Causes of natural gas geochemical differences in the high-temperature and overpressure zone of DF1-1 diapir belt, Yinggehai Basin

Natural gas geochemistry, fluid inclusion analysis and gas reservoir accumulation tracers, have been used to study natural gas geochemical characteristics, genetic types, filling period, cause of carbon-isotope partial inverse sequence and the causes of gas variability both vertically and horizontally in the high-temperature and high-pressure belt in the central diapiric zone of the Yinggehai Basin. In the high-temperature and overpressure area, natural gas is thermogenic coal-type gas, and the partial inversion of carbon isotopes of alkane gas is caused by the mixture of multi-period gases that are of different maturities. There are four periods of fluid filling in the high-temperature and overpressure area. The first two periods are mature or high-maturity alkane gas, while the second two periods are high-maturity or over-mature alkane gas and inorganic CO₂.The filling period of the high-maturity and over-mature alkane gas is earlier than that of the inorganic CO₂. The influence of different diapiric activity, together with different gas migration channel types and development is the main reason for gas differences in different high-temperature and high-pressure zones. Multi-periodic diapir activity and periodic occurrence of overpressure caused the gas chemistry differences inside and outside of the high-temperature and high-pressure zone in the diapir core of D1-1 and nearby D13-1.