THE ROLE OF SALT IN RESTRAINING THE MATURATION OF SUBSALT SOURCE ROCKS
Ulisses T. Mello (1) , Garry D. Karner (2), and Roger N. Anderson (2)
(1) Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão, Rio de Janeiro, RJ, CEP 21910,
Brazil.
Current address: IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.
(2) Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964, U.S.A.
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. In this paper, we
explore 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. We used
the finite element method to model the transient and steady-state conductive temperature
perturbations induced by salt layers, domes, and pillows. Our results indicate that the
modification of the thermal regime due to evolving salt domes may affect significantly
the maturation level of source rocks in the vicinity of the domes. Modeling 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 asymmetric, 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 drain very efficiently the heat from
below and from the side of the dome. These negative thermal anomalies may extend for
considerable depth beneath the base of the salt dome and may reach values of -85 *
C locally. Because of the large contrast of 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.
Our 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, we find 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
m.y). 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 the source rocks.