The seal bypass system of the northern Kattegat may contribute to the development of the bubbling reefs

The famous “bubbling reefs” in the Kattegat are, among other features, carbonate-cemented sandstone columns that rise to 4 meters above the seafloor. The carbonate cement is depleted in δ¹³C, indicating its formation through microbial methane oxidation. However, it is remarkable that the biogenic methane concentration in the Pleistocene sediments is particularly high in this specific area, which calls for an additional Methane source.

In this study, we test the hypothesis, that fluids from Triassic and Lower Jurassic hydrocarbon reservoirs are migrating upward, providing an alternative explanatory model for the origin of near-surface methane. This hypothesis is based on the observation that the bubbling reefs are located in a region underlain by Mesozoic hydrocarbon source rock.

High-resolution reflection seismic data collected by the University of Hamburg in 2013 and 2015 reveal a fluid bypass system similar to that described in the Skagerrak (Grob et al., 2020). Phase-reversed reflections and seismic attributes in certain sections of the Lower Jurassic and Upper Triassic (Gassum and Fjerritslev formation) indicate hydrocarbon accumulations. Localized vertical attenuation of reflection amplitudes and warped reflections suggest vertical migration pathways, commonly called pipes, partly related to faults north of the Sorgenfrei-Tornquist Zone. The present-day depth of the source rock is up to 900 m. However, before the inversion-related uplift in the Late Cretaceous – Paleocene, the source rock was about 1000 m deeper and, consequently, well within the oil and gas window.

We conclude that the assumption that thermogenic methane contributes to forming the bubbling reefs appears plausible. Since the isotopic signature also indicates biogenic gas, thermogenic gas cannot be the sole gas source.

 

 

References:

Grob, H., Seidel, E., Hübscher, C., 2020. Seismic amplitude and attribute data from Mesozoic strata in the Skagerrak (Danish-Norwegian North Sea): Indicators for fluid migration and seal bypass systems. Marine and Petroleum Geology 121, 104596, https://doi.org/10.1016/j.marpetgeo.2020.104596.

Petersen, H., Nielsen, L., Bistrup, T., Thomsen, E., 2003. Burial depth and post-Early Cretaceous uplift of Lower-Middle Jurassic strata in the Fennoscandian Border Zone based on organic maturity, Geological Survey of Denmark and Greenland Bulletin., https://doi.org/10.34194/geusb.v1.4686.