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Hydrocarbon inclusions trapped in quartz cement from Alwyn North field sandstones of well 3/9a-6 (North Sea) have been analyzed using microthermometric and microspectrofluorimetric methods. They are two-phased, with consistent gas–oil ratios, and mainly occur along the detrital grain–overgrowth boundary. Their homogenization temperatures vary from 60 to 129 °C with a great variability within a single fluid inclusion assemblage (up to 40 °C). The fluorescence colour of the hydrocarbon inclusions has been characterized using a microspectrofluorimeter which allows the analysis of very small inclusions (>1.7 μm) with good reproducibility. Hydrocarbon inclusions display large variations in their fluorescence colour, converted in chromaticity coordinates, even within a single fluid inclusion assemblage. Moreover, they are concentrically distributed within the detrital grain–overgrowth interface. According to the relationship between API gravity and fluorescence colour, established with reference to crude oils from Alwyn North field, this observation is interpreted as representing compositional variations. Such compositional variability in a single fluid inclusion assemblage could result from either the progressive entrapment of oils in which API gravity evolves through time, and/or the fractionation of hydrocarbons at the time of trapping. [Copyright &y& Elsevier]

Abstract: The Khushaym Matruk site in Central Jordan is investigated as a natural analogue of a deep geological repository for intermediate to high-level radioactive wastes to discuss the impact of high-alkaline solutions in an argillaceous formation. The studied site is characterised by Maestrichtian bituminous-rich marls, consisting of 66–83% of carbonates and 10–15% of clays of mixed-layers illite–smectite type (I–S), overlapped by a marble/cement unit formed by spontaneous in situ combustion of the sedimentary organic matter in sub-surface conditions. This high-temperature event produced marble and cementitious phases typical to those observed in industrial cements whose alteration generates high-alkaline solutions. Evidences of fluid circulations in Khushaym Matruk site are underlined by secondary minerals, mainly calcite and gypsum, neoformed in cracks and micro-cracks crosscutting the bituminous-rich marls, and in travertines capping them. The chemical and Sr isotopic data of these phases are characteristics of high-alkaline fluids generated by alteration of the cementitious phases and having percolated with partial equilibration in the underlying bituminous-rich marls. The propagation of these alkaline solutions by structural discontinuity paths in the clayey formation is underlined over distances greater than 5 m. Also, the mineralogical and petrographic characterisation of the bituminous-rich marls, far and close to the marble/cement unit, emphasizes slight variations in mineral contents, a few metres close to the marble/cement contact, with notably a decrease of I–S crystallinity, a change from beideillite to montmorillonite type and, calcite and zeolite recrystallisation. Together, textural disturbances of these rocks, underlined by micro-fissuration of the sedimentary matrix, are observed. A pervasive circulation of high-alkaline fluids in the argillaceous matrix is proposed to explain these changes and is underlined over a distance of a few metres under the contact with the marble/cement unit. Cathodoluminescence observations and U–Th radioactive disequilibrium dating applied on infilling calcite cracks indicate that these circulations were not continuous but occurred in several steps with a mean age of 110,000–130,000 years. [Copyright &y& Elsevier]

Deep argillaceous formations have petrophysical and hydrodynamic properties favourable to long-term radioactive waste confinement (very low intrinsic permeability, high sorption capacity,…). However, these properties may be modified by the development of discontinuities in the host-rock. The tectonic activity is responsible on the one hand for creating the fractures and on the other hand for reactivating them. Today, the calcite crystallisations in faults give evidence of paleofluid flows during the tectonic deformation. The microstructural study shows that faults were alternately and temporarily impermeable, permeable or “semi-permeable” during the tectonic activity. These “hydraulic states” were controlled by the nature and the architecture of the microstructures and by variations in the petrophysical properties of the rock in the core zone (CZ) and damage zone (DZ) of the faults. Within DZ, the structural fault permeability evolution is associated with (1) microcracking and (2) a probable ductile behaviour of the shales. Within CZ, the structural fault permeability is associated with the development of cavities generated by (1) dilation, (2) shearing and openings in extensional stepover and (3) microcracking in pre-existing calcite fillings. During the tectonic evolution, the development of a new structural porosity both in CZ and DZ gave up the faults permeable. The crystallisation sealing of the total structural porosity gave up the faults impermeable. But, when only the CZ was sealed, the fault was “semi-permeable”. Finally, we show that (1) the fluid transfers occurred principally from the DZ to the CZ, (2) the DZ constituted a “storage zone” in fluids for the CZ, (3) the DZ then remained longer permeable than the CZ and became permeable with weaker stress intensity and (4) the sealed discontinuities constituted zones of weakness (fracture reactivation with or without shearing) in the argillaceous material. [Copyright &y& Elsevier]