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Caltech

Geology Club Seminar

Thursday, November 29, 2018
4:00pm to 5:00pm
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Arms 151 (Buwalda Room)
Isotopic evidence of natural gas at equilibrium in petroleum fields and implications for natural gas formation
Nithya Thiagarajan, Division of Geological and Planetary Sciences, California Institute of Technology,

Thermogenic gas formed in petroliferous sedimentary basins has long been thought to be the product of kinetically controlled, irreversible and thermally activated catagenesis of buried organic matter.   Here we show that inter- and intra-molecular isotopic distributions among the small alkanes (C1-C5) in natural gases provide compelling evidence that a significant fraction of such gases approach thermodynamic equilibrium, either at the conditions of gas formation or the conditions of reservoir storage. This finding implies that a metastable cyclic equilibrium — a quasi-equilibrium state arising from networks of irreversible but interconnected reactions —controls the chemistry of many natural gases.  This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature and source substrate. This finding also implies that an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation.