Differences in the Sulfate–Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments

Chao Cao,Chengqiang Wu,Hongshuai Qi,Feng Cai,Shaohua Zhao

doi:10.3390/w13010068

Abstract

Different types of pockmarks, including single pockmarks, circular pockmarks, elongated pockmarks, chain-type pockmarks, and compound pockmarks, were identified in coastal areas around Fujian, China. The sediments associated with pockmarks were mainly silty clay to clay, with a small quantity of silt with fine sand. The sulfate content in the pore water in the sedimentary layers associated with pockmarks decreased with depth from the surface, whereas the free methane content increased with depth. The interaction between sulfate and methane is well known, but differences in the sulfate–methane transitional zone (SMTZ) were observed in different areas with different hydrologic characteristics. The sedimentary SMTZ of the offshore Zhe-Min mud wedge was shallow, at 50–70 cm below the seafloor. The sedimentary SMTZ was moderately deep (90–115 cm) in the central bay area and deep (180–200 cm) in the sandy area offshore. This variability in SMTZ depth reflects different amounts of free methane gas in the underlying formations, with a shallower SMTZ indicating a higher free methane content. The free methane had δ13C values of −26.47‰ to −8.20‰ and a biogenic hybrid genetic type. The flux of sedimentary gas from the pockmark surfaces, calculated according to Fick’s formula, was 2.89 to 18.85 L/m2·a. The shape, size, and scale of the pockmarks are directly related to the substrate type and the gas production of the underlying strata and thus vary with the sedimentary environment and development stage. Therefore, different types of pockmarks, in various phases of development, are associated with different sedimentary and dynamical conditions. A single circular pockmark is formed by a strong methane flux. As the intensity of methane flux weakens, the pockmark becomes elongated in the direction of the water flow because of long-term erosion induced by regular hydrodynamic forces. Finally, under a weak intensity of methane flux and the influence of complex hydrodynamic conditions, pockmarks merge to form large-scale compound pockmarks.

Highlights

Submarine pockmarks were first discovered in 1970 during a submarine oil and gas exploration process on the continental shelf of Nova Scotia in Canada [1]. These crater-shaped submarine depressions did not become widely known. It was not until 1987 that Hovland reported features described as hemp pits in North Sea sediments in authigenic carbonate cement and confirmed that pockmark formation is related to methane leakage events
Researchers gradually realized that pockmarks may indicate past and present submarine fluid activity
Pockmarks are craterlike depressions that appear in fine-grained sediments on the seafloor

Summary

Introduction

Submarine pockmarks were first discovered in 1970 during a submarine oil and gas exploration process on the continental shelf of Nova Scotia in Canada [1]. These crater-shaped submarine depressions did not become widely known. It was not until 1987 that Hovland reported features described as hemp pits in North Sea sediments in authigenic carbonate cement and confirmed that pockmark formation is related to methane leakage events. With the development of seismic exploration technology and marine acoustic detection technology, an increasing number of submarine pockmarks have been found globally, such as on the continental slope of northern Norway, the continental slope of equatorial West Africa, the Bering Sea, the North Sea, the continental shelf of western Canada, the Gulf of Mexico, the Black Sea, the East China Sea, and the South China

Results

Discussion

Conclusion