Abstract: The reservoir characteristics of the Suhongtu Formation in the Chagan Depression were studied using rock cores, thin sections, scanning electron microscopy, and mercury intrusion porosimetry in combination with well logging and seismic data, to identify the main factors affecting reservoir performance. Results of this study indicated that the primary reservoir spaces in volcanic rocks are gas pores, fractures, dissolution pores, dissolution fractures, and microscopic pores and fractures. Basalt gas pores, fractures and dissolution pores were broadly developed and distributed, and basalt is the main type of reservoir rock in the research area. Tuff fractures were also relatively well developed and were second only to basalt in terms of distribution, thus making tuff the second most important type of reservoir rock in the research area. The later the volcanic rocks are formed, the better is the preservation of their primary porosity and fractures. The explosive-effusive facies transition zone and parts of the upper section of the effusive facies have good petrophysical properties, and the filling of gas pores and fractures were the main cause of petrophysical degradation in volcanic rocks. Tectonic fractures and their associated weathering-induced leaching and dissolution greatly improved the reservoir performance of volcanic rocks. The clays generated during the alteration of volcanic minerals not only clog pore openings and degrade petrophysical properties, but also reduce the density of the rocks. These actions thus created the conditions for dissolution and alteration in subsequent periods.

Key words: Chagan Depression, volcanic rock, reservoir characteristics, main controlling factors, favorable areas