Influence of basement faults on the development of salt structures in the Danish Basin
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ABSTRACT Examination of growth-fault systems in offshore southeast Texas suggests two major growth-fault provinces. In one province, shale-withdrawal growth faults occur beneath the present inner-shelf region where an upstanding basement block restricted mid-Jurassic salt deposition. Salt withdrawal faults occur farther offshore beneath the outer-shelf and slope region where up to 9,800 ft (3,000 m) of mother salt was deposited in a deep salt basin. Our data indicate a four-stage sequence of growth-fault development. The sequence begins with a primary withdrawal stage of salt or overpressured shale. Primary withdrawal faults are generated in response to differential loading induced by progradational wedges or gravity spreading. Differential pressures displace salt and shale by pure shear deformation away from the zone of maximum overburden loading into massifs, reactive diapirs, and shortening structures. A secondary withdrawal stage occurs in the salt substrate province after the primary withdrawal fault has bottomed-out. No secondary withdrawal stage occurs in the shale province. Withdrawal of salt from massifs and pillows generates secondary withdrawal faults and collapse structures, and is accommodated by the growth of diapirs and increased shortening. An allochthonous salt sheet emplacement stage may subsequently occur to accommodate continued secondary withdrawal through emplacement of extensive salt sheets from overthrusted nappes and/or diapir close-up. Supra-allochthonous growth-fault systems may then be generated in a new phase of primary salt withdrawal. Complete four-stage growth-fault systems are referred to as megacycle structures.
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Data published during the recent past have improved understanding of initiation of salt and shale diapirism and of growth faulting on the central Gulf continental slope. Growth faults appear on diapir flanks during initial development, as well as along upper-slope depocenter flanks and the continental shelf edge. Rapid deposition, differential loading and subsidence on the upper slope and outer shelf enhances segmentation of salt ridge or massifs into individual diapiric spines, causing additional diapir-related growth faulting. Most growth faults originating on the slope remain active and, projected upward 5000-20,000 ft, provide the structural framework within which south Louisiana petroleum exploration takes place. Study of 31 piercement and 19 semipiercement salt domes plus 117 nonpiercement domes formed by salt and/or shale diapirs reveals important growth fault variations genetically related to diapiric structure type. Fault patterns associated with piercement and semipiercement salt domes are different and more complex than those on nonpiercement features. Counter-regional faults, commonly in compensating or crossing patterns, are far more common; fault splitting and crestal grabens are particularly common on semipiercement structures. Local growth faults related to differing flank subsidence rates around high-relief diapirs play a major role on these structures. In contrast, fault patterns are less complexmore » on nonpiercement diapiric structures. Counter-regional faults, compensating and crossing systems and splitting are less common; most major faults appear to be regional growth faults only indirectly related to diapir development. Implications for additional deep exploration diapiric structures exist.« less
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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.
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Abstract The stepped counterregional style of allochthonous salt is well developed and documented in the northern Gulf of Mexico. In this model, a tabular salt tongue or canopy is evacuated into isolated diapirs connected by counterregional (or landward-dipping) growth faults, with the withdrawal basin having the geometry of a basinward-dipping, monoclinal growth wedge. We suggest that a similar style dominates the deeper, supra-Louann level in large parts of the northern Gulf of Mexico, most notably significant portions of the deep shelf province. In our model, a basinward-dipping and -thickening minibasin subsides into Louann salt that is displaced into a basinward-leaning primary diapir. Salt withdrawal is the dominant process, although extension, contraction, and strike-slip movement can locally be important. The diapir eventually forms a counterregional weld as the salt moves into an allochthonous level. Subsidence of the minibasin relative to outlying areas is greatest at the diapir and decreases away from the diapir along counterregional faults until it is taken up only by folding beyond the fault tips. Depending on the orientation of a two-dimensional cut through such a system, the geometry can range from an asymmetric fault-bounded growth wedge to a faulted fold to a symmetric growth syncline. Two or more such systems with variable orientations in proximity to each other results in a wide variety of structural styles: repeated counterregional faults, growth synclines, faulted synclines, turtle structures, fault-bounded turtles, and horst blocks. Although four-way closures are certainly possible, structural traps tend to be three-way closures against welded feeders or counterregional faults.
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Salt tectonics
Echelon formation
Horst and graben
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