The Lower-Middle Ordovician carbonate rocks are studied by core observation, thin section observation, geochemical analysis to restore the diagenetic evolution history of the Yubei area, Tarim Basin. Dissolution, dolomitization, silicification and cataclasis are studied and the diagenetic evolution history is divided into four stages. The grained texture dominated limestone in the relative geomorphic high location exposes to the ground and undergoes penecontemporaneous dissolution due to the fluctuation of the sea level. Caves and pores with structural selectivity parallel to the sedimentary bed are generated by the penecontemporaneous dissolution. The dolomitization developed mainly in early diagenetic stage enhances the resistance of carbonates to compaction and pressure solution, which benefits the preservation of early pores and caves. The fractures formed during the Middle-Late Caledonian and Early Hercynian in this stage are mostly closed and filled due to complicated compaction and cementation. Hydrothermal activity in middle diagenetic stage damages the reservoir slightly by the presence of pyrite and dolomite with wavy extinction and saddle structure in the reservoir space. The late diagenetic stage is characterized by the silica and calcareous fluid activity, which fill the early space partially. The development degree of fractures formed during Late Hercynian and Himalayan epoch is weaker than early diagenetic stage. However, the fractures formed during late diagenetic stage keep open due to weak diagenetic transformation and become efficient migration channel and reservoir spaces in Yubei area.

Based on comprehensive analysis of field work, petrological and geochemical characteristics, this paper studies the characteristics and origin of giant patch dolomite of the Lower-Middle Ordovician HangGuletag Formation in Wuligezitag area, NE Tarim Basin, China. It can be seen in the field that the grayish yellow giant patch dolomite partially develops in the black thin-bedded micritic limestone, with an irregular boundary not controlled by sedimentation. The giant patches are several meters in size (the largest >10 m) which is larger than and quite different from the several-centimeter (decimeter) burrow-related dolomite or leopard fur dolomite. Microscopically, it consists of micro-finely and planare(s) dolomite crystals with a size near 100 μm. Besides, it is also characterized by its unique geochemical features: 1) δ¹³C_PDB (−1.34‰-−0.62‰) accords with the range of Early Ordovician seawater while δ¹⁸O_PDB (−8.01‰ -−4.79‰) is higher; 2) ⁸⁷Sr/⁸⁶Sr  (0.711000-0.711535, AVG=0.710863) is much higher than that of limestone and contemporaneous seawater; 3) average high Fe and Mn content (2001.32 and 601.73 μg/g, respectively) and low Sr and Ba (33.14 and 8.27 μg/g, respectively) content versus that of limestone; 4) dolomite has slightly negative Eu anomaly and similar REE composition and distribution patterns compared with limestone; 5) low order degree value (AVG=0.6). This study reveals that the dolomitization was related to fault and occurred in low-temperature and shallow-burial environment in Early-Middle Ordovician. Dolomitizing fluid was generated from the concentrated formation fluid in lower-middle member of HangGuletag Formation and underlying Tursaktag Group. Most Mg²⁺ came from the stabilization of quasi-stable carbonate minerals, and the transformation of clay minerals of (calcareous) mud in the limestone strata not only provided a little Mg²⁺ but caused the rise of ⁸⁷Sr/⁸⁶Sr. Dolomitizing fluid was likely to be richened in Xingdi-related normal fault and fractures which also acted as the main fluid pathways, and it finally caused the partial dolomitization of nearby limestone strata.