Little research regarding
the organic geochemistry of source rock
deposited in marine or transitional environment in the eastern deep-water
area of the Qiongdongnan Basin (QDNB) has been performed so far. Based
on the organic geochemical and petrological data of mudstone samples
collected from the eastern deep-water area in the QDNB, combined with
palynological data of rock samples in the shallow-water area, this
paper studies the paleoenvironment, hydrocarbon potential, and controlling
factors of Oligocene source rock in different sedimentary facies in
the eastern QDNB. Two distinct models of delta front source rock and
neritic source rock are proposedaccording to organic matter sources
and geochemical characteristics. Source rock in the delta front subface
is of medium to high organic abundance and gas-prone-type kerogen,
the geochemical biomarkers are characterized by high ratios of pristane/phytane,
oleanane/C30 hopane, T-bicadinane/C30 hopane,
and low ratios of C23 tricyclic terpane/C30 hopane,
C27 sterane/C29 sterane, (nC21 + nC22)/(nC28 + nC29). However, source
rock in the neritic face has medium organic richness and kerogen type
prone to generate both oil and gas; the biomarkers are characterized
by low ratios of pristane/phytane, oleanane/C30 hopane,
T-bicadinane/C30 hopane, and high ratios of C23 tricyclic terpane/C30 hopane, C27 sterane/C29 sterane, (nC21 + nC22)/(nC28 + nC29). During the Oligocene epoch, the paleoclimate in
the QDNB was subtropical warm-humid, and paleovegetation around and
within the sags was dominated by pteridophyte and angiosperm plants,
causing terrestrial higher plants to be the main organic matter source.
Source rock in the delta front subface is formed in oxidation fresh
or brackish water environment, whereas source rock in the neritic
face is formed in weak oxidization brackish or saline water conditions.
Organic matter abundance in the delta front subface is mainly controlled
by the terrigenous organic matter input, whereas organic matter abundance
in the neritic face is controlled by aquatic organic matter input
and influenced by the water redox environment.
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