Abstract Sharp-based marine shoreface sandstones interpreted as forced regressive deposits are a characteristic feature of the Gassum Formation in the intracratonic Danish Basin. Detailed process-based sedimentological and a high-resolution, sequence-stratigraphic interpretation of cores from closely-spaced wells has led to improved understanding of the erosional and depositional processes active during the formation of the sharp-based sandstones. Each sandstone shows an internal stacking of forced regressive shoreface units separated by thin muddy offshore facies. This stacked pattern records low-amplitude but widespread changes in relative sea-level during the overall progradation due to low depositional gradiednts. Laterally, the stacked forced regressive shoreface deposits show a seaward-dipping, shingled geometry indicating seaward displacement of the shoreline through stepwise, forced regressions during overall fourth-order relative sea-level fall. Thereby each sharp-based shoreface sandstone records deposition resulting from interaction of from two scales of superimposed relative sea-level fluctuations: a lower fourth-order fall responsible for the overall seaward shoreface displacement, and a higher fifth-order oscillation that resulted in repeated forced regression within the lower-order sequences. Although these stepwise, forced regressive deposits dynamically resemble ‘stranded’ parasequences, they differ from the conceptualized picture of ‘stranded’ parasequences as simple downstepping of forced regressive deposits, because of their gently dipping shingled geometry and distinctive deposition component resulting from intervening, high-order drowning. For both the fifth-order forced regressive units and the lower-order forced regressive sharp-based sandstones it is possible to differentiate between: (1) deposits formed during falling sea level as part of the forced regressive systems tract and (2) the last, forced regressive to progradational part formed at sea-level lowstand representing the lowstand systems tract. Accordingly, the sequence boundary, whether of high- or low-order, is placed below the last, forced regressive deposits and associated lowstand progradational deposits, but above the deposits formed during falling relative sea-level. Thus the sequence boundary is placed at the surface of subaerial exposure passing seaward into a marine regressive surface of erosion reflecting maximum regression. The basal, regressive surface of erosion below the fourth-order forced regressive systems tract is demonstrated to consist of coalesced fifth-order forced regressive surfaces. Therefore, the fourth-order regressive surface is a composite surface reflecting a series of forced regressions and intervening drowning and as such is diachronous. The basinwide dominance of sharp-based, forced regressive shoreface deposits in Upper Triassic of the Danish Basin is interpreted to reflect the interaction between low gradient and shallow palaeobathymetry, sediment supply and low-amplitude relative sea-level changes.

Abstract The Stenlille natural gas underground storage is located 70 km SE of Copenhagen and has been in operation since 1989. For safety reasons and to protect the environment it is necessary to monitor the storage carefully. Natural gas is being stored in an anticlinal structure with an expected gas storage capacity of about 3 billion Nm 3 (volume under ‘normal’ conditions) in the upper Triassic Gassum Sandstone Formation 1500–1600 m below the surface; it replaces saline formation water. So far, nineteen deep wells have been drilled on and around the structure. The 300 m thick clay sequence of the Lower Jurassic Fjerritslev Formation above the gas storage reservoir has acted as an efficient seal, since no sign of gas leakage has been observed in the monitoring well located in a sand stringer 15 m above the gas reservoir. Other monitoring wells have been located in order to check for possible lateral escape of natural gas. A baseline study on naturally occurring hydrocarbons performed before the natural gas storage came into operation indicated the presence of only trace amounts hydrocarbon gases in the subsurface of the Stenlille area. Results of analysis by the headspace and sorbed gas methods on drill-cuttings suggest that low-temperature thermal generation of hydrocarbon gases (δ 13 C 1 : −47 to −42‰; δ 13 C 2 : −34 to −30‰) has taken place in organic-rich marine shale below 1300 m. Low concentrations of dissolved methane (<0.5 mg/l) of bacterial origin (δ 13 C 1 : −90 to −62‰) were found in shallow groundwater that is used for water supply in the Stenlille area. After the start of injection of natural gas in 1989 (C 1 :C 2 :C 3 = 91:6:2; δ 13 C 1 : −47‰), no increase in methane concentration and no higher hydrocarbon gases were observed during the regular analysis of groundwater from 10 shallow wells located above the underground natural gas storage. However, a sudden increase in dissolved methane concentration from 0.02 to 27 mg/l was measured in a 130 m deep observation well after a minor gas leakage had been detected at a new deep drilling into the natural gas storage in 1995. Nonetheless, no increase in methane was observed in shallow groundwater at the same locality. Occasional higher concentrations of dissolved methane (up to 15 mg/l) were encountered in shallow observation wells in low permeability layers. Stable isotope analyses (δ 13 C 1 : −69 to −52‰) and radiocarbon dating show that the gas does not originate from the underground gas storage because the methane was less than 300 years old, but it may have formed due to local microbial activity.