The effect of pore size on cementation in porous rocks

Halite cementation in porous sandstone from the Lower Triassic Bunter Formation in North‐West Germany has been studied using measurements of porosity and permeability, before and after salt extraction, as well as from petrographic observations. The results show that in cemented sandstones there is a clear tendency for the larger pores to be halite‐filled while the smaller pores, which are responsible for the residual porosity, are left empty. Observations on cemented ‘pin‐stripe’ aeolian sandstones, in which laminations of fine and coarser sand grains alternate on a millimetre scale, indicate that this selective cementation is not due to differences in the brine composition in the pores. The implication, which is also supported by other observations, is that fluids in small pores can maintain a higher supersaturation with respect to crystallization.     

Safe Space: Towards a Reconceptualization

The term “safe space” dates to the late twentieth century women's movement, but it has since been used in many different contexts. In this paper, we review and analyze historical and contemporary “safe spaces”. These include “separatist” safe spaces in women's, anti‐racist, and feminist communities, “inclusive” safe space classrooms, and safe spaces in which (non‐human) objects are central. We argue that safe spaces should be understood not through static and acontextual notions of “safe” or “unsafe”, but rather through the relational work of cultivating them. Such an understanding reveals several tendencies. Namely, safe spaces are inherently paradoxical. Cultivating them includes foregrounding social differences and binaries (safe–unsafe, inclusive–exclusive) as well as recognizing the porosity of such binaries. Renegotiating these binaries is necessarily incomplete; a safe space is never completely safe. Even so, we encourage the critical cultivation of safe space as a site for negotiating difference and challenging oppression.     

Sub‐micron spongiform porosity is the major ultra‐structural alteration occurring in archaeological bone

Total pore volume and pore size distribution are indicators of the degree of post‐mortem modification of bone. Direct measurements of pore size distribution in archaeological bones using mercury intrusion porosimetry (HgIP) and back scattered scanning electron microscopy (BSE‐SEM) reveal a common pattern in the changes seen in degraded bone as compared to modern samples. The estimates of pore size distribution from HgIP and direct measurement from the BSE‐SEM images show remarkable correspondence. The coupling of these two independent approaches has allowed the diagenetic porosity changes in human archaeological bone in the >0.01 µm range to be directly imaged, and their relationship to pre‐existing physiological pores to be explored. The increase in porosity in the archaeological bones is restricted to two discrete pore ranges. The smaller of these two ranges (0.007–0.1 µm) lies in the range of the collagen fibril (0.1 µm diameter) and is presumably formed by the loss of collagen, whereas the larger pore size distribution is evidence of direct microbial alteration of the bone. HgIP has great potential for the characterization of microbial and chemical alteration of bone. Copyright © 2002 John Wiley & Sons, Ltd.     

Did Children in Medieval and Post‐medieval Poland Suffer from Scurvy? Examination of the Skeletal Evidence

Late‐medieval and post‐medieval writings report that scurvy was a widespread condition in medieval and early historical Poland. Archaeological and historical data indicate that the diet of children was based on foods poor in vitamin C and contained small amounts of raw plant products. Also, historians emphasise that in medieval and post‐medieval Poland, there were seasonal fluctuations in food availability, frequently accompanied by poor harvests. Both resulted in long periods of poor nutrition, which affected children most severely. The aim of this study was to investigate skeletal manifestations of scurvy in subadult remains from medieval and post‐medieval Poland. Following standards described by Ortner and colleagues, anatomical sites pathognomonic of scurvy in subadults (<17 years) were assessed for abnormal porosity and hypertrophic bone among skeletons excavated from three sites: Ostrów Lednicki (dated to the 11th–14th centuries AD), Cedynia (10th–14th centuries AD) and Słaboszewo (14th–17th centuries AD). In total, 3.6% of all examined children were found to bear traces of vitamin C deficiency. The prevalence of scorbutic lesions was 4.5% for Cedynia, 2.6% for Ostrów Lednicki and 3.6% for Słaboszewo. The majority of affected children were less than 7 years of age. Scurvy was likely more widespread in the living populations than it appears from the calculation of skeletal markers, because some individuals might have recovered or died before obvious traces became apparent. Also, in some children, scurvy might not have reached an advanced stage, identifiable in the skeletal material. The prevalence of scurvy reflects not only dietary patterns but also food storage and preparation techniques adopted in the Polish territories during the Middle Ages, which contributed to low intakes of vitamin C. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical models of petroleum migration via buoyancy-driven porosity waves in viscously deformable sediments

The apparent ability of petroleum to migrate rapidly through low permeability sediments in sedimentary basins has led to many questions about the manner of its transport. One possible explanation is suggested from observations of the compaction of viscously deformable porous media. These systems have been found in some cases to give rise to regions of locally elevated liquid fraction, in the form of fluid‐filled porosity waves that can ascend at rates much greater than that of the background flow. Previous research on the phenomenon has focused on its implications for magma transport, but recognition of the fact that the compaction of viscous porous media can take place in sedimentary basins has suggested the possibility that porosity waves could similarly be important for hydrocarbon transport. The purpose of the present study was to test this hypothesis by quantifying the transport that would occur as the result of porosity waves initiated during the conversion of kerogen to petroleum. A one‐dimensional numerical model was constructed solving equations for the mechanics of viscous compaction and for the kinetics of reactions describing the formation of petroleum from kerogen. The results showed that porosity waves would develop readily in viscously deformable regions of sedimentary basins, but would not necessarily provide enhanced transport over that of the background flow regime. In order for the waves to achieve this enhanced transport, they must develop high amplitudes, i.e. high porosities relative to the background porous medium. To achieve the high wave amplitudes, the background porosity must be very low in absolute terms. In addition, high kerogen contents are needed in the source layer, and the source layer needs to be buried rapidly to a high temperature region of the oil window. Considerable uncertainty exists as to the value of the matrix shear viscosity of sediments in basins. However, the wave volumetric transport capacity was not found to be significantly altered as a result of variations in the value of this parameter. The physical form of the waves was strongly altered by the matrix shear viscosity, with higher values leading to lower amplitudes and generation frequency, but higher wavelengths. Thus the waves become less recognizable physically at higher values of the matrix shear viscosity. As the waves ascend to higher stratigraphic levels, where the porosity is higher, they gradually lose their physical definition and become absorbed into the background.     

Bone changes in the human skull probably resulting from scurvy in infancy and childhood

Scurvy in children is poorly known in the palaeopathological literature despite biomedical references indicating that bone changes do occur in some cases and should be observable in human skeletal remains. Research on infants' and children's skulls in museum and anatomical collections in the USA and Europe reveals a complex of features on the external surface of the greater wing of the sphenoid and adjacent bone tissue that may be caused by scurvy. The lesions are bilateral and are characterized by porosity and, in some cases, hypertrophic bone formation. These abnormal bone changes are probably the result of a reaction to chronic haemorrhage in the deep vasculature associated with temporalis muscle function. While these lesions resemble those seen in anaemia and infection, their distinctive anatomical location and association with chewing should differentiate them, in most cases, from other disease conditions. These features are also associated with porous, hypertrophic lesions of the orbital roof and provide additional evidence that caution is needed in attributing porous lesions of the orbital roof solely to anaemia. © 1997 John Wiley & Sons, Ltd.     

Diagenesis of the Amposta offshore oil reservoir (Amposta Marino C2 well,Lower Cretaceous,Valencia Trough,Spain)

Samples from the Amposta Marino C2 well (Amposta oil field) have been investigated in order to understand the origin of fractures and porosity and to reconstruct the fluid flow history of the basin prior, during and after oil migration. Three main types of fracture systems and four types of calcite cements have been identified. Fracture types A and B are totally filled by calcite cement 1 (CC1) and 2 (CC2), respectively; fracture type A corresponds to pre‐Alpine structures, while type B is attributed to fractures developed during the Alpine compression (late Eocene‐early Oligocene). The oxygen, carbon and strontium isotope compositions of CC2 are close to those of the host‐rock, suggesting a high degree of fluid‐rock interaction, and therefore a relatively closed palaeohydrogeological system. Fracture type C, developed during the Neogene extension and enlarged by subaerial exposure, tend to be filled with reddish (CS3r) and greenish (CS3g) microspar calcite sediment and blocky calcite cement type 4 (CC4), and postdated by kaolinite, pyrite, barite and oil. The CS3 generation records lower oxygen and carbon isotopic compositions and higher ⁸⁷Sr/⁸⁶Sr ratios than the host‐limestones. These CS3 karstic infillings recrystallized early within evolved‐meteoric waters having very little interaction with the host‐rock. Blocky calcite cement type 4 (CC4 generation) has the lowest oxygen isotope ratio and the most radiogenic ⁸⁷Sr/⁸⁶Sr values, indicating low fluid‐rock interaction. The increasingly open palaeohydrogeological system was dominated by migration of hot brines with elevated oxygen isotope ratios into the buried karstic system. The main oil emplacement in the Amposta reservoir occurred after the CC4 event, closely related to the Neogene extensional fractures. Corrosion of CC4 (blocky calcite cement type 4) occurred prior to (or during) petroleum charge, possibly related to kaolinite precipitation from relatively acidic fluids. Barite and pyrite precipitation occurred after this corrosion. The sulphur source associated with the late precipitation of pyrite was likely related to isotopically light sulphur expelled, e.g. as sulphide, from the petroleum source rock (Ascla Fm). Geofluids (2010) 10 , 314–333     

Microstructures of Building Materials from Huaca De La Luna,Peru

ABSTRACT

The microstructures of building materials from the archaeological complex of Huaca de la Luna on the northern coast of Peru is assessed for the first time by a multiscale approach. The multiscale microstructural characterization used laboratory techniques including optical microscopy, scanning electron microscopy, X-ray microtomography, and both gas and powder pycnometry. Optical microscopy revealed the presence of organic matter and shell fragments and X-ray microtomography showed differences in the sizes and connectivity of pores. The BET (N₂ gas adsorption) specific surface areas showed greater presence of clayey matrix in the mortars. No significant differences among mortars and brick fragments mineralogy were detected by X-ray diffraction, so the larger amount of fine binding matrix in the mortars suggests that they were produced blending raw material with different grain sizes. The aim of this study is to provide data that can be used for a better understanding of the Moche culture, its construction techniques, and to support conservation strategies of the archaeological heritage.     

An examination of skulls from two British sites for possible evidence of scurvy

Skulls and mandibles from the Romano‐British site of Poundbury, Dorset and a medieval site at Abingdon in Oxfordshire were examined for porosity which has been considered to be indicative of the presence of scurvy. In addition, a number of skulls from various locations in Peru were examined. Virtually all the skulls had porosity at at least one site and in all cases the palate was most frequently affected. The rank order of affected sites showed some differences between the three samples but the most striking change was the frequency with which the cranial vault and maxilla were affected in the Peruvian skulls compared with the British. The differences were not statistically significant, however. None of the appearances was consistent with those of known cases of infantile scurvy and we doubt whether scurvy can be diagnosed on the basis of porosity in the skull and mandible unless it conforms to recognized clinical patterns. Copyright © 2003 John Wiley & Sons, Ltd.