Volcanic oil and gas reservoir characteristics and formation mechanism in Kalagang Formation of Malang sag in Santanghu basin

The petrographic characteristics, types of reservoir spaces and diagenesis of volcanic rocks, and formation mechanism of volcanic reservoir in Kalagang Formation of Malang sag in Santanghu basin were studied, by combining core, core wafer, scanning electron microscope log data, electric logging and geochemical analysis data. The results show that: (1) The hydrocarbon bearing volcanics in this region is mainly composed of lava (basalt and andesite) and volcanoclastic rock; (2) The volcanic phase contain mainly effusive facies, explosive facies and volcanic sedimentary facies; (3) The alkali metasomotism in the reservoir is obvious, the typical marks of which are albitization, arfvedsonization, laumontitization and pore-fracture filled with quartz by multi-stage; (4) The volcanics in Kalagang Formation was formed in alkaline environment, the reservoir space of which is mainly secondary dissolution porosity. The solution pore layers of laumontite in the reservoir are good reservoir, which reflects volcanic reservoir formation mechanism of alkali before acid in Kalagang Formation; (5) The interaction by layers of laumontite and layers of quartz-cementing, while the former ones are always distributed under the latter ones, is the favorable evidence of the upward migration and discharging silicon after the hydrocarbon alkali fluid metasomatized the volcanic rocks, which reflects the volcanic reservoir formation mechanism of 'metasomatism the lower layers and discharging in the upper layers' in Kalagang Formation. The research shows that alkali metasomotism is pervasive in Kalagang Formation, and it also reveals that the characteristics of hydrocarbon alkali fluid participation in the formation and development of volcanic reservoir, which is significant in further acquaintance of the formation mechanism in volcanic reservoir.