DETERMINING THE ROLE OF WATER IN HYDROCARBON-RICH CRUSTAL SYSTEMS

Water in the interconnected pore space or fractures within the shallow continental crust can play a critical role in the transport and chemical evolution of commercial hydrocarbon plays. In some systems water rock reactions produce abiogenic hydrocarbon that provide part of the energy chain within the deepest part of the crustal biosphere and in others water provides the medium for promoting biodegradation of hydrocarbons. Noble gases provide a powerful tool for identifying and quantifying these processes and, in some cases, provides temporal constraints on the system studied.

We review how noble gases are used to identify whether methane has been transported to natural gas fields dissolved within the groundwater or by buoyancy driven flow. By identifying how the water relates to the hydrocarbon phase, for example through open versus closed system equilibration, we show how it is possible to estimate the volume of water involved in hydrocarbon transport to provide basin scale hydrogeological context. We illustrate the role that groundwater plays in accumulating commercial quantities of radiogenic helium and discuss the limits of using average whole-crust steady state production rates to estimate hydrocarbon-related groundwater average residence time.

Within the crystalline basement there is growing evidence that some interconnected fracture fluids have residence times on the 10-100Myr time scale with one system [Holland et al., 2013] providing evidence for residence times >1Byr. These systems have a fluid chemistry known to support life and provide an exciting new understanding of how life in the deep crust may be nurtured and evolve, even if the planetary surface is inhospitable. Crucial in these calculations is building up evidence that these systems have accumulated radiogenic noble gases from in-situ rather than external production. Evidence for this is found within the isotopes of xenon with ^{124, 128,128}Xe/¹³⁰Xe showing deviations that correlate with recent predictions of the ancient atmosphere as well as ¹²⁹Xe/¹³⁰Xe excesses that are also difficult to account for without applying to an ancient fluid source.

Holland et al., 2013 Nature 497, 357-363