ALTERATION OF NATURAL-GAS DURING LEAKAGE FROM A NORTH-SEA SALT DIAPIRFIELD

Oil and gas are leaking from a salt diapir field in the Norwegian Sect or of the Central North Sea. The gas is being biodegraded as it migrat es to the surface. The seeping gas is pervasive in the sediments above the diapir and clearly visible on shallow and deep seismic records an d on wireline logs. An exploration well was drilled to test a Cretaceo us Chalk reservoir above the diapir and provided an opportunity to sam ple the gas along the seepage pathway. Carbon isotope ratios of gases released from sealed cuttings taken from the gas cloud interval above the diapir were determined to test the hypothesis that biodegradation of leaking hydrocarbons produces secondary methane with a biogenic iso tope signature. Generally high, but variable, concentrations of CO2 we re detected all the way from the surface to the salt diapir. This resu lts from biodegradation of hydrocarbons, and particularly oil. The sou rce of oxygen for this process is problematic since most of the migrat ion takes place through shales and there is no obvious aquifer source of dissolved oxygen. The effect of biodegradation of gases escaping fr om this structure results in a comparable increase in delta(13)C for b oth methane and ethane (up to 10 parts per thousand), with a lesser ef fect on propane and less still for butane and pentane. This conflicts with published data on such effects which suggest that methane and pro pane should be most affected, with little effect on ethane. Isotopical ly light methane was found only in the very deepest samples, within th e salt of the diapir itself, and this is thought to be biogenic gas tr apped in the salt at the time of deposition. No evidence was found tha t any ''secondary'' biogenic methane is being produced during gas seep age and biodegradation at this site.