dong Dong

Focused fluid flow in the Baiyun Sag, northern South China Sea: implications for the source of gas in hydrate reservoirs

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 1.043–1.047; 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 of thermogenic gas. A geological model governing 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 of thermogenic gas eventually contribute to the formation of the gas hydrates.

Tectonic contrast between the conjugate margins of the South China Sea and the implication for the differential extensional model

The extensional model of the South China Sea (SCS) has been widely studied, but remains under debate. Based on the latest high-quality multi-channel seismic data, bathymetric data, and other obtained seismic profiles, the asymmetric characteristics between the conjugate margins of the SCS are revealed and extensional model of the SCS margin is discussed further. Spatial variation of morphology, basement structure, and marginal faults are discovered among the SCS margin profiles. As for the NS-trending variation, the basement of northern margin displays in the shape of step downwards to the sea, while the basement of southern margin is composed of wide rotated and tilted blocks, without any obvious bathymetric change. The variation also exists in the development of marginal faults between the conjugate margins, and detachment fault system is identified on the southern margin. Along the southern margin from east to west, the Eastern and Southwestern Basins developed different structural units. Based on the tectonic contrast of the conjugate margins, differential extensional model is proposed to explain the spatial variation of the SCS structure, which introduces detachment faults controlling the evolution of the SCS. The upper crust above the detachment fault was deformed by simple shear, while the lower crust and upper mantle below the detachment fault was deformed by pure shear. Because of the different lateral transfer between the upper brittle faulting and the lower ductile extensional regions, there developed marginal plateau (Liyue basin) and outer rise (Zhenghe massif) on the lower plate margin of the Eastern Basin and the Southwestern Basin, respectively. The evolution of the present SCS may be influenced by the diachronous close of the paleo-SCS.

Formation and syn-rifting process of the Wan’an Basin, South China Sea

Based on seismic and drilling data, we calculated tectonic subsidence amounts and rates of the Wan’an Basin by backstripping. The genetic mechanism and syn-rifting process of the basin were analyzed in combination with the regional geological setting. The results reveal that the basin syn-rifted in the Eocene and early Miocene under the control of the dextral strike-slip Wan’an Fault Zone. The transtensional/extentional stresses along this fault zone may be attributed to seafloor spreading of the South China Sea (SCS) in multiple episodes. Extensive basal faults and some small initial rifts in the early Paleogene can be related to southeastward extrusion and clockwise rotation of the Indochina Block. During the Oligocene, the nearly N-S directed spreading of the SCS derived the transtensional stresses in a roughly NW-SE orientation. The basin subsided rapidly in the middle and north to form two major subsidence centers. In the early Miocene, the SCS spread again in a nearly NW-SE direction, resulting in rapid subsidence in the southern basin continuous extending until the period ∼16.3 Ma.

Geophysical Signature of the Shallow Water Flow in the Deepwater Basin of the Northern South China Sea

Shallow water flow (SWF), a disastrous geohazard in the continental margin, has threatened deepwater drilling operations. Under overpressure conditions, continual flow delivering unconsolidated sands upward in the shallow layer below the seafloor may cause large and long-lasting uncontrolled flows; these flows may lead to control problems and cause well damage and foundation failure. Eruptions from over-pressured sands may result in seafloor craters, mounds, and cracks. Detailed studies of 2D/3D seismic data from a slope basin of the South China Sea (SCS) indicated the potential presence of SWF. It is commonly characterized by lower elastic impedance, a higher Vp/Vs ratio, and a higher Poisson’s ratio than that for the surrounding sediments. Analysis of geological data indicated the SWF zone originated from a deepwater channel system with gas bearing over-pressured fluid flow and a high sedimentation rate. We proposed a fluid flow model for SWF that clearly identifies its stress and pressure changes. The rupture of previous SWF zones caused the fluid flow that occurred in the Baiyun Sag of the northern SCS.