## 5 Sedimentary Facies' Characteristics and Facies' Patterns of Carbonate Rock Section  
### Sedimentary Facies' Characterization of Carbonate Rocks  
#### 5.1.1 Single-Well Phase Characteristics  
At the turn of the Permian and Triassic periods, especially after the biological extinction event, the development ofolite limestone reached its peak worldwide. Oolite carbonate rocks appeared in the Eastern Tethys Ocean, Western Tethys Ocean, Cimmerian Block, Neo-Tethys Ocean, the Arabian Platform near the east side of Gondwana, the seamounts in the middle of the Panthalassa Ocean, and the shallow water areas near the ancient land on the east side [14]. From the perspective of oolite development areas around the world, the Mangeshlak Basin was also the distribution area of oolite carbonate rocks during the Permian-Triassic period. From the drilling core data of the D2 well located in the northern part of the basin, it can be seen that a large number of oolitic deposits were found in the middle and lower part of the Middle Triassic, including not only oolitic limestone but also oolitic dolomite. As the main structural component of carbonate rocks, oolitic has important referential significance. According to previous studies on the genesis of oolites, the environment in which oolites are formed requires the following main factors: 1 weak hydrodynamics; 2 clear water with few impurities; and 3 high seawater supersaturation [15]. The turbulent high-energy environment mostly forms bright crystalline oolitic limestone with large average particle sizes and poor sorting. In a low-energy and stable environment, more well-sorted limestone with a small average particle size is formed [16]. The environment in which oolites develop is a high-energy environment near the wave base behind islands, reefs, or barrier sandbars [17]. Figure 7: Sedimentary facies map of the Early, Middle, and Late Triassic Mangeshlak Basin. From the thin core slices of the well, the carbonate rocks of the Middle Triassic are mainly granular carbonate rocks such as oolitic limestone and oolitic dolomite with an oolitic structure. Marl, muddy limestone, gray dolomite, etc. are also developed. From bottom to top, in terms of the rock types of the formations, the lower part is more dolomite or dolomite limestone, and the upper part is more marl, limestone, and dolomite limestone. In terms of structural components, the middle and lower strata have oolitic structures. In general, the particle size of oolitic particles tends to decrease from bottom to top. In the lower energy muddy sedimentary sections, there are also bioclasts, which are often replaced by calcite [18]. In the microscopic thin sections, although the bioclasts are replaced by calcite, their basic outlines are retained. From the logging curve, the middle and lower part of the carbonate rock member is a stratum with a low gamma and high gamma interlayer, which is dominated by granular carbonate rock sedimentation. The curve is dominated by a \"box shape\", reflecting the predominance of high-energy granular sedimentation on the platform [19]. The upper part is dominated by a set of high gamma gray marks, and the curves are dominated by high-frequency jagged edges, reflecting the predominance of low-energy, fine-grained sediments [20] (Figure 8). The Early Triassic continued the sedimentary characteristics of the region in the Permian, and the seawater began to gradually expand into the basin. Therefore, in the early Triassic, the sedimentary clastic rocks near the source gradually transitioned to marl, quartz limestone, and limestone. Entering the Middle Triassic, seawater covered most of the basin, and the western half of the basin was submerged by seawater. However, due to the early \"filling\" sedimentation, the area showed a vast and flat terrain. Throughout the Middle  
Figure 8: Comprehensive bar chart of sedimentary facies in the Triassic carbonate rock section of Well D2.