Discovery and Recognition of the Central Diapiric Zone in Baiyun Depression, Pearl River Mouth Basin

The Baiyun depression is a sub-unit of the Pearl River Mouth basin. In the center of this depression, fuzzy zones on 2D seismic profiles and low magnetic anomalies have been analyzed. A diapiric zone has been revealed, with a WNW directional distribution, about 1 000 km2 planar area and about 8 km most pierced thickness. The diapiric structures display various types such as a turtleback-like arch, weak piercing, gas-chimney, fault (or crack) and seabed pit, originated in two huge mud-rich strata the Wenchang Formation (Eocene) and the Enping Formation (Upper Eocene-Lower Oligocene). There are deep-lake facies in the rift period of the depression; the secondary is the Zhujiang Formation (Lower Miocene) and Hanjiang Formation (Middle Miocene), which have shallow or deep-marine facies in down-warping period. In the depression, factors are favorable for diapirism such as rapid subsidence, fine-grain sediment mainly filling, and dextral tenso-shear stress-field since the Cenozoic. The structures indicate a high temperature-overpressure system ever developed. Closely associated and abundant bright-spots show natural gas has migrated vertically, and a dynamic system has existed for petroleum transported from the Paleogene to the Neogene and Quaternary.