Formation mechanism of volcanic reservoirs within a volcanostratigraphic framework: The case of the Wangfu fault depression in the Songliao Basin, China

We discuss the reservoir formation mechanisms in volcanic gas reservoirs within a volcanostratigraphic framework, with a focus on the Huoshiling Formation of the Wangfu fault depression in the Songliao Basin, China. The volcanic sequences are divided, from large to small, into the volcanic cycle (VC), cooling unit set (CUS) and cooling unit (CU). The CU is the basic volcanogenic stratigraphic unit and the CUS is a superposition of cooling units which may be recognized on seismic profiles. The Huoshiling Formation consists of two volcanic cycles and one volcanic-sedimentary cycle. The formation mechanisms of volcanic reservoirs include three processes based on the strata's evolution. First, CUs formed and primary pore spaces developed near cooling surfaces. Second, CUSs and VCs formed and parasyngenetic and secondary pore spaces developed around the crater or caldera, especially near the weathering surface. Lastly, numerous dissolved pores and structural fractures formed during burial and concentrated near the fault belt. CUs, volcanic edifices and structural systems controlled the formation of volcanic reservoirs. The overlapping crater-proximal crater region and the faulted area were centres of reservoir formation. In the Huoshiling Formation, CUS 6-CUS 8 and CUS 13 are the major horizons where volcanic reservoirs developed, forming dissolution reservoirs and structurally fractured reservoirs for natural gas accumulation.