The formation mechanism and four-stage evolution of volcanic primary reservoir spaces： a case study of Carboniferous volcanic rocks in Dixi area, Junggar basin

Abstract：Volcanic reservoir is usually of dual porosity medium character. The reservior spaces can be classified into two genetic categories： primary reservior spaces (PRS) and secondary reservior spaces (SRS), and most of the latter were generated from the fomer by late diagenesis. Based on extensive identification and description of casting thin section, this paper divided the PRS of Carboniferous igneous rocks in Dixi area into five types of primary pores (gas pores, residual amygdale, intergranular pores, intragranular pores and intercrystal pores) and four types of primary fractures (shrinkage joint, explosive fracture, intergravel crack and intercrystal crack). In combination with the influencing factors such as environment and intensity of eruption and texture and structure of rocks and according to the evolution features of magma at different stages, the formation of PRS is divided into four stages： "magma chamber" stage, outpouring surface stage, diagenetic stage of condensation and deuteric hydrothermal stage, of which the outpouring surface stage and diagenetic stage of condensation were the main periods to form primary apertures while the hydrothermal stage was the period when the number of apertures was reduced. The speed of crystallization constituted the key factor for the formation of different types of PRS. The study of the formation mechanism of igneous PRS can reveal the distribution of SRS and is of great significance in the prognosis of favorable reservoir zones.