Quartz‐Helium Method to Estimate Fluid Flow in Thick Aquitards, Gunnedah Basin, Australia

Abstract

The hydraulic integrity of aquitards is generally assumed and relies on a few core-scale permeability measurements, drill-stem tests, or textbook values. This approach is because hydraulic data across the full aquitard thickness is generally lacking. Proper assessment of aquitard integrity should be studied at the formation (spanning its entire thickness at a single point) or regional (formation properties at multiple locations throughout the basin) scale. One formation-scale approach uses environmental tracers and advection-dispersion modeling to constrain fluid flow rates. This study demonstrates the use of helium concentrations in quartz as a method of constraining the rate of fluid flow in a 520-m thick aquitard in the Gunnedah Basin, NSW, Australia. Quartz was separated from existing core samples in the Watermark and Porcupine Formations at depths from 750 to 1200 m. The helium was released from these samples by heating and select samples were impregnated with helium to determine the rate of helium diffusion through the quartz. One-dimensional advection-dispersion modeling of the helium profile accounting for diffusive helium exchange between quartz and pore water revealed, that (1) vertical fluid velocity has been on the order of 0.02 mm/year or less for tens to thousands of years, (2) helium is in equilibrium between quartz and pore water, and (3) the helium profile is transient indicating that helium concentrations in the underlying Maules Creek Formation has varied over geological time. Further modeling identified aquitard conditions (thickness and temperature) for which equilibrium exists, a precondition for deriving formation-scale permeability.

An Integrated Approach of Numerical Well Test for Well Intersecting Fractures Based on FMI Image

“…The Gunnedah basin is unconformably located on the Permian or older basement and partially unconformably covered by sedimentary and volcanic rocks of the Jurassic and Cretaceous Surat Basin sequence [36]. The large-scale magmatic intrusion occurred in Late Triassic to Early Jurassic [37,38]. Figure 1 shows the geological map of the basin and the position of the target well used for numerical simulation.…”

Mineral Crushing Methods for Noble Gas Analyses of Fluid Inclusions

“…Noble gases enclosed in rock minerals provide important insights about different evolutionary stages of minerals or rock formations and of geological processes, such as pressure-temperature conditions during ore/rock formation, the nature, provenance, and evolutionary history as well as ages of geofluids [25]. 40 Ar- 39 Ar chronology acts widely as a main application field (e.g., [26,27]). The main noble gas isotope ratios used for the identification of fluid inclusion origins in rock material representative of different geoscientific study areas (e.g., "crustal rock studies," "mantle rock studies," "ore rock studies," extraterrestrial rock studies," or "deepwater studies") are shown in Figure 1.…”

“…As a result, He isotope systematics are relatively well understood and have been described in many studies [28][29][30]. The combination of noble gas analysis and halogens of irradiated samples has been successful in studying 40 Ar/ 39 Ar age systematics [31]. Analysis of halogens (Cl, Br, I) in fluid inclusions can be also accomplished by measuring irradiationproduced stable noble gas isotopes.…”

“…Analysis of halogens (Cl, Br, I) in fluid inclusions can be also accomplished by measuring irradiationproduced stable noble gas isotopes. The method is based on sample irradiation and neutron activation of halogens that produce noble gas isotopes such as 38 ArCl, 80 KrBr, 128 XeI, or 39 ArK as proxy isotopes for 40 Ar- 39 Ar dating [32]. The main ultravacuum crushing method is made up of a Nupro valve type crusher (screw-type crushing systems) for noble gas analysis for 40 Ar- 39 Ar dating and is often combined with heating experiments.…”

“…The method is based on sample irradiation and neutron activation of halogens that produce noble gas isotopes such as 38 ArCl, 80 KrBr, 128 XeI, or 39 ArK as proxy isotopes for 40 Ar- 39 Ar dating [32]. The main ultravacuum crushing method is made up of a Nupro valve type crusher (screw-type crushing systems) for noble gas analysis for 40 Ar- 39 Ar dating and is often combined with heating experiments. The different crushing systems used in relation to the above-mentioned applications are summarized in Figure 2.…”