Distribution and geological controls of the seabed fluid flow system, the central‐western Bohai Sea: A general overview

Abstract

Thorough understanding of seabed fluid flow system is of great significance to geohazard identification and hydrocarbon exploration as it can reshape the seabed and act as an indicator of subsurface hydrocarbon resources. For the first time, an integrated study of side‐scan sonar, single‐ and multi‐channel seismic data and magnetic data reveals a complex fluid flow system composed of various seafloor expressions (i.e. pockmarks and mounds) and shallow fluid migration pathways in the central‐west Bohai Sea off northeast China. Gas chimneys, mud diapirs and a dense network of Quaternary faults are the main fluid migration pathways in the shallow subsurface. The gas chimneys can be classified into three categories (Type A formed by relatively rapid gas escape, Type B formed by episodic fluid expulsion and Type C formed by fluid escape from mud diapirs), based on their distribution and seismic character, implying variability in the formation processes. Sediment remobilization and basement‐involved faults contribute to deep fluid migration into shallow depths. As a seal for up‐moving fluids, the nature and thickness of Holocene marine sediments generally decide the permeability and overburden pressure that may control the distribution of pockmarks and mounds since they are almost distributed above relatively thin Holocene deposits (thickness <20 m) and localized coarse surface sediments. The results of the interpretation gain an improved understanding of the geological processes controlling the genesis and spatial distribution of gas chimney formation and show the significance of gas chimney classification. The distribution pattern of different types of gas chimneys may signify the difference of geological background and fluid flow process, like fluid migration through faults or flow of mobilized sediments, that is crucial for the evaluation of global petroleum systems and Carbon Capture and Storage studies.

Research on the characteristics of fluid escape near the seabed in Bohai Bay

“…The data in the study area are two-dimensional single-channel seismic data, lacking the constraints of other data types, and the plane distribution characteristics of the depressions on profiles are unclear, which will be uniformly interpreted as pockmarks. Duan et al (2022) confirmed that there are indeed pockmarks caused by fluid escape in the Bohai Sea. The pockmarks identified in the study area are divided into U-shaped pockmarks, Vshaped pockmarks and asymmetric pockmarks according to the morphology of the seismic profile (Figure 2).…”

“…Duan et al. (2022) confirmed that there are indeed pockmarks caused by fluid escape in the Bohai Sea. The pockmarks identified in the study area are divided into U‐shaped pockmarks, V‐shaped pockmarks and asymmetric pockmarks according to the morphology of the seismic profile (Figure 2).…”

“…Previous studies have proposed that the fluid sources in this area are composed of shallow biogenic gas and deep thermogenic gas (Duan et al, 2022). Biogenic gas is usually generated about 100 m below the seafloor.…”

Seabed fluid flow in the China Seas

“…Recently, we reveal a complex seabed fluid flow system composed of various seafloor expressions (i.e. mounds and pockmarks) and shallow fluid migration pathways in the central-west Bohai Sea based on an integrated study of side-scan sonar, single-and multi-channel seismic data and magnetic data (Duan et al, 2022).…”