Alteration of natural gas during leakage from a North Sea salt diapir field

Oil and gas are leaking from a salt diapir field in the Norwegian Sector of the Central North Sea. The gas is being biodegraded as it migrates to the surface. The seeping gas is pervasive in the sediments above the diapir and clearly visible on shallow and deep seismic records and on wireline logs. An exploration well was drilled to test a Cretaceous Chalk reservoir above the diapir and provided an opportunity to sample 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 isotope signature. Generally high, but variable, concentrations of CO2 were detected all the way from the surface to the salt diapir. This results from biodegradation of hydrocarbons, and particularly oil. The source of oxygen for this process is problematic since most of the migration takes place through shales and there is no obvious aquifer source of dissolved oxygen. The effect of biodegradation of gases escaping from this structure results in a comparable increase in delta(13)C for both methane and ethane (up to 10 parts per thousand), with a lesser effect on propane and less still for butane and pentane. This conflicts with published data on such effects which suggest that methane and propane should be most affected, with little effect on ethane. Isotopically light methane was found only in the very deepest samples, within the salt of the diapir itself, and this is thought to be biogenic gas trapped in the salt at the time of deposition. No evidence was found that any ''secondary'' biogenic methane is being produced during gas seepage and biodegradation at this site.