The Tarim basin experienced intensive magmatic/volcanic activities in its geological history. Both extrusive and intrusive igneous rocks were found in several field outcrops as well as discovered in many wells. Large volume of deep fluid was generated in the process of magma/volcanic activities. Numerous large abyssal faults, including basement faults and capcutting faults, and many small faults and fractures, which connect with the abyssal faults, serve as necessary conduits for deep fluid. Under the effects of deep fluid, the Lower Ordovician carbonates in the Tarim basin were remarkably eroded, and then many needlelike pores or pores of several microns in diameter were generated. Such pores were greatly significant to the improvement of reservoir properties. Contrast to the dissolution of carbonates, many types of minerals precipitated from the deep fluid and filled in the pores and fractures. The minerals include calcites, quartz, dolomite, fluorite, sphalerite, pyrite, and so on. In generally, two or more minerals occurred as mineral assemblage in one place. For example, quartz, dolomite and fluorite occurred together in well S15, calcite, fluorite and quartz in well Tz45, and sphalerite, pyrite and chlorite in well Tz12. In addition, some dolomite rocks recrystallized into white or shallow gray medium—coarse crystalline dolomite rocks under the effects of deep fluid. The typical recrystallized dolomite was found in well S15 and one outcrop in Bach. The intercrystalline pores as well as dissolution pores, generated during the recrystallization, are of great significance to the improvement of reservoir properties of dolomite rocks in the Tarim basin.