A Method for Processing Adsorbed Methane Stable Isotope Data from the Near Surface Based on Fractionation

Abstract

Molecular and isotopic fractionation of gases that are controlled by flow processes are observed in nature. Because gas flow in sedimentary basins is highly dynamic, reasonable fractionation occurs on all geochemical parameters that depend on flow. Isotopic ratios of methane adsorbed in near-surface sediments are often measured in geochemical surface exploration. It is apparent, however, that the rough data cannot be used directly to identify hydrocarbon sources in the subsurface because the isotopic composition of methane fractionates during the transition from the dissolved phase in which methane is transported into the adsorbed phase in which it is measured. This implies a knowledge of the physical processes involved in gas migration through near-surface sediments, which is derived here from one of the exceptional cases of prestationary fluid flow found in nature. This physical background is then applied to the more common stationary flow processes, in which isotopic fractionation reaches equilibrium conditions. d13C1 fractionates toward the negative and dDC1 toward the positive. The degree of fractionation depends on the methane flux in the subsurface and on the petrophysical properties of the sampled near-surface host sediment. Determination of the fractionation and the properties of the host sediment allows the methane flux from the subsurface to be calculated. The flux is observed to be stronger over gas accumulations and weaker over oil accumulations. The isotopic data processing method derives hydrocarbon fluxes and identifies source rocks from near-surface measurements and in this way allows the precise identification of hydrocarbon accumulations in the subsurface.