Light hydrocarbon gases in shallow sediments in the northern North Sea

Large amounts of data on hydrocarbons in shallow marine sediments (0-6 m below sea floor) have been gathered on the Norwegian sector in conjunction with surface exploration for petroleum seepage. In this paper some results from a re-investigation of five surveys is presented. The surveys include a total of 2136 shallow cores and cover mainly the Stord Basin and the Viking Graben areas offshore Western Norway. Adsorbed gases, interstitial gases, headspace gases, grain size, and inductively coupled argon plasma elemental data have been investigated (Table 1). The adsorbed methane ranges from 10 to 5000 ppb. It has a uni-modal distribution with a median concentration of 504 ppb (weight/weight). The interstitial methane ranges from less than 1 to 500 ppb. It has a uni-modal distribution with a median concentration of 13.4 ppb. There are two classes of headspace methane samples. A ''normal'' class comprising about 90% of the samples lies in the range 0.1 to 500 ppb with a median of 2.1 ppb. The second class lies in the range 500 to more than 100,000 ppb with a median of 6990 ppb, The ratio of unsaturated to saturated alkane homologues in the interstitial and headspace gasses is on average equal to one and show positive correlation. This rules out thermogenic hydrocarbons as a direct source for the headspace and interstitial gasses. The proportion of unsaturated hydrocarbons in the adsorbed gas is much lower (<1%), but also here a significant positive correlation between unsaturated and saturated alkanes is observed. The highest interstitial methane contents are found in sediments with high contents of coarse material. Principal component analysis show that the yields, dryness, and isotopic composition of adsorbed methane correlate positively with the contents of calcium and strontium, the Sr/Ca ratio in the sediments and the proportion of silt size material in the sediments. We infer from this that a major component in the adsorbed gas is cogenetic with carbonates in the fine fraction, probably resulting from in situ bacterial degradation of organic matter. Distinct and unquestionable regional headspace gas anomalies are found within the Quaternary Kleppe Senior (KSE) formation of soft, silty marine/glaciomarine clays. Outside of KSE only background levels of headspace methane is observed. Some of the most prominent headspace gas anomalies correlate very well with a shallow gas accumulation in a sand body just 10-20 m below sea floor. We conclude that gas found at the surface has migrated upwards from this sand body. The C-13/C-12 isotopic ratio for the anomalous methane (-51 to -100) ppt and its extremely dry characteristics are typical for a biogenic origin. We speculate that shallow biogenic gas may be a secondary form of carbon related to leakage from petroleum deposits at depth in this very prolific area.