As the main organic matter in sedimentary rocks and the main parent material for oil and gas generation, kerogen has become the object of great concern to petroleum geochemists after the modern oil generation theory was proposed.

1. Microscopic composition of kerogen

Using the transmitted light, reflected light, fluorescence and high-power electron microscope of the microscope, the organic microscopic composition of kerogen can be directly observed and further understood. Its original biological origin.

Using transmitted light and fluorescence methods, the microscopic components of kerogen from 20 samples in the Jinggu Basin were identified and calculated according to the type index method, and the kerogen types were divided. The content and description of each microscopic component are shown in Table 4-5.

Table 4-5 Microscopic component content and microscopic description of kerogen in the oil source rock of the Daniuquan Oilfield in the Jinggu Basin

The Daniuquan oil reservoir is a non-coal source rock The organic microscopic components are mainly saprolite group and chitin group, with a total amount of 70 to 95, and the content of the two is in a waxing and waning relationship with each other; the poor inert group, its content is only 0 to 5; vitrinite The group content varies greatly, ranging from 5 to 27. The organic matter type of the oil source rocks, calculated according to the type index method, is mixed kerogen, and type II1 is the majority, accounting for 57%, indicating a better parent material type for the Neogene oil source rocks in the Jinggu Basin. The chitin group is dominated by biodegradable terrestrial plants and contains more sporopollen assemblages. The fluorescence color under blue light excitation is that the degradation products fluoresce brown or brown-yellow, the sporopollen fluoresces yellow, and the saprolite group fluoresces yellow or dark yellow.

The microscopic components of the coal measure source rocks in the Jinggu Basin are mainly vitrinite, with a content of 39 to 81. Figure 4-3 shows the difference in distribution of microscopic components of coal measures and argillaceous source rocks. Observation under the microscope shows that even coal seam kerogen with organic matter type III contains a lot of bitumen and soluble hydrocarbon exudates, and forms an oil halo. This shows that coal in the background of Ro of 0.66 (lower coal seam) The possibility of producing oil exists in the Jinggu Basin.

Figure 4-3 Triangular composition of TMC of source rocks and coal seam microcomponents in Jinggu Basin

V—Vitrinite group; I—Inert group; S—Sapropelite group; E— Crutin group

2. Vitrinite reflectance of kerogen

As a residual basin, the maturity of organic matter in the mudstone of the Jinggu Basin is one of the important prerequisites for evaluating the current oil and gas potential of the basin. Vitrinite reflectance is a widely used maturity indicator and a reliable parameter for determining the maturity of oil source rocks. It can be seen from Table 4-5 that the muddy oil source rocks in the Jinggu Basin contain a certain amount of vitrinite. In the Niu 4 and Niu 7 fault blocks, the Ro value is about 0.50; while in the Niu 2 fault block (except for coal seams and Except for carbonaceous mudstone), the Ro value is generally around 0.50. This shows that Jinggu crude oil was formed under the background of vitrinite reflectivity maturity of 0.5, and its hydrocarbon evolution period is obviously at a low maturity stage. This is consistent with the characterization results of various chemical analysis data of crude oil mentioned in Chapter 3. consistent.

Figure 4-4 shows the variation of Ro value and Tmax value with depth in Jinggu Daniuquan area. Although the burial depth of the dark mudstone of the Sanhaogou Formation revealed by drilling in the Daniuquan area is mostly within 650m, and the burial depth of the oil layer is within 440m, the Ro value (from 0.4 to 0.4) shows the thermal evolution process of organic matter as the burial depth continues to increase. ~0.81) shows an increasing trend with the increase of burial depth. Around 650m, organic matter enters the mature stage.

The reflectance value of the samples from the coal-bearing strata is slightly higher than that of the argillaceous source rock, ranging from 0.66 to 0.70. Well Niu 3 outside the fault block is a special case. The reflectivity value of both the coal seam and the muddy source rock has reached 0.81, and the burial depth is only 207-226m, which is obviously not consistent with the geological conditions. The reason is probably that Well Niu 3 is close to the fault zone (shown in Figure 2-7), and the local thermal effects caused by tectonic movements increase the reflectivity value, and the coal and muddy source rocks should Some reflectivity differences have also been eliminated, or the original deeper strata have migrated to shallower locations due to structural uplift.

3. The elemental composition of kerogen

As a high molecular condensation polymer, kerogen is different from ordinary pure organic compounds, so it does not have a fixed and unified elemental composition. Because kerogen is mostly composed of C, H, and O elements, the diagrams used to explain the elemental analysis results mainly reflect the changes in the combination of these elements.

The elemental composition of kerogen is an important parameter to characterize the type of organic matter in source rocks, and its relative percentage content is closely related to the properties of kerogen. Table 4-6 shows the kerogen element analysis results in the Jinggu Basin. Except for the coal seams, the Niu 4, Niu 2 and Niu 7 fault blocks in the main oil-producing area of ??the Daniuquan Oilfield have H/C atomic ratios greater than 1, and the O/C atomic ratio is around 0.11. If the kerogen types are divided into standard humic type (Ⅲ2), humic type containing sapropel (Ⅲ1), mixed type (Ⅱ), humic type (Ⅰ2), standard sapropel type (Ⅰ1) Category 5, the corresponding standards for the H/C atomic ratio of the kerogen element composition are: <0.8, 0.8~1.0, 1.0~1.3, 1.3~1.5, >1.5. According to the kerogen H/C atomic ratio data in Table 4-6, except for the 134-138m sample from the Niu 4 well in the Niu 4 block of the Daniuquan Oilfield, which is type III1 kerogen, the other samples are all mixed type (II) and corrosion-containing The samples from the Niu 2 fault block are all mixed-type kerogen, except for the carbonaceous mudstone in the coal mine which is sapropel-containing humus type (III 1). The only samples from the Niu 7 fault block are One sample is mixed type kerogen; the H/C atomic ratio of the Niu 3 well and coal seam samples is in the range of 0.75 to 0.84, and they are all type III kerogen. This is consistent with the results obtained by the type index method of microscopic components. If the kerogen element analysis data points in the area are placed on the Fan classification diagram, as shown in Figure 4-5, the above results will be clearer. At the same time, it can also be seen from the figure that the coal and rock samples are biased towards the maturity division line with Ro of 1.0, while most other samples are close to the line with Ro of 0.5, which is basically consistent with the measured reflectivity results.

Figure 4-4 Ro value and Tmax value of rock samples in Daniuquan area change with depth

The composition characteristics of kerogen elements in source rocks in Jinggu Basin are shown in Table 4-6 .

4. Stable carbon isotope composition characteristics

The kerogen stable carbon isotope data of 20 samples analyzed in the Jinggu Basin have been listed in Table 4-6. The δ13C value of muddy source rock is -28.1‰~-29.7‰, while the kerogen value of coal seam samples is -27.3‰~-28.1‰, which is slightly heavier than muddy source rock. Compared with the carbon isotope composition characteristics of kerogen in other basins in China of the same geological age, the δ13C value of kerogen in this formation in the Jinggu Basin is somewhat negative. Since the carbon isotope composition of continental sedimentary organic matter is closely related to the depositional environment and organic matter type, the hydrocarbon evolution has little impact on the carbon isotope composition of kerogen. The kerogen still inherits the δ13C value characteristics of the original organic matter. Therefore, Daniu in the Jinggu Basin The lighter carbon isotope value of kerogen in the Sanhaogou Formation strata in the Yunnan-Guizhou Plateau may reflect the special combination of biological inputs in freshwater lakes on the Yunnan-Guizhou Plateau at that time. It is also consistent with the low-maturity oil produced in the Jinggu Basin, which has significantly lower negative values. are consistent.

Figure 4-5 Illustration of Fan’s classification of kerogen elemental composition

Table 4-6 Table of kerogen elements, isotope composition and infrared spectrum analysis parameters of Jinggu Basin

Regarding the reason why the carbon isotope composition of kerogen is slightly different from the kerogen types classified according to the kerogen O/C and H/C atomic ratios (the former classification type is slightly higher than the latter), it is probably due to the deposition of organic matter in the area. It is in the low maturity stage, and its characteristics are different from the classification standards of kerogen types according to the maturity stage.

5. Infrared spectral characteristics of kerogen

(1) Spectral characteristics

Kerogens of different types and maturity have different characteristics in the infrared spectra. The obvious difference, the position and relative intensity of its absorption peak, is a reflection of the atomic composition in the kerogen, its vibrational properties, and the relative abundance of the bonding properties, and represents the composition of the chemical groups condensed in the kerogen.

Type I and II kerogen have stronger absorption peaks at 2920cm-1, 2850cm-1, 1460cm-1, 1380cm-1 and 720cm-1, while type III kerogen and samples with higher maturity have stronger absorption peaks at 3030cm-1. 1. The absorption peaks at 1600cm-1, 860cm-1, 810cm-1 and 740cm-1 are strong, and samples with more oxygen atomic structures have stronger absorption peaks at 3400cm-1, 1700~1720cm-1, 1280~1050cm-1. .

The kerogen infrared spectrum of representative samples of source rocks in the Jinggu Basin is shown in Figure 4-6.

Figure 4-6 Representative infrared spectrum of kerogen in Neogene rocks in Jinggu Basin

The infrared spectrum characteristics of kerogen in Neogene source rocks in Jinggu Basin are:

1) Take the 250m sample from Well Niu 4 as a representative. The first peak group 2923cm-1 and 2852cm-1 have strong absorption, and the separation between the two is good; the second peak group 1703cm-1, 1604cm-1, 1460cm-1 and 1379cm-1 has moderate absorption; the third peak group 880cm -1, 750cm-1 and 720cm-1 are extremely weakly absorbed and cannot even be identified. In the second peak group, 1460cm-1, >1600cm-1, 1460cm-1>>1380cm-1, the 1710cm-1 absorption peak of the oxygen-containing gene appears as a shoulder on the spectrum next to the 1460cm-1 peak. This type of pattern is characteristic of type I kerogen.

2) Samples from 304 to 310m in Niu 2 well. The first peak group 2923cm-1 and 2852cm-1 are strong absorption, and the separation between the two is also good; the second peak group 1600cm-1, 1460cm-1 and 1379cm-1 are medium-strength absorption; the third peak group is weak absorb. But in the second peak group, 1600cm-1>1460cm-1>1380cm-1, and the 1710cm-1 peak is very weak. Such patterns are characteristic of mixed kerogen.

3) The spectrum characteristics of the 134-138m sample from Well Niu 4 are: the second peak group is strong absorption, the first peak group is medium-strength absorption, and the third peak group is still weak absorption. In the second peak group, the absorption intensities of 1600cm-1>>1460cm-1, 1460cm-1 and 1380cm-1 are similar, and the 1710cm-1 peak is not obvious. This type of spectrum represents the characteristics of type III kerogen of mudstone in the area.

4) Coal rock characteristic spectrum characteristics: the absorption intensity of the second peak group greatly exceeds the absorption intensity of the first peak group, and the third peak group is obvious, representing 880cm-1, 810cm-1, The 750cm-1 peak has a certain intensity. In the second peak group, 1600cm-1>>1460cm-1>>1380cm-1 and 1710cm-1 are not obvious.

(2) Infrared spectral parameters

Li Jinchao (1987) used 1460cm-1 (methyl, methine), The absorption intensities of the 1600cm-1 (aromatic core) and 1710cm-1 (carbonyl) bands form a triangle diagram to determine the kerogen type and evolution trend. Plotting the kerogen infrared spectrum measurement data (Table 4-6) in the area on the above triangle diagram (Figure 4-7), it can be clearly seen that the muddy source rocks in the Jinggu Basin are mainly mixed kerogen and the coal-measure strata Characteristic of type III kerogen.

Figure 4-7 Triangular distribution diagram of kerogen I1460cm-1, I1600cm-1 and I1700cm-1

Huang Difan (1987) based on the H/C atomic ratio and infrared 1460cm-1/ In the 1600cm-1 relationship diagram, the atomic ratio is greater than 1 and the corresponding 1460cm-1/1600cm-1 value is 0.45 to determine the dividing line between type I, type II kerogen and type III kerogen.

We also put the Jinggu Basin kerogen H/C atomic ratio data and 1460cm-1/1600cm-1 data points in the above relationship diagram (Figure 4-8). It can be seen from the figure that most of the points are distributed in Ⅰ , within the type II range, while the coal seam samples are located in the type III1 range, which is very consistent with the results represented by the Fan diagram; however, it is slightly different from the microscopic examination results, mainly manifested in the small number of type II2 kerogens classified by microscopic examination according to the above types. The classification methods are all classified into type III1. This is probably due to the bias in estimating the sample components near the dividing line between type III and type II2 when observing kerogen under a microscope.

Figure 4-8 Relationship diagram between kerogen H/C and I1460cm-1/I1600cm-1 in Jinggu Basin

In addition, the infrared spectrum 2920cm-1/1600cm-1 parameters were used to analyze the relationship between Jinggu Basin and kerogen H/C. The source rocks in the basin are divided into types, and the results are shown in Table 4-6. If the results are compared with the types classified by microscopy, the two results of type III kerogen are very consistent, while the type II kerogen classified by microscopy is quite different. Almost all of them are type I2 (a few are I1) cheese. root. Since the source rock in the Jinggu Basin is in the low-maturity stage, the infrared spectra of kerogen at 2920cm-1 and 2850cm-1 are also the strongest absorption in type II kerogen, so the 2920cm-1/1600cm-1 value is high, indicating that the type is getting better. It seems that the above parameters are greatly affected by maturity.

6. Pyrolysis analysis and maturity identification of source rocks

Pyrolysis analysis can be used to study the properties and maturity of source rocks. The results of pyrolysis analysis of cuttings and surface coal samples from 10 wells drilled in the Jinggu Basin are shown in Table 4-7.

(1) Identification of organic matter types

The hydrocarbon-generating parent material of Jinggu crude oil has been identified previously, and the pyrolysis analysis of the source rock can further confirm and deepen this understanding. .

The results of rock pyrolysis analysis are used to classify organic matter types, usually using S2/S3, hydrogen index (IH), hydrocarbon generation potential and degradation rate. According to the rock pyrolysis classification calculation in Table 4-8, although the calculation results are somewhat different due to different parameters, they still mainly show mixed types. Among them, the s segment II1 is better than type II2, while the N s segment reflects slightly worse organic matter properties. Vertically, it can be seen that type II1 organic matter was mainly developed in the middle and upper parts (167.73-321.27m section of Well Niu 4), that is, in the middle and late depositional period of the Sanhaogou Formation. At this time, as the lake waters of the Jinggu Basin expanded, lower aquatic organisms increased. , its original organic matter input becomes better accordingly. In the early and middle stages of the Sanhaogou Formation, more terrigenous debris input was consistent with its marginal depositional environment.

Table 4-7: Jinggu Basin oil source rock pyrolysis data table

Table 4-8: Type classification of source rock rock pyrolysis parameters in Jinggu Basin

Figure 4-9 is a diagram of the organic matter types classified by the hydrogen index and oxygen index of rock pyrolysis in the Jinggu Basin. It can be seen from the figure that except for one sample from the Niu 2 fault block, two samples from the Niu 4 fault block and the Niu 3 well sample belong to III Except for type kerogen, the rest of the samples are within the range of type Ⅱ and type Ⅰ kerogen, which is consistent with the results of the types classified in Table 3-8.

Figure 4-9: Hydrogen index (IH) and oxygen index (IO) classify the oil source rock types in Jinggu Basin

Figure 4-10: Pyrolysis hydrogen index and rock pyrolysis hydrogen index in Jinggu Basin Maximum pyrolysis temperature (Tmax) relationship diagram

In addition, observation of kerogen under a microscope is also an intuitive method to identify the type of organic matter. In the transmitted light observation of more than 100 sporopollen slices and dozens of kerogen slices studied in the Jinggu Basin, it can be distinguished that the component characteristics of kerogen are:

1) Sapropel group: including Phycoplasts and amorphous, algae bodies have a certain structure, the color is mostly light yellow, green-yellow or brown-yellow. Amorphous is mainly a product formed by sapropelization of the remains of algae or planktonic organisms. It has no obvious outline and structure. It is flocculent or cloud-like, light in color, mostly yellow to brown, transparent to opaque, and sometimes The specimen is dark in the middle but transparent at the edges, and is the main component of type II kerogen.

2) Chitin group: includes plant spores, pollen, leaves, epidermis, resin, etc., with lighter color, good transparency, irregular edges, curved shape, some with visible cell structure, and some with smaller Good hydrocarbon generation ability.

3) Inertinite and vitrinite: Inertinite is pure black, and vitrinite is mostly orange-red and brown. Both have clear outlines, straight or rounded edges, and mesh-like edges. They have poor oil-generating ability, but have the ability to generate energy.

(2) Maturity identification

1) Pollen color index

Judge the maturity of the source rock based on the color of the pollen contained in the kerogen degree has become one of the basic items in source rock evaluation. From the systematic analysis results of a well in the Jinggu Basin (Table 4-9), it can be seen that the color of Neogene sporopollen fossils gradually becomes darker from top to bottom in the Jinggu Basin: yellow → dark yellow → orange → light brown → brown. The pink color index (SCI) also gradually increases, and accordingly, the organic vitrinite reflectance (Ro) value also gradually increases, but it is still at a low maturity stage.

Table 4-9 Comparison of spore pink color change and Ro in a well in Jinggu Basin

2) Pyrolysis analysis index

From the Jinggu listed in Table 4-7 Judging from the Tmax values ??of 29 rock pyrolysis in the basin, 83.5 samples are less than 435°C. If the maturity is calculated based on mixed kerogen, these samples are all immature oil source rocks; the Tmax values ??of the remaining samples are between 436 and 441°C. . Still in the low maturity stage; the Tmax value of the coal seam sample is less than 425°C. This feature is clearly reflected in Figure 4-10. Most of the samples in the figure are located in the immature zone with Ro of 0.5.

As can be seen from Figure 4-4, the change of Tmax value with depth is the same as the change trend of Ro value with depth. As the burial depth increases, the Tmax value also increases, but it is about 650m above the layer. position, still at a low maturity stage.