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 conj unction 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 St ord Basin and the Viking Graben areas offshore Western Norway. Adsorbe d gases, interstitial gases, headspace gases, grain size, and inductiv ely coupled argon plasma elemental data have been investigated (Table 1). The adsorbed methane ranges from 10 to 5000 ppb. It has a uni-moda l 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 comp rising 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 th an 100,000 ppb with a median of 6990 ppb, The ratio of unsaturated to saturated alkane homologues in the interstitial and headspace gasses i s on average equal to one and show positive correlation. This rules ou t thermogenic hydrocarbons as a direct source for the headspace and in terstitial gasses. The proportion of unsaturated hydrocarbons in the a dsorbed 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 hig h contents of coarse material. Principal component analysis show that the yields, dryness, and isotopic composition of adsorbed methane corr elate positively with the contents of calcium and strontium, the Sr/Ca ratio in the sediments and the proportion of silt size material in th e sediments. We infer from this that a major component in the adsorbed gas is cogenetic with carbonates in the fine fraction, probably resul ting from in situ bacterial degradation of organic matter. Distinct an d unquestionable regional headspace gas anomalies are found within the Quaternary Kleppe Senior (KSE) formation of soft, silty marine/glacio marine clays. Outside of KSE only background levels of headspace metha ne is observed. Some of the most prominent headspace gas anomalies cor relate very well with a shallow gas accumulation in a sand body just 1 0-20 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 charact eristics 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.