Role of salt in restraining the maturation of subsalt source rocks

Abstract

The presence of salt within a sedimentary basin can potentially modify its temperature distribution and history. In particular, the positive thermal anomaly associated with the top of salt domes has attracted considerable attention in the past. The role of the lesser appreciated negative thermal anomaly associated with the base of salt domes in modifying the maturation history of subsalt source rocks is explored. The finite element method was used to model the transient and steady-state conductive temperature perturbations induced by salt layers, domes and pillows. The results indicate that the modification of the thermal regime due to evolving salt domes may significantly affect the maturation level of source rocks in the vicinity of the domes. Modelling the temperature structure of various salt structures has shown that, in general, the refraction of heat flow induces a dipole-shaped temperature anomaly; a positive anomaly located towards the top of the salt structure and a negative anomaly located towards its base. These dipole anomalies can be strongly asymmetrical, the degree of asymmetry depending on the shape of the salt structure and the proximity of the top of the salt structure to the surface of the basin. However, when the salt structure reaches the surface, the dipole-shaped temperature anomaly collapses to a monopole. Below the salt structure, all sediments, independent of their depth and lithology, are colder relative to a section with no salt. Similarly, salt domes that reach the surface very efficiently drain the heat from below and from the side of the dome. These negative thermal anomalies may extend for a considerable depth beneath the base of the salt dome and may reach values of −85°C locally. Because of the large contrast in thermal conductivity between the highly porous sediments and salt at lower temperatures, the efficiency of a salt dome to channel heat increases the closer the salt dome is to the surface. These results indicate that deep sedimentary basins containing salt are more prospective than basins without salt and/or salt diapirism. In addition to the structural traps associated with salt tectonics, salt domes and tongues connected to their source dissipate heat more efficiently and thus keep deeper regions of the basin relatively colder and potentially within the oil window for a longer time. This cooling effect is maximized when the top of the salt dome remains close to the surface of the sedimentary basin for a significant period of time and may be especially important for continental margins such as Brazil and offshore West Africa, where most of the source rocks lie beneath extensive evaporite deposits. In contrast, it was found that for the Gulf of Mexico basin, pre- and Early Tertiary salt diapirism and sheet emplacement may have caused significant delays in the maturation of subsalt source rocks in the deeper regions of the Gulf basin, but the maturation is likely to be relatively insensitive to the Late Miocene-Pliocene stage of salt mobilization because the time interval has been too short (< 6 Ma). In general, the earlier the deposition or emplacement of salt sheets, the larger the restraint in the maturation level because the thermal anomalies induced by the salt have more time to affect the maturation history of source rocks.