The structure of clastic rocks includes the characteristics of clastic particles (particle size, shape and particle surface structure), the characteristics of interstitial materials (including matrix and cement) and the relationship between clastic particles and interstitial materials (that is, the types of supports and cements).
1 3. 1. 1 debris particles
The characteristics of clastic particles include roundness, sphericity, particle size, sorting and surface structure.
Roundness refers to the degree to which the original edges and corners of debris particles are rounded. Roundness depends on particle size, physical properties and wear history. Within a certain distance, larger particles have better roundness than smaller particles; Limestone with less hardness has better roundness than limestone with greater hardness. Long-distance transportation or long-time wear is better than short-distance transportation or short-time wear. The medium and mode of transportation also have an influence on the roundness of particles. For example, particles are easier to round when transported in the wind than in water, while glaciers are less likely to round when transported.
Sphericity refers to the degree to which a particle approaches a sphere. Sphericity is related to the properties of particles themselves. For example, there is no cleavage of the timely particles, so the farther they are transported, the greater the sphericity, while the flaky mica is transported far away, but the sphericity is lower. Large spherical particles are easy to roll and transport, while small spherical flaky particles are easy to suspend and transport.
Particle surface structure is the morphological characteristics of particle surface, mainly observing the degree of surface polishing and surface etching marks. Because the surface structure of debris particles has certain significance in revealing erosion, transportation and deposition, especially the study of particle surface structure by electron microscope to identify deposition environment has attracted more and more attention.
1.3. 1.2 Characteristics of filler
The filler includes hetero groups and cement. Due to the different causes, they also show their own characteristics in structure. Heteromatrix is a fine interstitial component deposited with coarse debris in clastic rocks, and its particle size is generally less than 0. They are mechanical deposition products, not chemical precipitation components. The content and properties of heterobase can reflect the flow characteristics of transport media and the sorting of clastic components, so it is also an important symbol of the structural maturity of clastic rocks. At the same time, the content of miscellaneous alkali is also an important index of hydrodynamic strength. In high-energy environment, the content of heterobase is less and the sandstone is pure. On the contrary, in the low energy environment, the content of hetero groups is high, which indicates that the sorting ability is poor. Cementitious material is a chemical genetic material, and its structure is similar to that of chemical rock, which is determined by grain size, crystal growth mode and recrystallization degree. The common types of cement structures are amorphous and cryptocrystalline texture, obvious granular structure, embedded structure and autogenous enlarged structure. Cements are formed after the deposition of granular sediments, and their composition and structural characteristics mainly reflect the composition of intergranular solution during deposition and the physical and chemical conditions during diagenesis.
1.3. 1.3 cementation type and supporting structure
(1) cementation type
In clastic rocks, the contact relationship between cement and clastic particles is called cementation type. There are the following types: ① basement cementation, containing a large number of interstitial materials, in which debris particles float without touching each other, and interstitial materials are mainly heterogeneous, which represents the characteristics of rapid accumulation of high-density flow; (2) Pore cementation, in which debris particles form a scaffold, and most of the particles are in point contact, and the cement content is small, which is only filled in the pores between debris particles; (3) Contact cementation, that is, point contact or line contact between particles, with little cement content, distributed in the parts where debris particles contact each other; (4) Mosaic structure, under the compaction of diagenesis, especially when the compaction is obvious, the contact of debris particles in sandy sediments will be closer, and the contact of particles will develop from point contact to line contact or concave-convex contact.
(2) Supporting structure
The support structure can be divided into heterobase support structure and particle support structure: ① heterobase support structure with high heterobase content and particles floating in heterobase; (2) The particle supporting structure has contact points between particles, including point contact, line contact, concave-convex contact and sewing contact. From point contact to suture contact, it reflects the intensity and progress of diagenesis during buried diagenesis, and suture contact is a feature of deep diagenesis. It can be seen that understanding the cementation type and the nature of intergranular contact of clastic rocks can provide basis for the analysis of sedimentary environment and diagenetic stage.
Particle size distribution characteristics of 1. 3.2 and its environmental significance
The particle size of sediment is called particle size. The method to study the particle size and its distribution characteristics of various particle sizes of clastic sediments and clastic rocks is called particle size analysis. Particle size distribution can reflect the hydrodynamic properties and energy of sedimentary media, which is an important physical sign to distinguish sedimentary environment from hydrodynamic conditions, and is also of great significance to the evaluation of oil and gas sedimentary reservoirs.
Debris is is mainly transported by machinery, and its transportation and deposition are controlled by hydrodynamic conditions (such as medium, flow rate and velocity). After burial, the particles of clastic materials generally do not change much except that some of them are expanded or dissolved in time. Therefore, the particle size and distribution characteristics can be used to directly reflect the hydrodynamic conditions in the deposition process. The study of particle size distribution can provide the following information: ① Clarify the nature of transport media, such as wind, water, glacier, debris flow, turbidity current, etc. (2) Determine the energy status of the conveying medium, such as flow rate, strength, starting ability, etc. (3) Clear treatment methods, such as rolling, jumping and hanging; ④ Define the forms of precipitation, such as traction current and turbidity current.
The main method of 1 3.2. 1 particle size analysis
According to the difference of particle size and rock density, the following three methods are used for analysis respectively.
(1) direct measurement method
Generally used for conglomerate or gravel, the method is to directly measure the diameter or apparent diameter of gravel with measuring tools. In general, the gravel (particles larger than 2mm) in a certain area is not less than 100. It is used to analyze conglomerate, such as rivers, coasts, glaciers and proluvial.
(2) Sieve analysis method
Used for loose or poorly cemented gravel sandstone to siltstone. Sand samples are screened by a group of sieves with different mesh diameters to be divided into components with different particle sizes. Generally, it is better to select the diameter of sieve holes with the spacing of 1/4 φ, weigh the quality of each layer of sand and calculate its percentage content. Sieve analysis method is simple and accurate, and it should be noted that sampling should be in a complete sequence, and coarse, medium and fine sand should be sampled.
(3) Slice granularity method
Generally, it is used for dense rocks. The method is to directly measure the maximum apparent diameter of particles in rock slices with a micrometer under a microscope, and convert the measured values into φ values, and group them according to the interval of 1/4 to calculate the percentage of particles in each group. Each slice needs to count 300 ~ 500 particles.
The analysis results obtained by the above different methods may be biased. For example, the deviation between the flake particle size and the sieve particle size can reach 0. Above 25 φ, this is the result of slicing effect (slicing effect means that the apparent diameter of particles in the slicing of particle aggregates is smaller than their true diameter), which must be corrected. The correction equation of granularity regression proposed by Friedman (1962) is d = 0. 3864.000000005d is the apparent diameter φ value in the slice).
The influence of matrix in sandstone must also be considered when analyzing particle size by thin slice particle size method, that is, correcting impurity base. This method is to measure or estimate the alkali content of impurities with a microscope. Because of slicing and diagenesis, its value is generally high. Take 2/3 or 1/2 as the correction value, assume that it is X, and multiply each cumulative percentage by (100-x) as the real hundred of grain size.
1.3.2.2 Classification of granularity
Wooden-Wentworth standard is generally used to divide the particle size, which is a classification scheme in millimeters (mm). Later, Querubin (1934) proposed a logarithmic transformation called φ value (φ =-log2d, where d is the particle size). See table 1. 6 is the corresponding relationship between particle size (mm) and φ value.
Table 1. 6 Comparison of grading standards
1.3.2.3 Particle size curve and particle size parameters
According to the results of granularity analysis, various histograms and granularity curves can be compiled and various granularity parameters can be calculated.
1.3.2.3. 1 histogram and granularity curve
Histogram and granularity curve are both reference signs for sedimentary environment analysis. Commonly used granularity curves include histogram, frequency curve, cumulative curve and probability cumulative curve.
(1) histogram
It is the most commonly used graph for particle size analysis, and the abscissa is the particle size interval, and the ordinate represents the percentage content of particle size, making a series of interconnected uneven rectangular graphs (Figure 1. 49) left. The advantage of histogram is that it can reflect the characteristics of particle size distribution intuitively and concisely.
Figure 1. 49 histogram frequency table (according to Kru Ainitar, 1938)
(2) frequency curve
Is to connect the midpoint of the vertical and horizontal sides of each column in the histogram into a smooth frequency curve (Figure 1. 49 right), and the enclosed area is basically equal to the area of the histogram. The frequency curve can clearly show the characteristics of particle size distribution, sorting quality, symmetry (skewness) and cusp (kurtosis) of particle size distribution.
(3) Cumulative curve
It is a commonly used simple graph, with the cumulative percentage content as the ordinate and the particle size as the abscissa, and the cumulative percentage content of each particle grade is marked on the graph from one end of coarse particles. Connect the points with a smooth curve, which is the cumulative curve (figure 1. 50). The cumulative curve is generally in the shape of "S", from which we can see the quality of particle size sorting. When calculating particle size parameters, we can also read some particle size values corresponding to cumulative percentage. The shape of cumulative curve can be used to distinguish different sedimentary environments.
(4) Probability accumulation curve
It is also the cumulative curve of particle size, which is drawn on normal probability paper, and the abscissa represents particle size; The ordinate is the cumulative percentage, using the probability scale. The probability coordinates are not equidistant, but centered at 50%, and the upper and lower ends gradually increase accordingly, so that the thick and thin tails can be enlarged and displayed clearly. The particle size of clastic sediments in the probability curve is not a simple lognormal distribution, but consists of several lognormal subpopulations, generally including three subpopulations, which are represented by three straight lines on the probability diagram, representing three different basic treatment methods, namely suspension treatment, jumping treatment and rolling treatment (Figure 1. 5 1). The three sub-populations are called suspended sub-population, jumping sub-population and rolling sub-population respectively on the cumulative probability curve, and other parameters except the three sub-populations on the probability graph include intercept point, mixing degree, percentage content of sub-populations and ranking.
Figure 1.50 Three common particle size curves (according to Reineck et al. 1973).
Figure 1. Probability cumulative curve and subpopulation of 5 1 particle size distribution (according to Vischer, 1969)
Cut-off point: refers to the intersection of straight lines of two subpopulations, which is indicated by abscissa. The fine demarcation point (S demarcation point) is the intersection of suspended sub-population and jumping sub-population, which represents the coarsest particles that can be suspended. The coarse tangent point (T tangent point) is the intersection of the jumping sub-population and the rolling sub-population, which represents the coarsest particle that can jump.
Degree of mixing: When two subpopulations intersect in a straight line, some points at the boundary point are scattered and transitional, also called transitional zone, reflecting sedimentary differentiation.
Percentage of sub-populations: that is, the percentage of each sub-population in the total sample.
Sortability: expressed by the slope of the straight line segment of each subpopulation, that is, the inclination angle of the straight line segment. The number, particle size range, sorting and other parameters of the whole development are controlled by the regularity of sedimentary conditions and hydrodynamic conditions. The probability particle size distribution of various sedimentary environments is different (table 1. 7).
Table 1. 7 Particle size probability distribution characteristics of sandy sediments in different types of sedimentary environments
(Simplified by Vischer, 1969)
1.3.2.3.2 granularity parameter
Commonly used particle size parameters are average particle size (Mz), standard deviation (σi), skewness (SK) and peak shape (KG). There are two methods to calculate particle size parameters: ① Mathematical statistics, which is based on probability statistics and directly uses the percentage of each particle size obtained by particle size analysis. The commonly used calculation method is matrix method, which is more complicated and less used; (2) Graphic method: read the particle size at a certain accumulation percentage from the accumulation curve, and then use simple arithmetic formula to calculate various particle size parameters, including average particle size (Mz) and standard deviation (σi).
Mean particle size (Mz): indicates the average particle size of the sample and reflects the average kinetic energy of the conveying medium. The calculation formula is as follows
Lithofacies palaeogeography
The standard deviation (σi) indicates the sorting degree, that is, it reflects the dispersion and concentration state of particles, and the calculation formula is
Lithofacies palaeogeography
According to the calculation of a large number of samples collected in different environments, the sorting degree can be divided into seven grades: ① σ I < 0.35, and the sorting is excellent; (2) σ I = 0.35 ~ 0.50, and selected; ③ σ I = 0.50 ~ 0.70, and the separation is good; ④ σ I = 0.70 ~ 1, with medium ranking; ⑤ σ I = 1 ~ 2, with poor sorting; ⑥ σ I = 2 ~ 4, and the ranking is very poor; ⑦ σ i > 4, with poor separation.
Skewness, also called Skewness (SK), is a parameter used to express the symmetry of frequency curves. According to its symmetry form, it can be divided into three types: ① unimodal symmetrical curves, symmetrical curves with the peak as the symmetry axis, which are normally distributed, reflecting Mz (mean particle) =Md (median) =Mo (mode); (2) Asymmetric oblique curve, the curve is asymmetric, and the main peak is on the thick side, that is, the sediment is mainly composed of coarse components; ③ Asymmetric negative skewness curve, the curve is asymmetric, and the main peak is on the thin side, that is, the sediment is mainly composed of fine components. Skewness (SK) is calculated as follows
Lithofacies palaeogeography
The peak state, also called sharpness (KG), is used to indicate the sharpness or bluntness of the curve compared with the normal frequency curve. The calculation formula of peak state is
Lithofacies palaeogeography
Different sedimentary environments have different sedimentary control conditions, so their particle size distribution characteristics are also different (Table 1.8).
Table 1.8 Characteristics of Grain Size Parameters of Sandy Sediments in Different Environments
(According to Reineck et al., 1973)
1.3.2.4 scatter plot of granularity parameters
The scatter plot of granularity parameters is a graphical representation of granularity parameters. Friedman (1967) calculated the granularity parameters by analyzing the granularity of 355 samples in modern oceans, lakes and rivers, and then obtained the relationship diagram between various parameters, that is, there are *** 19 kinds of granularity scatter diagrams, and figure 1.52 is one of them. Discrete graph is a graph that comprehensively shows the characteristics of granularity parameters, which is more meaningful than single parameter. By compiling discrete maps with different parameters, sandy sediments with different origins can be distinguished. It can be seen from Figure 1.52 that although there is no obvious boundary between sands in different environments, the general change trend can be seen.
1.3.2.5 C-M diagram
C-M diagram is a comprehensive genetic diagram (figure 1.53) proposed by Passega (1957), and it is also a scatter diagram of granularity parameters. C value is the granularity value with the content of 1% on the cumulative curve; M is the particle size value of 50% content on the cumulative curve. He thinks that the two particle size parameters, C value and M value, can best reflect the transport and deposition capacity of the medium, so he uses these two parameters as the ordinate and abscissa of double logarithmic coordinate paper respectively to form a C-M diagram. A typical C-M diagram, as shown in figure 1.53, can be divided into NO, OP, PQ, QR, RS and T regions. Different profiles represent products of different deposits: ① No profile represents coarse-grained materials transported by rolling, and the C value is greater than1mm; ② The OP segment is mainly processed by rolling, and the rolling component and the suspended component are mixed. The value of c is generally greater than 800μm, while the value of m changes obviously. (3) PQ section is mainly suspended, with a small amount of rolling components, and the value of C changes while the value of M remains unchanged; ④QR section represents the gradual suspension section, and the gradual suspension transportation means that the particle size of suspended solids in the fluid gradually decreases from bottom to top, and the density gradually decreases, and the value of C changes in direct proportion to the value of M, so that this graph is parallel to the baseline of C=M; ⑤RS section is a uniform suspended section, and the C value changes little, but the M value changes greatly, mainly due to fine sand deposition; ⑥ The T-zone is suspended solids, and the m value is less than10 μ m. ..
Figure 1. 52 Discrete graphs of standard deviation (σi) and skewness (sk) (according to Friedman, 1979)
Figure 1. 53 C-M diagram of turbidity current and traction current deposit (according to Passega, 1964).
C-M maps are usually sampled systematically from a group of contemporaneous sequences, and the samples should be selected from the coarsest to the finest representative lithology. The number of samples in each C-M map is more than 20. Therefore, each C-M diagram can reflect the grain size characteristics of rocks in syngenetic stratigraphic sections with a thickness of several meters to several tens of meters.
1.3.2.6 environmental discrimination formula for granularity parameters
1964, according to Fokker's granularity parameters, using the granularity analysis results of modern aeolian dunes, shallow seas, beaches, deltas, rivers and turbidites, and applying the linear multivariate discriminant formula, Sahu obtained four comprehensive formulas (empirical formulas) to distinguish five common sediments: aeolian, beach, shallow seas, rivers and turbidites. The discriminant formula is as follows:
1)Y aeolian/beach =-3.5688mz+3.7016σ 2i-2.0766skl+3.1135kg (y
2)Y beach/shallow sea =15.6534mz+65.7091σ 2i+1071skl+18.5034kg (Y < 65.3650 is the beach, y < 65.3650 is the beach.
3)Y shallow sea/river (delta) = 0.2825mz-8.7604σ 2i-4.8922skl+0.0482kg (y >-7.4190 shallow sea, y)
4)Y river (delta)/turbidity current = 0.7215mz-0.4030σ 2i+6.7322skl+5.2927kg (Y > 9.8433 is river delta and y < 9.8433 is turbidity current).