Abstract: The Role of Salt in Restraining the Maturation Levels of Sedimentary Basins

Ulisses T. Mello, Garry D Karner, Roger N. Anderson

The thermal anomaly associated with slat masses modifies significantly the temperature structure and maturation history of sedimentary basins. In this work, we explore the role of the negative thermal anomaly nearby slat diapir roots in modifying the maturation history of subsalt source rocks.

Modeling of various evolving salt body shapes using finite element method has shown that, in general, the refraction of the heat flow induces dipole-shaped temperature anomalies. The positive anomaly is located on the top of the salt diapirs and the negative anomaly is close the salt base. These dipole temperature anomalies can be very asymmetric and its degree of symmetry depends both on the (1) shape of the salt mass and (2) on the proximity of the top if the salt body to the basin surface. Exceptions to the dipole-shaped temperature anomaly pattern are the salt layers and the salt domes that reach the surface. They produce monopolar temperature anomalies. Below the salt layers all the sediments independent of their depths and lithology are colder with respect to a section with no s lt. Similarly, salt domes that reach the surface drain very efficiently the heat from below and from the side of the diaper. These negative thermal anomalies may be as deep as 2-3 times the depth to the base of the diaper and they may reach values of -85°C locally.

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 go to positions closer 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 slat diapir during the salt piercement stage.

Our results indicate that deep sedimentary basins with salt are more prospective than basins without salt or without salt diapirism, not only because of the structural traps related to the salt tectonics but also because salt diapirism tends to keep the basin colder. This relatively colder basin evolution may be responsible for hydrocarbon generation and/or preservation in the deepest parts of salt basins. This relatively cooling effect may be responsible important in continental margin basins (e.g., Brazilian and West African margins) where most of the source rocks lie underneath the evaporite deposits. In the Gulf of Mexico, subsalt source rocks are predicted to be overmature in previous maturation modelings. The inclusion of salt diapirism causes significant delay in maturation lev ls of subsalt sediments in the deepest part of the Gulf of Mexico basin.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994