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 in
this study derive from wells, which are located around this area. This cross-section illustrates the occurrence of HC-bearing carbonate reservoirs
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
clear 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 log-evaluations of
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 over-pressured.



Now, focusing on the properties of Halite, deformation mechanisms involve, dislocation-creep, pressure-solution creep and in minor cases microcracking.
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 deformation mechanism.
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 study is
gamma-irradiation. The blue coloration is caused by defects in the sodium chloride lattice. Results are comparable to Cathode-luminescence: the
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 grain boundaries.



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 describe 3 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 and steeply dipping microcrack reveals a brownish solidified oil film within irradiated Halite.




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 recrystallization. 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 this grain-boundary.

The second type is characterized by an intracrystalline incorporation of isolated black droplets within large Halite crystals during dynamic recrystallization.



Evidence for dynamic recrystallization of Halite at the simultaneous presence of oil is indicated by deformed particles. The alignment of these droplets
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 inclusion.



Using subgrain size piezometry, the calculated maximum past differential stress for the Ara Salt around the carbonate reservoirs is smaller
than 2 MPa. This is in the range found worldwide…


Following the dilatancy boundary for Halite, derived from laboratory 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 SOSB, extensive 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 pressures.



Dynamic recrystallization continued after the main stage of HC-impregnation. Enhanced temperatures during subsequent burial converted
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 interior 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.