Basin scale natural gas source, migration and trapping traced by noble gases and major elements: the Pakistan Indus basin

摘要

He, Ne and Ar concentrations, He and Ar isotopic ratios, carbon isotopic ratios and chemical compositions of hydrocarbon gases were measured in natural gas samples from gas-producing wells in the Indus basin, Pakistan, where no oil has ever been found. ~3He/~4He ratios are in the range 0.01-0.06 Ra (Ra is the atmospheric value of 1.38 * 10~(-6)) indicating the absence of mantle-derived helium despite the Trias extension. ~(40)Ar/~(36)Ar ratios range from 296 to 800, consistent with variable additions of radiogenic argon to atmospheric, groundwater-derived argon. Rare gas concentrations show large variations, from 6 * 10~(-5) to 1 * 10~(-3) mol/mol for ~4He and from 3 * 10~(-7) to 3 * 10~(-5) mol/mol for ~(36)Ar. In general, ~(36)Ar concentrations are high compared to literature data for natural gas. CO_2 and N_2 concentrations are variable, ranging up to 70 and 20%, respectively. Mantle-derived he is not observed, therefore CO_2 and N_2 are not mantle-derived either. Hydrocarbon gas maturity is high, but accumulation efficiency is small, suggesting that early-produced hydrocarbons, including oil, were lost as well as mantle helium. This is consistent with the generally late, Pliocene, trap formation, and explains the high N_2 concentrations, since N_2 is the final species generated at the end of organic matter maturation. Based on δ~(13)C data, CO_2 originates from carbonate decomposition. Very elevated ~(20)Ne/~(36)Ar ratios are found, reaching a maximum of 1.3 (compared to 0.1-0.2 for air-saturated water and 0.5 for air), and these high values are related to the lowest rare gas concentrations. We suggest that this highly fractionated signature is the trace of the past presence of oil in the basin and appeared in groundwater. We propose a model where oil-water contact is followed by gas-water contact, both with Rayleigh distillation for rare gas abundance ratios, thereby generating the fractionated ~(20)Ne/~(36)Ar signature in groundwater first and transferring it to gas later. Assuming the gas-water contact occurred shallower than present reservoir depths, this model explains the generally high ~(36)Ar concentrations and low CH_4/~(36)Ar ratios compared to other studies on younger basins. It thus provides a historical perspective on fluid transfer in a sedimentary basin, where a gas accumulation may have been buried to greater depth since formation. Rare gas and major element data point to mixing between two gas pulses produced successively. The very CO_2-N_2-rich gases are terminal products of organic matter maturation which have been trapped after important migration. This gas was followed by a mover typical thermogenic gas which mixed with it.