Lithium enrichment mechanisms in oilfield waters of the Dongpu Depression, Bohai Bay Basin: insights from hydrochemistry and lithium isotopes

• Analyzed the sources of Li in formation brines of the Dongpu Depression. • Identified weathering of terrigenous rocks as the primary source of Li. • Demonstrated that evaporation and water–rock interaction are the key factors driving Li enrichment. • Revealed that Li sources and enrichment are modulated by multiple hydrogeochemical processes. Driven by the surging demand for lithium in the global renewable energy sector, Li-rich oilfield waters associated with oil, gas, and geothermal systems in sedimentary basins have emerged as a critical strategic resource, complementing salt lake and hard-rock deposits. However, the origin and enrichment mechanisms of lithium in these oilfield waters remain controversial and lack systematic understanding. This study investigates the Dongpu Depression of the Bohai Bay Basin, a typical Li-rich lacustrine basin. We systematically analyzed the hydrochemistry and lithium isotopic compositions of oilfield waters from different stratigraphic units (Es1 – Es3) and combined these with geochemical data from potential source rocks at the basin margin for source tracing. The lithium concentrations of the oilfield waters range from 4.33 to 94.56 mg/L (average 24.96 mg/L), with δ 7 Li values ranging from 4.2‰ to 17.0‰ (average 9.0‰). Our results indicate that lithium in the oilfield waters is primarily derived from the weathering of terrigenous materials during the paleolake stage and from water-rock interactions with surrounding strata during burial. Specifically, deep oilfield waters are characterized by high Li concentrations and low δ 7 Li values, controlled mainly by intense evaporative concentration during the paleo-lake period and Li desorption from clay minerals. In contrast, shallow waters exhibit lower Li concentrations and higher δ 7 Li values, resulting from dilution by meteoric water and fresh groundwater following late-stage tectonic uplift. Overall, lithium enrichment in the Dongpu Depression is the result of a multi-stage coupling process involving “terrigenous Li supply – lacustrine concentration – water–rock interaction – sealing and preservation – late-stage perturbation.” These findings provide critical theoretical support for lithium exploration in the Dongpu Depression and analogous sedimentary basins.