Accumulation stages and model of tuff gas reservoir: a case study of Huoshiling Formation in Dehui Fault Depression, Songliao Basin

The tuff gas reservoir in the Upper Jurassic Huoshiling Formation of the Dehui Fault Depression, Songliao Basin is expected to become the first tuff gas reservoir with industrial development value in China. However, due to the combined effects of multi-stage tectonic activities and multi-stage hydrocarbon generation and expulsion in Dehui Fault Depression, the accumulation process of the tuff gas reservoir is complex. Furthermore, the development history is relatively short, resulting in a current lack of understanding of the gas reservoir accumulation patterns. Using data including fluid inclusions, three-dimensional earthquake, cores, and cast thin sections, through petrographic analysis of tuff fluid inclusions and determination of homogenization temperature and salinity, combined with burial history and thermal evolution history simulation of the Dehui Fault Depression, the accumulation stage and time of tuff gas reservoir in the Huoshiling Formation were determined. A gas reservoir accumulation model was subsequently established. The results showed that the tuff reservoir of the Huoshiling Formation was mainly dominated by hydrocarbon-bearing saline inclusions. These inclusions were mostly found in micro-fractures and pores of quartz particles, showing clustered and banded distribution without distinct fluorescence characteristics. The homogenization temperatures of the inclusions showed bimodal characteristics, with peaks at 100 to 110 ℃ and 115 to 125 ℃. The tuff reservoir experienced two stages of natural gas injection in the middle Early Cretaceous (127 to 133 Ma) and late Early Cretaceous (106 to 114 Ma), with the main accumulation period in the late Early Cretaceous. Source rocks, reservoir and caprock combinations, and faults jointly controlled the enrichment of the tuff gas reservoir, with the gas reservoir mainly distributed in the upper part of the tuff. An accumulation model of "multi-source hydrocarbon supply, fault migration, and high-position enrichment" for the tuff gas reservoir in Huoshiling Formation of Dehui Fault Depression, Songliao Basin was established.