Diagenetic and Thermal History of the Jurassic-Tertiary Succession of the Zagros Mountains in the Dezful Embayment (SW Iran): Constraints from Fluid Inclusions

To constraint the diagenetic and thermal history
in the Mesozoic-Tertiary succession of the Zagros
Fold-Belt, a study was performed on fluid inclusions
trapped in intergranular, vug, and fracture-filling cements
in Jurassic to Miocene outcrop samples collected along the
Anneh and Fahliyan Valleys of Khuzestan Province,
southwest Iran. Petrographic observations showed at least
two systems of fractures that postdate intergranular
cementation. Two different types of oil-filled fluid inclusions
occur in the intergranular cements and in the first
fracture network, but they are absent in the second fracture
network. Microthermometry of fluid inclusions was used to
determine the temperature and salinity of fluids responsible
for mineral precipitation. Within intergranular cements and
calcite fillings the oldest fractures, precipitation occurred
from high saline fluids in a temperature range of 60–120C,
depending on the stratigraphic depth. The strong correlation
between fluid inclusions temperature and stratigraphic
depth suggests that the intergranular cementation and the
first fracture-filling event occurred prior to structural
deformation. Furthermore, this correlation indicates a relatively
constant paleogeothermal gradient at the time of
cement precipitation of about 28C/Km. On the contrary,
fluid inclusions trapped in cements within the second
fracture network lack oil and contain freshwater trapped at
low temperatures that do not correlate to the stratigraphic
position. This suggests second fracturing event occurred
after oil migration and during or after deformation of the
Zagros Fold-Belt, when the samples were in a near-surface
position.