Porosity measurements and form analysis of mineral grains in thin sections from oil- gas reservoir rocks using quantimet 720 and besi

Submicroscopic and image analyzer techniques have been applied for the measurement of the porosities of oil—gas—water reservoir rocks from thin sections. Backscattered electron scanning images (BESI) were made predominantly at low magnifications of × 30 and × 60. Porosity data from linear traverses were obtained and differences in porosities between two sandstone reservoirs and a carbonate reservoir were determined. A large number of the measured pores contained clayey material. The surface occupied by this material was subtracted from the apparent porosity measurements with the image editor of the Quantimet 720 to give the real porosity.

A sample from one of two sandstone reservoirs contained oil and could not be hardened using the common procedure for impregnation of the sample by a polyester resin. Gamma radiation was applied and hardening occurred in one weekend after 5 Mrad (50 kGy) had been absorbed. Hardening of this sample gave no problems, if the oil had been removed by monostyrene before a start was made with the impregnation process; such a sample was used for the present porosity study.

Form analysis was done of individual mineral grains using BESI and Quantimet. The grains were represented in a number of shape classes of a diagram used for shape classification. Form-separation of individual minerals was also possible.     

The characterization of the shape of mineral grains in thin sections of soils by quantimet and besi

Silt-sized mineral grains were analysed for their shapes or forms in thin sections of soils using backscattered electron scanning images (BESI) and an image analyzer (Quantimet 720). This work was done with an image-editor and required manual editing of the shape of mineral grains on the display screen of the Quantimet. Shape analysis of coarse and fine silt grains could thus be performed at magnifications of x 240 and x 480.

The shape of the mineral grains could be characterized by five parameters: area (A), perimeter (Pe), largest and smallest Feret's diameters (Fmax and Fmin), lobation ratio (Lr) and indentation ratio (Ir). A relation was found, however, between the APe² ratio and the Lr and Ir ratios. Consequently, shape analysis of mineral grains can be done by measuring three of the five parameters, i.e. A, Pe and Fmax/Fmin. These parameters, however, are still not be able to reproduce the shape of individual mineral grains and images of them therefore remain necessary.

Several of the 54 mineral grains were found to have different shapes but the same A/Pe² ratio. An explanation for this phenomenon is given by using a theoretical model in which two-dimensional forms change their shapes from a circle (A/Pe² = 79) to a line (A/Pe² = 1). Form separation of mineral grains with the same (A/Pe²) ratio can be done by using the Fmax/Fmin ratio.

The measurement of the shape of the mineral grains, the theoretical model and observation of the micrographs of the individual mineral grains, made it possible to draw a diagram with shape classes which can be used for shape classification purposes. Knowledge gathered in soil micromorphology on shape analysis of pores and soil aggregates has also been used to obtain boundaries of shape classes in the diagram. The present study is intended both as a proposal as to how two-dimensional shape classification can possibly be done, and as a work-model. The boundaries of the shape classes can easily be adjusted if data from forthcoming studies require it.     

The over-estimation of clay and the under-estimation of pores in soil thin sections

The amounts of clay visible in four thin sections from paleosol horizons (clay contents 9–32%) were measured by point counting. These sections were used as references for visual estimates, without point counting, of the amounts of clay in 14 further sections. The measurements and estimates were recorded on pore-free and gravel-free bases so they could be related to gravimetric analyses. Amounts of clay measured and estimated by area in these thin sections were between 1.6 and 3.1 times those determined gravimetrically. The reasons for these differences are discussed. Firstly, clay in soil thin sections contains much pore space beyond the resolution of the optical microscope and has an apparent density dependent on the amount of submicroscopic pore space. An edge effect also causes over-estimation of clay in soil thin sections and under-estimation of visible pore space. Components (clay, mineral grains, pores) are usually sloped at their borders with each other within the standard 25–30 μm (three-dimensional) thin sections. Clay, for instance, may overcap a pore at its border, hiding some pore space. The need for true two-dimensional images of soil pore patterns in image analysis is indicated. A nomogram has been constructed, using apparent density, to allow estimation of clay in thin sections of soils with any clay content. Coarse/fine ratios can be determined more accurately and estimates of clay can also be recalculated to provide illuvial to total clay ratios that are more meaningful. It is recommended that the micromorphological identification criterion for the argillic horizon be reassessed and that 4% illuvial to total clay in a soil thin section be adopted as the diagnostic cut-off.     

The characterization of microporosity in a ploughpan by submicroscopic and quantimet techniques

Ploughpans are usually recognizable in thin sections because of their massiveness (density) when compared with adjacent parts of the soil. When pores can be measured in the ploughpan such massiveness can be documented and compared with the porosity of the underlying soil profile. Such measurements, however, concern only meso- and macropores when done by Quantimet on micrographs obtained with the light microscope. This is due to the thickness of the thin section and the necessity to work with transmitted light. The measurement of the smaller mesopores may also be problematic with the light microscopy—Quantimet technique.

The introduction of backscattered electron scanning images allows to obtain micrographs of a very thin layer and this made it possible to measure micro- and mesopores by Quantimet. If only small magnifications are used, macropores can also be measured.

In the present investigation, micro- and mesopores were quantified by a combination of backscattered electron and Quantimet techniques and the macropores by light microscopy and Quantimet. It was demonstrated that macro- and mesopores were less frequent in a ploughpan than outside, whereas capillary or micropores increased somewhat.     

Quantitative analysis of trace and major elements in thin sections of soils with the secondary ion microscope (Cameca)

A series of experiments have been performed for a number of years to obtain quantitative microchemical analysis by secondary ion microscopy of materials in thin sections of soils. The first successful quantification was obtained from an area with a diameter of 300 μm in bauxite and done with an IMS 300 at Philips Research Laboratories. Such an area, however, is frequently too large for most soil samples which are extremely heterogeneous even on a microscale. A second generation IMS 3F was tested for soils at the Cameca factory in Paris. This instrument allows the quantification of trace and major elements of soil constituents in a spot with 1.5 μm diameter, using a computer program developed by Philips Research Laboratories. Calcite and clay from a Petrocalcic Xerochrept were used for this test. Ion spectra, ion images and linear traverses were made giving information on the nature and distribution of both major and trace elements. The possibility of investigating a larger number of chemical elements simultaneously in linear traverses is of considerable help in the study of thin sections. Various submicroscopic and other techniques are discussed to give some insight into the position of ion microscopy amongst these.     

土壤薄片中石英颗粒分布的分形特征

土壤中石英颗粒的粒度分布对揭示母质来源和模拟土壤结构等具有重要意义。本文根据石英的光学性质,利用数字图像处理技术提取土壤薄片中的石英颗粒信息,并应用分形维数来表征土壤薄片中石英颗粒的分布特征。采集免耕和翻耕处理下土壤样品并制作成土壤薄片,利用偏光显微镜和数字图像处理技术提取土壤薄片中石英颗粒,应用Sigma ScanPro5软件统计了石英颗粒的数量、大小和形状等信息,研究土壤薄片中石英颗粒的数量分形特征;并分析了其同石英颗粒数量、面积和平均半径等的关系。结果表明石英颗粒的数量分布具有明显的分形特征,分形维数在1.60～2.18之间,分形维数同薄片中石英颗粒的总面积和平均半径呈负相关。免耕处理下表层土壤石英颗粒的数量分形维数高于底层,而翻耕处理下则呈相反的趋势。免耕和翻耕措施下土壤薄片中石英颗粒的数量分形维数在0～5和10～15cm层次之间没有显著差异,但是在20～25cm层次,翻耕处理分形维数高于免耕处理,说明耕作措施会加速土壤矿物质的风化和破碎并改变石英颗粒在土壤剖面中的分布。     

Multi-orientation thin sectioning for the determination of 3-D morphology

A new soil thin sectioning technique which gives thin cross-sections (called sliver sections) of thin sections, is described. Optical analysis of soil thin sections and soil sliver sections, using plain, crossed polarized and circularly polarized light provides three-dimensional information on the micromorphology of clay coatings. This new technique is applied to a common type of clay plug and shows that these features consist of a stack of bowl-shaped clay coatings.     

Quantimet 720 analysis of porosities in backscattered electron scanning images made with different phototechniques

Three photo-techniques were used to make backscattered electron scanning images (BESI) of the same area in a thin section of soil using a scanning electron microscope (SEM). Subsequent porosity analysis with an image analyzer (Quantimet 720) showed that different results could be obtained in a number of diameter classes while the total porosity was virtually the same, a phenomenon which may be related to beam instability of SEM. Under such conditions, one needs a dependable photo-technique in which soil constituents on a BESI micrograph can be compared with the same constituents on the display screen of SEM and Quantimet. Such a photo-technique is suggested and can be used to obtain a limited number of measurable BESI micrographs.     

A comparative study of three methods of water removal prior to resin impregnation of two soils

The water in replicate samples from sub-surface horizons of a clayey and a fine-silty soil was removed either by oven-drying, freeze-drying or by acetone-replacement prior to resin impregnation. Pore space photograms (pores >60 μm diam.) from thin sections were analysed on a Quantimet 720 image analysing computer following each drying technique. During oven- and freeze-drying the clayey soil contracted considerably and subsequently satisfactory impregnation of these samples was either difficult or impossible. This macro-shrinkage caused a decrease in number and lengths of most planar pores and a decrease in size and number of all intra-aggregate pores. The loss in pore space and continuity obviously affected resin impregnation. The clayey samples in which water was replaced by acetone, and all samples of the silty soil, however pretreated, showed no measurable macro-shrinkage and all impregnated well. Oven-drying the silty soil appears to increase the porosity, causing an increase in size of all pores. A mechanism for this apparent enlargement is proposed. It is evident that acetone-replacement of the soil water prior to resin impregnation is the best of the three methods used here and should be seriously considered when image analysis is to be carried out on resin-impregnated blocks from soils of similar or related textures.     

A VERSATILE METHOD FOR TREATMENT OF CLAY SOILS FOR THIN-SECTION FABRIC STUDIES

In an earlier investigation three different methods had to be used for the treatment of laboratory-compacted clay soils prior to the preparation of thin sections for use in fabric studies. The three methods, viz., the Carbowax method, the Araldite-resin impregnation method, and the Aquax method, had limited application depending on the degree of saturation and the clay-mineral composition of the soil. A new method has been developed which can be applied to all clay soils irrespective of their mineralogy and degree of saturation. In this method, the pore water of the soil specimen is first replaced with monostyrene using a two-step diffusion process. Then the specimen is impregnated with an Araldite-resin mixture which can be cured at a temperature of 55°C. The method has yielded better thin sections than those obtained by any of the three previous methods. Pure montmorillonite soils can be treated with a greater degree of success than hitherto possible.     

PORE SPACE VARIABILITY IN A SUB-SURFACE HORIZON OF TWO SOILS

Image analysis was used to investigate the variability of pore space in soil thin sections. Two fine-textured soil profiles (or pedons) were sampled by inserting eight Kubiena tins (7.5 × 6 × 4 cm) in ledges at 50 cm depth. From each soil block three horizontal thin sections (7.5 × 6 cm) were made and photographed. Each photograph was then divided into nine ‘frames’ so that the voids in each of these frames could be measured using the Quantimet 720 Image Analysing Computer. The results show the variability of three specific pore-space characters and the consequent need to sample a number of large thin sections from several sample blocks for each soil horizon to obtain estimates of sample means with suitable accuracy.     

Submicroscopy and chemistry of heavy-metalcontaminated precipitates from column experiments simulating conditions in a soil beneath a landfill

The first results of a cooperative research project of IWGSUSM (International Working-Group on Submicroscopy of Undisturbed Soil Materials) are discussed. Wet chemistry of heavy metals which precipitated during column experiments, simulating conditions in a soil beneath a landfill, was studied. To compare these data of bulk chemistry with in situ microchemistry of the same heavy metals, thin sections were made. Using the normal technique of impregnation, problems are encountered in the hardening process. However, if gamma radiation is used, an absorbed dose of 5Mrad (50 kGy) was sufficient to harden the polyester resin.The first thin sections of the upper 30 cm of a sandy column have now been studied with SEM-EDXRA (scanning electron microscope—energy dispersive X-ray analysis) and initial quantification was tried out using EMA (electron microprobe analysis) and SEM-WDXRA (scanning electron microscope—wavelength dispersive X-ray analysis). The first results demonstrate that the heavy-metal-containing cutans (coatings on the walls of voids and on the surfaces of mineral grains) are brown and black when studied with the light microscope. The brown ones contain the smallest amounts of heavy metals and the black ones the largest amounts. Coatings of polluted precipitate are usually only present on part of the larger mineral grains; they can be concentrated in small bands, whereby smaller grains are often completely coated, or occur isolated on the surface of a small number of grains. Brown cutans also often contain fragments of black cutans. The distribution of Fe, Ni, Cu, Zn and Pb is extremely heterogeneous. Often this heterogeneity is already present in measurements of precipitates on the surface of one mineral. This indicates a rather extreme variety in composition of the pollutants, even on a microscale, and emphasises that submicroscopic techniques are certainly necessary to obtain detailed in situ information. The use of a step by step approach and the acquisition of numerous data should eventually allow a good understanding of the processes at work. STEM (scanning transmission electron microscope) — EDXRA measurements at magnifications larger than x 10,000 are of considerable help.     

Point counting pores and illuvial clay in thin section

Six soil thin sections containing various amounts of illuvial clay, previously estimated by ten operators, were point-counted four times by an additional experienced operator. The results indicate that a count of 6000 points over a section area of 25 × 25 mm is sufficient to quantify satisfactorily the pore space of the section. Replicated results for illuvial clay, however, differed widely for some sections. These differences are attributed both to difficulties in recognising illuvial clay and to orientation and variability of soil components. Variability of results for illuvial clay in this study was less than that reported for ten operators but, nonetheless, it is evident that for most sections the amount of illuvial clay cannot be estimated precisely by a single count of 6000 points. Suggestions are made for improving the reliability of estimates of illuvial clay in soil thin sections and an improved basis for reporting results is proposed.     

Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture

Total porosity and width, length and irregularity of elongated pores from thin sections, prepared from undisturbed Ap horizon samples taken from a zero versus conventional-tillage field experiment, were analysed by means of a Quantimet 720 System 23 image analysing computer. Both total porosity and elongated pore area were significantly higher in conventionally-tilled plots. On the contrary, both the total number of pores and the proportion of elongated pores were significantly higher in no-tilled plots. Thus, zero tillage reduced the size of pores. Conventional tillage caused an increase in the length of a few large, elongated pores which also showed very strong irregularity. In samples from no-tilled plots the total length of elongated pores was smaller, but these pores were more numerous and more regular than in samples from conventionally-tilled plots. Consequently, soil conditions for root growth were better.     

Seasonal changes in the structure of clay soils in relation to soil management and crop type. I. Effects of crop rotation at Cruden Bay, NE Scotland

Changes in soil structure and macroporosity were investigated using large thin sections and image analysis of photographs taken from fluorescent resin impregnated soil blocks. In this way porosity measurements were combined with visual assessment of the distribution and type of soil pores and aggregates. Three sites were chosen on a clay soil at Cruden Bay, Aberdeenshire, and sampled throughout two growing seasons in order to characterize various stages of a 7-year rotation.
The seasonal pattern of structural change depended on the crop. During a dry period in April and May 1980, a system of cracks developed on the grass site but not on sites sown to spring barley. The changes in structure on the grass sites could mainly be attributed to changes in water content, and the resulting cycle of shrinking and swelling.
After cultivation, structural changes resulting from settling were evident. Earthworm activity, especially after manure spreading, also had a pronounced effect on structure. Since many of these processes are cyclic in this environment, the resulting structural states showed a pattern of cyclic recurrence.     

A model system for studying the biochemistry and biology of the root-soil interface

Calcined attapulgite, a non-swelling clay mineral, has been used as a medium for plant growth when mixed with a nutrient solution in the proportion of 0.95 ml g^?1. Attapulgite is an ideal model “soil” for ultrastructural studies, enabling large intact thin sections through root, rhizoplane and soil. Transmission electron micrographs are presented which illustrate the value of attapulgite for in situ studies of rhizosphere populations, for the demonstration of enzyme activities in individual bacteria and for specific staining of extracellular polysaccharide.A standard fumigation-respiration technique widely used for estimating soil microbial biomass is shown to give unreliable results for rhizosphere samples and should not be used to measure microbial biomass in close asssociation with living roots. The addition of a dilute soil suspension to the attapulgite medium caused a stimulation of root growth without any increase in shoot growth.     

Towards systematic image analysis in the study of soil micromorphology

Operational laboratories for quantitative image analysis of soil thin sections are now possible. This paper describes the evolving methodology at the University of Guelph, Ontario, Canada, and its application in selected Canadian soils. The paper deals with the collection of data under transmitted, reflected and circularly polarized illumination. The perfect registration of all data is achieved simply by not moving the thin section. The protocol for analysis of differentiation of voids, organic matrix, clay coatings, carbonate and iron concretions is presented. The storage and retrieval of data from CD-ROMs for analysis and sharing is also outlined. Once the digital imagery is stored it is easily useable for more studies as a greater understanding of the value of data is recognized. The paper makes the case for much greater use of the soil thin sections that already exist in the laboratories around the world.     

MACRO- AND MICROMORPHOMETRIC INVESTIGATION ON SOIL-DEXTRAN INTERACTIONS

The effect of three microbial dextrans of defined molecular weight, on shrinkage and porosity inside clods has been studied on two Italian clay soils, a Fluvisol and a Vertisol. Shrinkage was determined on dried soil samples using an electro-optical apparatus. The same apparatus was also used to measure porosity and pore size distribution inside the clods. For this purpose large thin sections were prepared from cracked soil samples after shrinkage measurements. Dextrans influenced shrinkage and the size distribution of clods and cracks in the two soils. The major effect was found in the Fluvisol where the arrangement of cracks was changed visibly. The dextrans caused a marked increase in total porosity in the Fluvisol but had only a small effect in the Vertisol. In both soils the pore size distribution was changed resulting in a greater number of larger pores in the dextran treated samples.     

土壤微团聚体中矿物-有机复合体特征

以国家肥力网湖南祁阳红壤长期定位试验站的长期施有机肥和长期施化肥的土壤为研究对象,采用干筛法获得土壤微团聚体。利用同步辐射红外显微成像法研究土壤微团聚体中黏土矿物和有机官能团的原位分布图谱及其相关性。结果表明:土壤中黏土矿物(3 620 cm-1)和大分子有机物(脂肪,2 920 cm-1;蛋白质,1 650 cm-1;多糖,1 080 cm-1)呈高度异质性的分布规律。其中,黏土矿物和多糖有较为相似的分布模式;而黏土矿物和脂肪、蛋白类物质则呈现差异较大的分布模式。与长期施化肥处理的土壤微团聚体相比,长期施有机肥处理的土壤微团聚体中黏土矿物和大分子有机物呈现更高的分散性。此外,施有机肥和化肥处理土壤微团聚体样品中黏土矿物与有机官能团的决定系数(R2)均为:黏土矿物-脂肪黏土矿物-多糖黏土矿物-蛋白质,表明土壤微团聚体中黏土矿物和大分子有机物的亲和性有差异,且该差异不受长期施肥处理的影响。同步辐射微区域红外谱进一步表明,从土壤微团聚体外部到内部,黏土矿物的特征峰和大分子有机物的特征峰强度均逐渐增加。