The practice for exploring natural gas in Qiongdongnan Basin proves that coal measures and coal seams are the most important hydrocarbon source rocks.Based on the characteristic analysis of coal-bearing deposits in the Oligocene Paleogene by the seismic data

drilling core and logging data in Qiongdongnan Basin

the corresponding relationships between sedimentary facies and seismic facies are established

and 14 kinds of sedimentary assemblages are identified in the Oligocene of Qiongdongnan Basin

including the alluvial fan-tidal flat-lagoon

alluvial fan-tidal flat-coast

alluvial fan-sea bog sand ridge plain-coast assemblages.According to the micro-and macro-physicochemical properties of cored coal intervals

it is made certain that the Yacheng Formation coal seam was affected by seawater during its formative process and its sedimentary environment tended to be strong in reduction

indicating that the coal seam is developed in the offshore marsh environment.In view of the characteristic and environmental analysis for coal forming

four types of coal-forming sedimentary systems were classified

namely fan delta

braided river delta

tidal flat-lagoon and barrier-coast-sea marsh sand ridge plain systems.Among them

the thin coal seam in the fan delta system

prone to bifurcation

is mainly developed in the fan delta plains

abandoned lobes

and interlobe depressions;the coal seam in the braided river delta system

featuring thinness and instability

mainly grows in the braided river delta plains

diversion bays

and floodplains.The coal seam in the tidal-lagoon system is mostly developed in the swamps of the supratidal flats and the inter-tidal-channel bays of the upper intertidal flats

the tidal delta plains and the silted lagoons

and in areas of multiple tidal channels

the coal seam layers are relatively large in number

and characterized by thinness and larger spacing

while in areas of single tidal channels

the number of coal-bearing seams

complex in structure

is relatively small

but their thickness is relatively large

and their spacing is small.At last

the coal seam in the barrier coast-sea marsh sand plain system is mainly developed in coastal plain marsh areas

characterized by multiple layers and faster change in lateral

but thinness for each layer.