Based on comprehensive analysis of field work, petrological and geochemical characteristics, the authors study the sources of dolomitizing fluids and the models of dolomitization of the Lower Ordovician Penglaiba Formation in Tongguzibulong Outcrop, the northwestern margin of Tarim Basin, China. Four types of
dolomite are recognized: euhedral-subhedral powder crystallized dolomite, euhedral-subhedral fine crystallized dolomite, subhedral-xenotopic medium crystallized dolomite and subhedral-xenotopic coarse crystallized dolomite. Powder-fine crystallized dolomite is distributed in the lower part of Penglaiba Formation, and has cloudy center surrounded by clear rim. With residual sand texture, inter-crystal pores and inter-partical pores, medium-coarse crystallized dolomite is distributed in the upper part of Penglaiba Formation. The REE patterns of dolomite and contemporaneous limestone rich in LREE and deplete in HREE, present a trait of unobvious Ce anomaly and Eu negative anomaly, the values of C-O isotope locate in the scope of contemporaneous marine dolomite, and Eu negative anomaly. All these denote that the dolomitizing fluid is normal or slightly concentrated seawater. Besides, Powder-fine crystallized dolomite present a low value of Fe, Mn and a high value of Sr, Ba, formed by reflux seepage dolomitization in penesaline seawater. Cloudy center surrounded by clear rim texture and multi-rimmed texture are the results of over-dolomitization. Medium-coarse crystallized dolomite with residual grain texture, interbedded with sand limestone, are controlled by high frequency sea level change. With a higher value of Fe, Mn and a lower value of Sr, Ba when compared with powder-fine crystallized dolomite, medium-coarse dolomite formed by the early reflux seepage dolomitization and intensified by the subsequent burial recrystallization.

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.