Focused fluid flow systems of the Zhongjiannan Basin and Guangle Uplift, South China Sea

Abstract

In this article, we document a large number of focused fluid escape structures using high quality 2D seismic reflection data and multibeam bathymetry data from a poorly known area at the intersection of the northern South China Sea (SCS) and the western SCS. Three types of focused fluid escape systems are identified and described: mud volcanoes, pipes and associated pockmarks. The mud volcanoes occur singly or as clustered groups. The overpressure driving the mud volcanism is argued herein to be related to the generation of thermogenic hydrocarbons. The clustered distributions are related to localized tectonic uplift in the basin. Pipes mainly occur within the Guangle Uplift or accompany the mud volcano clusters. The pipes located within the Guangle Uplift are attributed to carbonate dissolution caused by hydrothermal fluids. Fluids ascended through these structures and were expelled at the palaeo-seabed or present seabed forming palaeo-pockmarks and present-day pockmarks. Some 'mega-pockmarks' show evidence of enlargement due to bottom currents. The marginal basins of the SCS are petroliferous, with attention gradually shifting to the deep-water area. Our results show that fluid migration must be taken into account when assessing seabed stability. This analysis also improves our understanding the petroleum geology in the study area, and is also useful for predicating where chemosynthetic ecosystems may be located.

Pockmark activity inferred from pore water geochemistry in shallow sediments of the pockmark field in southwestern Xisha Uplift, northwestern South China Sea

“…According to the discussion in Section 5.1, the sulfates in C9 and C19 increasing depth is due to the increasing contribution of DIC derived from organic matter degradation to the overall DIC pool in pore waters. The carbon isotopic compositions of methane in natural gases (ranging from À39.9& to À34.8&) from the QDN Basin indicate that the methane is mainly thermogenic (Huang et al, 2003), which is supported by seismic structure interpreted as deep-rooted feeder system in the study area (Sun et al, 2012b). Moreover, the high TOC content in sediments is in favor of the generation of biogenic gas (d 13 C 1 is typically more negative than À60& (Whiticar, 1999).)…”

“…Due to rapid sediment loading and the resulting undercompaction, the overpressure built up throughout much of the study area. Meanwhile, high geothermal gradients accelerated the maturity of source rock, and a large amount of hydrocarbon was produced and migrated towards the seabed driven by overpressure, thereby leading to pockmark formation (Huang et al, 2003;Sun et al, 2012b).…”

“…(Sun et al, 2011). The overpressure as the driving force for fluid eruption and seepage was mainly caused by both hydrocarbon generation and disequilibrium compaction triggered by high sedimentation rates in this area (Sun et al, 2012b). Nonetheless, the sulfate consumption-related biogeochemical processes in these mega-pockmarks as well as their current activity are poorly constrained.…”

“…Extensive pockmarks have been discovered in South China Sea (SCS), including in Zhongjiannan (ZJN) Basin, Qiongdongnan (QDN) Basin, Pearl River Mouth Basin, and offshore southwestern Taiwan (Chow et al, 2000;Sun et al, 2012aSun et al, , 2012bSun et al, , 2011Wang et al, 2010). Among these, the most typical pockmarks have been discovered in western Xisha Uplift with a density of ca.…”

Canyon-confined pockmarks on the western Niger Delta slope

“…In plan-view, pockmarks display a circular to elliptical geometry of less than 1 m to more than 300 m wide (e.g., Haskell et al, 1999;Hovland and Judd, 1988;Judd and Hovland, 2007;Pilcher and Argent, 2007;Cathles et al, 2010;Sun et al, 2013). In crosssectional view, they display variable morphology from U-shaped and V-shaped depressions to truncated cones (Newman et al, 2008).…”