Petroleum system evaluation of the world’s deepest (>5000 m) volcanic reservoirs, Sichuan Basin, China

Large-scale hydrocarbon accumulations have rarely been found in volcanic reservoirs around the world. This study documents large gas accumulations in Permian reservoirs in two areas of the Sichuan Basin, southwestern China, that include the deepest (>5000 m) volcanic reservoirs yet identified worldwide. Petroleum system elements differ in these deep volcanic accumulations between the Jianyang and Zhougongshan areas within the basin. In the Jianyang area, natural gas was derived mainly by the cracking of oil from Cambrian source rocks. Accumulations are overpressured stratigraphic traps spatially controlled by the distribution of a welded tuff reservoir with high porosity and high permeability. In contrast, low-porosity and low-permeability welded lapilli tuff and welded breccia reservoirs have lower gas saturations and are not productive. In the shallower Zhougongshan area, gas was generated by the cracking of kerogen in middle Permian source rocks. Regional tectonism formed normally pressured structural traps in fractured basaltic reservoirs. Multistage tectonism likely breached seals and allowed oil and gas to escape, reducing the scale of the accumulations in this area. Key factors influencing the large-scale accumulation and preservation of natural gas in the study area include development and preservation of adequate reservoir quality as well as tectonic history favorable to maintain seal integrity in brittle rocks. Our results provide guidance for the exploration for similar reservoirs in other areas.