The origin and migration of natural gas and the accumulation of gas hydrates within the Pearl River Mouth Basin of the northern South China Sea are poorly understood. Based on high-resolution 2D/3D seismic data, three environments of focused fluid flow： gas chimneys, mud diapirs and active faults have been identified. Widespread gas chimneys that act as important conduits for fluid flow are located below bottom simulating reflections and above basal uplifts. The occurrence and evolution of gas chimneys can be divided into a violent eruptive stage and a quiet seepage stage. For most gas chimneys, the strong eruptions are deduced to have happened during the Dongsha Movement in the latest Miocene, which are observed below Pliocene strata and few active faults develop above the top of the Miocene. The formation pressures of the Baiyun Sag currently are considered to be normal, based on these terms： 1） Borehole pressure tests with pressure coefficients of 1.043-l.047; 2） The distribution of gas chimneys is limited to strata older than the Pliocene; 3） Disseminated methane hydrates, rather than fractured hydrates, are found in the hydrate samples; 4） The gas hydrate is mainly charged with biogenic gas rather than thermogenic gas based on the chemical tests from gas hydrates cores. However, periods of quiet focused fluid flow also enable the establishment of good conduits for the migration of abundant biogenic gas and lesser volumes ofthermogenic gas. A geological model goveming fluid flow has been proposed to interpret the release of overpressure, the migration of fluids and the formation of gas hydrates, in an integrated manner. This model suggests that gas chimneys positioned above basal uplifts were caused by the Dongsha Movement at about 5.5 Ma. Biogenic gas occupies the strata above the base of the middle Miocene and migrates slowly into the gas chimney columns. Some of the biogenic gas and small volumes ofthermogenic gas eventually contribute to the formation of the gas hydrates.
Based on multi-beam bathymetric data and 2D high-resolution, multi-channel seismic prof＇des, combing ODPl148 drilling data, the morphology, internal sedimentary architecture, and evo-lution pattern of 17 deepwater canyons from the Middle Miocene to present are documented in the northern Baiyun （白云） sag （BS）, Pearl River Mouth basin （PRMB）, and northern South China Sea （SCS）. There exist six seismic architectural elements in these canyons, including basal erosive surfaces （BES）, thalweg deposits （TD）, lateral migration packages （LMP）, mass transport deposits （MTD）, can- yon margin deposits （CMD）, and drape deposits （DD）. According to the stratigraphical ages and geo-metrical features of these canyons, their formation and evolution processes are divided into three stages： （1） Middle Miocene scouring-filling, （2） Late Miocene lateral migration, and （3） Pliocene-Quaternary vertical overlay. An auto-cyclic progressive process of eroding and filling by turbidity currents results in the scouring-filling and vertical overlay; bottom currents are responsible for the remarkable asym-metry between the two flanks of canyons; and faults are inherent dynamic forces triggering these can yons. It is inferred that these canyons are caused by the double effects of turbidity and bottom currents under the control of faults as inherent dynamic forces.
