Universidade Fernando Pessoa

Porto, Portugal

Salt Tectonics Short Course

3- Growth of Salt Domes (cont.)

3.7- Geological Model of a Salt Dome (without significant overburden extension)

Trusheim’s model was advanced to explain salt structures observed in Central Graben (North Sea). In this model there is no significant extension of the overburden (fig. 79). The salt flows mainly by density contrast and differential pressure.

Fig. 79- Trusheim’s geological model explains certain salt structures without significant lengthening (extension) of the overburden. Halokinesis is the main mechanism. It explains not only the salt structures but the overburden depocenter as well.

In Trusheim’s model, three main geological stages can be recognized:

a) Mound Stage: the salt flowage creates a salt mound.

b) Salt Dome Stage; the upward salt flowage creates a salt dome and adjacent rim synclines (overburden depocenters).

c) Post-Dome Stage; the upper part of the salt dome flows laterally creating an overhang, enhancing the turtle-back structures.

The seismic lines illustrated in the next figures are examples of salt structures, which can easily be explained by the Trusheim’s model. Indeed, the tectonic disharmony, at the bottom of the salt layer, as well as prekinematic and synkinematic sedimentary intervals of the overburden are recognized without difficulty. Rim synclines are located around the salt structures, where the compensatory subsidence is higher, within the synkinematic overburden intervals.

Fig. 80- The sub-salt strata is mainly composed of Paleozoic sediments. The tectonic disharmony is roughly flat, which suggests an absence of significant tectonic stress. The overburden is composed by two principal intervals. The lower interval, in green colors, is prekinematic. The upper one is synkinematic and rim synclines are found around the salt dome.

Fig. 81- Here, the overburden here is composed of three intervals: (i) prekinematic, (ii) synkinematic and (iii) postkinematic. The sub-salt strata are Paleozoic sediments. The faults affecting the sub-salt strata can be explained by different hypothesis (see later). An extensional tectonic regime seems to have been active before the halokinesis took place.

3.8- Geological Model of a Salt Dome (with significant overburden extension)

In this model, proposed by Vendeville and Jackson (1992), salt domes are developed in association with a lengthening of the overburden, thereby, one cannot speak in halokinesis but in salt tectonics, since a significant tectonic stress is taken into account, as illustrated below (fig. 82).

Fig. 82- Geological model proposed to explain some diapiric salt structures when the overburden and the salt mother layer are lengthened by an extensional tectonic regime (1 vertical).

This geological model explains a lot of salt structures observed in the South Atlantic margins particularly in the offshore Angola. The authors considered three main evolutionary stages:

(i) Reactive Stage

The overburden is extended (normal faulting). Small anomalies at the top of the salt can locally extend the overburden by normal faulting.

(ii) Active Stage

The salt flows upward and arrives at the surface (or sea floor).

(iii) Passive Stage

This passive stage creates apparent diapirism. The salt does not deform the sediments of the overburden. Salt flows upward as sedimentation progresses on the condition that the salt dome is connected with the mother layer.

Fig. 83- With this tectonic mechanism, salt glaciers can occur. Indeed, when extension and sedimentation stops, if the diapir is connect with the mother salt layer, in surface, the salt can flow outward creating namakier, that is to say, salt glaciers (see fig. 85).

Fig. 84- This salt structure was shortened by a late reactivation of the fracture zone underline by the major fault (in blue). The normal faulting affecting the lower part of the overburden strongly contributes to the location and initiation of the salt flowage. Probably, at the onset of the overburden deposition, the fracture zone created a small salt anomaly, which initiated a local extension (normal faulting) of the overburden.

Fig. 85- The extension of the overburden initiated the flowage of the salt. Reactive and passive stages can be recognized during the evolution of the salt dome. On the other hand, it is likely that a late compressional tectonic regime slightly shortened the upper part of the salt dome.

Fig. 86- In spite of the late contraction of this salt structure, the extension of the overburden, the reactive and passive stages of the dome, and the superficial outward salt flowage (salt glacier) are recognized without difficulty.

3.9- Stretching Faults

3.9.1- Currie’s model

Salt movements induce local extensional tectonic regimes in the overburden characterized by 1 vertical. According to the Mohr’s theory, the sediments are lengthened by normal-faults striking parallel to 2, , as illustrated in Currie’s model (fig. 87). Salt movements induce local extensional tectonic regimes in the overburden characterized by 1 vertical. According to the Mohr’s theory, the sediments are lengthened by normal-faults striking parallel to 2, as illustrated in Currie’s model (fig. 87).

Fig. 87- In Currie’s experiment, the extension produced by the normal faulting is higher than the apparent shortening produced by the piston uplift. In addition, the westward fault displacements are balanced by the eastward displacements. In other words, the sum of the positive (right vergence) and the negative (left vergence) throws is zero.

3.9.2- Ellipsoid of Effective Stresses

Depending on the shape of the salt, the ellipsoid of effective stresses can be triaxial or biaxial:

a) When the ellipsoid is triaxial, 2 and 3 are different. The associated normal-faults (stretching faults) strike parallel to 2. This geological situation is found in association with elongated salt structures, as shown in fig. 88.

b) When the ellipsoid is biaxial, 2 is equal to 3. The local normal faults are parallel to 2. However, as s3 is equal to 2 , the normal-faults can be oriented in any direction; they are radial stretching faults.

In salt basins, these two geological situations are mainly associated with salt walls and salt circular diapirs, which, by definition, have different geometrical sections. The first have an elongate geometry, while the latter have a circular geometry.

Fig. 88- The tectonic sketches show different geometries of stretching faults. They are mainly related to the geometry of the associated salt body. Generally, as shown in the following figures, radial stretching faults are associated with salt domes and elongated stretching faults with salt walls or salt ridges.

Fig. 89- In this tectonic sketch, from the conventional offshore Angola, not far from Luanda, the salt bodies correspond to elongated salt walls. The associated normal faults strike parallel to the regional 2. They are more or less North-South. However, in detail, their geometry is slightly curvilinear; their hade increases in depth and their concavity indicates roughly the dip of the fault plane.

Fig. 90- This tectonic sketch from offshore Texas suggests three different fault patterns: (i) in blue, the growth-faults associated with the shale ridges, (ii) in red, the antithetic-faults associated with shale ridges and (iii) in green, the radial stretching-faults associated with the salt domes.

Fig. 91- This time contour map illustrates the tectonic sketch of a potential reservoir interval in the overburden of a salt dome. The time map, which was built using a 3D survey, suggests a local extensional tectonic regime characterized by a biaxial ellipsoid of the effective stresses. On the apex of the antiform, the normal-faults strike in all directions. They are radial stretching-faults.

Fig. 92- On this map of the sea floor, the faults overlying a salt structure are obvious. Three principal fault systems can be recognized. They correspond to extensional tectonic regimes induced by different geological features. Regionally, the curvilinear system striking roughly N-S seems to be dominant. It is associated with second-generation salt walls. All these fault systems are easily recognized on seismic lines as illustrated in fig. 93.

Fig. 93- On this seismic line (see location on fig. 92), the stretching faults developed in the overburden immediately above the salt structure are quite evident. These faults affect the sea floor creating an obvious bathymetric anomaly.

3.10- Dome Perturbation Wavelength

3.10.1- Dome wavelength

Taking a look to the aerial photo (fig. 94) and the map illustrated in fig. 95, where are located the principal salt domes of the Zagros Mountains, and adjacent Persian Gulf, it is easily to recognize the distance between the salt domes is more or less constant.

Fig. 94- In spite of the fact that the salt domes seem to be along major strike-slip faults, it is interesting to notice that the distance between them is more or less constant. This hypothesis is not refuted by the NIOC map illustrated in fig. 95.

Such a hypothesis begs two questions: (i) Is that a simple coincidence? and (ii) Is the salt dome distribution controlled by some physical principle?

The answer to these question was given by Goguel (1983), when he hypothesize that two domes can be formed, without any interference, only when they are fed by salt located within the double’s radius (fig. 96). The Goguel’s hypothesis is the application in Geology of the Rayleigh-Taylor instability in compressible Newtonian fluids, which can be summarized as follows: The Rayleigh-Taylor instability is an inherent instability in a layer of viscous fluid of uniform density overlying a compositionally less dense layer. Small perturbations in the horizontal interface become amplified at a rate represented by an eigenvalue Special value of a variable parameter, for which the solution of an equation is nontrivial) that depends on the thickness, density, and viscosity of every layer, size of initial perturbation, and time elapse.

Fig. 95- The cartography have the major salt domes in Zagros and Persian Gulf strongly suggests that the distance between them is more or less constant. In other words, the salt domes’ distribution seems to follow the Rayleigh-Taylor instability hypothesis.

The Rayleigh-Taylor instability physical principle points out:

a) When two Newtonian fluids, with different densities, are superposed and the denser is on the top, with time, the lighter will change to the top. (at room temperature and pressure, a rock clearly reacts to stress very different from a material such as water. If a stress is applied to a fluid, it begins to flow. When the stress is removed, the flow stops, but the fluid does not return to its undeformed configuration, and the deformation is said to be non-recoverable. The larger the applied stress, the faster the fluid flows, which suggests a relationship between the stress and the strain rate. This type of behavior is most simply idealized as a linearly viscous, or Newtonian, constitutive equation, and the appropriate one-dimensional equation for constant-volume deformation relates the normal deviatory stress Dn to the incremental shear strain per unit time en or the shear stress s to the incremental shear strain per unit time s)

b) The beginning of such an inversion starts with the development of small positive equidistant anomalies on the surface of the lighter fluid.

c) The equidistance between the anomalies is the double of the radius of each anomaly, which is function of:

(i) The density contrast between the fluids.

(ii) The height of the denser fluid.

Therefore, in salt basins, and particularly on the flat areas of these basins, the distribution of the salt domes seems to obey the Rayleigh-Taylor instability, as suggested in certain areas of the offshore Gabon (fig 97), in the Permian Eastern European Salt Basin (fig. 98) and in certain areas of the Gulf of Mexico (fig. 99).

Fig. 96- Assuming that the diapiric structures are connected with the mother layer, Goguel (1983) suggested the distance between salt domes is more or less constant. In physical terms, it can be said that there is a dominant wavelength (d) or an observed spacing of instability. The characteristic wavelength can change if the interface was initially deformed or if the Rayleigh-Taylor instability is influenced by others effects, such as differential loading, regional strain, thermal convection, or faulting, etc.

Fig. 97- The mapping of the salt structures in the offshore Gabon suggests that only locally the Rayleigh-Taylor instability can be invoked to explain the salt dome distribution (northern part of the map). The majority of the salt structures have a linear geometry (salt ridges), which strongly suggests an extensional tectonic regime with 2 striking NNW-SSW (roughly the direction of the shoreline).

Fig. 98- The mapping of the Permian salt of the Eastern European Salt basin suggests that the Rayleigh-Taylor instability was a main mechanism controlling the spatial distribution of the salt structures. Indeed, on this map, it difficult to recognize a particular lineation pattern on the distribution of the salt structures. The same is true, on the seismic lines of Caspian Sea (see fig. 16).

Fig. 99- In spite of the fact that in Gulf of Mexico the salt structure are partially associated with an extensional tectonic regime, in onshore, and particularly in the salt diapir provinces, the distance between the diapirs does not seem to be aleatoric. It is likely that the Rayleigh-Taylor instability had controlled the salt dome distribution.

3.10.2- Exploration Implications

The implications of the domes’ wavelength in petroleum exploration are quite important, particularly on the trapping hydrocarbon parameter. Indeed, the examples illustrated in next figures, clearly indicates that “Theory precedes Observation”, that is to say, that the knowledge of the Rayleigh-Taylor instability allows seismic interpreters to propose more consistent and less refutable geological interpretations (by definition a geological hypothesis must be refutable. The best geological hypothesis is the more difficult to refute. There is not such thing as a true geological hypothesis or theory).

Fig. 100- In Nordkap Basin, several years ago, this kind of conventional inductive interpretation was paramount. The majority of the oil companies stopped the HC exploration in the area, invoking a lack of significant structural traps. Seismic interpreters proposed so huge salt domes that they became potential targets for storage of HC or radioactive waste. At that time, interpreters ignored the Rayleigh-Taylor instability principle and the basic principles of salt tectonics. (see fig. 101).

In fact, several years ago, the hydrocarbon potential of the Nordkap basin (offshore North Norway) was considered as very weak. Such petroleum hypothesis was advanced on the basis of the absence of traps, particular structural traps with significant closed surfaces. The other hydrocarbon parameters were neglected. Indeed, taking into account the geographic location of the Nordkap basin, the killer hydrocarbon parameter was considered to be the size of the potential hydrocarbon accumulations. So, the search for petroleum traps was made on a wide seismic grid by seismic interpreters unknowing the geological setting of the area. The surprise came, when younger interpreters, with a good geological and physical background, took a quick look at few regional seismic lines of the area:

a) They recognized a lot of huge morphological traps by juxtaposition.

b) Their geological interpretations of the seismic lines, and particularly the pick of the limits of the salt, were completely different of those proposed previously, as illustrated in fig. 101

Actually, the conventional interpretations, as the one shown in fig. 100, which do not take into account the Rayleigh-Taylor instability, imply:

(i) A thickness of the salt domes (plus mother salt layer) higher than 15-20 km, which is much higher than the total thickness of the sedimentary basin.

(ii) An absence of evident trapping mechanism.

The alternative-interpretations proposed by explorationists having a good geological knowledge of the area and knowing the meaning of (i) the second law of the thermodynamics (Goguel’s law) and (ii) the Rayleigh-Taylor instability, is illustrated bellow (fig. 101).

Fig. 101- This alternative interpretation (compare with fig. 100) takes into account the fact that a seismic line is a time profile and the Rayleigh-Taylor instability principle. Potential traps by juxtaposition (sub-salt traps) are evident. Subsequently, the hydrocarbon potential of the basin becomes dependent mainly of the generating and reservoir petroleum sub-system.

Such an interpretation indicates that the majority of the diapiric structures are not in a dome stage (see later), but in a post-dome stage. They show well developed overhangs. Indeed:

- They are not cylindrical.
- Their stem has a relative small radius.
- They are roughly equidistant.
- Some of them are, or can be, completely disconnect of the mother salt layer.

On the other hand, this kind of interpretation suggests the possibility of huge morphological traps by juxtaposition under and against the salt of the overhang. Thus, at the hydrocarbon stand point, may be the Nordkap basin has no potential at all, but the killer parameter is not the trapping, but eventually the absence of a generating petroleum sub-system.

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