Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences
Authors: Korstgard J. A. , Lerche I. , Mogensen T. E. , Thomsen O. 
Published: July 2021

Korstgard, J. A., Lerche, I., Mogensen, T. E. & Thomsen, R. 0.: Salt and fault interactions in the northeastem Danish Central Graben: observations and inferences. Bulletin of the Geological Society of Denmark, Vol. 40, pp. 197-255. Copenhagen, 1993-12-30.
Salt structures in the Spgne Basin and the Tail End Graben, the northeastern part of the Danish Central Graben, have been studied using a 1/2-1 km spaced seismic grid together with publically available well data. The investigated area has been divided into three main basin areas: the northern part of the Spgne Basin, a plateau area; the southern part of Spgne Basin, a shallow halfgraben; and the Tail End Graben, a deep halfgraben. These basins are flanked by the Ringkpbing-Fyn High and the Manda! High. Pre-Zechstein, and/or Pre-Permian normal faults, together with the shape of reflectors in the northernmost Spgne Basin, indicate an Early, possibly Late Carbon­iferous, extension phase. The structural configuration of the sediments above the Zechstein salt pillows in the shallow halfgraben part of Spgne Basin, when compared with the structural configuration of the sediments in the much deeper Tail End Graben, indicates deeply buried salt pillows in the Tail End Graben. The salt structures fall into two basic groups, graben boundary salt pillows/diapirs and salt pillows/diapirs updip in halfgrabens. The evolution of these Zechstein salt structures is complex and intimately related to fault activity.
Development of Triassic depocentres was primarily controlled by halokinesis with large thicknesses of sediments being deposited in primary and secondary rim synclines in the Spgne Basin. Middle Jurassic rift initiation was associated with synrift sediments along the Coffee Soil Fault and Late Jurassic rifting caused a separation of the Spgne Basin and the Tail End Graben, and the formation of a ramp dipping to the south at the Coffee Soil Fault, which was· a pathway for sediments from the deeply eroded Ringkpbing-Fyn High footwall. This ramp has high sand potential. In the Early Cretaceous the Spgne Basin and Tail End Graben were separate depositional areas and the position of the Lower Cretaceous sediments were controlled by the Late Jurassic footwall uplift of the Manda! High, the southernmost part of the Spgne Basin, and the Ringkpbing-Fyn High. Upper Cretaceous deposits were the first sediments to overlay the Ringkpbing-Fyn High and the Manda) High footwalls, indicating a change of the structural framework and in the Late Cretaceous and Early Tertiary inversion occurred in the area with renewed halokinesis, especially in the Tail End Graben. Continued halokinesis in the Late Miocene is indicated by the position of Late Miocene channels.
Using vitrinite reflectance measurements from the Lulu-I well, drilled on top of a salt structure, it is possible to determine the excess maturity caused by the focusing of heat due to the higher thermal conductivity of salt. A method is presented for assessing the time of onset of diapirism and salt flow-speed. The method is based on calculation of the thermal anomaly surrounding a rising salt diapir. For a given salt speed, predicted vitrinite reflectance values are calculated and compared with the observed values at given depths. In this way salt migration rates are determined by forward modelling. The method can easily be tailored to thermal indicators other than vitrinite reflectance, thereby enhancing the resolution of the thermal history, and constraining both the onset of salt rise as well as the speed.
In addition geohistory, thermal history, source capacity and oil generation have been examined in the northwestern part of the Danish Central Graben using a one-dimen­sional fluid flow-compaction model. The burial history suggests that this part of the Danish Central Trough developed through three stages of subsidence, a Late Jurassic differential stage, a Late Cretaceous-Early Tertiary uniform stage and a Late Miocene­Quaternary uniform stage. Pseudo-wells "drilled" on seismic sections in areas without well data are used to improve the spatial distribution of wells. The palaeotemperature and palaeoheat flow have been modelled by inversion of vitrinite reflectance data. The inversion was carried out on wells with available vitrinite reflectance data and was based on known bottom hole temperatures, some temperature measurements with depth and the surface temperature. The thermal history assessed by inversion of vitrinite reflectance data gives a consistently cooler past; the available data's resolution of the thermal history is also discussed. The modelled maturation history of the Upper Jurassic shale in terms of vitrinite reflectance suggests that the shale reached maturity some 5-50 Ma ago. A geochemical study of the Upper Jurassic shales shows that these shales contain a mixture of type II and type III kerogens and have good to excellent source potential. Modelling of the hydrocarbon generation data indicates that the peak generation took place some time between 10 Ma BP and the present day. Possible migration paths are determined from modelled excess fluid pressure, and four areas of possible accumulation of hydrocarbons are indicated. The hydrocarbon potentials of the areas are evaluated and an area along the eastern boundary fault between the Tail End Graben and the Ringk!Zlbing-Fyn High is suggested as a target for further explora­tion.