Analytic Procedure and Results of an Isotope Geochemical Surface Survey in an Area of the British North Sea

Summary A sample preparation system for the determination of stable carbon isotope ratios in hydrocarbons, which are adsorbed in sediment samples, has been developed. The system consists of: (1) a degassing line, (2) a hydrocarbon separation and oxidation unit, and (3) an isotope mass spectrometer. As little as 25 µl CH 4 desorbed from a sediment sample is sufficient to achieve an accuracy of ± 1 parts/10 18 for δ 13 C determinations. Typical methane yields are 20 µl CH 4 200 g −1 sediment in the investigated area. Approximately 200 g sample material are routinely introduced into the degassing procedure. Natural degassing during or after sampling decreases the yields (i.e. the amount of hydrocarbons) at the samples. Laboratory experiments indicate that these effects can be minimized if the samples are rapidly degassed in a laboratory at the sampling site or quickly brought to low temperatures (≤ −80°C) immediately after sampling. As an example of the application of the method, the results of a geochemical reconnaissance surface survey covering the area of the Forties Field in the North Sea are discussed. Some 24 sediment cores have been taken from an area of about 5500 km 2 . Sediment samples from the lowermost part of the cores (≤2 m long) have been degassed and the amounts of methane, the molecular composition of the hydrocarbons, and the 13 C/ 12 C-isotope ratios of the methane have been determined. The geochemical data of the sediment gases, especially the δ 12 C 1 and the C 1 -yield data, favour that area for petroleum prospection, in which the large oil field is situated. The results indicate that methane migrates from deep source rocks to the surface and can be used as a guide to generating hydrocarbon source rocks and reservoirs.