Types of Paleogene Open Freshwater Lakes —— Pearl River Mouth and Beibu Gulf Ancient Lakes

(a) Evidence of opening freshwater lakes

Paleogene in Pearl River Mouth Basin includes Shenhu Formation, Wenchang Formation, Enping Formation and Zhuhai Formation. Zhuhai Formation is marine sediments, Enping Formation, Wenchang Formation and Shenhu Formation are mainly continental sediments, but they were intermittently influenced by seawater in the middle and late Paleogene. The lithology of these continental strata is mainly a set of sandstone and mudstone, and no carbonate rocks are found. In addition, according to the microfossil analysis results of 667 samples from 20 wells, calcareous microfossils in these strata are extremely rare, except for some shell fragments in some samples. Therefore, only one well SH 19- 1-3 was selected in the Pearl River Mouth Basin, and the carbon and oxygen isotopes of shell fragments in 7 samples of Shenhu Formation and Wenchang Formation were tested, and the strontium isotopes of gastropods in 2 and 3 samples of Wenchang Formation were analyzed.

Paleogene lakes in the Pearl River Mouth Basin are mainly distributed in the present Zhu 1 Hezhu 3 sag. According to a series of sample analysis, it is considered that the ancient lake at that time was a soft-water freshwater lake (see Table 7-2 above). The average values of carbon and oxygen isotopes in the Pearl River Mouth Basin are all negative (Figure 7-3 and Figure 7-4), and the analytical values of all samples in the table are in the third quadrant (Figure 7-6), which is an open freshwater lake. Another characteristic of the carbon and oxygen isotopic relationship between closed lakes and open lakes is that in open lakes, the change of carbon and oxygen isotopic composition of primary carbonate is independent. Therefore, the relationship between primary carbonate δ 18O and δ 13C can also be used to judge whether the lake is closed or open. Although there are only seven samples in the Pearl River Mouth Basin, if they are placed on the δ 18O and δ 13C scatter charts, they are scattered and have no correlation, indicating that the Pearl River Mouth Basin was an open ancient lake in Paleogene (Figure 7-7).

Only two samples in Wenchang group have 87Sr/86Sr dispersion of 0.7 1047 and 0.7 1053, respectively. As can be seen from Figure 7-2, it is higher than the ratio of Eocene seawater to Oligocene seawater at that time, and it is undoubted to confirm the lake.

When analyzing the Pearl River Mouth Basin, a peculiar phenomenon is that among more than 400 rock samples drilled in Well 18, only a small amount of shell fragments are found in the rock samples at the bottom of Wenchang Formation, which are considered to belong to Gastropoda, and a small amount may belong to Claudibranchia. There are only two explanations for the lack of ostracods or even fragments in such a large number of lacustrine strata. One is that the lake is large and deep, and the other is that the carbonate in the lake is unsaturated and alive. According to the analysis, the situation in the Pearl River Mouth Basin belongs to the second explanation, that is, the Paleogene ancient lake in the Pearl River Mouth Basin is a freshwater lake with low salinity, low hardness and low pH value. The results of carbon and oxygen isotope analysis and Sr, Ca and Mg trace elements analysis of the shells at the bottom of Wenchang Formation mentioned above all prove that the lake at that time was soft fresh water.

The Paleogene strata in the Pearl River Mouth Basin are generally developed in Pleistocene assemblage, in which Pleistocene content is absolutely dominant. Bifidobacterium. It belongs to the aquatic family of Chlorophyta, with special shape, flat star shape and easy identification. Dictyophora dictyophora is a freshwater algae with wide geographical distribution. The combination of Pershing algae is a sign of the existence of freshwater lakes. The Pearl River Mouth Basin was a freshwater lake in Paleogene, and phytoplankton developed in the lake, especially in Wenchang Formation (Figure 7-25), with the content exceeding 50%.

Figure 7-25 Comprehensive Map of Paleogene Planktonic Algae in Pearl River Mouth Basin

As mentioned above, the Paleogene phytoplankton in Beibu Gulf Basin is characterized by high content of non-marine dinoflagellate-Baise algae. Especially in Liushagang Formation, the content of freshwater green algae is less than that of Baise algae, and it is dominated by Puccinia and Vitis (Figure 7-26). The combination of Baise algae and coccidia should be a sign of low salinity freshwater lake (see below for details). The ancient lakes in Beibu Gulf Basin should be mainly fresh water, but the salinity and hardness of the lake water have increased, which should be hard water. In some short periods, it may reach brackish water or even brackish water. The water depth and stratification of ancient lakes are similar to those of the Pearl River Mouth Basin.

(2) Types and characteristics of mudstone

1. Pearl River Mouth Basin

As mentioned above, the Pearl River Mouth Basin was a freshwater lake with low salinity, low hardness and low pH value in Paleogene. This freshwater lake is not too deep, and it may be a shallow lake environment. Mudstone has high sandy content, and there are various insect holes in mudstone, which has strong disturbance structure and limited degree of hypoxia, and sometimes even a "strong biological disturbance layer" appears.

Fig. 7-26 backscattering characteristics and energy spectrum composition characteristics of organic matter in Wenchang Formation mudstone (LF 13-2- 1 well, 3220m)

At that time, the ancient lake had a certain degree of bedding, and the mudstone had a layered structure and was unstable. According to the sporopollen data, the Paleogene ancient lakes in the Pearl River Mouth Basin are tropical lakes. Compared with temperate lakes, tropical lakes are relatively stable, with small seasonal temperature difference and no seasonal backwater. Tropical lakes with less circulation can only stratify water bodies at a small water depth.

In the early stage of basin formation, volcanic activity was strong, and volcanic eruption materials fell into the lake. Due to the influence of pyroclastic rocks, the pH value of lakes is low, which is not conducive to the preservation of calcareous microfossils, so there are few calcareous fossils in mudstone. The types and characteristics of mudstone are as follows.

(1) pyroclastic mudstone

This is a special lithology in the Pearl River Mouth Basin, mainly distributed in Shenhu Formation. Gray-black, observed under the microscope, no clear stripe structure and well-defined biological wormholes were found. However, homogeneous mudstone is often mixed with volcanic crystal debris and glass debris.

(2) Mudstone containing dispersed organic matter

Although the organic matter in this kind of mudstone can not be observed in a fixed shape under high power electron microscope, only some parts rich in organic matter are scattered dark lumps or particles. The backscattering analysis samples of Wenchang Formation are all mudstone with dispersed organic matter. Energy spectrum analysis shows that the carbon content in the dark part is obviously higher (Figure 7-26). According to the number of dark parts, it should be possible to judge whether it belongs to mudstone rich in organic matter.

(3) Layered organic mudstone

This kind of mudstone looks like massive structure to the naked eye, but it shows clear texture under backscattering electron microscope. Dark striations are rich in organic matter and can be divided into continuous striations and discontinuous striations. These types are developed in Enping Formation, especially in coal-bearing section (Figure 7-27).

Fig. 7-27 Backscattering Characteristics of Organic Matter in Enping Formation Mudstone (Well HZ23-2- 1, 4206 m)

2. Beibu Gulf Basin

The overall characteristics of Paleogene mudstone in Beibu Gulf basin are similar to those in Pearl River Mouth basin, but there are also differences, which are as follows: ① thin carbonate rocks appear in source rock assemblage; ② Although the average content of CaCO3 is not high, the content of samples in individual horizons is high; ③ Some samples contain calcareous microfossils such as Ostracoda, phytoplankton and Charophyta. ④ It contains non-marine dinoflagellate represented by Baise algae (Figure 7-28), and Baise algae is considered as phytoplankton living in water with a certain salinity.

Red-gray banded silty mudstone similar to that in Bohai Bay Basin can also be seen in Liushagang Formation in Beibu Gulf Basin. With the naked eye, the lithology between prosodic units has little change, which is mainly manifested in the replacement of colors. The thickness of rhythmic layers of this kind of lithofacies is generally 3 ~ 8 cm, which should belong to centimeter-level sedimentary rhythm. Similarly, small biological crypt structures can be seen in the rhythmic layer, but they are not as clear as those in the Bohai Bay Basin. Under the thin-section microscope, it was found that there were filamentous, spherical, massive or cotton flocs in the red color.

Figure 7-28 Comprehensive Map of Paleogene Planktonic Algae in Beibu Gulf Basin

Silty mudstone with horizontal structure is the most common shale type in Paleogene in Pearl River Mouth Basin and Beibu Gulf Basin. The smooth observation of the core shows that it is similar to the sedimentary structure of Paleogene horizontal silty mudstone in Bohai Bay Basin, and the layered units are gray siltstone and gray mudstone respectively, and the thickness of both can reach several millimeters to several centimeters. Biological disturbance structures are developed in this kind of mudstone, and there are many types, including biological wormholes with backfill structure and lining, nearly vertical distribution, and vertical wormholes with funnel-shaped escape traces. Some cores are fully disturbed by organisms, which destroys the bedding structure of primary deposits and forms a "strong biological disturbance layer".

Under the thin film polarizer, the boundary between the two layers is clear or microwave-like, and the particle size is obviously different. For the Pearl River Mouth Basin, in the samples with high mud content, no carbonate cement was found under the microscope, only siliceous cement was found. However, some samples in Beibu Gulf Basin can be seen with internal clastic sandstone limestone or sandstone and calcareous mudstone mixed with mud gravel, which shows the difference of carbonate mineral composition between them.

The high-resolution observation under the backscattering electron microscope also shows that the stripe structure is clear (Figure 7-29), especially when it is further enlarged, a large number of filamentous and flaky organic matter is found in the fine-grained layer (Figure 7-29B, c), which may also be the main reason for the hydrocarbon generation of this kind of rock.

The energy spectrum analysis of mineral particles shows that calcite is found in some samples of Beibu Gulf Basin, but no carbonate minerals such as calcite are found in mudstone of Pearl River Mouth Basin. In addition, ostracods, molluscs, charophytes, fishbones and scales were found in some samples of Beibu Gulf Basin, but calcareous microfossils were not found in such lithologic cores, thin sections and backscattering electron microscopy in Pearl River Mouth Basin.

Figure 7-29 Backscattering Electron Microscope Image of Silty Mudstone with Horizontal Texture

The analysis of CaCO3 content in mudstone of the two basins shows that it is generally low, and the content in Beibu Gulf Basin is slightly higher than that in Pearl River Mouth Basin (Table 7-4). Among them, the average of Pearl River Mouth Basin is 3.45%, with the highest value of 5. 1%, the average of Shenhu Formation is 3.45%, Wenchang Formation is 2.8%, and Enping Formation is 4.3%. The average of Beibu Gulf Basin is 4.2%, and the highest samples of individual intervals can reach 15% to 20%, the average of Weizhou Formation is 9.3%, and the average of Liushagang Formation is 3.2%.

Table 7-4 Analysis Results of CaCO3 Content of Paleogene Mudstone in Pearl River Mouth Basin and Beibu Gulf Basin