CLASSIFICATION OF CARBONATE RESERVOIRS
Carbonate rocks as reservoirs of oil and gas confidently compete with terrigenous formations. According to various data, from 50 to 60% of the current world hydrocarbon reserves are associated with carbonate formations. Among them, the best quality reservoirs stand out - carbonate rocks of reef structures. Oil and gas production, large in volume, is produced from limestones and dolomites, including from the Paleozoic and Precambrian; the largest deposits have been discovered in Mesozoic and Paleozoic rocks, primarily in the countries of the Middle East. Large accumulations in Mesozoic reef structures have been discovered in the basin of the Gulf of Mexico (Golden Belt, Campeche, etc.). From reef limestones, record production rates were also obtained (tens of thousands of tons per day). There is some connection between the development carbonate reservoirs and increased carbonate accumulation in geological history, which is associated with the general cyclicity of geotectonic development and the periodicity of sedimentation.
Carbonate reservoirs are characterized by specific features:
1. Extreme incontinence, significant variability of properties, which makes it difficult to compare them.
2. Various diagenetic and catagenetic changes occur relatively easily in them.
3. The facies appearance of limestones, to a greater extent than in clastic rocks, affects the formation of reservoir properties.
4. In terms of minerals, carbonate rocks are less diverse than clastic ones, but in terms of structural and textural characteristics they have much more varieties.
5. In the process of studying the reservoir properties of carbonate strata, the genesis of deposits and the hydrodynamics of the medium play a decisive role in the formation of the structure of the void space, which can be more or less favorable for the formation of reservoirs and determines the nature of subsequent transformations.
6. Carbonate rocks are easily subject to secondary changes. This is due to their increased solubility. The influence of secondary transformations is especially great in rocks with a primary heterogeneous structure of the pore space.
7. By the nature of post-sedimentary transformations, carbonate rocks differ from terrigenous ones. First of all, it concerns sealing. The remains of bioherms from the very beginning are practically solid formations, and then compaction is already slow.
8. Carbonate silt can also quickly lithify, and in this case, peculiar fenestrial voids appear in it due to the release of gas bubbles. Small-clastic, lumpy-algal carbonate sediments are also rapidly lithified. The porosity is somewhat reduced, but at the same time, a significant amount of the pore space is “conserved”.
All types of voids are noted in carbonate rocks. Depending on the time of occurrence, they can be primary(sedimentary and diagenetic) and secondary(post-diagenetic).
In organogenic carbonate rocks, primary include intrashell voids, including those inside reef structures, as well as intershell voids. Some carbonate rocks may be of chemogenic or biochemogenic origin and form reservoir-type reservoirs. These include primarily oolitic, as well as limestones with inter- or intra-oolitic voids. Layered or massive limestones are characterized by pelitomorphic or cryptocrystalline, as well as crystalline structures. In crystalline, especially in dolomitic rocks, intercrystalline (intergranular) porosity is developed.
Carbonate rocks, to a greater extent than others, are subject to secondary transformations (recrystallization, leaching, stylolite formation, etc.), which completely change their physical properties, and sometimes their composition (dolomitization and razlomitization processes). This is the difficulty of identifying natural reservoirs, since the same rock under some conditions can be considered as a reservoir with very high properties, and in others, if there are no cracks, it can be a tire. The creation of secondary voids is facilitated by the processes of dissolution (leaching), recrystallization, mainly dolomitization and razdolomitization or stylolithization.
Certain processes affect differently depending on the genetic type of the breed.
Cementation can start very early and happen quickly, as is well seen in the example of beachrocks. Calcite cement crystallizes due to the evaporation of sea water flooding the beach and the partial dissolution of unstable minerals. Beach carbonate sand can harden in a few days. Similar almost instantaneous lithification occurred in past times. The further fate of the voids remaining in the frame of such a “lithicate” may be different.
Upon recrystallization there is a significant change in the structure and texture of rocks. In general, this process is directed towards an increase in the crystal size. If part of the substance is removed during recrystallization, the porosity increases. Unevenly recrystallized rocks have the highest secondary porosity. The growth of large crystals promotes the formation of microcracks.
The most effective influence on the formation of secondary voidness is exerted by leaching and metasomatism(mainly dolomitization). Dissolution during leaching manifests itself in different ways, depending on the greater or lesser dispersion of the particles that make up the rock. Finely dispersed components are more susceptible to this process. Solubility depends on the composition of minerals and waters: aragonite dissolves better than calcite, sulfate waters dissolve dolomite more actively, etc. An analysis of changes in porosity and porosity parameters, determined, among other things, by leaching, establishes their very clear connection with the structural and genetic types of rocks.
Dolomitization is one of the leading factors in the formation of reservoirs. The formation of dolomite is influenced by the ratio of magnesium and calcium in water and the total salinity. Higher salt concentrations require more dissolved magnesium. During diagenesis, dolomite arises from its precursors, such as magnesian calcite.
Primary diagenetic dolomitization is not essential for the formation of reservoir properties. Metasomatic dolomitization in catagenesis is more important for reservoir transformation. Magnesium supply is necessary for dolomite formation. Its sources may be different. During catagenetic processes at elevated temperatures, solutions lose magnesium, exchanging it for calcium of host rocks. The example of the Pripyat trough shows that a clear relationship is established between the composition of brines and the intensity of secondary dolomitization. In those stratigraphic zones where the Devonian carbonate rocks are most strongly dolomitized, the magnesium content in brines drops sharply, it is used to form dolomite.
During metagenetic dolomitization, an increase in porosity is especially noticeable, since the process takes place in a rock with a rigid skeleton, which is difficult to compact. The total volume of the rock is preserved, the voidness in it increases due to dolomitization.
reverse process breaking apart(dedolomitization) is especially common in near-surface conditions. It is most active in sections where dolomites contain interlayers of sulfates. When water seeps, magnesium in dolomites in solutions combines with the SO 4 2- radical and is carried out in the form of easily soluble MgSO 4 . There is an increase in the porosity of rocks.
But the transfer of sulfates by water often leads to opposite results in terms of the quality of reservoirs. Easily soluble CaSO 4 also precipitates easily and seals pores. It can also influence calcitization, which is often expressed in the build-up of regeneration borders and narrowing of the pore space.
Concluding the consideration of carbonate reservoirs, it is necessary to emphasize once again that, in comparison with clastic rocks, the structure of their pore space is extremely diverse. An undisturbed matrix has characteristics that are determined primarily by the primary structure, vugginess greatly changes these characteristics, and fracturing creates, as it were, two systems of voids superimposed on each other.
All this determines the need for a special classification of reservoirs. Such an estimated genetic classification of reservoirs was proposed by K.I. Bagrintseva (Table 2).
table 2
Estimated genetic classification of carbonate reservoir rocks
| Group | Class | Absolute transparency, D | Open porosity, % | Usable capacity and filtration properties | collector type | Textural and structural characteristic | |
| AND | I | 0,1-0,5 | 25-35 | high | Cavernous-porous | Biomorphic organs - detr., lumpy | |
| II | 0,5-0,3 | 16-35 | |||||
| B | III | 0,3-0,1 | 12-28 | medium | Pore, fissured-porous | Organic-detrital, relict | |
| IV | 0,1-0,55 | 12-25 | |||||
| V | 0,05-0,01 | 12-25 | Organic-clotty-detrital | ||||
| AT | VI VII | 0,0-0,001 0,001-0,0001 0,300-0,02 | 6-10 1-5 1-4,5 | low | Pore-cracks. cracks. vuggy-fractured | Pelithomophic-microgranular, clotted-water-detrital | |
The defining parameter of the proposed classification is permeability, limit values which are taken from analyzes of reservoir properties of rocks of various genesis and structural features. The minimum and maximum values of the estimated indicators (porosity, gas-oil saturation, etc.) are obtained from the correlation dependences between permeability, porosity and residual water. The relationship between residual water saturation and absolute permeability is most characteristic.
In the rocks, as the filtration properties improve, the amount of residual water decreases. Porosity can be different, while even high (more than 15%) values of open porosity occur in rocks with low filtration properties. The relationship between open porosity and residual water saturation is uncertain.
Low-porosity rocks always have a high water content, while highly-porous ones have a dual characteristic: well-permeable ones contain a small amount of water, and poorly permeable ones contain a significant amount (more than 50%). In the classification scheme, all reservoirs are divided into three large groups A, B, C, within which, in turn, classes are distinguished, characterized by different estimated parameters, lithological and structural features. Groups A and B are mainly represented by reservoirs of porous and vuggy-porous types, C - fractured and mixed types. The rocks of group A are dominated by primary voids, the dimensions of which are increased in the processes of subsequent leaching.
Group B rocks have developed sedimentation pore channels; leaching voids play a lesser role. The structure of the void space in the rocks of group A is much simpler than in group B, and it is most complex in group C. Small winding, poorly connected channels predominate here. Collectors of classes I and II in group A have mainly inherited high filtration and capacitance parameters. Classes III, IV, and V include detrital-organogenic and biochemogenic rocks with low primary reservoir properties. Secondary mineral formation, recrystallization, dolomitization, fracture, especially accompanied by leaching and material removal, improve their properties. In classes VI and VII, rocks of such chemogenic and biochemogenic varieties are distinguished, the petrophysical characteristics of which never reach high values. But here, to a greater extent than in the breeds of the higher classes, another factor is manifested - fracturing.
Carbonate rocks, as is known, are often composed of significant
Calcium and magnesium salts dissolved in waters served as carbonate rocks. At
excess amount of the latter in the aquatic environment, they begin to stand out in
sediment by a purely chemical means, or by absorption from the aquatic environment by living
organisms, these salts fall into the sediment in the form of carbonate skeletal
leftovers. Carbonate rocks: limestone, chalk, dolomite, calcareous tuff
Estimated - genetic classification.
In the classification scheme, all reservoir rocks are subdivided into
groups A, B, C, which combine seven classes of collectors that differ
from each other by estimated parameters, lithological and structural
features. Groups A and B are mainly represented by pore and
cavernous - porous types; group B - mixed and fractured reservoirs
Rocks - reservoirs, allocated to groups A, B, C, do not differ
only in terms of textural and structural features, but also in terms of the time of formation of the void space.
So, in the rocks of group A, mainly sedimentation pores are developed, the sizes of which are increased due to secondary leaching processes, sometimes to the size of caverns. There is no significant genetic difference between pores and caverns; their influence on reservoir properties is also unambiguous. Therefore, this group of reservoirs also includes reservoirs of cavernous-porous type. It is important that both permeability and capacity are determined by pore channels of various sizes. Group A is represented mainly by carbonates of organogenic and detrital origin, characterized by loose packing of the fragment and
fragments of various sizes and roundness. The cement is in
a small amount (up to 10%), forms crusts and
regeneration membranes around detritus, rarely fills pores, is represented
newly formed calcite crystals.
Group A contains two classes of rocks: permeability from 300 to 500 mD and
with a permeability of 500 mD or more. The content of bound water in them is negligible.
(from 5 to 20%), classes I and II are characterized by high usable capacity and
high filtering properties. Gas saturation coefficient of rocks I and II
high classes - 0.95 - 0.8. Reservoir type cavernous - porous and porous.
In the rocks of group B, sedimentary and relict-sedimentary pore channels are developed, but their sizes are sharply reduced, and leaching voids play a smaller role in the pore space. The main difference between the rocks of this group and the rocks of group A lies in the greater complexity of the processes of the structure of the pore space, which is due to the action of secondary processes.
Group B is represented by highly altered rocks of organogenic and
detrital origin, as well as fine - and medium-grained differences
chemogenic genesis. Organogenic and organogenic - detrital carbonates
characterized by varying degrees of cementation (cement 15 - 20% or more), unequal intensity of recrystallization (from weak to strong
recrystallized) and different packing densities of fragments.
The rocks of this group are characterized by significant secondary caltization,
the intensity of which determines the complex structure of the pore space:
morphology, size and shape of the pore channels, as well as the nature of their
relationships. The presence of pore channels and the predominance of narrow, strongly
winding causes a decrease in the permeability of these rocks from 300 to 10 mD.
Gradual complication of the structure of the pore space ( big number
small pores, strong tortuosity and roughness of pore channels, etc.)
caused the unequal influence of bound water on the change
effective parameters - capacitance and permeability. Just for collectors
group B is characterized by an inverse linear relationship between the residual
water saturation and permeability. They differ in average useful
capacity and average filtration properties. Coefficient
gas saturation of class III collectors 0.88 - 0.78, class IV) 0.84
-), 7; V class 0.8 - 0.62. Reservoir type is mostly porous, but V class
can be represented by a fractured-porous reservoir.
Carbonate rocks of group B are distinguished by the most complex character.
pore space. Small pore channels are developed, which have
tortuosity, poor communication. Characterized by isolated voids
leaching (cavities) and cracks of various orientations.
Group B is represented mainly by rocks of chemogenic and
biochemogenic origin, as well as strongly recrystallized,
altered post-sedimentary processes, organogenic rocks, in
whose shaped elements are practically indistinguishable. It's very dense
impermeable and most often low-porous rocks.
The pore space of chemogenic and biochemogenic rocks is extremely
heterogeneous and complex in structure: morphology, pore sizes, the form of their relationship are determined by the intensity of secondary processes. The pores are rounded, sometimes irregular in shape, located between the crystals or cross them. The pores are connected to each other through intercrystalline channels, the width and degree of tortuosity of which depend on the size of the cement crystals. The smaller the crystals, the thinner the gaps between them, and consequently, the narrower and more tortuous channels connecting the pores. Small pores are connected to each other along the thinnest (less than 5 - 10 microns) channels, which can be traced between the crystals in the main microfine-grained mass of carbonate. Communication of pore channels is difficult, they are often isolated, which determines their low filtration properties. Leaching and recrystallization voids are characteristic.
Breeds - collectors of this group are characterized by low useful capacity
matrices and low filtering properties - fractions and units of millidarcy.
Collectors of group B are characterized by a mixed type of hollow
space. It includes porous - fractured and fractured types of reservoirs.
The intensity of the development of cracks is of decisive importance for the classification of rocks
collectors or non-collectors.
Carbonate rocks as reservoirs of oil and gas confidently compete with terrigenous formations. According to various data, from 50 to 60% of the current world hydrocarbon reserves are associated with carbonate formations. Among them, the best quality reservoirs stand out - carbonate rocks of reef structures. Oil and gas production, large in volume, is produced from limestones and dolomites, including from the Paleozoic and Precambrian; the largest deposits have been discovered in Mesozoic and Paleozoic rocks, primarily in the countries of the Middle East. Large accumulations in Mesozoic reef structures have been discovered in the basin of the Gulf of Mexico (Golden Belt, Campeche, etc.). From reef limestones, record production rates were also obtained (tens of thousands of tons per day). One can note a certain connection between the development of carbonate reservoirs and the intensification of carbonate accumulation in the geological history, which is associated with the general cyclicity of geotectonic development and the periodicity of sedimentation.
Carbonate reservoirs are characterized by specific features:
1. Extreme incontinence, significant variability of properties, which makes it difficult to compare them.
2. Various diagenetic and catagenetic changes occur relatively easily in them.
3. The facies appearance of limestones, to a greater extent than in clastic rocks, affects the formation of reservoir properties.
4. In terms of minerals, carbonate rocks are less diverse than clastic ones, but in terms of structural and textural characteristics they have much more varieties.
5. In the process of studying the reservoir properties of carbonate strata, the genesis of deposits and the hydrodynamics of the medium play a decisive role in the formation of the structure of the void space, which can be more or less favorable for the formation of reservoirs and determines the nature of subsequent transformations.
6. Carbonate rocks are easily subject to secondary changes. This is due to their increased solubility. The influence of secondary transformations is especially great in rocks with a primary heterogeneous structure of the pore space.
7. By the nature of post-sedimentary transformations, carbonate rocks differ from terrigenous ones. First of all, it concerns sealing. The remains of bioherms from the very beginning are practically solid formations, and then compaction is already slow.
8. Carbonate silt can also quickly lithify, and in this case, peculiar fenestrial voids appear in it due to the release of gas bubbles. Small-clastic, lumpy-algal carbonate sediments are also rapidly lithified. The porosity is somewhat reduced, but at the same time, a significant amount of the pore space is “conserved”.
All types of voids are noted in carbonate rocks. Depending on the time of occurrence, they can be primary(sedimentary and diagenetic) and secondary(post-diagenetic).
In organogenic carbonate rocks, primary include intrashell voids, including those inside reef structures, as well as intershell voids. Some carbonate rocks may be of chemogenic or biochemogenic origin and form reservoir-type reservoirs. These include primarily oolitic, as well as limestones with inter- or intra-oolitic voids. Layered or massive limestones are characterized by pelitomorphic or cryptocrystalline, as well as crystalline structures. In crystalline, especially in dolomitic rocks, intercrystalline (intergranular) porosity is developed.
Carbonate rocks, to a greater extent than others, are subject to secondary transformations (recrystallization, leaching, stylolite formation, etc.), which completely change their physical properties, and sometimes their composition (dolomitization and razlomitization processes). This is the difficulty of identifying natural reservoirs, since the same rock under some conditions can be considered as a reservoir with very high properties, and in others, if there are no cracks, it can be a tire. The creation of secondary voids is facilitated by the processes of dissolution (leaching), recrystallization, mainly dolomitization and razdolomitization or stylolithization.
Certain processes affect differently depending on the genetic type of the breed.
Cementation can start very early and happen quickly, as is well seen in the example of beachrocks. Calcite cement crystallizes due to the evaporation of sea water flooding the beach and the partial dissolution of unstable minerals. Beach carbonate sand can harden in a few days. Similar almost instantaneous lithification occurred in past times. The further fate of the voids remaining in the frame of such a “lithicate” may be different.
Upon recrystallization there is a significant change in the structure and texture of rocks. In general, this process is directed towards an increase in the crystal size. If part of the substance is removed during recrystallization, the porosity increases. Unevenly recrystallized rocks have the highest secondary porosity. The growth of large crystals promotes the formation of microcracks.
The most effective influence on the formation of secondary voidness is exerted by leaching and metasomatism(mainly dolomitization). Dissolution during leaching manifests itself in different ways, depending on the greater or lesser dispersion of the particles that make up the rock. Finely dispersed components are more susceptible to this process. Solubility depends on the composition of minerals and waters: aragonite dissolves better than calcite, sulfate waters dissolve dolomite more actively, etc. An analysis of changes in porosity and porosity parameters, determined, among other things, by leaching, establishes their very clear connection with the structural and genetic types of rocks.
Dolomitization is one of the leading factors in the formation of reservoirs. The formation of dolomite is influenced by the ratio of magnesium and calcium in water and the total salinity. Higher salt concentrations require more dissolved magnesium. During diagenesis, dolomite arises from its precursors, such as magnesian calcite.
Primary diagenetic dolomitization is not essential for the formation of reservoir properties. Metasomatic dolomitization in catagenesis is more important for reservoir transformation. Magnesium supply is necessary for dolomite formation. Its sources may be different. During catagenetic processes at elevated temperatures, solutions lose magnesium, exchanging it for calcium of host rocks. The example of the Pripyat trough shows that a clear relationship is established between the composition of brines and the intensity of secondary dolomitization. In those stratigraphic zones where the Devonian carbonate rocks are most strongly dolomitized, the magnesium content in brines drops sharply, it is used to form dolomite.
During metagenetic dolomitization, an increase in porosity is especially noticeable, since the process takes place in a rock with a rigid skeleton, which is difficult to compact. The total volume of the rock is preserved, the voidness in it increases due to dolomitization.
reverse process breaking apart(dedolomitization) is especially common in near-surface conditions. It is most active in sections where dolomites contain interlayers of sulfates. When water seeps, magnesium in dolomites in solutions combines with the SO 4 2- radical and is carried out in the form of easily soluble MgSO 4 . There is an increase in the porosity of rocks.
But the transfer of sulfates by water often leads to opposite results in terms of the quality of reservoirs. Easily soluble CaSO 4 also precipitates easily and seals pores. It can also influence calcitization, which is often expressed in the build-up of regeneration borders and narrowing of the pore space.
Concluding the consideration of carbonate reservoirs, it is necessary to emphasize once again that, in comparison with clastic rocks, the structure of their pore space is extremely diverse. An undisturbed matrix has characteristics that are determined primarily by the primary structure, vugginess greatly changes these characteristics, and fracturing creates, as it were, two systems of voids superimposed on each other.
All this determines the need for a special classification of reservoirs. Such an estimated genetic classification of reservoirs was proposed by K.I. Bagrintseva (Table 2).
table 2
Estimated genetic classification of carbonate reservoir rocks
| Group | Class | Absolute transparency, D | Open porosity, % | Usable capacity and filtration properties | collector type | Textural and structural characteristic | |
| AND | I | 0,1-0,5 | 25-35 | high | Cavernous-porous | Biomorphic organs - detr., lumpy | |
| II | 0,5-0,3 | 16-35 | |||||
| B | III | 0,3-0,1 | 12-28 | medium | Pore, fissured-porous | Organic-detrital, relict | |
| IV | 0,1-0,55 | 12-25 | |||||
| V | 0,05-0,01 | 12-25 | Organic-clotty-detrital | ||||
| AT | VI VII | 0,0-0,001 0,001-0,0001 0,300-0,02 | 6-10 1-5 1-4,5 | low | Pore-cracks. cracks. vuggy-fractured | Pelithomophic-microgranular, clotted-water-detrital | |
The defining parameter of the proposed classification is permeability, the limiting values of which are taken from analyzes of the reservoir properties of rocks of various genesis and structural features. The minimum and maximum values of the estimated indicators (porosity, gas-oil saturation, etc.) are obtained from the correlation dependences between permeability, porosity and residual water. The relationship between residual water saturation and absolute permeability is most characteristic.
In the rocks, as the filtration properties improve, the amount of residual water decreases. Porosity can be different, while even high (more than 15%) values of open porosity occur in rocks with low filtration properties. The relationship between open porosity and residual water saturation is uncertain.
Low-porosity rocks always have a high water content, while highly-porous ones have a dual characteristic: well-permeable ones contain a small amount of water, and poorly permeable ones contain a significant amount (more than 50%). In the classification scheme, all reservoirs are divided into three large groups A, B, C, within which, in turn, classes are distinguished, characterized by different estimated parameters, lithological and structural features. Groups A and B are mainly represented by reservoirs of porous and vuggy-porous types, C - fractured and mixed types. The rocks of group A are dominated by primary voids, the dimensions of which are increased in the processes of subsequent leaching.
Group B rocks have developed sedimentation pore channels; leaching voids play a lesser role. The structure of the void space in the rocks of group A is much simpler than in group B, and it is most complex in group C. Small winding, poorly connected channels predominate here. Collectors of classes I and II in group A have mainly inherited high filtration and capacitance parameters. Classes III, IV, and V include detrital-organogenic and biochemogenic rocks with low primary reservoir properties. Secondary mineral formation, recrystallization, dolomitization, fracture, especially accompanied by leaching and material removal, improve their properties. In classes VI and VII, rocks of such chemogenic and biochemogenic varieties are distinguished, the petrophysical characteristics of which never reach high values. But here, to a greater extent than in the breeds of the higher classes, another factor is manifested - fracturing.
CRACKED MANIFOLDS
According to the formation of the void space, fractured reservoirs differ from other types. To determine the fracture voidness and permeability, there are special methods. As already mentioned, there are macro- and microcracks with an opening of more or less than 0.1 mm, respectively. Macrocracks are usually studied, described and measured in the outcrop field, and microcracks - under a microscope in thin sections, often of an enlarged size. A necessary element in the study of fractures is the determination of their orientation both in space (vertical, horizontal, inclined), and in relation to the reservoir (along the layering, across the layering, diagonal) and to structural forms (longitudinal, transverse, radial, etc.).
In genetic terms, lithogenetic and tectonic cracks are distinguished (Table 3).
Table 3
The main types of cracks in sedimentary rocks
Lithogenetic cracks according to their confinement to certain stages, they are subdivided into diagenetic, catagenetic, hypergenetic. Tectonic cracks differ in the causes that cause them: oscillatory movements, folded and discontinuous dislocations. Some types of fractures can pass into others, but in principle an experienced geologist will always distinguish between lithological and tectonic fractures. As a rule, lithological fracturing adapts to the structural and textural features of the rock. The cracks branch out and go around individual grains; in general, their location is chaotic. The surface of the crack walls is uneven.
Tectonic cracks they are more straightforward, they take less into account the structural and textural features of the rocks, the surface of their walls is smoother and sometimes passes into slickensides.
Different rocks are prone to fracturing to varying degrees. Marls and pelitomorphic limestones have the greatest ability to crack, followed by siliceous rocks, shales, and sandstones. The least fractured salts. It is noted that there is a certain relationship between the thickness of the layers and the intensity of fracturing - with the same composition in thicker layers, the distance between the cracks is greater.
Observations from space, aerial photographs, descriptions of outcrops show that there are fissures and fissure zones of different scales. Elements of a very large planetary system of jointing are distinguished, possibly confined to the joints of large tectonic blocks of the earth's crust. These fractured zones are the basis of the so-called lineaments on the Earth's surface. One of the large lineament zones can be traced from the Urals, through Central Asia goes to the Persian Gulf region and further to Oman (Ural-Oman lineament). Other smaller lineaments, reflecting zones of increased fracturing, are known in the Eastern Ciscaucasia. The identification and mapping of such zones is a paramount task, especially in practical terms.
An important issue is the implementation of cracks. They can be free and partially or completely filled with any substances precipitated from the solutions circulating in them. Most often, fractures are filled with carbonate minerals, quartz, sulfates, clay material (often impregnated with bituminous substance) and residual products of hydrocarbon conversion (black-bitumen fractures). Drop-liquid oil is often found on the walls of cracks.
The main elements of fractures during measurements are their orientation (in space, in relation to layers, etc.), their length and openness. In addition, we can talk about the density and density of cracks. When determining the density, the number of cracks of one system per unit length perpendicular to this system of cracks is taken into account. For macrocracks, the length unit is 1 m, for microcracks (determined in sections) - 1 mm. Under the density of cracks, the total number of all systems per unit volume or per unit area (outcrop surface, section area) is taken.
The void space of fractured reservoirs is divided into two categories. On the one hand, these are pores and other voids in the rock matrix (in blocks not disturbed by cracks), on the other hand, the volume of the cracks themselves, the caverns associated with them, etc. The property of block rocks (matrices) is determined in the usual way. The volume of fractures is usually not large, but due to the relative simplicity of the structure and the predominant straightness of fractures, filtration through them can be very effective.
Fracture voidness is the ratio of fracture volume to rock volume:
where b is crack opening (average statistical distance between crack walls); 1 - their total length in the sample; S - study area.
The dependence of fracture permeability on openness and fracture voidness is expressed by the relation:
K t \u003d 85 000 b 3 m T,
where b - crack opening, mm; t t - fractured voidness, fractions of a unit; K t - fracture permeability, µm 2 .
The above relation is valid for those cases when the surfaces of the crack walls are perpendicular to the filtration surface. In the presence of several systems of cracks and their different orientation with respect to the filtration flow, different numerical coefficients should be applied.
In addition to studying in samples (macrofracturing) and in thin sections (microfracturing), fracturing is also studied by geophysical and hydrodynamic methods, by photographing borehole walls, but each of these methods has its own errors.
The degree of rock fracturing and, therefore, the allocation of the corresponding zones in the section can be made on the basis of acoustic logging (AK) data.
UNCONVENTIONAL MANIFOLDS
Rocks whose role in oil and gas content is still small compared to those described above include strata composed of clayey, siliceous, volcanogenic, intrusive, metamorphic rocks, etc. They can be divided into two groups. In some, oil and gas potential is usually syngenetic, in others it is associated with the arrival of hydrocarbons from neighboring strata.
1. In clayey rocks natural reservoirs arise in the process of catagenesis. The very occurrence of voids is associated with the generation of oil and gas hydrocarbons and the restructuring of the structural and textural features of the mineral matrix of the rock. One of the characteristic examples is the clay sequence of the Bazhenov formation in Western Siberia. The deposits of the Bazhenov Formation differ from the underlying and overlying rocks by the increased content of organic matter (from 5 to 20% or more) and the increased content of silica. The rocks have a reduced density (2.23-2.4 g/cm 3 ) compared to the underlying and overlying strata. According to T.T. Clubova, in sedimentogenesis, the formation of microblocks covered with a film of sorbed organic matter occurred. Collomorphic silica, enveloping aggregates of clay minerals, creates complex complexes on their surface with the participation of organic matter and silica (so-called organosilicon "shirts" appear). The processes of transformation of clay minerals and release of bound water lead to the formation of small layer-by-layer cracks. Deconsolidations occur at a certain depth of the zones. Due to the growth of internal pressure, some sections of the rock are penetrated by a system of cracks along the surface of the "shirts". When opening the rocks of the Bazhenov formation, as a rule, deconsolidation and abnormally high reservoir pressure are noted.
As a result, zones with increased reservoir properties appear, bounded on all sides by less altered and permeable rocks. Often, these areas are in no way connected with the structural and tectonic features of the region. So, apparently, reservoirs were formed in the Bazhenov carbonate-siliceous-argillaceous strata of the Upper Jurassic in Western Siberia (Salymskoe field, etc.). Reservoirs in the Maikop clayey series of the Stavropol region (Zhuravskoye deposit, etc.) could have been formed in a similar way.
It can be concluded that in these reservoirs the formation of reservoir properties and generations of petroleum hydrocarbons coincide in time. Some tectonic processes also contribute to the increase in rock fracturing. When oil is extracted from such rocks, the fractures close, thus, bazhenites and other similar rocks are reservoirs, as it were, of a “single use”. They cannot be injected with gas or oil, as is done in the construction of underground storage facilities for other types of rocks.
2. Processes run differently in siliceous strata biogenic origin. At the first stages of sedimentation of the initial stages of diagenesis, an “openwork” organogenic structure is formed from the shells of flint-building organisms. Subsequently, the transformation of the organogenic structure is closely related to the transformation of amorphous forms of silica (opal) into crystalline forms. When opal A transforms into opal CT, a globular microtexture appears and an interglobular type of collector is formed. With an increased content of sapropelic OM with an increased catalytic role of surface-active silica, the processes of hydrocarbon generation begin. Collectors for them have already been prepared in the same strata, their properties are high (porosity reaches 40%). Oils in biogenic-siliceous strata are considered to be oils of early maturation. With a further increase in catagenesis, dehydration occurs, the transition of silica into other mineral forms - chalcedony, and then quartz. Fracturing develops in the rocks, the associated system of fractures contributes to the formation of a reservoir of a reservoir or massive type with a fractured reservoir. There are several fields on the California shelf where the siliceous rocks of the Monterey Formation of the Miocene are commercially oil-bearing. The largest is the Point Arguello field. On Sakhalin, two deposits have also been discovered in such strata. Reservoirs appear in a similar way in siliceous-argillaceous-carbonate-rich OM, the so-called domanicoid strata.
3. collectors in rocks of igneous and metamorphic origins have been known for a long time. In particular, oil has been found in volcanic rocks, in secondarily altered porous lavas and tuffs, and in Mexico, Japan, and other places. Oil and gas in tuffs, lavas, and other varieties are associated with voids that were formed by the release of gas from lava material or with secondary leaching. The oil content of these rocks is always secondary.
4. In volcanic rocks Muradkhanly field was discovered in Western Azerbaijan. Oil deposits in the rocks of the volcanogenic complex of the Eocene age were discovered in Eastern Georgia. Accumulations of oil are known in metamorphosed basement rocks in Algeria, in altered serpentinites in Cuba, etc. Oil inflows are obtained from the weathering crust of granite-metamorphic rocks occurring in the cores of the Mesozoic uplifts in the Shaim region of Western Siberia. In the Oimash area in the South Mangyshlak, oil was obtained from the zone of secondary altered granites.
5 . However, a real boom was caused by the discovery of oil in granite-gneiss rocks on the shelf of Vietnam (the White Tiger field, etc.). These rocks are involved in the structure of deposits, their massifs are covered with tertiary sedimentary rocks, granite bodies are introduced into sedimentary rocks. The occurrence of reservoir properties in them is associated with metasomatism and leaching as a result of hydrothermal activity, with the phenomena of contraction (shrinkage) during cooling, with crushing into zones of tectonic disturbances. As a result of the action of solutions, leaching of feldspars, large caverns are formed in the rocks.
As a result of the impact of the above processes, sub-horizontal and sub-vertical zoning appeared in the distribution of permeable areas and three types of voids developed: fractured, fractured-cavernous, and porous. The main volume of voids in the magmatic reservoir belongs to microcracks and microcaverns. The main void space of tectonic origin is associated with fracturing, cataclasis and mylonitization, as a result of which the rocks are crushed into rubble. Contraction shrinkage on cooling led to the creation of contraction voids. The porosity of rocks in most cases does not exceed 10-11 %. The permeability of the matrix is low, but as a result of the development of vugginess and fracturing in general, the permeability reaches hundreds of millidarcies. Improved reservoir zones provide hundreds of tons of oil inflows.
Given the need to compare the main parameters of the two leading groups of reservoirs - clastic (granular) and carbonate - the authors propose a general classification of these reservoirs (Table 4). It is based on a comparison of the original classifications, it takes into account both the structural features of the rock and, in part, their composition. Classes are distinguished mainly by the value of open porosity, while its boundaries, as well as the permeability in the classes, are very wide (respectively 10-20%, 100-1000 mD). This shortcoming can be eliminated by introducing subclasses depending on the development of specific rock varieties in a given area with their material-structural characteristics and parameters.
For example, in class 2, subclass 2a can be distinguished with well-sorted low-cement sandstones and 2b - with sandstones containing an increased amount of cement and, accordingly, with reduced capacity and especially permeability. In class 4, weakly altered pelitomorphic and fine-grained limestones have satisfactory storage capacity, but low permeability. This can also include lumpy leached limestones or stromatolites, which have enhanced properties. Enlarged classes are useful for identifying general trends in property changes over large areas of parts of the section.
Table 4
General classification of collectors
Carbonate rocks as reservoirs of oil and gas confidently compete with terrigenous formations. According to various data, from 50 to 60% of the current world hydrocarbon reserves are associated with carbonate formations. Among them, the best quality reservoirs stand out - carbonate rocks of reef structures. Oil and gas production, large in volume, is produced from limestones and dolomites, including from the Paleozoic and Precambrian; the largest deposits have been discovered in Mesozoic and Paleozoic rocks, primarily in the countries of the Middle East. Large accumulations in Mesozoic reef structures have been discovered in the basin of the Gulf of Mexico (Golden Belt, Campeche, etc.). From reef limestones, record production rates were also obtained (tens of thousands of tons per day). One can note a certain connection between the development of carbonate reservoirs and the intensification of carbonate accumulation in the geological history, which is associated with the general cyclicity of geotectonic development and the periodicity of sedimentation.
Carbonate reservoirs are characterized by specific features:
1. Extreme incontinence, significant variability of properties, which makes it difficult to compare them.
2. Various diagenetic and catagenetic changes occur relatively easily in them.
3. The facies appearance of limestones, to a greater extent than in clastic rocks, affects the formation of reservoir properties.
4. In terms of minerals, carbonate rocks are less diverse than clastic ones, but in terms of structural and textural characteristics they have much more varieties.
5. In the process of studying the reservoir properties of carbonate strata, the genesis of deposits and the hydrodynamics of the medium play a decisive role in the formation of the structure of the void space, which can be more or less favorable for the formation of reservoirs and determines the nature of subsequent transformations.
6. Carbonate rocks are easily subject to secondary changes. This is due to their increased solubility. The influence of secondary transformations is especially great in rocks with a primary heterogeneous structure of the pore space.
7. By the nature of post-sedimentary transformations, carbonate rocks differ from terrigenous ones. First of all, it concerns sealing. The remains of bioherms from the very beginning are practically solid formations, and then compaction is already slow.
8. Carbonate silt can also quickly lithify, and in this case, peculiar fenestrial voids appear in it due to the release of gas bubbles. Small-clastic, lumpy-algal carbonate sediments are also rapidly lithified. The porosity is somewhat reduced, but at the same time, a significant amount of the pore space is “conserved”.
All types of voids are noted in carbonate rocks. Depending on the time of occurrence, they can be primary(sedimentary and diagenetic) and secondary(post-diagenetic).
In organogenic carbonate rocks, primary include intrashell voids, including those inside reef structures, as well as intershell voids. Some carbonate rocks may be of chemogenic or biochemogenic origin and form reservoir-type reservoirs. These include primarily oolitic, as well as limestones with inter- or intra-oolitic voids. Layered or massive limestones are characterized by pelitomorphic or cryptocrystalline, as well as crystalline structures. In crystalline, especially in dolomitic rocks, intercrystalline (intergranular) porosity is developed.
Carbonate rocks, to a greater extent than others, are subject to secondary transformations (recrystallization, leaching, stylolite formation, etc.), which completely change their physical properties, and sometimes their composition (dolomitization and razlomitization processes). This is the difficulty of identifying natural reservoirs, since the same rock under some conditions can be considered as a reservoir with very high properties, and in others, if there are no cracks, it can be a tire. The creation of secondary voids is facilitated by the processes of dissolution (leaching), recrystallization, mainly dolomitization and razdolomitization or stylolithization.
Certain processes affect differently depending on the genetic type of the breed.
Cementation can start very early and happen quickly, as is well seen in the example of beachrocks. Calcite cement crystallizes due to the evaporation of sea water flooding the beach and the partial dissolution of unstable minerals. Beach carbonate sand can harden in a few days. Similar almost instantaneous lithification occurred in past times. The further fate of the voids remaining in the frame of such a “lithicate” may be different.
Upon recrystallization there is a significant change in the structure and texture of rocks. In general, this process is directed towards an increase in the crystal size. If part of the substance is removed during recrystallization, the porosity increases. Unevenly recrystallized rocks have the highest secondary porosity. The growth of large crystals promotes the formation of microcracks.
The most effective influence on the formation of secondary voidness is exerted by leaching and metasomatism(mainly dolomitization). Dissolution during leaching manifests itself in different ways, depending on the greater or lesser dispersion of the particles that make up the rock. Finely dispersed components are more susceptible to this process. Solubility depends on the composition of minerals and waters: aragonite dissolves better than calcite, sulfate waters dissolve dolomite more actively, etc. An analysis of changes in porosity and porosity parameters, determined, among other things, by leaching, establishes their very clear connection with the structural and genetic types of rocks.
Dolomitization is one of the leading factors in the formation of reservoirs. The formation of dolomite is influenced by the ratio of magnesium and calcium in water and the total salinity. Higher salt concentrations require more dissolved magnesium. During diagenesis, dolomite arises from its precursors, such as magnesian calcite.
Primary diagenetic dolomitization is not essential for the formation of reservoir properties. Metasomatic dolomitization in catagenesis is more important for reservoir transformation. Magnesium supply is necessary for dolomite formation. Its sources may be different. During catagenetic processes at elevated temperatures, solutions lose magnesium, exchanging it for calcium of host rocks. The example of the Pripyat trough shows that a clear relationship is established between the composition of brines and the intensity of secondary dolomitization. In those stratigraphic zones where the Devonian carbonate rocks are most strongly dolomitized, the magnesium content in brines drops sharply, it is used to form dolomite.
During metagenetic dolomitization, an increase in porosity is especially noticeable, since the process takes place in a rock with a rigid skeleton, which is difficult to compact. The total volume of the rock is preserved, the voidness in it increases due to dolomitization.
reverse process breaking apart(dedolomitization) is especially common in near-surface conditions. It is most active in sections where dolomites contain interlayers of sulfates. When water seeps, magnesium in dolomites in solutions combines with the SO 4 2- radical and is carried out in the form of easily soluble MgSO 4 . There is an increase in the porosity of rocks.
But the transfer of sulfates by water often leads to opposite results in terms of the quality of reservoirs. Easily soluble CaSO 4 also precipitates easily and seals pores. It can also influence calcitization, which is often expressed in the build-up of regeneration borders and narrowing of the pore space.
Concluding the consideration of carbonate reservoirs, it is necessary to emphasize once again that, in comparison with clastic rocks, the structure of their pore space is extremely diverse. An undisturbed matrix has characteristics that are determined primarily by the primary structure, vugginess greatly changes these characteristics, and fracturing creates, as it were, two systems of voids superimposed on each other.
All this determines the need for a special classification of reservoirs. Such an estimated genetic classification of reservoirs was proposed by K.I. Bagrintseva (Table 2).
table 2
Estimated genetic classification of carbonate reservoir rocks
According to the lithological composition, two main types of reservoirs are distinguished - terrigenous (sandy-silty) and carbonate. In addition, there are reservoirs associated with volcanic-sedimentary, clayey and rare crystalline rocks.
Terrigenous reservoirs take the main place among others: 58% of the world's proven oil reserves and 77% of gas are associated with them. Suffice it to say that in such a unique basin as the West Siberian, almost all gas and oil reserves are located in terrigenous clastic reservoirs. Lithologically, terrigenous reservoirs (sands, sandstones, siltstones) are characterized by granulometry - grain size.
The capacitive-filtration properties of terrigenous deposits are very different. The porosity of oil-bearing sandy reservoirs averages 15-20%, the permeability is usually tenths and hundredths, rarely a few square micrometers (µm 2).
The reservoir properties of terrigenous rocks are determined by the structure of the pore space, intergranular porosity. Clay minerals, in general, clay content worsen reservoir properties.
Carbonate reservoirs ranked second in importance. They account for 42% of the world's oil reserves and 23% of gas reserves.
Carbonate reservoirs are fundamentally different from terrigenous ones in that, firstly, they contain only two main rock-forming minerals - calcite and dolomite. Secondly, in carbonate reservoirs, oil and gas filtration is mainly determined by cracks and caverns. The main processes that form the void space in carbonates are associated either with biogenic accumulation, or with leaching and karst formation, or with tectonic stresses that led to the formation of a developed network of cracks, microcracks, etc.
The largest deposits located in the Persian Gulf basin are associated with carbonate reservoirs, in many oil and gas basins USA and Canada, in the Caspian basin.
collectors found in volcanic and volcanic-sedimentary rocks. They are represented by effusive rocks (lavas, pumice) and volcanogenic-sedimentary (tuffs, tuff breccias, tuff sandstones). Reservoirs in effusive rocks are associated in most cases with ultramafic rocks. Voids in them arose during the degassing of erupted magma or in the process of erosion, tectonic crushing, etc. There are deposits in Cuba associated with tuff sandstones, the Kelebia deposit in Yugoslavia is in rhyolite-type effusives. Reservoir properties of volcanic rocks are often associated with the secondary change of rocks, the occurrence of cracks. In general, these reservoirs are poorly studied.
Clay collectors. Oil and gas fields associated with clay reservoirs have been known for a long time in the USA, in California in the Santa Maria basin as early as the beginning of the 20th century. The reservoirs are represented there by siliceous, bituminous clays of the Upper Miocene.
Among the clay reservoirs, a special place is occupied by bituminous clays of the Bazhenov formation in Western Siberia. At the Salymskoye, Pravdinskoye and other deposits, Bazhenov clays occur at depths of 2750-3000 m at reservoir temperature of 120-128ºС, have a thickness of 40 m. Age - the Volgian and Berriasian (Jurassic and Cretaceous). Oil flow rates - from 0.06 to 700 m 3 / day. The problem of clay reservoirs is very interesting not only in connection with the nature and genesis of voids, but also from the point of view of studying the origin of oil and the formation of deposits.
Impermeable rocks - "tires". Seals, or seals, are rocks that prevent oil, gas, and water from escaping from a reservoir. They overlap the reservoir from above (in traps), but they can also replace the reservoir along strike, when, for example, clays replace sandstones upstream of the formation.
The concept of "tire" is relative, because if the tire does not pass liquid (oil and water), then at the same time it can pass gas through itself, which has a lower viscosity. At the same time, with large pressure drops, liquids will be filtered through impermeable rock - a tire.
According to the development area, regional and local tires are distinguished. For example, Kynovskie (Timan) clays are a regional seal, a seal of Devonian deposits throughout the Volga-Ural basin.
According to the lithological composition, the caps are represented by clayey, carbonate, halogen, sulfate and mixed types of rocks. The most fully studied clay tires
(T.T. Klubova), then carbonate.
The best quality tires are rock salt and plastic clays because they don't have cracks. In rock salt, due to its plasticity, there are no open voids and cracks, filtration channels, so it is an excellent screen for the movement of oil and gas. But if there is an admixture of sandstone in rock salt, then gas filtration is possible into post-salt deposits. Gypsum and anhydrite have worse shielding properties than rock salt.
clay tires most often found in terrigenous oil and gas complexes. Their screening properties depend on the composition of minerals having different absorption capacities.
As immersion occurs, dehydration of clays occurs, their plasticity decreases, and the fracturing of rocks increases. Sometimes clay - argillite - turns into a fractured reservoir. An example of such a collector is bazhenov suite Upper Jurassic of Western Siberia. Fine-grained limestones and dolomites they also screen, serve as a cover for oil deposits, but the admixture of a small clay and sandy material worsens their screening properties several times.
At depths of more than 4.5 km, thick strata of rock salt and sulfate-halogen rocks with high plasticity can serve as reliable "tires".
Enhances the screening properties of the tire, the excess of water pressure in the formation above the tire, making it difficult to vertical migration; inverse relationship, i.e. excess water pressure in the reservoir under the seal, on the contrary, worsens the shielding quality of the seal over the deposit.
Thus, the screening properties of tires depend on the lithology of rocks, tectonic, hydrogeological conditions, on the properties of oil, gas, pressure gradient and other factors.
When studying reservoir properties oil and gas bearing complexes, an important parameter is the hydraulic conductivity, which characterizes the filtration properties of the reservoir: To etc· h/μ - where To pr - permeability coefficient, m 2; h– collector thickness, m; μ – dynamic viscosity, Pa s.
The physical value of the hydraulic conductivity parameter shows the ability of the reservoir to pass a liquid of a certain viscosity per unit time with a pressure drop
0.1 MPa. Formation transmissibility information is obtained from field surveys (pressure recovery curves or indicator curves), but this is often not possible. Then, for each well, information about the permeability of the reservoir, the effective thickness of the reservoir, the viscosity of the reservoir oil is inscribed on the location plan, and hydraulic conductivity isolines are built using these data.
