ABSTRACT
Approximately 350 sediment samples were taken in the central part of the North Sea, and hydrocarbon composition, methane (C1) and ethane (C2) yields, and carbon isotope composition of the methane (δ13C1) adsorbed in the sediment were determined. Methane generation or oxidation by bacterial activity can not be deduced from the analytical data. This fact is explained by the assumption of a threshold of methane concentration in sediments below which methane oxidizing bacteria are inactive, and that this threshold was not reached in the area investigated. Background gases in the surface sediments, characterized by low C1 yields and δl3C1 > − 37 ppt can be distinguished from thermal sediment gases which are obviously derived from a hydrocarbon generating source rock in the subsurface. Source rock data, obtained from several wells situated on a cross section through the investigated area, were compared to the geochemical patterns of the sediment gases and used to define profile sections with “good” (δl3C1 ≈ − 40 ppt, yield C1 ≈ 217 ppb) and “poor” source rock properties (δ13C1 ≈ -33 ppt, yield C, ≈ 24 ppb). An assessment of the source rock potential of the whole area under investigation has been carried out using the geochemical data of the surface sediment gases and taking into account that the C1 and C2 yields are generally higher in clay sediments than in sand samples. The number of producing wells was related to the number of all wells (271) in the areas characterized by surface geochemistry as good or poor source rock areas. The comparison showed that the relative number of producing wells is higher in areas geochemically characterized as good. Hydrocarbons in surface sediments of “good” North Sea areas are not generated by bacteria at shallow depth; they are formed thermally from the organic carbon of deep source rocks and migrate to the surface. Therefore they are indicative of thermal hydrocarbon generation in the subsurface.
