Abstract Grains within siliciclastic muds are deposited either as flocs, in which grains are generally <~10µm, or as single grains: “sortable silt,” generally > 10µm. When clay-size (<2µm diameter) particles form >30% of mudstones, pore-size distributions are controlled mainly by the interaction of phyllosilicates; these materials are ‘matrix-supported.’ Pores associated with clay-size particles are typically <20 nm, even at shallow burial. When clay-size particles comprise <~30% of the grain-size distribution, a second, much larger pore system is observed, controlled by the amount and size of sortable silt; these mudstones are ‘framework-supported.’ Compaction of these silt-rich materials occurs mainly by the loss of the largest pores, but large pores still exist up to high effective stresses in the absence of chemical compaction. Mercury injection porosimetry (MIP) gives information about pore-throat size and pore connectivity and thus provides useful data with which to estimate permeability. Models based on generally flat pore shapes can estimate the permeability of homogenous mudstones to ± a factor of 3 of the true value, but cannot be used for heterogeneous, laminated mudstones, which exhibit highly anisotropic permeabilities. As MIP gives information about pore throats and microscopy gives information about pore bodies, the two techniques generate different results. Both are required, along with other techniques such as small-angle neutron scattering and low-pressure gas sorption, in order to fully appreciate the complexity of mudstone pore systems.

Abstract Although shale gas systems constitute a new target for commercial hydrocarbon production, only a little attention has been paid to the evolution of these unconventional systems with increasing thermal maturation. This study reports the characterization of samples of the Lower Toarcian (Lower Jurassic) Posidonia Shale from northern Germany at varying levels of thermal maturity. Observations were made using an original combination of focused ion beam-scanning electron microscopy (FIB-SEM) and transmission electron microscopy (TEM). The paper documents the formation of microfracture-filling bitumen in close assotiation with kerogen residues with increasing maturity. Porosity evolves from mostly submicrometric interparticle pores in immature samples to intramineral and intraorganic pores in overmature (gas mature) samples. This intraorganic nanoporosity has most likely come about by the exsolution of gaseous hydrocarbon and been hydrocarbon wet during the thermal maturation processes. Although FIB-SEM and TEM images are small compared to field size, this study emphasizes the need for nanoscale imaging to better constrain hydrocarbon generation processes in gas shale systems.

Abstract Sediments are a major component of the ocean system and the composition of, and chemical processes acting upon, these sediments are controlled by many factors. In this chapter we outline the major types of marine sediment (lithogenous, biogenic, hydrogeneous and authigenic), discuss the sources of these and their chemical and mineral characteristics. The most abundant minerals are (oxyhydr)oxides, clay minerals, sulfides and carbonates, and all of these have been studied extensively, using a range of mineralogical techniques. A major aim of this chapter is to provide a framework by which the mineral transformations that take place upon, or just below, the sea-floor can be understood. There are significant thermodynamic, kinetic and biological controls on these transformations, and the interaction of these plays a major role in element cycling between the oceans and the lithosphere, and trace element-enrichment on the sea-floor and carbon burial and remineralization. Further, we conclude that more research is required on the interactions between minerals and bacteria within marine sediments, and on the role of amorphous oxide minerals delivered from land in early burial reactions and mineral precipitations. Oceans cover ∼70% of the Earthâ∈™s surface so that the processes operating within the marine realm and upon ocean floors play a marked role in element and nutrient cycling, biological productivity, ultimately exerting an important influence on global climate. This chapter will focus on the distribution, composition and mineralogy of sediments that are actively accumulating on the sea floor, both in the shallow marine and deep ocean realms. Further, it will describe the geochemical and mineralogical processes operating in these sediments.

Abstract Recently deposited sediments comprise a diverse mixture of detrital minerals and amorphous or poorly crystalline materials, biogenic material, organic matter and pore water. The mixture is inherently unstable and progresses towards equilibrium through a series of diagenetic reactions ( Garrels, 1986 ), ultimately generating the mica–chlorite–quartz (–Fe oxide–organic carbon) mineralogy typical of low grade metamorphic phyllites. Diagenesis begins immediately upon sedimentation, initially involving change of the most reactive components: organic matter, oxyhydroxides, dissolved oxidants such as oxygen and sulfate, and poorly crystalline or amorphous aluminosilicates. Over the past 40 years, the chemistry and mineralogy of recent sediments has been extensively studied. Early mineralogical descriptions have been superseded by combined studies of interstitial water chemistry and mineralogical changes in a wide range of sedimentary settings. These have generated qualitative and increasingly quantitative insights into the nature and rates of early diagenetic reactions and the processes of authigenic mineral formation. Classification of sediment components according to origin generates the framework of this chapter: Detrital material is generated by continental weathering processes and is transported to the sediment by water (and air). Detrital material comprises the vast bulk of lacustrine, estuarine and nearshore sediments and therefore exerts a major control on their physical and, to a lesser extent, their chemical properties. Although any mineral can be supplied to sediments as part of the detrital load, the most common detrital phases are clay minerals and quartz. Biogenic material is produced in the water column and mainly comprises calcareous and siliceous microfossils, plus organic matter.

Abstract Quartz occurs widely as a cement in pre-Tertiary North Sea reservoir sands. Fluid inclusion and oxygen isotope data show that much of the cement formed in the Tertiary from 18 O-rich waters with variable, but often high salinities. The volume of this type of water is limited and is unlikely to be more than the void space of the sedimentary basin. The mineralizing fluids also show considerable compositional heterogeneity over relatively small areas and depth ranges. The composition and compositional heterogeneity of the mineralizing fluids limit the mechanisms by which silica transport and quartz cementation could have occurred. Meteoric recharge is ruled out. Mass balance considerations strongly suggest that compaction-driven flow is extremely unlikely to have led to the type of pervasive quartz cementation observed in the pre-Tertiary North Sea section. The compositional heterogeneity of the mineralizing fluids suggests that large-scale convective overturn is also an unlikely transport mechanism. It is probable that the silica required for quartz cementation was supplied locally and not by large-scale fluid flow. We believe that the fluid data restrict potential silica transport mechanisms to: (1) reservoir-scale convection and (2) diffusion. However, significant mass transport by either of these mechanisms has yet to be demonstrated in sedimentary basins.