In the last talk of our session I will present an example from the
South Oman Salt Basin, showing that Halite as a soft sediment can
loose its property as a seal, when geological conditions are right.
This study is about the Evolution of Halite and Solid Bitumen in the
Petroleum System of the SOSB at RWTH Aachen and is financially
supported by Petroleum Development Oman.
Understanding the style of deformation and the rheology of soft
sediments, like Halite, is of wide importance like for the
long-term storage of radioactive waste or the exploration of HC`s. The
rheological behavior of Halite is under most geological
conditions fully non-dilatante and ductile, whereby porosity and
permeability remains near zero and crystal-plasticity is
accompanied by dynamic recrystallization. In general, in the subsurface
Halite has a very low porosity, which is not
interconnected and filled with brine or gas. Therefore, because of its
ductility, Halite is known to have the best rheological
properties for being a seal for HC`s. But as you will see, there is
more variability in the rheology of salt, which has a major
influence on the properties for salt as a seal! In the following we
will consider how good a seal is Salt.
The SOSB is a large evaporite/carbonate basin, here
colored in pink, which is located in the deep subsurface between 3000 m
and 5500 m. It formed
near the end of Neoproterozoic and its sedimentary infilling ended with
cyclic carbonates and evaporites of the Ara Group. Samples investigated
this study derive from wells, which are located around this area. This
cross-section illustrates the occurrence of HC-bearing carbonate
enclosed by large, domal bodies of Ara Salt. A schematic cross-section
shows the 6 carbonate/evaporite cycles
building up the Ara Group. HC-bearing
Halite samples investigated in
this study derive from wells exploring these carbonates for gas and
oil. In general, the habit of Halite is popular for its
transparent appearance, like this core sample deriving from the SOSB.
But, in this study we investigated black-stained Halite, like this core
sample drilled about 10 m below the base of a carbonate
reservoir in a depth of 4000 m.
This diagram gives an overview about the formation
pressures in the
SOSB. Evidence for over-pressured in-situ stresses derive from
production wells. While reservoirs with hydrostatic
formation pressures are arranged around the blue trend line, a lot of
reservoirs distributed along
the orange line, are
Now, focusing on the properties of Halite, deformation
involve, dislocation-creep, pressure-solution creep and in minor cases
1. A characteristic microstructure for dislocation-creep – see on the
right hand side – is the presence of subgrains. Dynamic
recrystallization of salt is very often associated with this
2. During pressure solution the highly stressed part of a grain goes
into solution and precipitates on the less stressed part.
3. Dilatancy is the inelastic increase in volume during deformation
under applied differential stress due to microcracking.
The first and second process usually occurs together during “normal”
salt tectonics, while dilatancy is only achieved under lithostastic
fluid pressures in the deep subsurface.
To investigate these deformation mechanisms we combine
at least 2
techniques in halite microtectonics. One of these techniques for this
gamma-irradiation. The blue coloration is caused by
defects in the sodium chloride lattice. Results are comparable to
internal microstructure is revealed in
samples which are otherwise colorless. In this example the thin section
shows white polygons which are
subgrains and the dark lines are
Another technique is the subgrain size piezometry. The
curved line is a
laboratory-calibrated line. If we measure the subgrain diameter
in our irradiated salt sample, we can say something about the value of
deviatoric stress during deformation.
To characterize the occurrence of the dark salt, we
different types of microstructures:
At first HC`s occurs along microcracks and grain-boundaries.
At second intracrystalline droplets of Solid Bitumen.
And furthermore oil inclusions.
In the first example a detail view of a linear arranged
dipping microcrack reveals a brownish solidified oil film within
Here, the grain-boundaries of Halite crystals - joining in a triple
junction - are stained brownish. If we now zoom into that triple
junction...(see next image)
… we recognize under reflected light a filling of the grain-boundaries
by bright reflective non-crystalline material, typically for the
properties of solid bitumen. After oil-influx, enhanced temperatures
led to the in-situ precipitation of solid bitumen out of oil.
This micrograph reveals several stages of dynamic
Obviously, the arrangement of subgrains occurred at first, followed by
a first stage
of grain-boundary migration, which stopped here and
left behind fine-grained remnants of dolomite. Finally, the
grain-boundary migrated into the right
subgrain-rich crystal. During
that process, HC-particles have been picked up and accumulated along
The second type is characterized by an intracrystalline
of isolated black droplets within large Halite crystals during dynamic
Evidence for dynamic recrystallization of Halite at the
presence of oil is indicated by deformed particles. The alignment of
from upper right to the lower left is thought to be a
former grain-boundary at which oil was injected during brittleness of
the salt. After that stage grain-
boundary migration continued from
right to left, shown by thinned particles to the left.
At least, the presence of oil aligned in “healed
cracks” also indicates
a stage of brittle deformation within the Ara Salt. The small
micrograph proves the presence of oil by gas bubbles within the oil
Using subgrain size piezometry, the calculated maximum
differential stress for the Ara Salt around the carbonate reservoirs is
than 2 MPa. This is in the range found worldwide…
Following the dilatancy boundary for Halite, derived
experiments by Popp et al. 2001, effective stress must be extremely
reduced to achieve
dilatancy in salt. Therefore in the Ara Salt around
the carbonate reservoirs, dilatancy is only possible at near-zero
effective stress. This means that in the
deep subsurface the fluid
pressure must be lithostatic.
Integrating these data into the tectonic history of the
deformation with dynamic recrystallization of the pre-cambrian
Ara Salt occurred in an early stage of evolution.
After generation of HC`s within the carbonates, oil
migrated from these
reservoirs into the Ara Salt. This happened during a stage of
dilatancy as pressures within the Salt changed to lithostatic fluid
Dynamic recrystallization continued after the main
HC-impregnation. Enhanced temperatures during subsequent burial
the oil into solid Bitumen. Therefore, when Halite is allowed to
dilate, it can become much more permeable than intact Halite under
“normal” geological conditions. It will loose its sealing capacity!
To extend this study, we collected Halite samples from
northern Oman, where the Ara Salt pierced the surface squeezing up
isolated carbonate platforms. Within these Salt domes…
… some outcrops show layered Halite with black bands. Partly, this Salt
incorporates black material, which has a fetid smell when
broken. Further investigations of these microstructures could show,
that oil leakage occurred over a wide distance in the SOSB.