Geochemical effects of incremental high-pressure squeezing on pore waters from argillaceous aquitards

Document available in extended abstract form only. The chemistry and stable isotopes of pore waters in aquitards provide valuable information on geochemical reactions, transport mechanisms, age and origins of groundwaters. High-pressure squeezing is one of the available methods used to obtain pore water from unaltered core samples of argillaceous formations, especially when in situ pore water sampling at significant depths is not possible. Low pressure squeezing (<20 MPa) is an effective method of obtaining representative samples from near-surface aquitards (e.g., lacustrine, marine, and clay till deposits; < 80 m below ground (BG)). Collecting pore water samples from argillaceous aquitards (e.g., siltstones, mud-stones, and shales) buried to greater depths may require higher squeezing pressures. Limited literature on the topic suggests that ionic concentrations of pore water can be adversely affected by elevated pressures (>20 MPa in consolidated and plastic clay-rocks and >200 MPa in indurated argillaceous rocks). Concentrations of dissolved ions, including chloride and sodium decrease as squeezing pressure is increased, while calcium and magnesium have been shown to increase. The cause of these changes has been attributed to anion exclusion effects due to the negatively charged surface area of clays and the expulsion of double-layer water. Besides, cations could also be affected by cation exchange reactions due to carbonate dissolution enhanced by oxidation processes. This study examines the effects of high-pressure squeezing on the concentrations of dissolved ions and stable isotopes of pore waters from argillaceous aquitards. Testing was conducted on core samples collected from consolidated Cretaceous clay-stones in the Williston Basin, Saskatchewan, Canada (K2 - 41 m to 260 m BG; n = 2), and consolidated Cretaceous clays from the Great Artesian Basin, South Australia, Australia (NB2 - 42 m to 289 m BG; n = 3). The core samples were squeezed at progressively increasing pressure increments from a minimum of 13 MPa to a maximum of 150 MPa. The bulk pore water samples collected at each increment were analyzed for major ions and stable isotopes δ18O and δ2H. In conclusion, these particular clayey core samples showed measurable changes in pore water concentrations resulting from increasing squeezing pressure above 20 MPa. A linear relationship was observed between decreasing concentrations of chloride and sodium and increasing pressure. The degree of change in concentration of these ions may be related to certain physical properties of the sediment, such as mineralogy and clay content. Isotopes δ18O and δ2H of the squeezed pore waters also decreased with increasing pressure, but not in all samples and not in the same linear fashion as that for chloride and sodium