Control of volcanic lithofacies on play fairways: A case study of the Huoshiling Formation in the Dehui faulted depression, southern Songliao Basin, China

Volcanic reservoirs demonstrate strong heterogeneity and substantial variations in productivity due to the complexity of volcanic eruption and lithology. The main types of reservoir space are not clear, and the dominant lithofacies distribution, particularly the favorable areas for high-quality reservoirs, remains to be determined. In this paper, the Huoshiling Formation in the Dehui faulted depression, Songliao Basin is taken as an example to carry out the multi-scale joint characterization of its pore throat structure, establish a reservoir evaluation standard that considers both the gas content and seepage capacity, and perform reservoir evaluation and play fairway mapping under facies control. The results show that the storage space types of the gas-bearing reservoirs in the faulted depression can be ascribed into three categories and six subcategories according to the pore throat and pore characteristics. In terms of pore sizes, volcaniclastic lava rank the first, followed by volcaniclastic rocks, sedimentary volcaniclastic rocks and volcanic lava. The comprehensive evaluation parameter ($\Phi \bullet K\bullet S_g$, where $\Phi$ is porosity, K permeability, and S_g gas saturation) of high-quality reservoirs are all greater than 0.1. The volcanic reservoirs in the Stage-Ⅲ strata are the highest in quality and largest in area of play fairways. The thermal debris flow sub-facies developed at Stage Ⅲ are mainly seen along the western strike-slip fault zone in the Debei sub-sag and the southwest Nong'an tectonic belt, while those developed at Stage I are distributed along the central and eastern fault zones in the southeastern Baojia sub-sag. The favorable layer evaluation and favorable area delineation under facies control will be of certain reference significance for subsequent exploration and development of volcanic gas reservoirs.