Geological features and evaluation techniques of deep-seated volcanics gas reservoirs, Songliao Basin

The significant breakthrough of natural gas exploration in deep-seated volcanic rocks of Songliao Basin, China, demonstrates that the volcanic rocks as reservoirs can form natural gas accumulations where excellent source–reservoir–caprock assemblages exist. This paper reveals several features of volcanic gas accumulations: (1) Two types of gas reservoirs, including hydrocarbons and non-hydrocarbons, are developed; the former is organic and usually trapped in shallower volcanics associated with sub-deep-seated faults, while the latter is mainly inorganic and accumulated in deep-seated volcanics associated with large-scale deep-cut basement faults. (2) Lower Cretaceous Shahezi Formation coal series source rocks, Yingcheng Formation volcanics, and Denglouku Formation mudstone form good source–reservoir–seal associations that control the major volcanic plays. (3) Volcanic crater and eruptive facies spread along deep-seated faults, so discordogenic faults control the distribution of large-scale volcanic gas reservoirs. (4) Fracture zones, accompanied with large-scale faults, control the distribution of high-yield zones. (5) Effusive volcanic rocks in the deep parts of sags have huge thickness and touch extensively with source rocks, so the gas accumulation potential is promising if reservoirs exist. Evaluation techniques can be summarized in three steps: (1) Determining the distribution of volcanic rocks by joint inversion of gravitational, magnetic, and seismic data. (2) Predicting the distribution of volcanic reservoirs by multiple methodologies. (3) Predicting the distribution of gas-bearing volcanic rocks by integrated methods, including gas detention.