How to distinguish limestone from dolomite?

The structural features are manifested in the fracture of the rock: micro-grained ones have an earthy fracture and stain your hands (chalk), and strong ones have a porcelain-like or conchoidal fracture, medium-coarse-grained ones have a crystalline sparkling fracture. 4. Texture (massive, layered, biogenic, textures of replacement, dissolution, stylolites, sutures, etc.) 5. Impurities (clay content, sand content, etc.) 6. Porosity, cavernousness, fracturing 7. Features of the sample (degree and nature of saturation, fracture, etc.) Clastic carbonate rocks are described as terrigenous. Marl is a carbonate rock with a clay content from 25 to 50%, or clay rocks with a carbonate content of 25-50%. With a lower content of carbonate matter, the clays are calcareous or dolomitic, and with a low clay content (5-25%) – clayey limestones or dolomites.

Carbonate rock groups

1. Organogenic

2. Granular and oolitic, i.e., chemogenic

Within groups, rock types are distinguished according to shape, size and ratio of structural elements (shells, crystals, fragments, etc.): Biohermic: corals, bryozoans, algae, etc., which are intravitally buried reef-type formations – skeletal remains of attached organisms. They are relief (with significant height) and, most often, large organogenic formations on the seabed of a short horizontal extent. Whole shell: large-shelled or shell rocks, which include: brachiopods, pelecypods, cephalopods, gastropods, trilobites, etc. and small-shelled: foraminifers (fusulinids, nummulinids), ostracods, etc. Detrital or detritus (organogenic-clastic) limestones consist of fragments of skeletal remains, unrounded or from spicules of calcareous sponges that make up the skeleton, separated when they die:, etc. There are both homogeneous and mixed – polydetritic (crinoid-brachiopod, etc.). Chalk is currently considered an organogenic rock composed of coccoliths and foraminifera, significantly modified by silt eaters and granulation already in the sediment.

Part 2. Study of reservoir rocks and tires

Reservoir rocks are called rocks that can contain liquids and gases and release them during development. According to the conditions of formation, they are terrigenous and carbonate. Getting to know a reservoir rock sample terrigenous type, you should carefully consider its structure. The rock consists of individual grains. Light grains are fragments of quartz, pink ones are feldspars. Debris of different sizes. Among them, the largest ones stand out, 2-3 mm in diameter. Between them are smaller ones of various sizes and shapes. The shape of the fragments is different: from acute-angled irregular shapes to well-rolled, almost round grains. From the theoretical course it is known that sandstone is a sedimentary rock. Its origin is associated with the destruction, transport and deposition of igneous rock fragments. Therefore, here we see grains of quartz and feldspars. These are products of granite destruction. The size of the grains indicates the distance from the source of destruction of granites. Larger fragments accumulate closer to the source of destruction, while smaller ones are transported over long distances. The roundness of the grains also depends on this. Near the source of drift, the grains will be acute-angled. When transported for a long time by water currents or wind, they become rounded and rounded. Quartz and feldspar grains are usually firmly “soldered” to each other with cement. Sometimes they are weakly cemented and crumble when rubbed with the fingers. The cementing material is clay, carbonate or silicate particles. Among them you can see sparkles of mica. If a solution of hydrochloric acid is dropped onto a fresh sandstone fracture surface, the presence of carbonate material in the cement can be determined. Its presence will manifest itself in the form of hissing air bubbles on the fracture surface. Thus, after a thorough examination of a rock sample (based on its visible features), it is possible to determine the name of the rock that makes up the terrigenous reservoir. It can be siltstone, sand, sandstone, gravel. It is possible to preliminarily determine the quality of this rock as a reservoir rock, knowing that porosity and permeability depend on roundness, sorting of fragments, the nature of the placement of grains, the degree of their cementation, the quality of cement, etc. It is known that porosity is the presence of free space in a rock, and permeability is the ability of rocks to pass liquids or gases through themselves. Studying the sample carbonate reservoir (limestone, dolomite), it should be remembered that its formation occurs in an aquatic environment, at great depths. Carbonate rocks can also be of chemical and organic origin. Therefore, it is necessary to carefully study the structure of the rock, that is, the presence of grains, or crystals, or fragments of shells cemented by clayey, siliceous or carbonate material. You can also distinguish limestone from dolomite using a solution of hydrochloric acid. Dolomite will only react with hydrochloric acid in powder form. After determining the name of the rock, its structure, impurities, texture (layering), you should proceed to a careful study of the free space in the rock. These can be pores, cavities, cracks. If possible, it is necessary to measure their dimensions (length, width, diameter), determine the shape, the direction of the predominant propagation of cracks, then calculate the approximate amount of free space in an area of ​​u2bu2bI cm 4 and, if possible, establish the presence of interconnected pores (cavities or cracks). To examine rock samples, it is recommended to use a magnifying glass with XNUMX-XNUMXx magnification. Reservoir rocks may contain residues of heavy hydrocarbons, and then they will be dark in color, have a specific odor, stain your hands, etc. Signs of the presence of gas in the rock can be visually detected only by the smell in a fresh fracture or after placing a sample in water immediately after its split. The dark color of the rock, which has no visible signs of oil or gas, indicates the presence of bituminous substances in it. Such a rock could be oil and gas producing. Their study is important for determining the degree of oil and gas potential of a particular area of ​​the earth’s crust and the migration routes of hydrocarbons. After getting acquainted with the reservoir rocks, they move on to studying tire rocks. Seal rocks are practically impermeable rocks that prevent the migration of hydrocarbons in the earth’s crust and contribute to the preservation of already formed accumulations. It is known that the caprocks are clayey, carbonate and halogen sedimentary formations. Clay rocks can be determined by their very fine grain size, layering, and “fatness” to the touch. Clays consist of the finest fragments of destroyed rocks of various compositions, transported into a water basin and deposited at great depths. As a rule, the fragments have the form of flat flakes several microns in size. The color of the clays is dark – brown, chocolate, black. This depends on the composition of the flakes, impurities and geochemical conditions of the environment in which the clayey material accumulated. The distance between clay and mica flakes in the rock is negligible, however, given the large number of flakes, the volume of “free” space in the clays is large, therefore the porosity of the clays is high. The clay porosity coefficient reaches 50%. However, clays act as tires, since they are practically impenetrable, because the finest pores in the clays do not communicate with each other. Clay rocks are loose, viscous, but when they dry, they crack, split along bedding planes and across, and under certain conditions: they can contain liquids or gases, that is, serve as a reservoir (for example, the Bazhenov formation in Western Siberia). There are argillic, pellitic and other clay covers. Carbonate tires – these are limestones, dolomites of various origins, without signs of free space in them. They are dense, often clayey, often silicified. Tire rocks halogen type visually easy to distinguish from other types of breeds. These are light-colored rocks with a crystalline structure, dense and strong. These include gypsum, anhydrite, and rock salt. They were formed by precipitation from brines (highly mineralized waters) in shallow reservoirs connected to the sea (lagoons). The best halogen tire and the most common in nature is considered to be a tire made of rock salt. A sample of rock salt differs from other rocks in its regular cubic crystal shape and bitter-salty taste. Laboratory work № 3. Construction of a geological profile. Geological profile sections are compiled at all stages of geological prospecting and exploration work and are constructed both from geological survey data and from well drilling data. The goal of the work is to master the skills and methods of constructing geological profile sections based on drilling data. A geological profile is a graphic representation of the structure of a section of the earth’s crust in a vertical sectional plane. Geological profile sections reflect the geological structure of the selected section of the earth’s crust, show the peculiarities of the conditions of occurrence of rocks and identified accumulations of oil and gas, the nature of changes in rocks in the section of the field, the position of gas and water-oil contacts. In the presence of disjunctive dislocations, geological profile sections make it possible to establish the nature of the disturbance and its main parameters. In combination with structural maps, geological profile sections give an idea of ​​the nature of the structure of the territory not only along the section line, but also in area. Geological profile sections based on drilling data are constructed when a sufficient number of wells have been drilled within the study area to complete this work. Depending on the goals and objectives, the direction and scale of construction are chosen. The direction of the profile section line is chosen, as a rule, across the strike of the structure, taking into account the use of the maximum number of wells. If it is impossible to choose a straight direction according to the well location diagram, you can lay it by connecting the wells along a broken line. It is also allowed to transfer wells located away from the selected direction to the profile line, taking into account corrections for the depth and absolute elevation of each stratigraphic horizon. After choosing the direction, we proceed to choosing the scale of construction. The horizontal and vertical scales should be taken the same, in this case the distortion in the image of geological boundaries in the drawing will be minimal, sometimes due to geological conditions (for example, small angles of incidence of layers within platform areas) or depending on the goals and objectives of the construction, the vertical scale is chosen larger than the horizontal . Further work on constructing a geological profile section is carried out in the following order. A horizontal line is drawn corresponding to the zero value or sea level, and to the left of it a vertical scale of absolute depths (vertical scale) is drawn. Vertical lines depict wellbores located in a certain sequence from south to north, west to east, taking into account the selected horizontal scale and the “drift” of the wells to the profile line. Upwards from the base (zero) line are plotted on the altitude scale of wellheads. The resulting points are connected by a smooth curve showing the terrain in the selected section; boreholes are shown with signs along the line of the earth’s surface. Their faces are marked vertically from the surface with a limiting horizontal stroke. The position of geological boundaries in wells can be determined in two ways. In the first case, the depths of the roof or base of each stratigraphic unit in each well are plotted from the level of the earth’s surface on a scale. In the second case, after preliminary calculation (AO = A – H, where AO is the absolute elevation of the roof or bottom, A is the altitude of the wellhead, H is the depth of the roof or bottom of the sediments), the absolute elevations of the roof or bottom of all stratigraphic units. The points obtained in both cases are connected by a smooth curve. The drawing is prepared according to the standard scheme (Fig. 2). It is recommended to use graph paper for construction. The age of rocks in the drawing is shown by color and index. The initial data for constructing a geological profile section are given in tables 4a – 4d. It is recommended that the distance between wells on a scale of 1:10 be equal to 000 – 400 m. The vertical scale is equal to the horizontal. Scale: horizontal 1: 10000 vertical 1 : 10000 Fig. 2. Geological profile section along the line of wells 3 -1 Crushed stone is a bulk material that is obtained from rock or waste from the metallurgical industry, as well as from gravel mining. There are several types of this building material, one of which is dolomite crushed stone. It is obtained from a mineral called dolomite, which belongs to the class of carbonates (the rock contains more than 95% of this substance). The size of the fraction of such crushed stone is 3–300 mm. The definitions of “crushed limestone” and “dolomite crushed stone” are often confused. This is probably due to the fact that both rocks contain calcite. In fact, it is quite simple to distinguish between them: if the volumetric content of dolomite is less than 75%, it is dolomitic limestone, more than 75% is limestone dolomite.

Classification and characteristics

1. The mass of 1m3 of dolomite crushed stone is 2650 kg.
2. Hardness index – 3,5 – 4 units.
3. The average weight of 1m3 is 1450 kg. This characteristic is important when calculating the properties of concrete mixtures.
4. The flakiness of dolomite crushed stone is about 10–35%. This value reflects the flatness or elongation of the stones. These indicators affect compaction density and drainage properties. A cube-shaped stone, with a flakiness of up to 10%, is advantageous to use in the production of concrete, and a flatter stone is used as a cushion in the construction of highways.
5. The amount of dust formations in dolomite crushed stone should not exceed 2%, clay and soil inclusions – no more than 0,25%.
6. The degree of frost resistance of dolomite crushed stone is F150.
7. Dolomite raw materials are characterized by high adhesion to most binders used in construction: gypsum, cement, polymers from the acrylic group, petroleum bitumen. Thanks to this property, it can be used as crushed stone for foundations or as backfill in the production of roofing felt.

Pure dolomite crushed stone is usually colorless or white. At the same time, the presence of inclusions of various minerals makes it possible to distinguish such types as yellow or gray crushed stone from dolomite. For example, the presence of warm shades in the mixture means that the rock contains iron hydroxide and clay particles. Gray, bluish-gray shades indicate the presence of manganese, strontium and barium oxide additives in the mixture. There are also natural samples of yellow-pink, pale pink, greenish and bluish shades. When chipped, such a stone has a glassy, ​​pearlescent or matte sheen. The technical characteristics of dolomite crushed stone make it possible to use it in construction, for example, as a filler for foundations.

Dimensions.

Dolomite crushed stone has certain dimensional characteristics.

According to the size of the fraction, it is divided into 3 groups:

• 1st group, 5–20 mm;
• 2nd group, 20–40;
• 3rd group, 40–70.