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.

The interactions between carbonate and H2S saturated acid fluid at various temperatures and pressures in-situ conditions were simulated using hydrothermal diamond-anvil cell equipment combined with Raman spectroscopy. The heating process is from room temperature to 230?C and then the system is cooled to room temperature again. Experimental results clearly demonstrate that carbonate minerals present much precipitation from room temperature to 140?C and little precipitation from 140?C to 230?C. Carbonate trends to precipitate with the increase of temperature and pressure, and dolomite is more stable than calcite and limestone. But in the cooling process carbonate suffers from little dissolution. So in the burial process, carbonate trends to precipitate, and the rapid closed burial and slow uplift process is beneficial to form high quality reservoirs in the deep closed condition. Fault and magmatic hydrothermal activities may break the closed system, which needs further study.

Based on the chemical thermodynamics, the dissolution response mechanism of the calcite and dolomite with the increase of depth in the CO₂-beared fluid and H₂S-beared fluid was studied by the method of numerical simulation. The result indicates that the dissolution of calcite is always more evident than dolomite in same conditions. The amount of gas confined in the dissolution system is another important factor which influences the dissolution of carbonate mineral except the temperature, pressure, partial pressure of the acid gas and the component of fluid. When the amount of confined gas is relative large, with the depth increases, the dissolution amount curve shows an increase followed by a decline. And the dissolution pore in the dolomite is more developed than that in the calcite. When the amount of confined gas is relative short, both minerals’ dissolution amount curves declines tonelessly.