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

He, Ne and Ar concentrations, He and Ar isotopic ratios, carbon isotopic ra tios and chemical compositions of hydrocarbon gases were measured in natura l gas samples from gas-producing wells in the Indus basin, Pakistan, where no oil has ever been found. He-3/He-4 ratios are in the range 0.01-0.06 Ra (Ra is the atmospheric value of 1.38 x 10(-6)) indicating the absence of ma ntle-derived helium despite the Trias extension. Ar-40/Ar-36 ratios range f rom 296 to 800, consistent with variable additions of radiogenic argon to a tmospheric, groundwater-derived argon. Rare gas concentrations show large v ariations, from 6 x 10(-5) to 1 x 10(-3) mol/mol for He-4 and from 3 x 10(- 7) to 3 x 10(-5) mol/mol for Ar-36. In general, Ar-36 concentrations are hi gh compared to literature data for natural gas. CO2 and N-2 concentrations are variable, ranging up to 70 and 20%, respectively. Mantle-derived He is not observed, therefore CO2 and N-2 are not mantle-derived either. Hydrocar bon gas maturity is high, but accumulation efficiency is small, suggesting that early-produced hydrocarbons, including oil, were lost as well as mantl e helium. This is consistent with the generally late, Pliocene, trap format ion, and explains the high N-2 concentrations, since N-2 is the final speci es generated at the end of organic matter maturation. Based on delta(13)C d ata, CO2 originates from carbonate decomposition. Very elevated Ne-20/Ar-36 ratios are found, reaching a maximum of 1.3 (compared to 0.1-0.2 for air-s aturated water and 0.5 for air), and these high values are related to the l owest rare gas concentrations. We suggest that this highly fractionated sig nature is the trace of the past presence of oil in the basin and appeared i n groundwater. We propose a model where oil-water contact is followed by ga s-water contact, both with Rayleigh distillation for rare gas abundance rat ios, thereby generating the fractionated Ne-20/Ar-36 signature in groundwat er first and transferring it to gas later. Assuming the gas-water contact o ccurred shallower than present reservoir depths, this model explains the ge nerally high Ar-36 concentrations and low CH4/Ar-36 ratios compared to othe r studies on younger basins. It thus provides a historical perspective on f luid transfer in a sedimentary basin, where a gas accumulation may have bee n buried to greater depth since formation. Rare gas and major element data point to mixing between two gas pulses produced successively. The very CO2- N-2-rich gases are terminal products of organic matter maturation which hav e been trapped after important migration. This gas was followed by a more t ypical thermogenic gas which mixed with it. (C) 2000 Elsevier Science B.V. All rights reserved.