Review of volcanic reservoir geology in China

Volcanic reservoirs are widely distributed in more than 40 basins in 13 countries and have become an important target for oil and gas exploration. The study of volcanic reservoirs is becoming a hot research topic. After decades of research in China, especially within the last 20 years, numerous achievements have been attained including the study of void space, petrophysical characteristics, distribution pattern and reservoir origin. The research shows that the volcanic void space can be divided into 11 types and 27 subtypes. Volcanic rocks can be rich in primary vesicles, shrinkage fractures and explosive fractures, which are only found in volcanic rocks. Generally, the porosity and permeability values of volcanic rocks in basins are low, and pore throat values are small. Sometimes, sweet spots occur. Volcanic reservoir formation correlates with burial depth. In China basins, the porosity and permeability values of pyroclastic rock and tuffite buried above a depth of 3 km are higher than those of lava and welded pyroclastic rocks, while these values are reversed below a depth of 3 km. In general, all kinds of lithologies can bear hydrocarbons in basins, but only certain lithologies can bear oil and/or gas in specific blocks. The distribution model of the reservoir correlates the volcanic stratigraphic units; for example, it identifies the “good upper flow crust and poor lower flow crust” pattern formed by the lava flow and lava dome and finds the porosity and permeability values in the lava flow to be higher than those in the lava dome. The porosity and permeability values of the crater and near crater belt of the volcanic edifices are better than those of the proximal belt and the distal belt. Most favorable reservoirs are located within 200 m below the eruptive interval unconformity boundary or tectonic unconformity boundary. Release of volatiles, cooling and quenching, pre-burial weathering and devitrification are the important processes of volcanic reservoir formation. The deformation of lava during compaction processes is small, while that of pyroclastic rock is significant. The high content of unstable components in acidic fluid can provide the material for alteration and/or dissolution. A volcanic reservoir in a basin is the result of the above types of diagenesis and forms from a complicated origin process. The reservoir evolution process becomes more complicated when volcanic strata have undergone uplift and re-burial. With an increase in burial d