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 δ¹³C_CO2 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.

不像在琼东南盆地相邻陵水Ya'nan下陷和更远莺歌海盆地，在松南-宝岛凹陷大多数气体的典型的煤型气体具有δ轻得多¹³ Ç ₁和δ ^13个Ç _2个值，其是在范围分别为-54.68‰〜-33.68‰和-31.03‰〜-23.50‰。差异归因于以下几个方面：（1）优先裂解¹² C- ^12产生的较轻气体较低温度下粘土矿物催化下的脂肪酸脱羧和缩聚反应中的C键，以及（2）凌水组第一段上覆土层中的腐泥土比例较大，相应的Ro小于0.6％。天然气分为三种成因类型：（1）灵水组第一段页岩上生的生物热催化过渡带气；（2）灵水组下段和崖城组产生的热催化气；（3）混合气。生物热催化过渡带气显示打火机δ ¹³ Ç ₁（δ ¹³ C ^ ₁ <-44‰），更宽范围的δ ¹³ C ^ ₂（> -31‰），相对较低的干系数（0.65-0.91），和丰富的有机CO ₂与δ ¹³ Ç _CO2范围从-28.9‰至-7.61‰。热催化气体是典型的煤型气体（δ ¹³ C ^ ₂ > -28‰）具有较高的成熟度和主导无机CO ₂。混合气体是上述两种气体的混合物。与莺歌海盆地的凝析油相比，n-烷烃单异构体和研究区的凝析油全油同位素更轻，类似于生物热催化过渡带气体。归因于低成熟页岩中藻类比例增加和陆生高等植物减少的混合投入，陆地生物标志物如齐墩果烷，卡丁烷，双cadinane等的含量非常低。根据质谱图和C _7的特征分析油源相关性生物热催化过渡带天然气和混合气中轻烃的正庚烷，异庚烷值与研究区低成熟页岩中丰富的腐殖质呈遗传关系。这项研究提供了新的认识，以认识由先前被忽略的松南—宝岛凹陷陵水组第一段及琼东南盆地深水附近的低成熟页岩所产生的潜在的天然气和凝析油资源。