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

Abstract

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 H 2 O leachates were most probably contaminated during coring, which induced abnormally high metal and REE contents. The leachates considered free of pollution yield 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ratios than the associated pore waters, which suggests a crystallization earlier than the recent flushing of the upper sediments by surface-type waters. • Pore waters of gas-bearing sands from Peciko reservoir in Indonesia were reconstructed. • The method includes the 87 Sr/ 86 Sr ratios and the major, metal and REEs of H 2 O leachates. • The H 2 O leachates with high metal and REE contents were contaminated during coring. • Only a few sand units seem to be connected at intermediate depths. • The minerals dissolved by HCl are older than recent fluid supplies in the upper sediments.