Organic matter enrichment mechanisms of terrigenous marine source rocks in the Qiongdongnan Basin
The Oligocene terrigenous marine source rocks in the Qiongdongnan Basin serve as a significant suite of source rocks in deep-water areas.We analyze their formation mechanisms from the aspects of the basin's tectonic framework,paleoclimate,sedimentary facies,and delta scale.The results indicate that the tectonic framework of the basin generally determines the source rock types.Specifically,the terrigenous organic matter of the Yacheng Formation,deposited during the rifting stage,predominantly accumulates offshore and forms coal-measure source rocks.In contrast,the terrigenous organic matter from the Lingshui Formation(E3l)forms terrigenous marine source rocks via fluvial-deltaic transport.The 3rd member of the Yacheng Formation(E3y3)and the 2nd member of the Sanya Formation(N1s2)witnessed the most luxuriant terrigenous higher plants,followed by the 1st to 2nd members of both the Yacheng and Lingshui formations(E3y1 to E3y2,and E3l1 to E3l2).These contributed significantly to an abundant supply of terrigenous higher plants.Variations in sedimentary facies zones determine the differences in terrigenous organic matter,whose differential deposition leads to the formation of significantly varying terrigenous marine source rocks.Favorable areas for the terrigenous marine source rocks include delta fronts to inner neritic zones,along with submarine fans.Large-scale deltas facilitate the transport of terrigenous organic matter with higher biotic fluxes,thus improving the organic matter abundance of source rocks.As a result,significant and characteristic gas source rocks are formed in the sea area.
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