| Author's Name: HU Yingjie, LIU Xingzhou, HUANG Shuangquan, CAI Guogang, WANG Yanshan,
LI Jinyou, YANG Guangda, HOU Tuo, ZHANG Dongwei |
| In 2022, a significant gas breakthrough was achieved in the key exploration well (Kuitan-1) located in the
Kuihuadao structural belt of the eastern offshore, Liaohe Depression, with high-productivity gas flows in Dongying
Formation, Shahejie Member 3, and Jurassic Xiaodonggou Formation. It is worth noting that there are significant
differences in methane carbon isotope composition (δ13C1) between the Mesozoic and Cenozoic natural gas formations.
Determining the origin of natural gas and its source rock is significant for assessing the potential of natural gas resources
and selecting exploration targets. This paper systematically analyzes geochemical data such as the components and stable
carbon isotope composition of natural gas samples from the three gas-bearing strata. The results indicate that there are
organic thermogenic gas and inorganic gas in the eastern offshore: (1) The natural gas in the Dongying Formation and
Shahejie Formation belongs to the coal-type organic thermogenic gas, mainly consisting of methane, the dryness coefficient
is at the range of 0.789-0.949. δ13C1 is mainly around -35‰. Natural gas originates from the mature mudstone in the middle
and lower part of Shahejie Formation in Gaizhoutan sag, and accumulates during the late Dongying-Minghuazhen periods.
According to the empirical formula, Ro of gas source rock ranges from 0.77% to 1.59%. There is a clear indication of
migration effects for natural gas in the middle and shallow Dongying Formation, with clearly higher Ro of gas source rock
than the mudstone at the same depth. In the deep Shahejie Member 3, it accumulates in situ or with short distance migration,
indicated by the similar or slightly higher Ro of gas source rock than the mudstone at the same depth. (2) The natural gas in
Jurassic Xiaodonggou Formation is of inorganic origin. It has an average dryness coefficient of 0.991 and is characterized by
extremely heavy methane carbon isotope (δ13C1＞-20‰). It’s inferred that the natural gas originates from Fischer-Tropsch
synthesis in the deep crust or mantle, migrates and accumulates along the deep-seated fault with strike-slip activity during
the late Dongying period. These natural gas reservoirs of two genetic types have obvious vertical separation and different
accumulation processes. Plays around the Shahejie mature source rocks in the Gaizhoutan sag and the Yingkou-Tong'erbu
strike-slip zone are favorable to organic thermogenic gas and inorganic gas respectively. |
