A combined laboratory and modelling analysis of the evolution of fluid overpressure and fluid escape structures in the Mediterranean Salt Giant Basin

Abstract

The Messinian Salinity Crisis (5.97 to 5.33 Ma) led to rapid deposition and loading of thick evaporite units below what is now the Mediterranean Sea. Observations of large-scale methane venting, visible in the form of eruptive pockmark features in the paleo-seabed topography, indicate that high pore fluid overpressures were generated during that period fracturing the sediments, including the evaporites, of the Mediterranean basin. In this thesis, I present a quantitative assessment that links sea level fall, salt deposition, fluid overpressure development, and generation of fluid escape structures in the Mediterranean Salt Giant Basin. This thesis is divided into three scientific studies: (i) overpressure development from basin inception to present-day for basin-centre and marginal basins in the Western Mediterranean, (ii) quantification of gas overpressure and sea-level fall that triggered a possible fluid escape at the onset of the Messinian Salinity Crisis (MSC) in the Eastern Mediterranean, and (iii) laboratory investigation of effective pressure controls on the elastic and hydromechanical properties relations in evaporites. These studies use new data for evaporites from laboratory experiments performed in this thesis including permeability measurements from pore pressure transmission (PPT), mercury injection porosimetry, elastic wave and resistivity measurements in brine injection experiments, and 3-D X-Ray micro-CT (XCT) imaging. Evaporites have long been recognised as impermeable seals that create some of the world’s highest subsalt reservoir pressures. However, studies show that salts can retain open pore spaces and connected pore-fluid pathways. To reduce uncertainty on fluid properties of evaporites and increase the predictive ability of overpressure models applied to salt basins, I undertook laboratory measurements of evaporite properties (density, porosity and permeability) on seven high quality, high purity, and intact, and two fractured salt rock core sample covering Pre-Cambrian to Miocene evaporite basins across the globe (Pakistan, Tunguska Basin, Russia, NW Lancashire, UK, Sicily, Italy). These properties were measured for confining pressure ranging from 5 to 50 MPa, representing shallow and deep loading states of stress, equivalent to ~236 to 2395 m below ground. The results for intact salt rock show low absolute porosity below 1.2 %, permeability strongly influenced by stress state, and permeability below 10-20 m2 once cracks close. Fluid overpressure modelling was applied in the Liguro-Provençal and Algero-Balearic basins of the Western Mediterranean, and the Levant Basin of the Eastern Mediterranean. For the Western Mediterranean, I show that rapid sediment loading of low permeability evaporites from the MSC generated high overpressure up to 11.2 MPa within the evaporites and throughout pre-Messinian sequences. The high overpressure within the evaporites would have been sufficient to hydro fracture them and generate vertical fluid release features. The connection between the formation of the observed pockmarks in the Eastern Mediterranean and gas overpressure is uncertain. Hence, I test if the large crater pockmarks observed at the base of the Messinian evaporites may have been caused by fluid migration from methane gas accumulates in Miocene sediment towards the seafloor, triggered by sea-level drop at the beginning of the MSC. Our results show that the pockmarks were most likely caused by tensile fracturing of shallow Miocene sediment and subsequent gas migration when sea-level fell between 50 to 400 m, compatible with the observed enhanced erosion observed in the deep water canyons of the Levant margin. At a basin scale, this discharge of gas may have led to major emissions into the atmosphere. The presence of structural discontinuities is an important factor that may lead to uncontrolled dissolution events during caverning, important for underground hydrogen (energy) storage (UHS) in salt formations. I designed laboratory based dissolution tests on intact and fractured salt rock to demonstrate with geophysical signature that even small structural discontinuities may significantly impact the dissolution patterns. Our results show that pre-existing fractures can give rise to rapid dissolution irrespective of fluid pore pressure or confining pressure.

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Identifiers

ORCID for Hector Marin Moreno: orcid.org/0000-0002-3412-1359

ORCID for Jonathan Bull: orcid.org/0000-0003-3373-5807

ORCID for Lisa Mcneill: orcid.org/0000-0002-8689-5882

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