Intracrystalline Porosity Research Articles

Diagenetic history and porosity evolution of the Middle Permian clastic-carbonate mixed system, Indus Basin, Pakistan: Implications for reservoir development

This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin, Pakistan. Multiple data sets including outcrop, petrography, cathodoluminescence, scanning electron microscopy (SEM), mineralogy, and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis. The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend. The low values of δ18O of the target strata, relative to average values of the Permian carbonate, hints to diagenetic alteration in the strata. The data sets used in this study reveal modification of the strata in four environments, that is, i) early marine diagenesis indicated by micritization, pervasive dolomitization and isopachous fibrous cements, followed by ii) meteoric dissolution, and iii) shallow burial diagenetic processes including the precipitation of blocky cement, compaction of skeletal and non-skeletal allochems, and stylolites, and iv) a deep burial environment, characterized by pressure solution, and micro-fractures. The clastic intervals host subangular to subrounded quartz grains, floating textures, and almost complete absence of deleterious clay minerals, consequently resulting in the preservation of primary porosity. The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity. The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution. The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction, precipitating sulfides within the pores. The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit, and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies. However, both intervals show very low permeability values probably due to isolated moldic pores and intense micritization. Therefore, clastic intervals may provide an opportunity to serve as a moderate reservoir; however, the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.

Hierarchical Zeolites Prepared Using a Surfactant-Mediated Strategy: ZSM-5 vs. Y as Catalysts for Friedel-Crafts Acylation Reaction.

Hierarchical ZSM5 and Y zeolites were prepared through a surfactant-mediated strategy with NH4OH changing the duration of the treatment and the amount of CTAB surfactant and taking as reference multiples of the critical micellar concentration (CMC). The materials were characterized using powder X-ray diffraction, N2 adsorption isotherms at -196 °C, and SEM and TEM microscopy. The catalytic performance was evaluated in Friedel-Crafts acylation of furan with acetic anhydride at 80 °C. The alkaline surfactant-mediated treatment had different effects on the two zeolites. For ZSM5, the CTAB molecular aggregates can hardly diffuse inside the medium-size pores, leading mainly to intercrystalline mesoporosity and increased external surface area, with no positive catalytic impact. On the other hand, for large-pore Y zeolite, the CTAB molecular aggregates can easily diffuse and promote the rearrangement of crystal units around micelles, causing the enlargement of the pores, i.e., intracrystalline porosity. The optimized Y-based sample, treated for 12 h with a CTAB amount 32 times the CMC, shows an increase in product yield and rate constant that was not observed when a higher amount of surfactant was added. The reuse of spent catalysts upon thermal treatment at 400 °C shows a regeneration efficiency around 90%, showing good potentialities for the modified catalysts.

Synthetic Control of the Defect Structure and Hierarchical Extra-Large-/Small-Pore Microporosity in Aluminosilicate Zeolite SWY.

The SWY-type aluminosilicate zeolite, STA-30, has been synthesized via different routes to understand its defect chemistry and solid acidity. The synthetic parameters varied were the gel aging, the Al source, and the organic structure directing agent. All syntheses give crystalline materials with similar Si/Al ratios (6-7) that are stable in the activated K,H-form and closely similar by powder X-ray diffraction. However, they exhibit major differences in the crystal morphology and in their intracrystalline porosity and silanol concentrations. The diDABCO-C82+ (1,1'-(octane-1,8-diyl)bis(1,4-diazabicyclo[2.2.2]octan)-1-ium)-templated STA-30 samples (but not those templated by bisquinuclidinium octane, diQuin-C82+) possess hierarchical microporosity, consisting of noncrystallographic extra-large micropores (13 Å) that connect with the characteristic swy and gme cages of the SWY structure. This results in pore volumes up to 30% greater than those measured in activated diQuin-C8_STA-30 as well as higher concentrations of silanols and fewer Brønsted acid sites (BASs). The hierarchical porosity is demonstrated by isopentane adsorption and the FTIR of adsorbed pyridine, which shows that up to 77% of the BASs are accessible (remarkable for a zeolite that has a small-pore crystal structure). A structural model of single can/d6r column vacancies is proposed for the extra-large micropores, which is revealed unambiguously by high-resolution scanning transmission electron microscopy. STA-30 can therefore be prepared as a hierarchically porous zeolite via direct synthesis. The additional noncrystallographic porosity and, subsequently, the amount of SiOHs in the zeolites can be enhanced or strongly reduced by the choice of crystallization conditions.

DEVELOPMENT OF FIBROUS CALCITE VEINS RELATED TO HYDROCARBON GENERATION AND OVERPRESSURING IN ORGANIC‐RICH SHALE SOURCE ROCKS: THE VACA MUERTA FORMATION, NEUQUÉN BASIN, ARGENTINA

Fibrous calcite bed‐parallel veins (BPVs) are a typical feature of the Upper Jurassic – Lower Cretaceous Vaca Muerta Formation in the subsurface of the Neuquén Basin (Argentina). The formation is considered to be the main source rock in the basin as well as an important unconventional play. This study examines the growth of BPVs through an analysis of core from three wells located along a transect extending for some 150 km from the NE Platform near the basin margin in the east to the Agrio fold‐and‐thrust belt at the Andean deformation front in the west. The main objective is to integrate fluid inclusion data with the palaeothermal and palaeopresure evolution obtained from a regional‐scale 2D basin and petroleum systems model to examine the timing of fracture development and its relationship with hydrocarbon generation in the Vaca Muerta Formation through time.The apertures of BPVs were measured in more than 360 m of core from three wells (wells A, D and E). This data was combined with optical petrography to investigate the number of calcite cementation events, and the temperature of cement precipitation based on fluid inclusion data. The organic geochemical and mineralogical characteristics of the Vaca Muerta source rock were also analysed. The integrated results were incorporated into a poro‐elastic basin model to investigate the impact of horizontal shortening due to Andean compression on pore pressure development and fracturing in the Vaca Muerta Formation. This framework allowed the timing of BPV formation to be determined together with possible mechanisms governing overpressure conditions through time.Near the Andean deformation front in the west of the modelled section where the Vaca Muerta Formation is in the wet gas window (well D) and dry gas window (well A), BPVs are characterized by two or more generations of calcite fibres indicating multiple growth phases. Calcite which precipitated during cementation event 1 (E1) in the internal zones of BPVs consists of crystals oriented perpendicular to fracture walls, indicating perpendicular vein opening. Calcite precipitated during cementation event 2 (E2) in the outer zones of BPVs includes curved and oblique crystals. During this phase, shear occurred between the opening vein walls as a result of horizontal shortening. Cementation event 3 (E3) is characterized by an equant mosaic of calcite crystals which preserve intracrystalline porosity. E1cements formed between 110 and 90 Ma with trapping temperatures of ∼112 °C (upper Vaca Muerta, well A) and ∼125 °C (lower Vaca Muerta, well D). Fracturing resulted from disequilibrium compaction and from volumetric expansion due to primary cracking of kerogen within the oil window. E2 cements record a trapping temperature of ∼159 °C and formed between 70 and 55 Ma (lower Vaca Muerta, well D) during maximum burial of the Vaca Muerta Formation, synchronous with the secondary cracking of retained liquid hydrocarbons and the beginning of Andean compression. E3 cements (upper Vaca Muerta, well A) have a trapping temperature of ∼162 °C, and formed between 65 Ma and 53 Ma synchronous with the generation of thermogenic gas.By contrast, in the east of the modelled section in the less deformed foreland area of the Neuquén Basin where the Vaca Muerta Formation is in the early oil window (well E), BPVs are composed of a single generation of calcite fibres (E1)with a trapping temperature of ∼118 °C. The E1 cement is characterized by calcite crystals which are oriented perpendicular to fracture walls with no evidence of shearing. According to model simulations, cementation here occurred between 64 Ma and 53 Ma during maximum burial and was related to overpressures which resulted from both disequilibrium compaction and primary transformation of kerogen into oil.The data presented suggests that in some intervals of the Vaca Muerta Formation, a slight increase in TOC content is accompanied by an increase in vein thickness, with the highest number of cementation events occurring towards the Andean deformation front in the west of the study area compared to the foreland in the east.

Phase Formation, Microstructure and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites, (I): Effect of Sintering Temperature

We have studied the densification, phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni0.30Cu0.20Zn0.50+zFe2-zO4-(z/2) with 0 ≤ z ≤ 0.06 sintered at temperatures from 900 °C to 1150 °C. The shrinkage is shifted from 1000 °C for z = 0 towards lower temperatures and reaches its maximum rate at 900 °C for z = 0.02. Stoichiometric ferrites show regular growth of single-phase ferrite grains if sintered at Ts ≤ 1100 °C. Sintering at 1150 °C leads to the formation of a small amount of Cu2O, triggering exaggerated grain growth. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites coexisting with a minority CuO phase after sintering at 900–1000 °C. At Ts ≥ 1050 °C, CuO transforms into Cu2O, and exaggerated grain growth is observed. The formation of Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability of the ferrites increases with sintering temperature up to a maximum permeability of µ = 230 for z = 0 or µ = 580 for z = 0.02, respectively, at Ts = 1000 °C. At higher sintering temperatures, the permeability decreases, which is due to the formation of a microstructure with intra-crystalline porosity in large grains, and a non-magnetic Cu oxide grain boundary phase.

One-step versus two-step dolomite calcitization (dedolomitization): differences and inferences

Polycrystalline calcite pseudomorphs after dolomite can exhibit mosaic or drusy fabric caused by replacement of the dolomite by calcite (one-step dedolomitization) or dolomite dissolution followed by calcite cementation (two-step dedolomitization), respectively. This study discusses the differences between the two types and interprets the characteristics of the fluid causing them. Conclusions drawn here are founded on a microscopic and petrophysical examination of an outcrop of Tuwaiq Mountain Formation, central Arabian Peninsula. Petrographic examination shows that dedolomite has distinct microfabrics in each part of the section. The lower part of the section is not affected by dolomite calcitization, the middle part is dominated by replacement fabrics, and the upper part is dominated by dissolution and/or cementation fabrics. Petrophysical examination shows a wider range of porosity and permeability values for the upper part of the section, relative to the middle and lower parts. One-step dedolomitization is characterized by preservation of the external morphology of the parent dolomite, confinement to the volume previously occupied by the dolomite, development of intracrystalline porosity and permeability that allows the fluid to maintain contact with the reaction front, and no significant effect on porosity of the original rock. Two-step dedolomitization is characterized by corrosion of the external morphology of the parent dolomite, association with other forms of calcite cementation, development without intracrystalline porosity and permeability, and positive or negative effects on the porosity. While one-step dedolomitization appears to occur within a stagnant zone of fluid circulation, the two-step dedolomitization is considered to reflect an active fluid-flow system.

Properties and microstructures of lightweight alumina containing different types of nano-alumina

The key challenge in designing lightweight wear linings for industrial furnaces lies in the fabrication of porous materials with the smallest possible open porosity and pore size. In this study, in order to increase the migration velocity of the grain boundary, alumina sol and nanosized α-Al2O3 were introduced for their high temperature grain boundary superplasticity. Investigations on the effect of various types of nano-alumina additives on pore repartition and some properties of sintered lightweight alumina were carried out. The introduction of nano-alumina increased the surface stress and accelerated the division of pores. The addition of alumina sol and nanosized α-Al2O3 showed similar effects on the physical properties of lightweight alumina. Compared to samples without addition of nano-alumina, samples containing alumina sol and nanosized α-Al2O3 exhibited the increased closed porosities (from 2.8% to 7.1% and 9.2%, respectively), decreased open porosities (from 12.4% to 10.5% and 5.2%, respectively), lower thermal conductivities (decreasing amplitudes of 16% and 9%, respectively) and increased intracrystalline porosity. Furthermore, the addition of different types of nano-alumina led to microstructural differences in the samples. Abnormally grown grains, with a size of approximately 100–200grain μm, containing numerous smaller intracrystalline pores inside were observed in the sample added alumina sol. The sample containing nanosized α-Al2O3 exhibited a homogeneous distribution of grain sizes. The different grain growth rates, accompanying with various types of nano-alumina, were responsible for the differences in microstructure.

Focused Ion Beam Tomography Reveals the Presence of Micro-, Meso-, and Macroporous Intracrystalline Regions Introduced into Calcite Crystals by the Gastropod Nacre Protein AP7

Intracrystalline modification of calcium carbonates by macromolecules is a fascinating process that offers insights into potential pathways for modifying the material properties of inorganic solids. Recently, we reported on the induction of intracrystalline nanoporosities within calcite by the nacre layer intracrystalline protein, AP7 (Haliotis rufescens). In this report we revisited this AP7-mediated phenomenon and tracked time-dependent intracrystalline porosity formation during in vitro mineralization using FIB/SEM serial milling. We find that AP7 induces intracrystalline nanoporosities as early as 1 min of elapsed assay time. Quantitation of pore regions confirms that average cross-sectional volume (ACSV), average void volume (AVV), and percent porosity parameters increase over time, leading to the formation of porous calcite crystals with a high surface-to-volume ratio. FIB serial milling, SEM imaging, and 3-D tomography revealed the presence of unexpected semicontinuous channels and cavities in the subsurface regions of a representative 60 min assay crystal. The random locations of these intracrystalline features are limited to the top and sides of the calcite crystal, which correspond to the sites of AP7 protein phase deposition during mineral formation. This random porosity distribution was also documented for protein-containing voids within nacre aragonite tablets in situ. In some instances we observed geometric relationships between adjacent channels and cavities. Interestingly, all three IUPAC-defined material porosity categories (micro-, meso-, and macro-) were represented in the AP7-treated crystals. Thus, the deposition of AP7 protein phases onto calcite surfaces induces surface nanoparticle nucleation and subsurface multiscale intracrystalline porosities and interconnected channels.

Fabrication of high-surface-area γ-alumina by thermal decomposition of AACH precursor using low-temperature solid-state reaction

In this paper, systematical investigation has been made for the new simple synthesis pathway of catalytic alumina with high surface area through thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), which has been prepared by a low-temperature solid-state reaction in non-aqueous system. The results indicate that the molar ratios of NH4HCO3/Al influence crystallinities and the morphologies of AACH. Well-crystallized AACH with nanorod morphology and low-crystallized AACH with cluster structure can be obtained with different molar ratios NH4HCO3/Al. The γ-Al2O3 product with high surface area (511m2/g) can be obtained from the calcination of AACH at 500°C. Moreover, the results also indicate that crystallinities and morphologies of AACH affect their thermal decomposition behaviors. In addition, the formation of high-surface-area is possibly attributed to intracrystalline porosity of as-prepared γ-Al2O3.

Polycrystalline apatite synthesized by hydrothermal replacement of calcium carbonates

Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH 4) 2HPO 4 solution. Scanning electron microscopy of the reaction products showed that the transformation of aragonite to apatite is characterised by the formation of a sharp interface between the two phases and by the development of intracrystalline porosity in the hydroxyapatite phase. In addition, electron backscattered diffraction (EBSD) imaging showed that the c-axis of apatite is predominantly oriented perpendicular to the reaction front with no crystallographic relationship to the aragonite lattice. However, the Ca isotopic composition of the parent aragonite, measured by thermal ionization mass spectrometry was inherited by the apatite product. Hydrothermal experiments conducted with use of phosphate solutions prepared with water enriched in 18O (97%) further revealed that the 18O from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of 18O with Raman spectroscopy was possible because the incorporation of 18O in the PO 4 group of apatite generates four new Raman bands at 945.8, 932, 919.7 and 908.8 cm −1, in addition to the ν 1(PO 4) symmetric stretching band of apatite located at 962 cm −1, which can be assigned to four 18O-bearing PO 4 species. The relative intensities of these bands reflect the 18O content in the PO 4 group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the 18O distribution between aqueous phosphate and water, we could show that the concentration of 18O in the apatite product is linked to the degree of 18O equilibration in the solution. The textural and chemical observations are indicative of a coupled mechanism of aragonite dissolution and apatite precipitation taking place at a moving reaction interface.

Improvement of retention capacity of ETS-10 for uranyl ions by porosity modification and their immobilization into a titanosilicate matrix

The retention capacity of ETS-10 titanosilicate for uranyl ions was improved by generation of additional intra-crystalline porosity with formation of an extended silica-rich surface area. Chemical bonds between uranyl groups and [SiO4] tetrahedra were found. By calcination at 800 °C, the uranium-loaded ETS-10 transforms into a inert titanosilicate matrix that encapsulates uranium.

New insights into the ultrastructure, permeability, and integrity of conodont apatite determined by transmission electron microscopy

New crystalline structures have been observed in argon ion-milled conodont elements from a diverse suite of Ordovician taxa (‘Cordylodus robustus’, Drepanoistodus suberectus, Panderodus gracilis, Plectodina? sp., Aphelognathus sp., Periodon aculeatus), using transmission electron microscopy (TEM). Electron diffraction patterns of albid tissue reveal that the component crystals are extraordinarily large, in the order of hundred(s) of microns. These large albid crystals show typical cancellate porosity, although a distinctly lamellar structure has also been observed within a large albid crystal positioned between hyaline lamellar and cancellate albid tissues. There is a distinct absence of ‘interlamellar space’ within all hyaline tissues examined, which are characterized by a polycrystalline matrix of micron-scale elongate crystals that are both strongly aligned and tightly bound within a broader lamellar structure. Optical opacity, caused by light scattering within large (≥ 0.5 µm) pores, is also a feature of both albid and polycrystalline lamellar crown tissues. Accordingly, conodont hard tissues are differentiated by crystal size and shape, as well as inter- and intracrystalline porosity. These new observations highlight the structural complexities of conodont histologies and the need for more comprehensive investigations particularly of transitional crown tissues, which are not well defined by terms typically used in the literature. Their histological structures are interpreted to be a product of in vivo crystallization and thus provide new insights into the relative porosity, permeability, and inherent integrity of the tissues as well as their growth relationships. Accordingly, these data not only have implications for earlier histological and palaeobiological interpretations of conodont hard tissues but are also fundamental in determining their chemical integrity, which is crucial for characterizing palaeoseawater composition and palaeoenvironmental change. The potential for conodont apatite to retain primary chemical information depends on crystal size and permeability, so the large albid crystal domains are consistent with parallel geochemical studies that suggest that cancellate albid crown is more resistant to diagenetic modification.

Intracrystalline Alteration of Low-Magnesian Calcite Cement in the Devonian Pillara Formation, Canning Basin, Western Australia

ABSTRACT Intracrystalline alteration is present in approximately 25% of the low-Mg calcite cement throughout the lowermost 160 m of the Middle to Upper Devonian Pillara Formation in the Emanuel Range, Canning Basin, Western Australia. A younger, luminescently and chemically distinct calcite phase disrupts primary zonation in older cement and results in irregular, mottled cathodoluminescence (CL). The younger calcite is always in optical continuity with host calcite. Continuous records of Mg, Fe, Mn, and Sr abundances were measured across concentrically zoned, non-CL and yellow-CL cement altered by orange-CL calcite using a new microprobe method designed for high spatial resolution and rapid collection of minor-element abundances in calcite cement. Minor-element composition of the crosscutting, orange-CL phase, designated as cement VIII, differs significantly from that of the older, zoned cement, designated as cement VIa. Mg abundance in cement VIa is variable, ranging from 700 to 4600 ppm, with a mean of 2760 ppm, whereas that of cement VIII is relatively constant at about 400 ppm. Mn in the yellow-CL zones of cement VIa is variable, 400-7720 ppm, with a mean of 4025 ppm. Mn in cement VIII is relatively constant at 1420 ppm. Fe and Sr were not detected in cement VIa. Abundances of Fe and Sr are highly variable in cement VIII. Fe ranges from Low-Mg calcite traditionally is regarded as a stable carbonate phase with little tendency toward intracrystalline alteration of chemistry or microstructure. Significant elemental variations measured over small-scale intervals within time-equivalent zones in cement VIa indicate that some parts of the crystal have different chemical potential, which could provide the thermodynamic drive for intracrystalline alteration. Alteration could take place either by a one-step, iterative process of recrystallization or by a two-step, dissolution-cementation process. Lack of micro-collapse features in host calcite, in combination with lack of minor-element memory between host and secondary calcite, support alteration by recrystallization at a high water-rock ratio. However, most alteration of ceme t in the Emanuel Range probably originated by dissolution-cementation, on the basis of the combination of: (1) lack of minor-element memory effect between host and secondary calcite, and (2) rare occurrences of concentrically zoned secondary calcite in which the zonation is parallel to the contact with the host cement. Creation of intracrystalline porosity by partial dissolution of metastable parts of low-Mg calcite could provide conduits for fluid and element transport that have been unrecognized in modeling porosity changes and fluid movement through carbonate strata.

Role of Anhydrite Diagenesis in the Creation of Porosity in Middle Permian Dolostone Reservoirs: ABSTRACT

In middle Permian dolostone reservoirs, the most abundant pore type is secondary intercrystalline porosity. Generally ranking second in abundance, but first in many instances, are pores created by dissolution of anhydrite that occurred in the Permian. Much anhydrite was emplaced diagenetically during and following early dolomitization, as nodules, blocky porphyroblasts, and cement, and as combination cement and replacement. Generally, anhydritization was followed by an influx of a low-salinity fluid, which dissolved anhydrite - creating tertiary porosity - or altered it to gypsum, hemihydrate, silica, calcite, or fluorite. Dissolution of anhydrite nodules and porphyroblasts creates distinctive molds with angular and stair-step outlines. Dissolution of anhydrite, which was emplaced as cement and replacement, enhances the original primary and secondary pores by the increment that was replaced. This dissolution also produces voids with angular and stair-step outlines. Replacive anhydrite tends to nucleate within organic-rich components, such as burrows, stromatolites, skeletal grains, ooids, and peloids. Many dolomite crystals have cloudy organic-rich centers, which represent the original replacive crystals, and clear rims of epitaxial dolomite cement. Anhydrite preferentially nucleates within the organic-rich cores, and subsequent dissolution produces hollowed dolomite crystals or intracrystalline porosity. Intracrystalline pores constitute the predominant pore types within many intervals. Anhydrite commonlymore » replaces skeletal grains or ooids, and the replacement of such anhydrite produces a mold that replicates the original shell (biomold). It is possible to determine how many biomolds actually record dissolution of replacive anhydrite. The best Permian reservoirs contain intercrystalline porosity in conjunction with pores created by dissolution of anhydrite and grains.« less

Zeolites treated with silicon tetrachloride vapour. Part 2.—Sorption studies

Adsorption isotherms for nitrogen, water, n-butane, isobutane, benzene and cyclohexane have been measured on a series of samples of zeolite Y dealuminated with silicon tetrachloride vapour at various temperatures. Differential heats of adsorption, differential entropies, activity coefficients and virial coefficients have been calculated from the isotherms. The nature of the intracrystalline porosity and the status of the sorbed molecules and extra-framework aluminium have been considered in the light of the results.

Morphologic Variations of Calcite Crystals in Waterfall Travertine Deposits, Arbuckle Mountains, Oklahoma: ABSTRACT

A motley assortment of low-magnesian calcite morphologies occurs in travertine deposits in Oklahoma. These morphologic variations result from (1) precipitation, (2) dissolution, and/or (3) neomorphism in this nonmarine environment; analogous morphologic variations in both marine and nonmarine strata may likewise indicate nonmarine processes. Commonly precipitated crystals include hexagonal prisms and rhombohedrons (2-150 µm long), and bladed to fibrous forms (0.02-2 mm long, many revealing triangular cross sections). Many of these crystals contain inclusion-defined growth layers that dissolve preferentially, leaving abundant intracrystalline porosity. This porosity parallels crystal outlines, imparting a nested appearance to the crystals. Partial dissolution also creates parallel spikes (4-30 µm long, 1-10 µm wide), and parallel ribbon crystals (30-150 µm long, 3-12 µm wide) that repeatedly narrow and widen, and occasionally twist. In addition to precipitational and dissolutional forms, aggradational neomorphism produces columnar crystals, commonly exceeding 8 mm in length. These crystals orig nated as elongate spar around filamentous cyanophyte tufts and were transformed subsequently into ragged-edged columnar crystals at the expense of overlying micritic crystals. Morphologic variations in calcite crystals often have been attributed to ionic concentrations (particularly Mg and SO4) of ambient waters. In this study, however, the low concentrations of both Mg (averaging 12 ppm) and SO4 (approximately 16 ppm) may be interpreted as supporting theories relating precipitational morphologies to growth rates rather than to poisoning ions. In any case, recognition of similar morphologic suites resulting from precipitation, dissolution, and/or neomorphism may aid in the identification of nonmarine processes in marine and nonmarine strata. End_of_Article - Last_Page 281------------

Intracrystalline Porosity--A Newly Discovered Pore Type in Dolostone Reservoirs and Implications for Dedolomitization and Pseudospiculites: ABSTRACT

Intracrystalline porosity consists of hollow dolomite crystals. It forms when anhydrite, which had replaced cores of dolomite crystals, was dissolved. Significant volumes of intracrystalline porosity have been preserved in subsurface Permian dolostones of the Permian basin and in outcrops of Lower Cretaceous dolostones of Texas. Anhydrite can be seen replacing organic-rich cores of dolomite crystals in Permian and Jurassic (Smackover) dolostones. Dolomite crystals, which replace original carbonate, commonly appear because of organic inclusions. After dolomitization, continued flux of the dolomitizing fluid causes precipitation of clear rims on the cloudy crystals. Anhydrite tends to replace the organic-rich cores, leaving the clear epitaxial rims to form pill box structures. When anhydrite was dissolved, many hollow dolomite crystals collapsed and fragments became transported as vadose internal sediment. Obtuse and acute angles of these hollow dolomite rims superficially resemble sponge spicules, and pseudospiculites are layers of vadose internal sediment in which numerous fragments of hollow dolomite rims have been deposited in solution channels. Hollow dolomite crystals and pseudospiculites constitute evidence for disconformity and also represent subtle indications of vanished sulfates. In many instances, calcite cement was precipitated within intracrystalline pores and is misinterpreted as partial dedolomitization. Such calcite cement can be distinguished from dedolomite, because the calcite is not syntaxial with dolomite rims, as it would be in dedolomitization. In some Permian dolostone intervals, intracrystalline porosity is the predominant pore type. Molds formed by dissolution of replacement porphyroblasts and nodules of anhydrite also commonly were formed along with intracrystalline pores and all represent tertiary (third order) voids. End_of_Article - Last_Page 491------------

Petrography and diagenesis of the taormina formation, gela oil field, sicily (Italy)

The Taormina Formation (the reservoir rock of the Gela Oil Field, Sicily) consists of a thick dolomite sequence of Middle and Late Triassic age. On the basis of petrographic examination, this formation can be subdivided into three facies: White Dolomite, Tan Dolomite and Stromatolitic Dolomite. The White Dolomite is medium-grained, sometimes vuggy, with molluscan and oolitic relics. It seems to be indicative of a shallow-water, probably high-energy environment; sediments were later dolomitized. The Tan Dolomite is formed by very fine to medium-fine dolomite with a few molluscan and oolitic relics. The layers of fine dolomite, interbedded with anhydrite streaks, are thought to have been deposited in a restricted, probably lagoonal, environment. The Stromatolitic Dolomite is characterized by a cyclic succession of intraformational breccia, intraclastic dolomite, stromatolitic dolomite and crystalline dolomites. Geopetal fabrics and desiccation structures (bird's eyes) are frequently found. This facies, having lithologic and sedimentologic features similar to the Dolomia Principale of northern Italy, is believed to represent a carbonate shelf environment ranging from subtidal and intertidal to supratidal. A complex sequence of diagenetic processes may be observed in the Taormina Formation. Some of the main modifications observable in many layers of stromatolitic dolomite are due to syngenetic dolomitization, succession of early lithification-erosion-resedimentation, deposition of dolomite cement, leaching, and desiccation. Diagenetic dolomitization, deposition of dolomite cement and leaching can be generally observed also in the White and Tan Dolomite. Dedolomitization and subsequent partial solution of calcite occurred in all facies of the Taormina Formation during very late stages of diagenesis (epigenesis). Dedolomitization was probably caused by deeply penetrating, sulphate-rich ground waters during early tectonic movements in Jurassic time. Partial solution of calcite, probably due to corrosive action of CO 2 related to igneous activity, occurred later. A considerable amount of early porosity (dolomitization; vugs due to desiccation; leaching; solution of nonreplaced calcite) was almost destroyed during the subsequent diagenetic stages by deposition of dolomite cement, pressure solution, etc. A certain amount of secondary porosity was produced by vadose diagenesis (epigenesis), and by partial solution of calcite (intercrystalline and intracrystalline porosity) subsequent to dedolomitization which, according to Tatarskiy (1949), reduces porosity.

Sorption and intracrystalline porosity in organo-clays

A comparison has been made of the sorptive and swelling properties of organo-montmorillonites in which the exchange ions were n-alkylammonium and n-alkyldiammonium. and of organo-hectorites in which these ions were n-alkyldiammonium. The sorbates studied were permanent gases and vapors such as benzene, n-heptane, acetonitrile, and α-ω-diaminopropane. Other penetrants were investigated less fully. Those parts of the isotherms of the gases at 78°K. which were associated with interlamellar sorption were very rectangular and readily gave interlamellar saturation capacities. Interlamellar intrinsic porosities were calculated and from them the theoretical saturation capacities. For the hectorites the experimental and theoretical capacities were in approximate agreement; for the montmorillonites it was observed that when long-chain organic cations were present only part of the interlamellar porosity was accessible, the inaccessible fraction rising rapidly for the longest chains. Net sorption volumes were also calculated for the variously swollen organo-clays which showed them, when swollen, to have sorption capacities fully comparable with the most porous molecular sieve zeolites. Three types of hysteresis loop have been observed among the sorption isotherms and their causes indicated. With all the forms of organo-clays α-ω-diaminopropane gave complexes having d(001) of about 14.3A; for benzene d(001) was 15.0A; and for acetonitrile two series of complexes were prepared with d(001) ∼16.3 and ∼19.9A, respectively. Thus the d(001) spacing is determined by the penetrant and is little influenced by the valence or size of the organic exchange ion. Although there was strong swelling with acetonitrile, ethyl and propyl cyanides did not swell the organo-clays appreciably. In addition to physical imbibition, acetonitrile and α-ω-diaminopropane were in part chemisorbed by reaction with the exchange cations.