Classification and evaluation of micro pore structure of volcanic rock reservoirs: A case study of the Carboniferous volcanic reservoirs in Xiquan area, eastern Junggar Basin

The studies of the characteristics of volcanic reservoirs in Xiquan area of the Junggar Basin are relatively weak. The pore types and pore assemblages as well as the micro pore structural features of the Carboniferous volcanic reservoirs in Xiquan area were classified and evaluated in this paper according to hydrocarbon yield, and also based on the observation and description of cores from 25 drilling wells, and the identification and quantifying of casting thin sections under microscope, measurement of SEM, porosity and permeability and capillary pressure. Four types of volcanic reservoirs were identified based on the thin-section porosity and the superior pore assemblage. The type Ⅰ pore assemblage is the vesicles+enlarged dissolution vesicles and dissolution pores+fractures. The type Ⅱ includes vesicles+structural fractures+dissolution fissures. The type Ⅲ is vesicles+dissolution pores, while only isolated fine pores developed in the type Ⅳ reservoir. The thin-section porosity reduces in turn from the type Ⅰ to the type Ⅳ. Different micropore structure of the volcanic reservoirs indicated that the type Ⅰ is an excellent reservoir with well-developed large pores and coarse throats. The type Ⅱ and type Ⅲ are better to medium favorable reservoirs with developed large to medium pores and medium to fine throats. While the type Ⅳ is a tight reservoir due to its fine pores and fine to micro throats. Finally, a volcanic reservoir classification and evaluation criterion for the study area was established on the basis of the above study and integrated with well logging data.