Migration characteristics and migration-accumulation patterns of natural gas along faults in Pinghu Slope of Xihu Sag, East China Sea Basin

Faults are the main pathways for natural gas migration in Pinghu Slope of Xihu Sag, East China Sea Basin, and have obvious controlling effects on differential hydrocarbon accumulation. Clarifying the migration characteristics and migration-accumulation patterns of natural gas along faults can effectively guide the prediction of favorable hydrocarbon migration-accumulation areas in Xihu Sag and similar regions. Therefore, by integrating data from well logging, seismic surveys, and analytical test data including natural gas composition, carbon isotopes, inclusion homogenization temperatures, and abundance, the migration characteristics of natural gas along static faults were systematically studied, and the migration-accumulation patterns along static faults were further clarified. Results showed that there were two stages of hydrocarbon accumulation for natural gas in Pinghu Slope, namely, the depositional period of Miocene Yuquan Formation-Liulang Formation and the depositional period of Pliocene Santan Formation (or Pleistocene Donghai Group) to present. Faults were basically in static state during natural gas accumulation periods, and inclusion abundance values within fault zones ranged from 1% to 5%, indicating continuous migration of natural gas along static faults. The equivalent maturity of natural gas in Pinghu Slope was 1.08% to 1.23%, which was much higher than the maximum maturity of local source rocks, indicating that the gas source mainly came from high-maturity source rocks in adjacent sags. Hydrocarbon migration pathway tracing results showed that natural gas in A and B structures mainly migrated along fault strikes; natural gas in H and J structures mainly migrated vertically along faults and laterally across faults; natural gas in C and F structures mainly migrated along fault strikes and laterally across faults. Based on the above characteristics, three types of natural gas migration-accumulation patterns along faults were determined: "stepwise", "strike-parallel", and "strike-stepwise composite" patterns.