Geophysical signatures associated with fluid flow and gas hydrate occurrence in a tectonically quiescent sequence, Qiongdongnan Basin, South China Sea

Abstract

Interpretation of high-resolution two-dimensional (2D) and three-dimensional (3D) seismic data collected in the Qiongdongnan Basin, South China Sea reveals the presence of polygonal faults, pockmarks, gas chimneys and slope failure in strata of Pliocene and younger age. The gas chimneys are characterized by low-amplitude reflections, acoustic turbidity and low P-wave velocity indicating fluid expulsion pathways. Coherence time slices show that the polygonal faults are restricted to sediments with moderate-amplitude, continuous reflections. Gas hydrates are identified in seismic data by the presence of bottom simulating reflectors (BSRs), which have high amplitude, reverse polarity and are subparallel to seafloor. Mud diapirism and mounded structures have variable geometry and a great diversity regarding the origin of the fluid and the parent beds. The gas chimneys, mud diapirism, polygonal faults and a seismic facies-change facilitate the upward migration of thermogenic fluids from underlying sediments. Fluids can be temporarily trapped below the gas hydrate stability zone, but fluid advection may cause gas hydrate dissociation and affect the thickness of gas hydrate zone. The fluid accumulation leads to the generation of excess pore fluids that release along faults, forming pockmarks and mud volcanoes on the seafloor. These features are indicators of fluid flow in a tectonically-quiescent sequence, Qiongdongnan Basin.

The morphologies and genesis of mega-pockmarks near the Xisha Uplift, South China Sea

“…The high amplitude is interpreted to be caused by incision and/or coarse sediment infill. In the adjacent Qiongdongnan Basin fluid expulsion structures are widespread (Wang et al, 2010). Volcanism has been active since late Miocene, and we can identify many modern volcanoes within the study area and adjacent areas based on seismic data.…”

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

“…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.…”

Geophysical Indicators of Gas Hydrate in the Northern Continental Margin, South China Sea

“…Bottom simulating reflectors (BSRs) identified from seismic reflection profiles are conventionally interpreted as indicators for gas hydrate beneath seafloor [3]. Gas hydrates-associated BSRs have been recognized from the seismic data of other geophysical studies, and their presences have been validated by drilling or coring either in accretionary wedges [4][5][6][7] or in the continental margin of the world [8][9][10][11]. The Hikurangi Margin, east of New Zealand's North Island, is a large marine gas hydrate province.…”