Velocity Structure Revealing a Likely Mud Volcano off the Dongsha Island, the Northern South China Sea

Abstract

The Dongsha Island (DS) is located in the mid-northern South China Sea continental margin. The waters around it are underlain by the Chaoshan Depression, a relict Mesozoic sedimentary basin, blanketed by thin Cenozoic sediments but populated with numerous submarine hills with yet less-known nature. A large hill, H110, 300 m high, 10 km wide, appearing in the southeast to the Dongsha Island, is crossed by an ocean bottom seismic and multiple channel seismic surveying lines. The first arrival tomography, using ocean bottom seismic data, showed two obvious phenomena below it: (1) a low-velocity (3.3 to 4 km/s) zone, with size of 20 × 3 km2, centering at ~4.5 km depth and (2) an underlying high-velocity (5.5 to 6.3 km/s) zone of comparable size at ~7 km depth. MCS profiles show much-fragmented Cenozoic sequences, covering a wide chaotic reflection zone within the Mesozoic strata below hill H110. The low-velocity zone corresponds to the chaotic reflection zone and can be interpreted as of highly-fractured and fluid-rich Mesozoic layers. Samples dredged from H110 comprised of illite-bearing authigenic carbonate nodules and rich, deep-water organisms are indicative of hydrocarbon seepage from deep source. Therefore, H110 can be inferred as a mud volcano. The high-velocity zone is interpreted as of magma intrusion, considering that young magmatism was found enhanced over the southern CSD. Furthermore, the origin of H110 can be speculated as thermodynamically driven, i.e., magma from the depths intrudes into the thick Mesozoic strata and promotes petroleum generation, thus, driving mud volcanism. Mud volcanism at H110 and the occurrence of a low-velocity zone below it likely indicates the existence of Mesozoic hydrocarbon reservoir, which is in favor of the petroleum exploration.

Analysis of acidolysis hydrocarbon anomaly in the Chaoshan Depression of the Northern South China Sea and its response to reservoir properties

“…Some of phenomenon of oil-gas leakage is indirect evidence, such as mud volcanoes and hydrates [3], [12], it can be observed that the acid-extraction hydrocarbon methane is abnormally developed in the seafloor sediment above the reservoir.…”

“…It is divided into 6 secondary tectonic units (Figure 1), namely eastern sag, western sag, northern slope, middle slope, western slope, and middle low uplift, with an overall NNE direction distribution. Well LF35-1-1 has revealed organic-rich Middle to Upper Jurassic strata [1], indicating good hydrocarbon generation potential and the conditions for the formation of various types of oil-gas reservoirs [2][3][4][5][6][7][8][9][10][11][12][13][14]. However, due to multiple tectonic events, strong uplift and erosion have occurred, resulting in the development of fault structures.…”

Acid-Extracted Hydrocarbon Anomalies and Significance in the Chaoshan Depression of the Northern South China Sea

“…The Dongsha Sea is an area in the northeast of the South China Sea comprising of thick Mesozoic strata, and the Chaoshan Depression is the largest residual depression in the area (Figure 1). The depression has undergone six stages of tectonic evolution in the Mesozoic and Cenozoic eras: (1) In the late Triassic rifting period, semi enclosed bay began to develop in the Mesozoic basin in the northeast of South China Sea; (2) During the Jurassic depression period, the largest scale transgression occurred, and the basin expanded to its maximum extent; (3) In the Late Jurassic tectonic uplift period, compaction fold and structural unconformity occurred under compression; (4) In the Early Cretaceous re-subsidence period, volcanic debris, lava flows, and continental margin debris began to rapidly fill and deposit in the basin; (5) In the Late Cretaceous tectonic inversion period, under the influence of four episodes of the Yanshan movement, the Chaoshan Depression was uplifted and denuded again, forming the second regional unconformity; and (6) In the Neogene regional thermal subsidence period, the central basin of the South China Sea reached its peak of expansion during the Miocene, and the Cenozoic basins in the northern continental margin of the South China Sea successively entered a stage of post-rift depression evolution; large-scale transgression also occurred, mainly accepting coastal and shallow marine deposits [33][34][35][36][37][38]. The DS-A structure is located in the M-low bulge and is a fault anticline structure (Figure 1), controlled by NE-, NNE-, and NW-trending faults.…”

Advanced Progress of the Geo-Energy Technology in China

“…Yan et al ( 2021) [8] used ocean bottom seismic data to perform first-arrival tomography, which showed two obvious phenomena: (1) a low-velocity (3.3 to 4 km/s) zone with a size of 20 × 3 km 2 (centering at ~4.5 km depth) and ( 2) an underlying high-velocity (5.5 to 6.3 km/s) zone of comparable size at ~7 km depth. MCS profiles showed highly fragmented Cenozoic sequences that covered a wide chaotic reflection zone within the Mesozoic strata below hill H110.…”