Free gas accumulations in basal shear zones of mass-transport deposits (Pearl River Mouth Basin, South China Sea): An important geohazard on continental slope basins

Free gas is an important trigger of instability on continental slopes, and resulting mass-wasting strata can potentially form competent seals to hydrocarbon accumulations. This work uses two high-quality 3D seismic volumes to investigate fluid accumulations at the base of mass-transport deposits in the Pearl River Mouth Basin, South China Sea. In parallel, IODP/ODP borehole data are used to document the petrophysical character of mass-transport deposits formed in similar continental-slope environments to the South China Sea. The interpreted data show gas accumulations as comprising enhanced seismic reflections that are discordant, or vertically stacked, below mass-transport deposits with chaotic seismic facies. Gas was accumulated in basal shear zones of mass-transport deposits in response to differences in capillary pressure and porosity. Free gas in Zone A covers an area of at least 18 km2. In Zone B, the free gas is sub-circular in plan view and covers an area of 30.58 km2 for a volume of sediment approaching 1.5 km3. This work is important as it shows that vertical migration of gas is not significant in mass-transport deposits from the Pearl River Mouth Basin, but up-dip migration along their basal shear zones is suggested in multiple locations. As a result, free gas can pinch-out laterally to extend 1–2 km beyond these same basal shear zones. As a corollary, we show that free gas accumulations below mass-transport deposits comprise an important geohazard and should be taken into account when drilling continental-slope successions in both the South China Sea and continental margins recording important mass wasting. Strata charged with free gas form weak layers, hinting at a novel trigger of retrogressive slope failures on continental slopes worldwide

Contourite porosity, grain size and reservoir characteristics

Acknowledgements Many people are to thank for the collection and release of the data used in this study. In particular, we thank the captain, officers and crew, and the scientific and technical shipboard parties of the different IODP expeditions utilised. We each thank our respective institutes for their ongoing support. Xiaohang Yu acknowledges financial support from the National Natural Science Foundation of China (No. 41976067).Peer reviewedPostprin

Runup of landslide-generated tsunamis controlled by paleogeography and sea-level change

Abstract: Pre-Holocene landslides and tsunami deposits are commonly observed on continental margins and oceanic islands. However, scarce evidence has thus far linked pre-historic submarine landslides to particular tsunami events. This work focuses on an 839 km3 submarine landslide that occurred in the South China Sea at 0.54 Ma. Bathymetric restorations show that the paleoshoreline at 0.54 Ma was 180–580 km to the south of its present-day location. In such a setting, the tsunami triggered by the landslide at 0.54 Ma was able to generate larger waves with shorter arrive times when compared to an equivalent landslide-generated tsunami under present-day conditions. This observation proves that tsunamis generated by submarine landslides during sea-level lowstands caused catastrophic damage to the South China Sea coast in the past, and so will do in future sea-level lowstands. This study stresses the importance of restoring paleoshorelines for detailed analysis of historic landslide-generated tsunamis

True volumes of slope failure estimated from a Quaternary mass-transport deposit in the northern South China Sea

Submarine slope failure can mobilize large amounts of seafloor sediment, as shown in varied offshore locations around the world. Submarine landslide volumes are usually estimated by mapping their tops and bases on seismic data. However, two essential components of the total volume of failed sediments are overlooked in most estimates: a) the volume of sub-seismic turbidites generated during slope failure and b) the volume of shear compaction occurring during the emplacement of failed sediment. In this study, the true volume of a large submarine landslide in the northern South China Sea is estimated using seismic, multibeam bathymetry and ODP/IODP well data. The submarine landslide was evacuated on the continental slope and deposited in an ocean basin connected to the slope through a narrow moat. This particular character of the sea floor provides an opportunity to estimate the amount of strata remobilized by slope instability. The imaged volume of the studied landslide is ~1035±64 km3, ~406±28 km3 on the slope and ~629±36 km3 in the ocean basin. The volume of sub-seismic turbidites is ~86 km3 (median value) and the volume of shear compaction is ~100 km3, which are ~8.6% and ~9.7% of the landslide volume imaged on seismic data, respectively. This study highlights that the original volume of the failed sediments is significantly larger than that estimated using seismic and bathymetric data. Volume loss related to the generation of landslide-related turbidites and shear compaction must be considered when estimating the total volume of failed strata in the submarine realm

Enrichment conditions of Hetaoyuan Formation shale oil in Biyang Depression, China

Abstract In this paper, the data of thin section, organic petrology, geochemistry, lithology and porosity were used to investigate enrichment conditions of Hetaoyuan Formation shale oil in the Biyang Depression. The results show the shale oil is a kind of low-maturity oil with medium density and low viscosity, and main factors controlling shale oil enrichment include total organic content (TOC), formation pressure, lithofacies and effective reservoirs. Shales in the fifth organic-rich interval are developed widely with large thickness (reach up to 150 m), high TOC content (2.0–4.68%), better organic matter types (type I and II1) and appropriate thermal maturity (usually larger than 0.5%), which are beneficial to generate much hydrocarbon, and volume expansion causes overpressure in source rock, especially in sections with low hydrocarbon expulsion efficiency due to less fractures. Free hydrocarbons S 1 values increase with TOC contents, and significant overpressure correlates with the larger OSI (S 1/TOC × 100) values and porosity. Well-developed calcareous shales and argillaceous shales with silty shale interlayers have better porosity, and effective pores (throat diameters greater than 10 nm) contribute to 47.5% of the total volume

Post-rift tectonic reactivation and its effect on deep-water deposits in the Qiongdongnan Basin, northwestern South China Sea

The post-rift evolution of extensional basins is traditionally thought to be dominated by thermal subsidence due to cessation of the major fault activity during the post-rift stage. The Qiongdongnan Basin, which is located in the northwestern continental margins of the South China Sea, has exhibited significant deviations from typical post-rift characteristics. In the basin, a distinct tectonic reactivation occurred since the Late Miocene (11.6 Ma). Three notable aspects of the observed tectonic reactivation during the post-rift stage include, (1) pre-existing fault reactivation, (2) multiple large-scale magmatic intrusions, and (3) rapid post-rift subsidence. During this period the basin infill significantly changed in depositional environments shifting rapidly from littoral-neritic to bathyal-abyssal environments since Late Miocene. The pre-existing fault activity along the No. 2 fault of the basin resulted in the formation of initial shelf breaks and led to the development of continental slope. In addition, the pre-existing faults along the Central Depression zone created a small sub-basin with distinctive axial negative topography characteristics formed between structural highs. These geomorphological changes led to the formation of the Central Canyon. Large-scale magmatic intrusions occurred along the fault zone in the Central Depression of the basin during the post-rift stage. Those deviations, as evidenced from pre-existing fault reactivation, magmatic intrusions, and rapid post-rift subsidence in the Qiongdongnan Basin is believed to be related to the Hainan Plume event

Late Pleistocene- Holocene architecture and sedimentary processes on the glacially influenced SW Grand Banks Slope off Newfoundland

Complex inter-relationships between alongslope and downslope sediment dispersion exist on glaciated continental margins and vary widely along continental margins depending on sediment supply and bottom current strength. In eastern Canada, proglacial sedimentation rates are relatively high on the SW Grand Banks Slope compared to the sediment starved SE Grand Banks margin, but relatively low compared to the glacially dominated Scotian margin off eastern Canada. As on other parts of the Canadian margin, its late Quaternary sediment architecture has been constructed by interacting alongslope and downslope processes. These include sediment transported by downslope glacial meltwater discharge, alongslope bottom currents and ice-rafting. Based on the analysis of sediment cores going back to 24 ka (Heinrich event 2), this study investigates fine-grained sedimentary facies and the development of variable depositional patterns on the glacially influenced SW Grand Banks Slope off Newfoundland (eastern Canada). Both turbidites and contourites show stratification, but differ in internal structure, the presence of IRD, and the nature of their upper and lower boundaries. Sandy contourites are mostly massive, occurring either as lenses or as part of the ideal bi-gradational vertical sequence with mottled silt-mud. Glacial silty contourites have distinct rhythmic laminae with the long axis of IRD parallel to bedding. Regional scale thickness variations allow discrimination between hemipelagites and muddy contourites. Depositional architecture is built through temporal and spatial coupling of the diverse sedimentary processes. During the last glacial maximum and early deglaciation, turbidity currents fed either by meltwater or oceanographic processes flowed in canyons, and a contourite depositional system developed between the canyons. The two systems interacted on inter-canyon ridges, where contourite sedimentation was not completely overwhelmed by energetic turbidity currents. In the Holocene, alongslope processes became dominant, building a drift with clearly variable thickness, in part related to seabed morphology. A conceptual model is proposed to present the key elements of depositional processes in this depositional system, and a similar evolutionary history can be expected on other distal glacial margins

The segmentations and the significances of the Central Canyon System in the Qiongdongnan Basin, northern South China Sea

The submarine canyons as the important element of the source to sink have attracted the widespread interests in studying their morphologic features, stratigraphic frames, depositional architectures, as well as the related depositional model, hydrodynamic simulation, and hydrocarbon exploration. The Central Canyon System, a large axial submarine canyon, in the Qiongdongnan Basin is developed in Neogene passive continental margin of northern South China Sea, which is paralleled to the shelf break with an "Sshaped" geometry and an NE-NEE orientation. Based on the integrated analysis of high-resolution 2D/3D seismic data and well log data, the whole canyon could be divided into three segments from west to east through its distinct morphological and depositional architecture characteristics, the head area, the western segment and the eastern segment. The canyon shows the classical U-shaped morphology in seismic profiles, and the infillings are composed of a suit of turbidite channel complex in the head area. In the western segment, the canyon demonstrates the sinuous geometry and multiple-shaped morphology in seismic profiles. Four complexes of turbidite channel and mass transport complex (MTC) are observed, which could constitute into two stratigraphic cycles. The canyon in the eastern segment shows V-shaped morphology with steep flanks and a narrow and straight course, which is composed of collapse deposits in the flanks and the sheet sand-MTC complex. The sediment supply, northern continental slope system, paleo-geomorphic characteristics and tectonic setting in the Qiongdongnan Basin are considered as the controlling factors on the development and evolution of the Central Canyon System, each of them have different influences in the three segments. The turbidite channel in the head area was triggered by the abundant sediment supply from western source together with the fault activity at 5.7 Ma of the Red River Fault. The evolution of the canyon in the western segment should be the combined effects of the turbidite channel from western source, the mass transport complex from the northern continental slope, and the paleo-seafloor geomorphology. In the eastern segment, the canyon should be constrained by the semi-closed subbasin in eastern Qiongdongnan Basin corresponding to the tectonic transformation at about 11.6 Ma. This unique submarine canyon in the Qiongdongnan Basin is suggested to be characterized by axial gravity flow, ascribing to different gravity deposits originated from different sediment supplies and the tectonic activities. (C) 2013 Elsevier Ltd. All rights reserved