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

摘要

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 delta C-13(1) and delta C-13(2) values, which are in the range of -54.68 parts per thousand similar to -33.68 parts per thousand and -31.03 parts per thousand similar to -23.50 parts per thousand, respectively. The differences are attributed to the following aspects: (1) lighter gas from the preferential cracking of C-12-C-12 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 delta C-13(1) (delta C-13(1) -44 parts per thousand), a wider range of delta C-13(2) ( -31 parts per thousand), a relatively lower dry coefficient (0.65-0.91), and abundant organic CO2 with delta C-13(CO2) ranging from -28.9 parts per thousand to -7.61 parts per thousand. Thermal Catalytic Gas is typical coal-type gas (delta C-13(2) -28 parts per thousand) 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 lowmature shale, contents of terrestrial biomarkers such as oleanane, cadinane, bicadinane, etc, are very low. Oilsource correlation analyzed from characteristics of mass chromatograms and C-7 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.