Volcanic facies and their reservoirs characteristics in eastern China basins

In these years, with more and more volcanic oil and gas fields being discovered and developed, the volcanic rocks reveal a great petroleum potential in the eastern basins of China. There are five volcanic facies identified in the study area, which include volcanic conduit facies, explosive facies, effusive facies, extrusive facies, and volcanogenic sedimentary facies. The subaerial eruption usually happened in Mesozoic and Paleocene, and subaquatic eruption in Eocene. The upper subfacies and top autoclastic brecciation of effusive facies of subaquatic volcanic rocks and pyroclastic flow subfacies of explosive facies of subaerial volcanic rocks are the most favorable volcanic reservoirs. The intermittent belt formed between two times of volcanic eruptions is most effective for reservoirs both in subaquatic and subaerial volcanic rocks. Their main porosity types are interclast porosity, interflow laminar porosity, vesicular and gas pipes porosity, intercrystalline sieve of moldic porosity, secondary dissolution porosity, and tectonic fracture. Developed between pre-emplacement stage and final cooling, the primary porosity may lead to high porosity and permeability, and the secondary porosity usually developed upon them. The porosity of volcanic rocks was less influenced by the compaction and the burial depth.