Terrigenic helium in brackish groundwaters of Kuwait, probable influences from hydrocarbon resources

Abstract

The presence of helium in groundwater can shed light on its age, migration patterns, and potential recharge locations. The 4He, 3He/4He, 20Ne, and major ions concentrations in groundwater samples were analysed. He concentration in groundwaters of the Kuwait group aquifers ranged from 3.82 × 10−8 to 1.33x−6 cm3STP/g and that of Dammam formation ranged from 9.97 × 10−8 to 1.62 × 10−6 cm3STP/g. The distribution of He was inferred to be influenced by geological structures and it was found that He decreased with Cl concentration and δ18O with few exceptions. The δ18O signatures reflect mixing between aquifers. The higher He fluxes were recorded during the Last Glacial Maximum, but they rose steadily over time. Most of the north Kuwait samples, dating from 1650 to 348 yBP, had a tritiogenic component similar to the present atmosphere, as indicated by RA (Air normalized He ratios) values, which was inferred to be due to the decay of tritium derived from meteoric impact. Magnitude 2–4 earthquake epicenters occurred more frequently in areas with lower RA values. Lateral flow was predominant with insitu (He derived from rock matrix along the flow) and exogenous sources (vertical upward cross formational flow, atmospheric input and from hydrocarbon reservoirs). Groundwater samples near oil fields were found to contain more terrigenic He, suggesting its migration to shallow waters through fault planes. The presence of more He in brackish groundwater at a shallow depth may be indicative of an abundant source beneath. He excess (Heex) and air corrected He excess (ΔHe%) are indicators of non-neuclogenic and radiogenic He sources, such as the exsolution from adjoining hydrocarbon fields through structurally weaker planes.