Stringers in Salt Formation

? Definition:

Stringers are layers or fragments of rock mainly consisting of minerals other than halite (e.g., anhydrite, carbonate, claystone) that are embedded in salt (halite) and have different mechanical and flow properties than the salt. The stringers are sometimes also referred to as floaters or floating blocks, stringers are commonly over-pressured.

? Importance:

The carbonate ‘stringer’ play is one of the oldest petroleum occurrences, and it constitutes one of the most complex and unconventional deep oil and gas plays, like in the south Oman salt basin.

? Lithology and depositional environment:

Consist of depositional cycles of (carbonate- evaporite or clastic-carbonate-evaporite ) sedimentation in a tectonically active basin, Each cycle contains several isolated platforms (claystone, carbonate, gypsum/anhydrite, halite, and K-Mg salt rocks) formed during transgressive to highstand accommodation conditions.

Stringers have a variable average thickness (10–150 m thick) and a complex structure dominated by boudinage and folding. They are commonly broken into mappable fragments of varying sizes, but are most likely still arranged along the deformed boundary of salt.

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? Types of deformation and mechanisms controlling the movement of salt stringers:

The movement of salt is driven by tectonics in combination with passive diapirism and deformation conditions (stress and strain) and styles (brittle or ductile) during halokinesis.

The mechanical behavior of both carbonate and anhydrite is different from that of salt, Salt tends to be (ductile) while carbonates and anhydrite stringers tend to be more (brittle), causing a highly complicated deformation pattern that includes shear zones, folding, and boudinage.

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? Stringer Fluids:

Fluids inside stringers may contain brine, natural gas (mainly methane), oil, wet gas, condensate and sour gas (H2S). In addition, varying inorganic gases like carbon dioxide (CO2), nitrogen (N2), and hydrogen (H2) may be present in salt formations.

The intra-salt stringers in (the south Oman salt basin) were regarded as a self-charging hydrocarbon system.

There are the grounded “stringers” located at the base of the South Oman Salt Series. Recent geochemical work suggests a contribution of pre-salt source rocks in these stringers, which can be fed or not by pre-salt source rocks via a dense fault network.

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? Mechanisms of overpressure in the stringer:

1- Rapid compaction during burial leads to non-hydrostatic pore pressures and sealing by impermeable salt.

2-Pressure- and temperature-induced changes in the density of pore fluids.

3- Dewatering of gypsum during transformation to anhydrite may add to the overpressure in stringers with brine as pore fluid.

4- Hydrocarbon generation leads to additional overpressures if the stringers have moved to the oil or gas maturation window during their burial history.

5- The deformation and deflation of stringers, including fracturing, faulting, folding, and boudinage, is prone to overpressures, due to enhanced permeability along interconnected fracture networks.

Reference and credit:

1. Hisham A. Al-Siyabi
2. TNO Advisory Group

3. Peter A. Kukla1, Lars Reuning1, Stephan Becker1, Janos L. Urai1, and Johannes Schoenherr2 Search and Discovery Article #41112 (2012) Posted December 31, 2012

4. Hamood Al-Habsi

5. Petroleum Development Oman (PDO)