Observation of Catastrophic Degassing from Mantle-Crust in Yinggehai Basin, South China Sea

To lower the CO₂ risk for hydrocarbon exploration in the west continental shelf of Yinggehai basin, South China Sea, we do attempt not only to know the CO₂ origins, but also to make an understanding of the degassing processes from the mantle and crust. Based on the stable carbon isotope ratios of CO₂ alone, the organic and inorganic CO₂ can be successively distinguished, but the formation conditions and mixing processes for inorganic CO₂ are still not clear. The relationships between lg[R(= ³He/⁴He)/R_a (=1.386×10^-6)] and CO₂ content (%), CO₂/³He and δ¹³C_CO2 have been employed, respectively, to obtain that the CO₂ gases in the reservoirs can be classified into three categories: (1) organic CO₂ with very low contents but contaminated by mantle-derived helium; (2) inorganic CO₂ gases with lower to higher contents being mixtures of crustal CO₂ with mantle-derived CO₂, the mantle-contributed percentage being in the range of 0 %-27 %, and (3) mainly crust-derived inorganic CO₂ gases being characterized by high contents (more than 50 %) and indicating the crustal addition by metamorphism of rich-in carbon rocks in basement. Nevertheless, some CO₂/³He ratios of organic CO₂ fall into the range 10⁸-10¹⁰, which made us inquire whether the CO₂/³He = (1-10) × 10⁹ can be the unique signature of magmatic CO₂ or not. All the observation of plutonic activities, fluid inclusion measurements in gas reservoirs, pre-stack depth/time seismic sections and the satellite infrared remote photography taken from Yinggehai basin, South China Sea, during Chichi earthquake in Taiwan on September 21, 1999, supports that the degassing processes are in a discontinuous mode, which may be triggered by igneous intrusion or extrusion, or earthquakes. In the central diapir zone of the basin, at least 3 to 4 orders of discharge of across-formational thermal fluid flows through fractures can be determined in different scales. The mantle degassing process may have a strong effect on overpressured system forming and outgassing in crust. However, it is very difficult to estimate the transferring rates for a special fractured zone at a specific time interval.