Abstract: Natural gases in the northern South China Sea are predominantly derived from the Late Oligocene-Early Miocene marine shales. The organic matter enrichment and its controlling factors are key points for the natural gas exploration in the Qiongdongnan Basin. The organic matter enrichment from the Oligocene to the Early Miocene marine shales from fifteen wells in the Qiongdongnan Basin was found using the data of total organic carbon measurement and Rock-Eval pyrolysis. During the early Oligocene, the Middle Yacheng Formation shales are enriched in terrigenous organic matter, which was thought to be caused by the flourished tropical/subtropical plants in the more humid environment. During the Late Oligocene-Early Miocene, there is also a terrigenous organic matter enrichment. Analyses of plant-derived biomarkers, palynogical records in shales in combination with regional global climate variations suggest the terrigenous organic matter enrichment was in association with warming and humid climate in the Late Oligocene-Early Miocene, during which higher plants flourished and the East Asian summer monsoon coupled with annual precipitation intensified. The significant extra precipitation brought by intensified East Asian summer monsoon was beneficial for the large amount of scattered plant organic matter enrichment and hence the terrigenously-enriched marine shales were more widely deposited in shallow marine environment. The finding of terrigenous organic matter enrichment in the Late Oligocene-Early Miocene marine source rocks was beneficial for the deep-water natural gas exploration in the deeply buried Ledong Sag and the Lingshui Sag in the Qiongdongnan Basin.

Key words: palaeovegetation, palaeoclimate, terrigenously-dominated marine shales, organic matter enrichment, deep-water gas exploration