa lake had its minimum extension. Mudstones units formed during the humid parts of these climate cycles when the central playa lake expanded in size. Internal lithological variation and thereby reservoir properties (sandstone-mudstone ratio) in the sandstone units are related to 100,000 years eccentricity variation in climate as well as to obliquity and precession variation. There is very limited core evidence on facies characteristics in the Rødby-1 well, but comparison with core data from the Tønder area in southern Jutland suggests that the sandstone unit in the Volpriehausen Member is composed of alternating aeolian and ephemeral river deposits with facies change mainly related to climate variation. A similar sedimentary facies pattern is probably also characterizing the sandstone unit of the Detfurth Member. In the Solling Member only fluvial facies are present, and it is suggested that the lithological variation primarily reflects climatic induced variation in fluvial style and transport capacity. Mudstones units that separate the sandstone units are interpreted to have formed on sabkhas and in large playa lakes.