Lijun Mi

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.

Morphology, seismic characterization and sediment dynamics of the Baiyun Slide Complex on the northern South China Sea margin

A large submarine slope failure, the Baiyun Slide Complex, has been discovered in the northern South China Sea. We describe the slide complex morphology, the seismic character of its structural elements and the slide evolution based on high-quality seismic reflection and multi-beam bathymetry data. The Baiyun Slide Complex has three major slide scars that show differences in headwall and sidewall geometry, the nature of the basal shear surfaces and the internal architecture of the deposits. From these observations, we propose a four-phase emplacement model. An extrapolation of the post-slide drape thickness (60 m) gives a rough age estimate for the mass transport events of 0.3 Ma. Pore pressure models for the unfailed continental slope in the vicinity of the Baiyun Slide are based on porosity measurements at nearby Ocean Drilling Program Site 1146. They show that excess pore pressure in slope sediments is anomalously high at a depth around 93 m, most probably as a consequence of a dramatic increase in sedimentation rates over the past 1.8 Ma. This excess pore pressure is proposed to be the major preconditioning factor for the slide initiation, possibly aided by volcano-tectonic activity and gas hydrate dissociation. The unfailed slope is stable under static conditions. However, a near-field earthquake of Mw 5 would suffice to induce a slope instability at c. 93 m depth.

Baiyun Slide and Its Relation to Fluid Migration in the Northern Slope of Southern China Sea

A large-scale submarine landslide (Baiyun Slide) covering an area of 10,000 km2 was identified from the multibeam bathymetric data, high-resolution 2D and 3D seismic data acquired in the Baiyun Sag, Pearl River Mouth Basin, northern South China Sea (SCS). Numerous polygonal faults are also found below the translational domain of the Baiyun Slide. Enhanced reflections, bright spots and pull-down reflection have been illustrated from the 2D and 3D seismic data, indicating the presence of gas. The headwall scarps of the slide are located stratigraphically above the sediments where the amplitude anomalies are identified. The focused fluid flow maybe leak from the gas reservoir and migrate upward into the base of the Baiyun Slide. Though the triggering mechanism of the Baiyun Slide is still poorly known, the fluid trapped below the slide will reduce the strength of the sediments and trigger the slope failure. We propose a conceptual model of the relationship between fluid migration and slope stability.

Crustal structure and extension mode in the northwestern margin of the South China Sea

Combining multi-channel seismic reflection and gravity modeling, this study has investigated the crustal structure of the northwestern South China Sea margin. These data constrain a hyper-extended crustal area bounded by basin-bounding faults corresponding to an aborted rift below the Xisha Trough with a subparallel fossil ridge in the adjacent Northwest Sub-basin. The thinnest crust is located in the Xisha Trough, where it is remnant lower crust with a thickness of less than 3 km. Gravity modeling also revealed a hyper-extended crust across the Xisha Trough. The postrift magmatism is well developed and more active in the Xisha Trough and farther southeast than on the northwestern continental margin of the South China Sea; and the magmatic intrusion/extrusion was relatively active during the rifting of Xisha Trough and the Northwest Sub-basin. A narrow continent-ocean transition zone with a width of ∼65 km bounded seaward by a volcanic buried seamount is characterized by crustal thinning, rift depression, low gravity anomaly and the termination of the break-up unconformity seismic reflection. The aborted rift near the continental margin means that there may be no obvious detachment fault like that in the Iberia-Newfoundland type margin. The symmetric rift, extreme hyper-extended continental crust and hotter mantle materials indicate that continental crust underwent stretching phase (pure-shear deformation), thinning phase and breakup followed by onset of seafloor spreading and the mantle-lithosphere may break up before crustal-necking in the northwestern South China Sea margin.

Mass transport deposits and processes in the north slope of the Xisha Trough, northern South China Sea

Triple mass-transport deposits (MTDs) with areas of 625, 494 and 902 km2, respectively, have been identified on the north slope of the Xisha Trough, northern South China Sea margin. Based on high-resolution seismic reflection data and multi-beam bathymetric data, the Quaternary MTDs are characterized by typical geometric shapes and internal structures. Results of slope analysis showed that they are developed in a steep slope ranging from 5° to 35°. The head wall scarps of the MTDs arrived to 50 km in length (from headwall to termination). Their inner structures include well developed basal shear surface, growth faults, stepping lateral scarps, erosion grooves, and frontal thrust deformation. From seismic images, the central deepwater channel system of the Xisha Trough has been filled by interbedded channel-levee deposits and thick MTDs. Therefore, we inferred that the MTDs in the deepwater channel system could be dominated by far-travelled slope failure deposits even though there are local collapses of the trough walls. And then, we drew the two-dimensional process model and threedimensional structure model diagram of the MTDs. Combined with the regional geological setting and previous studies, we discussed the trigger mechanisms of the triple MTDs.

Continent-Ocean Transition at the Northern Margin of the South China Sea

The northern margin of the South China Sea (SCS) has particular structural and stratigraphic characteristics that are somewhat different than those described in typical passive margin models. The differences are due to poly-phase tectonic movements and magmatic activity resulting from interaction of Eurasian plate, Philippine Sea plate and the Indian-Australia plate. Based on several crustal-scale multi-channel seismic reflection profiles and satellite gravity data across the northern SCS margin, this article analyzes the structures, volcanoes, and deep crust of the continent-ocean transition zone (COT) at the northern margin of the SCS to study the patterns and model of extension there. Results show that the COT is limited landward by basin-bounding faults near shelf-slope break zone, and bounded by seaward-dipping normal faults near the oceanic basin in our seismic lines. The shallow anatomy of the COT is characterized by rift depression and structural highs with igneous rock and may include a volcanic zone or a zone of tilted fault blocks at the distal edge. Gravity modeling revealed that a High-Velocity Layer (HVL) with a thickness of 0.8–6 km is commonly present in the slope below the lower crust in the mid-northern margin of the SCS. Our study shows that the HVL is only located in the eastern portion of the northern SCS margin, based on the available geophysical data. We infer from this that the presence of an HVL is not required in the COT at the northern SCS margin. The magmatic intrusions and HVL may be related to partial melting caused by decompression of passive, upwelling asthenosphere which resulted primarily in post-rifting underplating and magmatic emplacement or modification of the crust. Based on this study we propose that an intermediate mode of rifting was active in the mid-northern margin of the SCS with characteristics that are closer to those of the magma-poor margins than those of volcanic margins.