Tracing methane sources in shallow waters of the North Sea: Biogenic signatures and hydrocarbon transport near abandoned wells

Abstract

Numerous abandoned wells in the North Sea may provide gas, particularly methane (CH ₄ ), migration pathways by penetrating shallow, biogenic gas accumulations. However, related sampling campaigns have been largely contradictory and lack a substantial database. Research cruise MSM98 targeted the abandoned wells B18‐1, DOK 1‐X, B11‐2, THOR and D‐1 and known sedimentary gas ebullition sites at the eastern Dogger Bank, using, e.g., measurements of CH ₄ and other light hydrocarbons in the water column above. Gas samples collected with a Remotely Operated Vehicle at the ebullition sites were composed of CH ₄ , traces of ethane (C ₂ H ₆ ; 50–125 ppm) and propane (C ₃ H ₈ ; 1 ppm). Values of δ ¹³ C‐CH ₄ of −74‰ vs. VPDB and −53‰ for δ ¹³ C‐C ₂ H ₆ in conjunction with δ ² H‐CH ₄  = −201‰ vs. VSMOW indicate a biogenic origin of both types of hydrocarbons, with CH ₄ originating from microbial CO ₂ ‐reduction. The δ ¹³ C‐C ₂ H ₆ values are similar to data from sedimentary gas seepage in areas that contain high ratios of terrigenous organic matter. Of the abandoned well sites, none seems to have a prominent input of CH ₄ into the water column. The dissolved CH ₄ , C ₂ H ₆ and C ₃ H ₈ , which had a thermogenic fingerprint and temporarily occurred in the working area, originated from lateral net transport of water masses. The overall inventory and CH ₄ fluxes were significantly altered by vertical mixing and the lack of stratification. Our study indicates that leakage from abandoned wells is insignificant to the methane budget in the German Exclusive Economic Zone in the Central North Sea.