Ulisses T. Mello
Petroleo Brasileiro Inc.
Rio de Janeiro
Roger N. Anderson Garry D. Karner
Lamont-Doherty Earth Observatory of Columbia University
Palisades, N.Y.
The thermal positive anomaly associated with the top of salt diapirs has attracted significant attention in modifying the temperature structure and history of a sedimentary basin. Here we will explore the role of the negative thermal anomaly beneath salt in modifying the maturation history of the source rocks in subsalt sediments.
The existence of anomalously high temperatures around the top of salt diapirs has been known for some time.12 Two factors are responsible for these temperature anomalies: (1) the difference between the thermal conductivity of the salt and surrounding sediments (Fig. 1), and (2) the geometry of the salt bodies.
At room temperature, salt thermal conductivity may be up to five times greater than the thermal conductivity of porous sediments. The refraction of heat flow associated with salt diapiric geometry enhances the temperature anomalies originated by the thermal conductivity difference.
Diapiric salt structures are analogous to optical lenses that focus the light rays (heat flow) more or less effectively depending on their shapes and refraction indices (thermal conductivity).
However, few studies have been published on the temperature distribution around salt structures due to the mathematical and geological complexities involved in the thermal analysis of moving and deforming salt diapirs. Numerical modeling of steady state temperature distribution for simple shapes of salt domes, detached diapirs, and surrounding sediments showed that a maximum positive temperature anomaly of 200 C. could be induced above salt diapirs.34
In addition, a negative temperature anomaly of 200 C. is also predicted close to the root of the salt dome.45
Organic matter maturation is believed to follow temperature dependent chemical reactions. Therefore, any temperature anomaly associated with salt masses affects the nearby maturation of potential source rocks. The level of maturity of source rocks close to salt diapers will differ from that predicted based on regional trends. The impact of the thermal anomaly on a given point will depend on the duration and distance of the thermal anomaly to this particular point. Consequently, the maturation history of source rocks in salt basins is closely related to the salt motion history, implying that a transient thermal analysis is necessary to evaluate the true impact on maturation of the thermal anomalies associated with salt diaperism.
VITRINITE KINETICS
Vitrinite reflectance is the most widely used thermal indicator of the maturity level of organic matter.
Although vitrinite reflectance is characteristic of organic matter with low amounts of hydrogen (type III), it is a useful proxy with which to estimate the thermal maturity level of the other organic matter types.
The thermal maturity of potential source rocks is estimated using a vitrinite reflectance model based on the kinetic equations defining the conversion of organic matter to hydrocarbons.
Our vitrinite maturation model uses the Arrhenius first order parallel reaction approach distributed over 20 activation energy bands. The predicted vitrinite reflectance (% Ro) is related to the transformation ratio of the organic matter.8
In the following discussion of maturation levels we will use the terminology and the vitrinite reflectance (% Ro) values suggested by Waples.9 The values of % Ro: 0.6, 1.0, 1.3, 1.75, 2.0, 2.2, and 4.8 represent approximately the onset of oil generation, peak of oil generation, end of oil generation, upper limit for occurrence of oil with API gravity less than 400, upper limit for occurrence of oil with API gravity less than 500, upper limit for occurrence of wet gas, and last known occurrence of dry gas, respectively.
SALT IN EVOLVING BASINS
The temperature and maturation calculations begin with the forward modeling of basin evolution. The newly deposited sediments are represented by rows of elements that lay, on the top of the previously deposited sediments.
The amount of the sediment mass to be deposited in each time step is determined earlier by the back stripping technique. The salt movement is done kinematically and the salt deformation history is based on the geological reconstruction of the section.
This approach has the advantage of reproducing exactly the geologist's view of the salt evolution, but it has the disadvantage of not providing the deformation field history of the salt. There is no unique deformation path between the initial "non deformed" shape and the final deformed shape.
Therefore, to constrain the deformation path we used velocity fields that satisfy the Laplace and the bi harmonic equation (Table 1) and, consequently, conserve mass. This implies that the solution is valid for an incompressible fluid with very large viscosity undergoing steady flow between the time steps. This kinematic approach has been used extensively in aerodynamics and recently in graphic computing.10 11
Internal salt flow is obtained by assuming that the salt undergoes two dimensional channel flow and that the salt moves with a parabolic velocity distribution in the horizontal direction in a channel of variable thickness.12
The elements within the salt body are remeshed when they are excessively deformed by the salt motion. Outside of the salt body the elements compact as the sediments are buried. The salt velocity field is calculated for successive salt configurations and then the finite element mesh is updated during the time steps.
SALT AND TEMPERATURE
Steady state solutions are very useful to introduce concepts related to heat conduction involving salt masses with various shapes.
In addition, they are the end members of our thermal transient analysis. The geometry of the salt masses is an important factor in controlling the movement of heat flow within sedimentary basins. Common salt shapes include salt layers, salt pillows, salt domes, and mushroom shaped domes that are both connected and disconnected from the salt source layer.
Fig. 2 displays the temperature distribution profile of a shale section with a salt layer or sheet located between the depths of 4 5 km. The dashed line shows the temperature profile if no salt were present.
The effect of salt can be seen: (1) the temperature gradient is higher shallow in the section and is a function of sediment compaction and (2) the temperature gradient is lower in the salt layer, a function of the high salt thermal conductivity.
The area between the solid and the dashed lines represents the temperature anomaly induced by the presence of the salt layer in the section. As expected, the temperature in the salt layer (4 5 km) increases more slowly compared with the shales.
Below the bottom of the salt layer a difference of approximately 200 C. is maintained for all subsalt sediments, independent of their depths and lithology. Although this concept is simple, it has powerful implications on maturation of source rocks below salt sheets.
Any source rock below salt layers or sheets is subject to lower temperatures than present. Although 200 C. does not seem to be a large anomaly it may affect significantly, the maturation history of nearby source rocks. For example, a rule of thumb predicts that the rate of kinetic reactions for hydrocarbon generation doubles with every 100 C. increase of temperature.9
In addition, the effect of this negative anomaly is cumulative in time since maturation depends on time and temperature. As a result, we may interpret that the presence of salt causes a "cooling" effect when compared to a basin without salt. This cooling effect induced by the salt emplacement is proportional to the thickness of the salt layer.
All the sediments below the salt sheet are subject to a non transient cooling effect that lasts as long as the salt layer remains. It should be noted that in this case e presence of salt does not alter in any way the temperature field in sediments above the salt layer.
Fig. 3 displays the temperature distribution around a salt diapir connected to a salt sourcing layer. We observe three main f at res of the temperature field: (1) the temperature gradient is higher in the sediments above the salt diapir; (2) as expected, the temperature gradient is low within the salt body; and (3) the isotherms are depressed below the salt base.
Since we analyzed the effect of a salt layer in a shale section in Fig. 2, now we will focus our attention to the temperature disturbance induced only by diapiric salt shape rather than the salt source layer.
In order to do this we subtracted the temperature field of the salt layer section (Fig. 2, dashed line) from the temperature distribution around the salt dome (Fig. 3A) to obtain the temperature anomaly (Fig. 3B). Consequently, Fig. 3B displays the anomalous temperature distribution related to the salt diapirism with respect to the regional trend that has only the salt layer.
Fig. 3 displays a dipole-shaped thermal anomaly with maximum magnitudes of + 250 C. on the top of the salt diapir and 250 C. close to the base of the diapir source layer.
It should be added that the total negative anomaly with respect to a section without salt is about 450 C. because of the additional negative anomaly ( 200 C.) associated with the salt source layer.
Two additional observations should be emphasized: (1) the negative anomaly associated with the salt diapir tends to vanish with depth in contrast to the negative anomaly associated with the salt source layer that is constant and pervasive to subsalt sediments; (2) the wavelengths of the anomalies are not symmetric as predicted for cylindrical salt domes because of the dome diameter variation with depth.
This is an important point for the investigation of interference patterns of thermal anomalies associated with multiple domes. Salt domes that have their bases broader than their tops cause the temperature anomalies to interfere more constructively below the base of the domes than above their tops. This is because wavelength of the negative anomalies is broader.
Due to constructive interference the negative anomaly close to the salt base is almost double ( 400 C.) the magnitude of a single negative anomaly (-250 C.).12 Individual positive anomalies need to be closer than one dome diameter to interfere constructively.
Usually, salt domes tend to be separated by more than two diameter radii. Consequently, positive anomalies close to the top of the salt tend to be more restricted than the broader negative temperature anomalies that can extend for larger area beneath the salt diapir root.
SALT DOME HEAT DRAINS
Temperature anomalies associated with salt diapirs that have their tops close to the surface or even outcrop (e.g., Iran and Gulf of Mexico 13) present a dramatically different thermal comportment from the anomalies previously examined.
Fig. 4 displays the temperature distribution associated with a salt dome that reached the surface and is still connected to the salt source layer. The geometric shape of this dome was inspired by the diapirs from the East Texas salt basin. 13
Since this dome outcrops, there is no high thermal gradient above the top of the salt dome and, therefore, the temperature anomaly is monopolar in contrast to the dipolar pattern predicted earlier.
The magnitude of the negative anomaly reaches 850 C. locally, and this relative cooling affects more than six times the diapir radius. It acts as a very efficient heat drain, collecting and channeling the heat around and underneath the salt body and delivering it to the surface.
The vertical heat flow on the top of salt is approximately 150 mWm 2 (Fig. 5), almost three times the assumed basement heat flow (55 mWm 2). Beside the dome the vertical heat flow drops to values close to 28 mWm 2, showing how effectively this structure collects heat from the side.
The regional cooling effect can have a profound impact on hydrocarbon source rocks near such salt structures. This thermal behavior raises the question of how close to the surface a salt dome has to be to effectively drain heat in large amounts from the basin.
Fig. 5 displays the vertical heat flow at surface as a function of the minimum depth to the top of the salt dome. These results suggest a rule of thumb that predicts that the maximum negative anomaly is reduced 10 150 C. for every 250 m of increase in the depth to the top of the salt diapir (assuming that salt is surrounded by shales).
The heat draining mechanism described above may explain why many oil prolific salt basins still produce large amounts of hydrocarbons today when thermal calculations that did not include these effects indicate that most of the known source rocks should be overmature.
MATURATION RESTRAINED
No direct maturation estimates were done using the steady state temperature distributions associated with the above salt structures because maturation is time dependent and the salt structures were unrealistically assumed stationary and in equilibrium.
To investigate the impact of moving salt on maturation we used a hypothetical burial history based on a small area of the Santos basin located at the Brazilian Atlantic continental margin.
Sedimentation is assumed from the Neocomian (135 million years ago) to the present day and a salt layer with initial thickness of 1.75 km is postulated to be deposited in the time interval between 115 million to 112 million years before present. As before in the steady state analyses, we consider only two lithologies in the modeling, salt and shale.
The transient temperature distribution within and around a salt dome during its evolution shows that the isotherm patterns are intermediate between those discussed above (Figs. 2, 3). The temperature isotherm patterns change with time according to salt motion.
Initially, when the salt body is a layer, all the isotherms are parallel to each other and the temperature gradient is inversely proportional to thermal conductivity of the sediments. As the salt moves and forms a salt pillow, with large wavelength, the isotherms show the effects of heat flow refraction.
The isotherms above salt are approximately horizontal and sub parallel. The concentration of heat flow on the top of the salt pillow causes the temperature gradient in this area to be higher than laterally. Within the salt the isotherms have a concave upward shape (e.g., Fig. 3).
Keeping in mind that the isotherms are potential surfaces perpendicular to the heat flow, this isotherm shape shows how the salt collects heat both laterally and from the bottom of the salt layer.
Below the base of the salt pillow the isotherms have a concave upward shape but with smaller amplitude than within the salt pillow. The depression of the isotherms below the salt base is maximum directly below the salt diapir.
As the salt is buried, it domes upward, changing the amplitude and wavelength of the salt diapir. As the diapir wavelength decreases the salt restricts its lateral area of heat collection, but at the same time it concentrates more heat on its top. Our modeling shows that after the development of diapiric shapes there is always a temperature depression below the salt diapir!
Fig. 6 demonstrates the role of the salt in keeping areas beneath salt colder and, therefore, restraining the maturation of source rocks. The solid lines in this figure show the calculated reflectance of vitrinite (% Ro) within and around the evolving salt dome, whereas the dashed lines show the same domal evolution except they, are from shale diapirs. Thus, the dashed lines represent the maturation levels predicted for geological sections without salt.
Following the deposition of salt, the first effects of the salt restraining the maturation are immediately detectable. For example, at 112 million years before present, just after salt deposition and before diapirism the depth to the top of the oil window is close to 5 km with salt, whereas it is around 4.3 km without salt.
The cumulative "cooling" effect on maturation associated with the salt in the pillow stage causes the pattern of the iso maturation curves with depth to be quite different from the shale counterpart. As expected, the shaly section has iso maturation curves that are essentially flat while the salt dome section presents iso maturation curves with concave upward shape below the salt body.
In Fig. 6A, the dashed lines (without salt) show that at 49 million years before present subsalt sediments are overmature for oil. If the main source rock is located only in the presalt sediments, like many Brazilian and West African basins, all the oil would be gone before the deposition of the best reservoirs associated with the Middle to Late Tertiary turbidites.
In contrast, the solid lines (calculations with salt) show that the uppermost part of the presalt sediments remains in the oil window until the present day time. At present day (Fig. 6B) the iso-maturation curves present very distinct maturation levels within and below the salt dome region. These figures show clearly how the effects of the temperature anomalies around the salt diapir are "accumulated" in maturation levels through time.
In general, the shaly section presents always higher maturation levels than salt, except on the top of the diapir where the heat flow is concentrated. The differences in the maturation levels increase with depth and, consequently, with the age of the sediments.
In the section with salt the sediments deposited earlier were exposed to depressed temperatures for longer and, therefore, they display reduced or restrained maturation levels when compared to a similar basin without salt or without salt diapirism. These results strongly suggest that deep sedimentary basins with salt diapirism present may be far more prospective, longer than basins without salt.
CONCLUSIONS
The modeling of various diapiric salt geometries has shown that the refraction of heat flow induces a dipole-shaped temperature anomaly within and around the salt structures. A positive anomaly is located on top of the salt diapirs whereas a negative anomaly is located beneath the base.
The symmetry of the temperature anomalies depends on both the (1) geometry of the salt diapiric structure and (2) the proximity of the diapir to the basin surface.
Exceptions to the dipole-shaped temperature anomaly pattern are the salt layers and the salt domes that actually reach the surface. They produce monopolar temperature anomalies. Below salt sheets, all subsalt sediments are colder independent of depth when compared to basins without salt or without salt diapirism.
Similarly, salt domes that reach the surface drain very efficiently the heat from below and from the side of the diapirs. Because the thermal conductivity of the sediments depends on temperature the closer to the surface the salt body is the higher is its thermal conductivity and the more effective is the dissipation of the heat channeled by the salt structure.
Therefore, the faster the salt diapirs move to positions close to the basin surface, the colder the area around the base and underneath that salt body will be. The fast movement of the salt in direction to the surface would also minimize the "thermal damage" (heating) of the sediments on the top of the salt diapir during the salt piercement stage.
These results suggest the need for accurate basin history reconstructions involving salt tectonics because it is critical to estimate accurately the depth to the top of the salt structures through time. The amount of heat drained from the basin is very sensitive to the depth to the top of the salt structure. Therefore, the changes in depth to the top of salt diapirs can affect significantly the maturation history of source rocks in the vicinity of salt diapiric provinces.
Our results indicate that deep sedimentary basins with salt bodies are more prospective than basins without salt, not only because of the structural traps related to the salt tectonics but also because, in general, the salt diapirism keeps basins colder. This relatively colder basin evolution may be responsible for hydrocarbon generation and/or preservation in the deepest parts of salt basins.
The relative cooling effect induced by the salt emplacement and diapirism may be especially important in continental margin basins (e.g., Brazilian and West African margins) where most of the source rocks lie underneath the evaporate deposits. In the Gulf of Mexico, subsalt source rocks are predicted to be overmature in previous maturation modeling. Our results indicate that the inclusion of salt diapirism causes significant delay in maturation levels of subsalt sediments in the deepest part of the Gulf of Mexico basin.
Unfortunately, there is no data set publicly available that includes temperature measurements and geochemical data. We are looking for data to test these fascinating predictions. We hope our work contributes to increasing the momentum of subsalt exploration around the world.
ACKNOWLEDGMENTS
The authors thank Neal Driscoll, John McGinnis, Carlos Pirmez, and Lincoln Pratson for comments and discussion of this work. The help of Joy Allen is very appreciated for editing and improving the figures of this paper. This work was supported by grants from the Conselho Nacional de Pesquisa CNPq, Petroleo Brasileiro Inc. (Petrobras), and the U.S. Department of Energy Cooperative Agreement DE FC22 93BC14961.
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