Shallow Salt Structures Research Articles

Seismic stratigraphic framework and depositional history of a large Upper Cretaceous and Cenozoic depocenter off southwest Nova Scotia, Canada

Interpretations of the Late Cretaceous to Quaternary geological history of the North American Basin rely on a seismic and lithostratigraphic framework developed predominantly from data along the northeastern United States margin. Prior to this study, attempts to extend this framework northward to the Nova Scotia margin were hindered by correlation issues across seamounts, limitations in data coverage, and significant complications caused by widespread canyon erosion and shallow salt structures. These challenges are overcome using modern seismic reflection data along the southwestern Nova Scotia margin, in the Shelburne sub-basin, where Upper Cretaceous through Quaternary sequences manifest four distinct phases in the interplay between down-slope and along-slope depositional and erosional processes. The first phase, spanning the Late Cretaceous to Late Eocene, is characterized by widespread periods of gully erosion interspersed with periods of pelagic and hemipelagic, carbonate-rich sedimentation. Extensive failure of upper slope regions related to the Montagnais meteorite impact also occurred during this time. The second phase, spanning the Oligocene to Late Miocene, is dominated by widespread erosion, deposition by gravity flow processes, and sediment re-working by bottom currents. Marked erosion and/or non-deposition along the mid-to lower slope is attributed both to down-slope and along-slope processes. The third phase, spanning the Late Miocene to Late Pliocene, is dominated by bottom current deposition including stacked sequences of giant sediment waves and large contourite drifts along the slope. The final phase, from the Late Pliocene to present, exhibits a return to predominance of down-slope gravity flows with deposition focused below the upper slope. This four phase history of depositional and erosional processes is similar to that documented south of the study area along the U.S. margin.

Detecting a salt dome overhang with magnetotellurics: 3D inversion methodology and synthetic model studies

Detecting a salt dome overhang is known to be problematic by seismic methods alone. We used magnetotellurics (MT) as a complementary method to seismics to investigate the detectability of a salt dome overhang. A comparison of MT responses for 3D synthetic salt models with and without overhang shows that MT is very sensitive to shallow salt structures and suggests that it should be possible to detect an overhang. To further investigate the resolution capability of MT for a salt dome overhang, we performed a 3D MT inversion study and investigated the impact of model parametrization and regularization. We showed that using the logarithms of the conductivities as model parameters is crucial for inverting data from resistive salt structures because, in this case, commonly used Tikhonov-type stabilizers work more equally for smoothing the resistive and conductive structures. The use of a logarithmic parametrization also accelerated the convergence and produced better inversion results. When the Laplace operator was used as a regularization functional, we still observed that the inversion algorithm allows spatial resistivity gradients. These spatial gradients are reduced if a regularization based on first derivatives in contrast to the Laplace operator is introduced. To demonstrate the favorable performance when logarithmic parametrization and gradient-based regularization are employed, we first inverted a data set simulated for a simple model of two adjacent blocks. Subsequently, we applied the code to a more realistic salt dome overhang detectability study. The results from the detectability study are encouraging and suggest that 3D MT inversion can be applied to decide whether the overhang is present in the shallow salt structure even in the case when only profile data are available. However, to resolve the overhang, a dense MT site coverage above the flanks of the salt dome is required.

Salty groundwater flow in the shallow and deep aquifer systems of the Schleswig–Holstein area (North German Basin)

In the Schleswig–Holstein area, salty groundwaters are not always spatially related to the presence of shallow sat structures. Furthermore, hydrochemical data point to instable salinity profiles and to the occurrences of deep brines close to the surface. Therefore, complex interaction between shallow and deep solute migration must occur. Numerical simulation of fluid flow, mass and heat transport have been carried out in order to understand the role of shallow salt dissolution and young geological features on groundwater flow as well as to investigate the interrelationships between shallow and deep aquifer systems. For these purposes, a shallow (− 500 m) and a deep profile model (− 5 km) have been constructed. The results indicate that different flow regimes coexist within the study area. Shallow brine migration is strongly controlled by the geological features of the basin such as salt flank, glacial channels and sand layers. Furthermore, shallow salt dissolution is the major cause for gravitational convection in the deeper aquifers. This source of salinization from above leads to the formation of instable salinity profiles at 2 km depth. Further interactions between shallow and deep fluid flow exist in the eastern part of the basin where brine upconing is due to both topography-driven flow and thermohaline convection. The simulations also showed that the hydraulic conductivity of the stratigraphic units influences the brine regime on a regional-scale. Gravitational convection is likely to occur in permeable units located near shallow salt structures, while thermohaline convection persists within thick permeable areas in which salt dissolution is not the dominant process. The presented study provides new insights into fluid basin processes. The described flows could develop in any geothermal basin hosting salt diapirs which pierce shallow aquifer systems.

The effect of allochthonous salt evolution and overpressure development on source rock thermal maturation: a two-dimensional transient study in the northern Gulf of Mexico Basin

Sequential two-dimensional forward modelling of source rock maturation along a north–south 600 km megaregional profile across the northern Gulf of Mexico Basin illustrates the coupled influence of allochthonous salt evolution and overpressure development on source rock maturation. Model results indicate that allochthonous salt evolution strongly affected local and regional source rock maturation, while overpressure development had a minor impact. Within the lower slope portion of the profile (south of the Sigsbee escarpment to the Tabular Salt/Minibasin province), localized thermal lows associated with salt stock development locally retarded the maturation of source rocks, while the slow development of overpressures effected little influence on the maturation. Within the centre of the profile (offshore Louisiana shelf), however, the evolution of allochthonous salt sheets and shallow salt structures locally and regionally lowered the oil generation window. The timing of overpressures in this region, related to the progradation of the Cenozoic marine depocentre, may have accelerated the maturation of middle Eocene source rocks, although the deeper Turonian source rocks had reached maturation at the time of greatest overpressure development. In the northern portion of the profile (southern Louisiana), the Terrebonne salt sheet lowered the oil generation window and retarded the maturation of the middle Eocene source rocks.

Abstract: Spatial Variations in Formation Water Salinities, South Pelto and South Timbalier Areas, Eastern Louisiana Continental Shelf 

ABSTRACT Spatial variations in pore fluid pressure, temperature, and salinity were determined from borehole logs along a 100-km long north-south transect from the Louisiana coastline to the shelf edge in the South Pelto and South Timbalier outer continental shelf areas to help identify driving forces and pathways of regional fluid flow and solute transport, information potentially useful also in identifying pathways of hydrocarbon migration. The Pleistocene to Upper Miocene sediments were deposited in fluvial, deltaic, and open marine environments and hence contained waters of fresh to brackish to normal marine salinities, <1 to 35 g/L, at the time of their deposition. Most of these sediments, however, now contain hypersaline fluids having salinities of up to 140 to 170 g/L. Exceptions are shallow Upper Pleistocene fluvial and deltaic sediments that contain fluids less saline than sea water, even though this section is now overlain by seawater. These shallow brackish fluids probably represent the remnant of a regional freshwater lens formed during the last low-stand of sealevel. A regional plume of water having salinities in excess of 140 g/L dips gulfward to the shelf edge at depths of 3 to 5 km below the seafloor. The locus of this plume is within the sandier Pliocene section. Potential sources of dissolved salt within this plume include updip shallow salt structures and the underlying remnants of an allochthonous salt sheet at depths of approximately 6 km. Regional faults in the area may have acted as vertical conduits for upward transport of brines. Down-dip fluid flow was presumably in part gravity-driven. The distal end of this regional salinity plume near the shelf edge is now highly overpressured, and fluid flow should be in the opposite direction, i.e., updip. Hence, this part of the regional salinity structure was probably established prior to over pressuring. The results of this study establish the dynamic nature of formation water flow which has occurred in the outer continental shelf. Further work is now needed to quantify the magnitude of the driving forces and rates of these processes.

Bryant Canyon Fan System: An Unconfined, Large River-Sourced System in the Northwestern Gulf of Mexico

The high-relief sea floor of the northwestern Gulf of Mexico slope is pockmarked by salt-rimmed minibasins and dissected by channels and canyons. Analysis of multichannel seismic reflection data reveals that Bryant Canyon and its fan are an unconfined, fluvially sourced system associated with the ancestral Mississippi River. Bryant Canyon eroded an extensive bypassing cut on the slope and delivered sediments to the unconfined area of the base of slope where they formed a fan. The canyon incorporates several minibasins and shallow salt structures. The canyon cross sections are easily recognizable in the minibasins, but are partially or completely obliterated by salt tectonism where the canyon incised the shallow salt structures. Salt appears to have encroached upon the can on after abandonment and restored the sea floor to its precanyon, low-relief profile. We interpreted three depositional units in the Pleistocene section of the Bryant Canyon Fan. The lower units are characterized by erosional surfaces or channels and mass-transport deposits. The top unit consists of low- to moderate-amplitude reflections with moderate continuity, suggesting relatively uniform deposition. The widespread occurrence of erosional surfaces and channels in the lower units may suggest extensive canyon activity during sea level falls. Decreased depositional energy from lower units upward indicates abandonment of the canyon or eastward shift of the ancestral Mississippi River. The Bryant Canyon Fan system contains several potential reservoir facies, including turbidites, trapped in high-relief minibasins during periods of less extensive canyon activity, that may exhibit good reservoir quality and massive sand occurring in canyon fill. Channel deposits and distal, unchannelized turbidites of the fan also may contain reservoir-quality sand. Mass-transport deposits with moderate- to high-amplitude reflections and levee deposits close to channels probably contain sand.

Distribution of shallow salt structures, lower slope of the northern Gulf of Mexico, USA

Shallow salt structures on the lower slope of the northern Gulf of Mexico are interpreted to be the manifestation of a unique geological situation: an unusually thick original salt layer mobilized and deformed by episodic Cenozoic sedimentary loading into a variety of shallow subsurface forms. Salt deformation, observed on seismic data, increases in intensity and complexity from the relatively simple shallow salt canopies of the Florida Escarpment area to partitioned and deformed shallow salt masses of the offshore Texas area. Salt ‘tongues’ (extensive lateral salt bodies in the shallow subsurface, with demonstrable sources) are observed from the Mississippi Fan area to the western edge of the lower slope. Salt tongues vary in style from subhorizontal, very shallow, laterally persistent tongues that comprise the frontal lobe of the Sigsbee Escarpment to adjacent undulatory, landward dipping tongues which rise obliquely through the sedimentary section. The salt tongues form a trend which defines the southern limit of shallow salt in the northern Gulf of Mexico. Shallow salt features landward of this trend are interpreted as earlier formed salt dome and tongue systems, the original shape and distribution of which were modified by partitioning of a precursor shallow salt feature by sedimentary loading. The morphology of the prominent Sigsbee Escarpment is due entirely to lateral and basinward movement of asymmetric salt masses. Lateral displacement of salt in these salt tongues is of the order of only tens of kilometres. The occurrence of dome-like features and withdrawal basins landward of the salt tongues suggests that the tongues originate from or in association with these domes.

Salt Piercement in Shallow Gulf Coast Salt Structures

Data from 16 salt structures of the Gulf Coast province have been analyzed to (1) document some of the piercement features on shallow Gulf Coast salt structures and (2) investigate some of the mechanisms and controls of the salt-piercement processes. The process of piercement by vertical displacement of overburden by salt has been the primary subject of this study. Accordingly, the investigations have concerned mainly the upper section of alternating sandstone and shale beds that are in contact with the salt and show a discordant relation with the salt core. The two general types of piercement processes recognized on shallow Gulf Coast salt domes are (1) piercement by rupture of the overburden through fracturing, faulting, and tilting of the overburden upward and outward in segments, with rise of the salt being enhanced by erosion, and (2) piercement by vertical displacement of overburden ahead of the growing salt mass, accompanied by peripheral faulting surrounding the salt dome. The discordant relation commonly observed in the upper parts of shallow salt structures between the salt plug and the enveloping strata is the result of piercement by vertical displacement of overburden by salt. The discordant relation can be produced by piercement of each bed contemporaneously with deposition or by postdepositional piercement of a thick section of overburden. Postdepositional piercement and vertical displacement of overburden as much as 2,400 ft thick can be documented in the Gulf Coast. Piercement by vertical displacement is most likely where the salt has a plug shape and is growing by a process of plug flow. The plug shape evolves from a broad salt mound or pillow in the later stages of salt dome growth. Piercement by rupture and outward tilting of segments of overburden is most likely in the earlier stages of salt-dome growth, when the salt mass is in the broad mound or pillow shape or is changing from the pillow to the plug shape.

Emba Salt-Dome Region, U.S.S.R., and Some Comparisons with Other Salt-Dome Regions

Although oil has been produced since 1908 from salt domes in the Emba area, which lies between the south end of the Ural Mountains and the north shore of the Caspian Sea, the geological situation in that region has escaped the attention of many American geologists. Salt-dome geologists of the Gulf Coast region have frequently had their attention directed to the salt structures of Germany and of Roumania, whereas greater resemblance to the Gulf Coast is to be found in the Emba district, where more than 100 salt domes are known in an area about the size of the Gulf Coast exclusive of South Texas. Future discoveries may bring the total to 300 or more salt domes, including deep ones. Only six domes are being exploited but many others have shown evidence of accumulations of oi and gas. Production has been found in Permo-Triassic, Jurassic, and Cretaceous strata, but chiefly in Jurassic sands. The oil reserves appear to be commercially important but do not rank with those of the Gulf Coast. The Emba mother salt series is of Permian age--probably Upper Permian. The overburden is much thinner than that of the Gulf Coast, consisting chiefly of Mesozoic strata, with local patches of Tertiary sediments, and a blanket of Quaternary (Caspian) beds in the coastal area. Many of the shallow salt structures are larger than any Gulf Coast shallow domes. Future exploration will probably reveal an increasingly larger percentage of deep domes. More is known about pre-salt strata in the Emba region than in the Gulf Coast because of the outcrops in the southern Urals. The Upper Carboniferous and Permian sections have been rather closely correlated by several investigators with the Pennsylvanian and Permian sections of the Mid-Continent region, largely on the basis of the ammonites contained. Development has been slow in the Emba district due largely to the remoteness of the area, the long severe winters, and the thinness of the sands as compared with the prolific Tertiary sands of the Caucasus belt. The tectonic setting of the Emba domes is discussed and compared with that of the salt domes of the Gulf Coast, of the North German basin, of Persia (Iran), and of the Carpathian foothills belt in Roumania.