Top Of Salt Research Articles

Mesozoic dissolution tectonics on the West Central shelf, UK Central North Sea

3-D seismic mapping of the Upper Jurassic Kimmeridge Clay Formation on the West Central Shelf in the Central North Sea reveals a complex fault array which is constrained by seismic interpretation and well control to be of late Jurassic/early Cretaceous age. Fault shapes in plan-view range from linear to circular. Linear fault lengths are 200–300 m to 5 km, the strongly curved and circular faults range in diameter from 100–1000 m. Fault trends are apparently random and display no correlation in location or trend with basement (sub-Zechstein) structures. There is, however, a strong link between this fault pattern and the structure of the top Zechstein (top salt) surface. Linear faults occur at the edges of elongate salt walls and the circular faults lie directly above structures which have been interpreted here as tall, steep-sided salt chimneys. The salt chimneys are present only in the thick, elongate minibasins of Triassic sediment which lie between the salt walls. It is argued that salt dissolution controls the timing, location, orientation and shape of the late Jurassic/early Cretaceous faults. A model is provided to account for the development of both salt walls and chimneys. We suggest that early Triassic karstification of the Zechstein evaporites led to development of an array of circular collapse features. During the ensuing episode of Triassic halokinesis which led to minibasin subsidence and salt wall growth, salt passively ‘intruded’ the circular collapse features within the subsiding minibasins to form narrow salt chimneys. The resulting array of salt walls and chimneys was subject to dissolution during subsequent subaerial exposure and the late Jurassic marine transgression of the basin (creating the observed fault array), prior to sealing of the salt from circulating groundwater by compaction of the Upper Jurassic and Lower Cretaceous shales which blanket the area.

A systematic technique for the sequential restoration of salt structures

A method is described for the sequential restoration of cross sections in areas of salt tectonics where deformation is confined to the salt and higher layers. The subsurface geometry evolves with time through the interaction of various processes: sedimentation, compaction, isostatic adjustment, thermal subsidence (if present), faulting, and salt withdrawal/ diapirism. The technique systematically calculates and removes the effects of each of these processes during specified time intervals defined by the interpreted horizons. It makes no assumptions about salt kinematics and generally results in the area of the salt layer changing through time. The method is described for restoration of extensional terranes, but it is also suitable for areas of contractional salt tectonics with only minor modifications. After converting an interpreted seismic profile to depth, the top layer is stripped off and the underlying section is decompacted according to standard porosity-depth functions. A deep baseline, unaffected by compaction or deformation, is used to restore any isostatic compensation or thermal subsidence. Isostasy is calculated according to the Airy model, and differential sedimentary loading across a section is shown to be approximately balanced by changes in salt thickness so that the load is evenly distributed. After these processes have been reversed, the resulting geometry and the seismic data are used to create the sea-floor template for structural restoration. Fault offsets are removed and the layers down to the top salt are restored to this template, while the base salt remains fixed. The resulting space between the restored top salt and the fixed base salt defines the restored salt geometry. In addition, the difference between the sea-floor template and a fixed sea level provides a measure of the change in water depth (ignoring eustatic changes in sea level). The technique is applied to an interpreted seismic profile from the eastern Green Canyon/Ewing Bank area of offshore Louisiana. The section is characterized by a variety of salt structures, including salt rollers, a diapiric massif, a remnant salt sheet, and a salt weld, which are shown to have derived from an originally continuous salt sheet which has been modified by sedimentary loading. Early loading created vertical basin growth that was accommodated primarily by salt withdrawal and associated diapiric rise through the process of downbuilding. Once the salt weld formed, continued sedimentation was accommodated by a lateral increase in basin size caused by down-dip extension on listric growth faults.

3-D Salt and Sub-salt Imaging Strategy: A Case History from the Gulf of Mexico

The purpose of this paper is to demonstrate 3-D prestack depth migration as an ultimate technique for imaging structural targets associated with salt diapirism, and compare it with other migration strategies ? 2-D prestack, and 2-D/3-D poststack depth migrations. The structural problem is to image sedimentary bed terminations, bottom salt boundaries and undersalt layers around two large salt masses in the Gulf of Mexico. A large number of oil and gas reservoirs in the Gulf of Mexico is controlled by salt structures. Typical salt flank plays have been mapped quite successfully with time migration strategy. However, as exploration moves into more complicated targets around salt overhangs and subsalt traps, depth migration strategy becomes necessary. The first task is to build a detailed 3-D velocity-depth model. To accomplish this, the following robust approach has been chosen. Using a velocity-depth model for the overburden (sedimentary section) based on edited stacking velocities, 3-D poststack depth migration was performed to verify its accuracy. Iterative 3-D poststack depth migration then followed to define the salt masses. Top salt reflectors were interpreted in 3-D and constant salt velocity was assigned to the half-space below the top-salt. Again, 3-D poststack depth migration was performed and base-salt events were interpreted. Finally, the velocity model was updated to contain complete salt bodies and 3-D poststack migration was rerun to check results. This loop of three iterations was repeated until convergence was achieved ? the input model matched the result of the depth migration. Given the velocity model, a single output line was selected and 3-D dynamic raytracing was performed to link output image points with surface shot-geophone locations. Finally, 3-D Kirchhoff depth migration was run in 3-D prestack, 2-D prestack and 2-D poststack modes to produce comparison sections along the output line.