Paleosol is the soil formed under the relatively stable climate and geomorphic environment in the past, and is sometimes called soil fossil. Its development was interrupted by the change of the climate or geomorphic environment that formed the soil, or was buried by other sediments in the later geological process. Paleosols were mainly formed in Quaternary, occasionally found in Paleogene-Neogene strata, and older paleosols were also found.

There are many definitions of the meaning and types of paleosol. At present, the classification of ancient soil is generally based on the systematic classification system of modern soil, that is, the ancient soil is classified by using diagnostic layers and diagnostic characteristics, supplemented by the overall chemical properties of ancient soil and other indicators. According to the formation age and preservation environment, paleosols can be simply divided into buried paleosol, exhumed paleosols and residual paleosols (Nettleton et al., 2000). The residual paleosol was formed in the past environment and landscape, and there was no soil buried by young sediments. From the initial formation stage to modern times, it is influenced by the surface environment. Buried paleosol was also formed under paleoenvironmental conditions, but was subsequently buried by sediments for a long time. Buried and bare paleosol refers to the soil that was buried after it was formed under paleoenvironmental conditions, but re-emerged from the surface due to the erosion of the upper overburden. Residual paleosols and buried bare paleosols are exposed to the earth's surface, which are influenced by late pedogenesis and are the result of superposition of different pedogenic environments, so it is difficult to restore paleosols. The influence of late pedogenesis on buried paleosol is relatively small, and the recorded paleoenvironmental information is relatively simple and well preserved, so there are many studies on it.

2. Paleosoil identification

Because the appearance of paleosols in sedimentary sequence is very similar to that of sediments or sedimentary rocks, many paleosols were once considered as gray, red or green mudstone and could not be recognized. How can an ordinary geologist identify ancient soil in the field without training in soil science? R et al. allack( 1997) and R et al. allack et al. (1998) put forward three main diagnostic characteristics that distinguish ancient soil from sedimentary rocks: life remains, soil layer and soil structure.

Root trace is the most important trace of life preserved in ancient soil. Root traces provide a diagnostic basis for rocks exposed to the atmosphere and "occupied" by plants to produce soil. Roots are scattered in the upper part of the soil layer. Most of the roots gradually taper downward and diverge (Figure 1-3), so that they can be distinguished from hidden points. On the other hand, some roots are obliquely distributed on the clay layer of soil, and some types branch upward and grow soil. When the original organic matter is preserved, the plant roots are most easily identified, which mainly occurs when the paleosol is formed in the flooded and anoxic lowland environment. In the red oxidized paleosol, the root trace mainly has tubular characteristics and contains different substances from the surrounding paleosol.

Figure 1-3 The original organic matter of the roots of plants in ancient soil has been partially replaced by iron oxide.

The existence of soil layer is the second characteristic to identify ancient soil. The top of the uppermost horizontal layer of paleosol is usually strongly cut by erosion surface, but the soil layer will show gradient changes in structure, color or mineral composition when it enters the parent rock downward.

Biological disturbance (destruction), moisture, dryness and other soil biological disturbance processes will destroy the original structure of parent rock and form a typical soil structure in ancient soil. Soil structure refers to the arrangement and combination of soil particles (including aggregates). In field identification, it usually refers to structures with different shapes and sizes that can be separated from each other. According to the shape, soil structure can be divided into three types: block, sheet and column. According to its size, development degree and stability, it can be divided into granular, massive, massive, prismatic, prismatic, columnar and flaky structures. Figure 1-4 shows the Miocene paleosol. Although these soils are red, paleosols can have many different colors and characteristics (R et allack, 1997). In addition, micromorphological characteristics, oxide characteristics and chemical properties, mineral characteristics, particle composition, soil organic matter and element geochemical indicators are also helpful to further identify and diagnose paleosols.

Figure 1-4 red paleosol under sandstone

3. Significance of paleosol research

The formation of soil is controlled by five factors: climate, biology, parent material, topography and time. Palaeosoil is the product of past environmental conditions, which inevitably contains rich information of paleoenvironment and paleoclimate, paleovegetation, paleohydrology and so on. Therefore, by studying ancient soil types and characteristics, and then comparing the same or similar modern soil types and characteristics, according to the soil-forming environment corresponding to these modern soils, we can infer and reconstruct the ancient environment in the formation period of ancient soil. Studying paleosols formed before Neogene can reconstruct the history of global paleoclimate change, and reveal the evolution process of O2 partial pressure of Precambrian paleoatmosphere and c O2 concentration level of paleoatmosphere after CAMBRIAN. Palaeosoil has multiple information such as space and time, which can reflect the palaeogeomorphology, palaeogeomorphology and palaeohydrological characteristics of a basin or region.

With the wide application of sequence stratigraphy in petroleum exploration, it is found that paleosol plays an irreplaceable role in determining sequence boundaries and lateral correlation, finding sedimentary discontinuities and micro-karst landforms on carbonate platforms, and determining the trend of delta paleotopography (Lin Youling, 200 1). In addition, paleosol is also of great significance in the restoration of paleovegetation, paleoenvironment and paleoclimate, and in the study of paleogeomorphology of sedimentary basins (Kraus, 1999). Recently, some scholars have discovered high-yield oil layers in calcareous paleosol of Paleogene Shahejie Formation in Jiyang Depression (Zhou et al., 2009), which not only provides an important basis for finding out the sedimentary discontinuity at the end of the fourth member of Shahejie Formation in Jiyang Depression, but also expands the field of reservoir exploration in Shengli Oilfield.