Small normal faults are well developed in the Miocene in Jinghai Sag of the Pearl River Mouth Basin and they are characterized by close spacing

highly variable strike

limited trace length

small throw

steep dip

crosscutting and polygonal geometry on plane view.Comprehensive study reveals that they are non-tectonic origin.A polygonal fault system (PFS) consists of small normal faults formed due to periodical pressure release induced by shallow overpressure.The overpressure is mainly caused by differential compaction

aquathermal pressuring and chemically driven volumetric contraction (syneresis) due to rapid sediment deposition.During the Eocene and Oligocene

Jinghai Sag experienced lithospheric stretching

followed by thermal subsidence in the Early and Middle Miocene

with the deposition environment evolved from lacustrine-neritic facies to bathyal-abyssal facies.The poor development of growth faults which play a key role in connecting the source rock of lacustrine-neritic facies and reservoir of bathyal-abyssal facies resulted in the absence of vertical hydrocarbon migration pathways.Polygonal faults are highly developed in the Miocene Zhujiang and Hanjiang Formations in Jinghai Sag and can act as hydrocarbon migration pathways

thus are highly significant for hydrocarbon accumulation during post-rift thermal subsidence.