Abstract: Based on the data of core, thin section, scanning electron microscopy and the petrophysical property, this paper presents the characteristics of Harjiawu volcanic reservoir of the Carboniferous in Malang sag, and obtains the following understandings. ①The lithology is dominated by basalt, andesite and transitional lithology; ②The main lithofacies is overflow facies, explosion and volcanosedimentary facies are secondary; ③The volcanic reservoir are of medium-low porosity and low-super low permeability; ④Primary porosity is not developed because of its underwater eruption, however, the pores, vugs and fractures often superimposed on the original storage space after the transformation of the post-magmatic diagenesis and woven together form the effective storage spaces; ⑤The phenomena such as albitization, arfvedsonitization and Sideritization prove existence of hydrocarbon alkali fluid in the Carboniferous volcanic rocks and the extensive development of alkali metasomatism; ⑥Devitrification, weathering, tectonism, metasomatism and dissolution have significant effects on the volcanic reservoir modification, which are the main diageneses for forming secondary pores and the key factors for forming effective reservoirs of the Harjiawu formation.

Key words: Santanghu basin, Malang sag, Harjiawu formation, volcanic reservoir, alkaline diagenesis, secondary pore zone