Fluid Flow,
Overpressures and Fracture Sealing Events in Mesozoic Limestones,
Jebel Akhdar Dome, Oman Mountains
Hilgers, Christoph1,
David
Kirschner2,
Jean-Paul Breton3, Janos Urai1 (1)
RWTH Aachen, Aachen,
Germany (2) Saint Louis University, Saint Louis, MO (3) BRGM Oman Branch,
Muscat,
Sealed fractures are frequently described to have acted as
important fluid conduits, although studies considering fracture formation, vein
growth history and regional scale variations are rare. We studied veins hosted
in autochthonous Mesozoic limestones of the Jebel Akdhar dome, Oman,
using meso- and microstructural
analyses and stable isotope geochemistry. This reservoir rock displays
different sealing and overpressure events, which display the complex fluid
history during subsidence and exhumation. At least six fracture sealing events
were identified in the dome. Early sets of extension veins formed at
supra-hydro-static fluid pressure and low differential stresses <40 MPa during burial and are truncated by bedding parallel
veins. Evidence of exhumation is given by normal faults, which contain two
different phases of sealing events. Late thrusts displace the normal faults and
represent a late compression phase. Samples of these different vein sets and
their host rocks were analyzed in the stable isotope laboratory at Saint Louis University to provide information on
fluid-rock interaction in the dome and the scale(s) of fluid movement. Oxygen
isotope values range from ca. 16 to 29 per mil; carbon isotope values range
from 0 to +4 per mil. The initial results from approximately 120 analyses are
consistent with a rock-buffered system during formation of the earliest veins,
and a more open system that allowed external fluids to infiltrate the dome
during normal faulting and late thrusting. Late fluid infiltration is in
accordance with mechanical constraints of high differential stresses during
late faulting events and suggests drained conditions.