Helium Isotopic Constraints on Terrestrial Degassing and the Transcrustal Transport of Mantle Methane. Second Annual Report 1 July 1988-30 June 1989

摘要

The research is directed towards an evaluation of the crustal degassing process, its length- and time-scale, and its geologic and tectonic controls. The improved understanding of the crustal porefluid transport processes determined in the project will provide new insight with regard to the traditional gas resources and the techniques employed to assess their potential, their genesis and the mechanisms of their accumulation and transport to geologic traps. The accumulation of He and Ar in the Great Artesian Basin (Australia) indicate that a large crustal degassing flux dominates the accumulation of these gases in the reservoir system. These fluxes are equivalent to the whole crustal production rate from U, Th and K. The fluxes appear to be decoupled and discontinuous. The magnitude of these fluxes is confirmed with an atmospheric balance. The results of porefluid sampling and analysis at the Cajon Pass scientific drillhole indicate that the mechanisms of crustal fluid mass transport are indeed discontinuous and very rapid compared to predicted and expected Darcy flow models. The decoupling and the discontinuity of these fluxes is probably controlled by mechanical and tectonic processes although the inhomogeneity of U and Th distribution contribute significantly to an enhancement of He release over Ar release.