An elemental and 87Sr/86Sr isotopic restitution of contaminated pore waters from sandy sediments of the Peciko gas field (Mahakam Delta, Indonesia)

Pore waters of hydrocarbon-bearing sand layers from gigantic Peciko gas reservoir located to the South of the Mahakam Delta in Indonesia were reconstructed and studied by applying the restored pore-water technique. The method is based on the Sr isotopic compositions of rock chips leached with de-ionized water aided, here, by the associated contents of major, metal and rare-earth elements (REEs). The soluble minerals were also leached with dilute hydrochloric acid and analyzed for their Sr isotopic compositions. The main founding is that many of the recovered H2O leachates were most probably contaminated during coring, which induced abnormally high metal and REE contents. The leachates considered free of pollution yield 87Sr/86Sr ratios that increase progressively with depth, which suggests a recent supply of surface waters to the upper pore waters. It seems also that only a few sedimentary horizons are interconnected at intermediate depths. Furthermore, the minerals dissolved by dilute HCl yield quite systematically lower 87Sr/86Sr ratios than the associated pore waters, which suggests a crystallization earlier than the recent flushing of the upper sediments by surface-type waters.

Automated summary

The study on pore waters from hydrocarbon-bearing sand layers in the Peciko gas reservoir in Indonesia revealed that the recovered leachates were likely contaminated during coring. The Sr isotopic ratios of the leachates increased with depth, indicating recent surface water supply to the upper pore waters. Additionally, minerals dissolved by dilute HCl showed lower Sr isotopic ratios than the associated pore waters, suggesting earlier crystallization prior to recent surface water flushing of the sediments.