RECOGNIZING LINKS BETWEEN NATURAL GAS STABLE ISOTOPES AND TECTONIC PROCESSES IN THE APPALACHIAN (USA) AND INDUS (PAKISTAN) BASINS

The stable isotopes of natural gas components, including noble gases, provide information regarding the origin, thermal history, migration, and mixing of fluids in sedimentary basins. Isotopes also reflect rock fluid responses to tectonics. Two case histories, one from the Appalachian basin USA and one from the Kirthar fold belt of the Indus basin in Pakistan, are instructive.

Paleozoic gases produced in northeast Pennsylvania are overmature. Hydrocarbons exhibit full carbon isotope reversals (δ¹³C₁ > δ¹³C₂ > δ¹³C₃). Carbon isotope reversals result from alteration effects during deep burial. Hydrogen isotopes reversals (δ²H_C1 > δ²H_C2) due to high-temperature hydrogen isotope exchange between CH₄ and water also characterize most of these gases. However, some Marcellus Formation gases produced at the Allegheny Structural Front initially exhibit normal hydrogen isotope distributions (δ²H_C1 < δ²H_C2) which shift to reversed distributions over time. One plausible explanation for the observed δ²H variations is retrograde isotope exchange during cooling of subsurface temperatures during episodes of uplift. Experimental evidence shows that the rate and direction of hydrogen isotope exchange between methane and water is highly sensitive to the rate of exhumation during uplift, particularly as the rocks pass upward from maximum burial through temperatures between 130 and 160°C. Hydrogen isotope trends in Marcellus gases produced along the Allegheny Structural Front may reflect uplift and cooling variations related to individual structures and to the kinematics of discrete faults and folds. Noble gas systematics support this interpretation.

Natural gases produced from Cretaceous Pab Sandstone traps in the Kirthar fold belt contain variable concentrations of N₂ and CO₂ respectively attributed to organic matter maturation and thermal carbonate decomposition. Nitrogen and noble gas isotope systematics confirm that the N₂ is of late catagenetic organic origin. However, the CO₂/³He ratios of the gases range from 9.31 x 10⁸ to 3.66 x 10⁹, values that are within or below the MORB range of 1 x 10⁹ to 1 x 10¹⁰. CO₂ produced from the Pab Sandstone reservoirs is unequivocally derived from a mantle source. Igneous intrusives mapped within Cretaceous source rocks south of the gas fields are the likely source of magmatic CO₂.