A new genetic type of natural gases and origin analysis in Northern Songnan-Baodao Sag, Qiongdongnan Basin, South China Sea

Unlike the typical coal-type gas of neighboring Lingshui and Ya'nan Sags in the Qiongdongnan Basin and farther Yinggehai Basin, most gases in the Songnan-Baodao Sag have much lighter δ13C1 and δ13C2 values, which are in the range of −54.68‰∼-33.68‰ and −31.03‰∼-23.50‰, respectively. The differences are attributed to the following aspects: (1) lighter gas from the preferential cracking of 12C-12C bond in aliphatic acid decarboxylation and polycondensation under catalysis of clay minerals in lower temperature, and (2) greater proportion of sapropelinite in strata overlying 1st member of Lingshui Formation with corresponding Ro less than 0.6%. The natural gases are classified into three genetic types: (1) Bio-thermocatalytic Transition Zone Gas generated from shale overlying 1st member of Lingshui Formation, (2) Thermal Catalytic Gas generated from lower Lingshui Formation and Yacheng Formation, (3) Mixed gas. Bio-thermocatalytic Transition Zone Gas shows lighter δ13C1 (δ13C1 <−44‰), a wider range of δ13C2 (>-31‰), a relatively lower dry coefficient (0.65-0.91), and abundant organic CO2 with δ13CCO2 ranging from −28.9‰ to −7.61‰. Thermal Catalytic Gas is typical coal-type gas (δ13C2 >−28‰) with higher maturity and dominating inorganic CO2. Mixed gas is mixture of above two gases. Compared with the condensate in the Yinggehai Basin, n-alkane mono-isomer and whole oil of condensate in study area are isotopically lighter, which are similar to the lighter Bio-thermocatalytic Transition Zone Gas. Attributed to mixed input of increasing proportion of alga and decreasing terrigenous higher plants in low-mature shale, contents of terrestrial biomarkers such as oleanane, cadinane, bicadinane, etc, are very low. Oil-source correlation analyzed from characteristics of mass chromatograms and C7 system, n-heptane, isoheptane value of light hydrocarbons of Bio-thermocatalytic Transition Zone Gas and Mixed gas show genetic relation with abundant sapropelinite in the low-mature shale in study area. This study provides a new insight to recognize potential gas and condensate resources generated from the previously neglected low-mature shale overlying 1st member of Lingshui Formation of Songnan-Baodao Sag and adjacent deep waters in Qiongdongnan Basin.