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Thoughts and steps of compiling lithofacies palaeogeographic map
The study of lithofacies palaeogeography is mainly based on the detailed analysis of the sedimentary environment of single profile and sedimentary profile, counting various parameters that can reflect the sedimentary environment, and finally compiling the lithofacies palaeogeography map. Different schools of lithofacies palaeogeography have different theoretical bases, such as palaeostratigraphy (Liu Hongyun,1955; Lu, 1965), geotectonics (Zhang Wenyou,1959; Wang Hongzhen, 1985), single factor comprehensive mapping method (Feng Zengzhao, 1977, 2004), sedimentary facies mapping method (Liu Huanjie et al., 19 1). The lithofacies palaeogeographic map or sedimentary facies palaeogeographic map drawn by different methods have their own characteristics. Among them, the comprehensive mapping method of single factor analysis provides quantitative basis for the study of lithofacies palaeogeography.

The Late Paleozoic coal-bearing rock series in the study area is dominated by clastic rocks, limestone marker beds are developed, strata division and correlation are clear, and coal drilling is controlled in the area. These characteristics are not only suitable for the detailed study of sedimentary environment and sedimentary model, but also conducive to the statistics of various paleogeographic parameters, so they are more suitable for lithofacies palaeogeographic mapping (Zhang Pengfei et al., 1997).

Lithofacies palaeogeography analysis should first pay attention to lithofacies research, sedimentary environment characteristics, single profile environment analysis and paleocurrent analysis (Liu Baojun, Zeng Yunfu,1985; Zhang Pengfei et al., 1993), in addition, various objective and quantitative statistics reflecting sedimentary environment parameters (i.e. single factor) are also essential in the mapping process. The study of actual data is the basic work of compiling lithofacies paleogeographic map. Pay attention to analyze each borehole, choose a section purposefully, continuously expand the scope of understanding, and finally make a plan. Using this method of point to area and step by step, the whole mapping process can be accurate, reliable and accurate, and finally a lithofacies palaeogeographic map which conforms to the objective reality and can reflect the distribution of sedimentary environment can be compiled.

(1) Statistical index of lithofacies palaeogeography mapping

Lithofacies palaeogeographic map is actually a comprehensive map that analyzes, understands, induces and abstracts various parameters of palaeogeography.

We counted all kinds of parameters that can reflect the paleogeographic characteristics, and drew various parameter isolines, which reflected the paleogeographic characteristics at that time from different angles. It should be pointed out that the lithofacies palaeogeographic map is mainly based on the isoline map of sand-mud ratio and other maps.

The paleogeographic parameters adopted this time include stratum thickness, sandstone thickness, sandstone percentage, sand-mudstone ratio, coal seam thickness, mudstone thickness percentage of each layer of coal seam, limestone thickness (content and number of layers) and so on. See Table 2.5 for the statistical method of each parameter and the geological significance of its contour map. Among them, limestone thickness, sand-mudstone ratio, sandstone content and coal seam thickness are the most important basis for drawing the final lithofacies palaeogeographic map.

Table 2.5 Statistical methods of various parameters in lithofacies palaeogeographic analysis and their significance

(2) Steps of compiling paleogeographic map

The first step is to compile the histogram of lithofacies and sedimentary facies.

The second step is to draw the sedimentary profile.

The third step, data statistics, analysis, selection, drawing various parameter diagrams.

The fourth step, comprehensive analysis, drawing paleogeographic map.

The paleogeographic map should reflect the distribution of land and sea, the distribution of paleogeographic units, the direction of transgression and the direction of provenance at that time. The map should also have scale, north direction and general modern geographical indications (such as provincial boundaries and major cities, etc.). ), and the map should be neat and beautiful.