The paleogeomorphology of the catchment basin plays an important role in the lake system, so the restoration of the paleogeomorphology of the catchment basin is an important aspect of paleolimnology. The so-called ancient topography is to determine the ancient height, which has two meanings: absolute ancient height refers to the ancient height from the sea level at that time, that is, the ancient altitude, and relative ancient height refers to the height difference in different places and the degree of topographic fluctuation.
Pre-Quaternary paleotopography reconstruction is mainly based on sedimentary strata, and sediments are usually preserved in negative topography. For example, the ancient depth of a basin can be reconstructed from sediments and fossils. As for the normal topography of the denuded area, it is difficult to leave a direct geological record, so we can only guess for a long time and cannot rebuild it. Geology can answer the question "How deep is the water" to varying degrees, but it can't answer the question "How high is the mountain". It is much more difficult to rebuild the ancient height than the ancient depth. In recent years, the development of earth science has begun to provide a way for the reconstruction of ancient height, and the method of reconstructing ancient height by material balance is one of them.
The method of reconstructing ancient height by material balance is a computer simulation method, and its basic idea is to reverse the process of sediment filling, that is, to "dig" the sediments accumulated in the lake basin in each geological period in sequence, and "return" them to the catchment basin according to the identifiable characteristics, and then through a series of correction treatments, the paleotopographic map of the catchment area in each period can be obtained. Its principle is "mass conservation": it is assumed that the material source and sedimentary area in the study area are in a closed system during the deposition and transportation of debris, and the quality of denudation should be equal to the quality of sediment. This method was established on the basis of studying modern marine sediments [such as the Gulf of Mexico (Hay et al., 1989) and Beihai Basin (Wold, 1992)], and was simplified and modified according to the characteristics of inland lake basins and existing data when it was applied to restore the ancient height of ancient lake catchment basins in oil-gas-bearing basins.
1. Time step
The time range studied is divided into several time periods, and each time length (such as I to J) is called time step.
2. Definition of catchment area
The definition of catchment basin is an inevitable condition for the "regression" of sediments in the basin On this basis, the whole research area is divided into several grids, and the data in each grid is the most basic unit of paleotopography reconstruction.
3. Initial surface morphology
The initial surface is one of the important boundary conditions for reconstructing ancient topography by material balance. The reconstruction of the ancient topography of the river basin into the sea by Hay et al. takes the modern topography as the starting surface; It is also possible to choose the initial surface of intercropping at a specific time according to limited objectives (such as only studying ancient lakes) (such as the end of Dongying Formation as the initial surface in this study).
4. Selection of erosion datum
Terrain height above the erosion datum is the most critical factor to control the erosion rate of clastic materials, so the selection of erosion datum directly affects the reconstruction results of paleotopography. The study of ocean basins should be based on the global sea level height and its changes, while the study of inland lake basins that are not directly affected by sea level changes should be analyzed in detail.
5. Lithologic stratigraphic histogram
The thickness and distribution scale of sediments in a certain time unit determine the quality of materials returning to the source area in this time period, and also determine the height that the source area should increase in this time unit. According to the contour maps of strata in each period, an average thickness is assigned to each grid, thus the lithostratigraphic histogram of each grid is established.
On the basis of the above data collection, the ancient height can be reconstructed by using certain mathematical formulas, and a series of corrections such as balanced decompression can be made. See Cheng Xinrong et al. (1993) for specific methods and formulas.
It should be acknowledged that the sedimentary record is only one aspect of the evolution of paleo-height, and on the other hand, it is an independent evidence of crustal tectonic fluctuation, including isotopic chemical evidence of crystalline minerals. In the absence of such data, we take the paleovegetation reflected by sporopollen and the paleodepth reflected by ostracods as a reference supplement to explore the reliability of reconstructing paleoheight by material balance method.
(2) Environmental magnetism
Environmental magnetism is a new discipline that rose in 1980s. It mainly restores its paleoenvironment by studying the magnetic characteristics of sediments. At present, this method has been widely used in the study of Quaternary soil, rivers, lakes and marine sediments, but there are few precedents for its application in pre-Quaternary continental sediments. This study has explored and tried this and achieved some results.
Sediments (sedimentary rocks) are mainly composed of minerals. From the magnetic point of view, minerals can be divided into three categories: ① diamagnetic minerals: minerals that do not show magnetism and produce extremely weak magnetic field in the presence of external magnetic field are called diamagnetic minerals. Such as quartz, feldspar and calcite. ② Paramagnetic minerals: minerals that are magnetic when there is an external magnetic field. Common chlorite, pyrite, siderite, epidote, biotite and so on. ③ Ferromagnetic minerals: Some minerals show magnetism and become ferromagnetic minerals in the absence of external magnetic field. Common minerals are magnetite, maghemite, hematite, goethite and lepidocrocite. The composition and content of these minerals determine the magnetic characteristics of sediments, which are closely related to the geology and environment of their source areas, the physical, chemical and biological conditions of sedimentary media and diagenesis. Environmental magnetism reflects the changes of mineral composition, particle size and arrangement by testing the magnetic parameters of sediments (sedimentary rocks), thus revealing the changes of sedimentary environment.
Magnetic parameters commonly used in environmental magnetism include magnetic susceptibility (including volume magnetic susceptibility and mass magnetic susceptibility), frequency magnetic susceptibility ratio, isothermal saturation remanence, demagnetization parameters and so on. Commonly used detection instruments include MS2 portable magnetic detector, dual-frequency magnetic susceptibility probe, rotating magnetometer and pulse magnetometer.
Environmental magnetism has been widely used in the study of deep-sea strata to loess profile because of its simple testing instrument, fast data acquisition and large quantity, especially for those strata without biological fossils, which can provide high-resolution stratigraphic division and correlation scheme. The magnetic susceptibility curve of loess profile shows a very regular glacial cycle, which reflects the climate cycle. In the core testing of deep-sea formations such as ocean drilling, environmental magnetism has become a routine project in stratigraphic work, and even developed into magnetic susceptibility logging. At the same time, environmental magnetism is of great significance to the study of sediment provenance, sedimentary rhythm, paleoclimate and diagenesis, and is an effective method to study the paleoenvironment of oil-bearing basins. Shu's article (1993) introduces the principle of this method in detail.
(3) Backscattering electron microscope imaging technology.
Backscattered electron imaging (BSEI for short) is a technology that built-in backscattered electron probe and image analysis device in scanning electron microscope to observe, analyze and photograph samples with high resolution. Its basic principle is: when the incident electron beam contacts the atoms in the target area, it will collide elastically and produce backscattered electrons; The number of backscattered electrons (called backscattering coefficient η) is mainly related to the atomic number of the target region. When the number of atoms is high, the η value is large and the image is bright; When the atomic number is low, the η value is small and the image is dark (Belin, 1992). As for shale, the atomic numbers of various mineral particles (such as pyrite, quartz, feldspar, clay minerals, carbonate minerals, etc.). ) and the relationship between mineral particles and organic matter is different, so the backscattered electron microscope image can clearly reveal the relationship between them. If the atomic number of mineral particles is higher than that of organic matter, the color of mineral layer in the image is bright, while the color of organic matter layer is dark.
Compared with other shale research methods, the biggest advantage of BSEI technology is its high resolution. X-ray photography mainly studies the structure of shale. When the texture thickness is less than 200 microns, it cannot be clearly displayed by X-rays. The maximum resolution of optical microscope is 65438 0 μ m, and it can't be distinguished when the composition particles of shale are less than 65438 0 μ m.. The resolution of backscattering electron microscope can reach 0.0 1 ~ 0. 1 micron (Belin, 1992). In addition, BSEI, as a technology developed on the basis of SEM, can not only highlight the contrast between different components of shale, but also clearly identify the shapes of mineral particles, organic matter and paleontological fossils at high magnification. Finally, BSEI technology can be used together with energy spectrum analyzer (EDS) for qualitative or semi-quantitative analysis of mineral composition.
Since the 1990s, this technique has become the most commonly used method in shale research, and it has been used to analyze the fabric and composition of sediments in many modern and ancient sedimentary studies, and then to study the ancient environments such as ancient oceans, ancient climates and ancient lakes (Table 3-2). The backscattering electron microscope imaging analysis of shale in this study is based on the cooperation with Manchester University. The instrument used is Joel 6400 scanning electron microscope. See Pike et al. (1996) for the sample treatment method.
Table 3-2 Application Examples of Backscattering Electron Microscope Imaging Technology
(4) Analysis of sedimentary rhythm
Rhythm or periodicity is a common phenomenon in sedimentary rocks all over the world, because the sedimentary process is the superposition of periodicity and events (Einsele et al., 1982). In lacustrine sedimentary system, sedimentary rhythm is one of the most common phenomena, especially in lacustrine source rocks. Studying sedimentary rhythm can not only extract paleoclimate, paleolake chemistry and paleoproductivity, but also provide an important basis for understanding hydrocarbon generation conditions and mechanisms of lacustrine source rocks. Sedimentary prosodic analysis includes prosodic identification, genetic research and spectrum analysis.
The scale of lake stratigraphic rhythm ranges from seasonal or even shorter periods to astronomical periods of 100 thousand years and 100 thousand years. Therefore, there are many methods to identify rhythm, including sedimentology, geochemistry, environmental magnetism, microfossils and logging geology. The most easy to identify is the striation, which is only judged by the naked eye. Some rhythms are most easily distinguished by magnetic susceptibility or logging curves; Sometimes it can only be identified by the results of micropaleontology or geochemical analysis. The rhythm of Shahejie Formation in Dongying Lake is mainly identified and determined by characteristics such as color, carbonate content and magnetic susceptibility.
The formation of lacustrine stratigraphic rhythm can be caused by the change of lake basin water body, the change of catchment basin environment, sedimentation itself (such as turbidity current) or diagenesis (Einsele et al., 1982). In addition to the measurement of rhythm thickness and frequency estimation, the analysis of mineral and chemical components in rhythm, the petrology analysis under polarizing microscope and even the high-resolution analysis of micro-stratification by backscattered electron scanning electron microscope are all important ways to study the causes of rhythm. Specifically, the analysis of microfossils (including spore pollen) and the observation and statistics of trace fossils are also effective methods to reveal the causes of rhythm.
Finding out the main period of sedimentary rhythm through spectrum analysis is an important aspect of understanding its genesis and one of the contents of high-resolution stratigraphic work. The time series of strata (such as susceptibility curve or carbonate content curve) can be obtained by Fourier transform or Walsh transform, thus revealing the main period of rhythm. Of course, the time span of the whole set of strata is the premise to find the age length of the main cycle.
See Wang's article (1993) for the research methods and principles of spectrum analysis and overall sedimentary rhythm.
(5) Numerical simulation method of sediment deposition.
The transformation of earth science from qualitative to quantitative, from phenomenon description to mechanism exploration makes the role of numerical simulation increasingly obvious. For paleolimnology, a comprehensive and exploratory subject, it is particularly important to test the existing assumptions and point out the links to be identified through numerical simulation. Paleolimnology studies lakes as a complete system, and it is necessary to use quantitative methods as far as possible to reveal the relationship between various factors. At the same time, paleolimnology involves the fluid layer, even the atmosphere and water flow in the modern fluid layer are usually approached by numerical simulation because of their diversity.
There are many kinds of numerical simulation, and three kinds are mainly used in the study of ancient lakes.
1. flow field simulation
By using the method of simulating the surface circulation according to the wind field in oceanography, the lake flow in ancient lakes can be numerically simulated. According to the boundary conditions such as contour line and water depth of the lake basin at that time, we can give a certain wind field, study the surface circulation in different sedimentary periods by numerical simulation, and check it with sedimentary records.
2. Paleotopography simulation
Using fossils as a sign of relative water depth, semi-quantitative simulation of ancient water depth can be carried out by computer drawing. As an attempt of numerical simulation of ancient height of Huishui basin, according to the numerical simulation method of sedimentary filling developed for a long time in basin analysis, the ancient height of Huishui basin is obtained by stripping method, which is the "material balance reconstruction ancient height method" introduced earlier.
3. Geochemical simulation
Quantitative discussion of sedimentary geochemical processes by numerical simulation is a new topic in international academic circles in the 1990s. For example, according to the origin of carbonate/mudstone rhythmic stripes in source rocks, a mathematical model of primary carbonate chemical deposition can be established (Mei Hongming, 1996).
In addition, computer drawing and other methods can also be used to estimate ancient productivity. At present, computer numerical simulation has been widely used in Quaternary paleoenvironment research, and is mainly used in basin analysis in petroleum geology. In fact, paleolimnology, like paleooceanography, has a broad prospect by introducing quantitative methods for numerical simulation.