Chronological and geochemical constraints on the origin and timing of carbonate-cemented sandstones in the Middle Jurassic Zhiluo Formation, northeastern Ordos Basin, China

Abstract

The northeastern Ordos Basin experiences significant natural gas dissipation. As key geological archives for deciphering hydrocarbon migration-dissipation processes, the genesis and timing of carbonate-cemented sandstones (CCSs) from the Zhiluo Formation (northeastern Ordos Basin) remain controversial and lack systematic investigation. In this study, we conducted a comprehensive investigation of CCSs, integrating petrographic characterization, in situ rare earth element (REE) and U-Pb geochronological analyses, coupled with C-O-Sr isotope and fluid inclusion studies. The CCSs represent non-syndepositional diagenetic products with three stages: Calcite 1 (147.4–125 Ma), Calcite 2 (119.78–100.9 Ma), and Calcite 3 (71.84–61.9 Ma). Calcite crystallinity progressively increases from Calcite 1 to 3, with distinct geochemical signatures across stages. The Calcite 1 generation predominantly originated from chemical precipitation during supergenic diagenesis, with diagenetic fluids dominated by alkaline-enriched external surface water. The formation of Calcite 2–3 was controlled by the amalgamation of deep-sourced hydrothermal fluids and hydrocarbon migration-dissipation. Notably, Calcite 2 also records superimposed supergene alteration involving terrigenous inputs, which serves as an intermediate diagenetic product. Hydrocarbon inclusions, which are ubiquitously hosted within Calcites 2–3, exhibit high thermal maturity, and the gas within inclusions has genetic affinity with Upper Paleozoic coal-derived natural gases from the Hangjinqi-Sulige field. This genetic linkage directly and compellingly indicates that Upper Paleozoic gas migration-dissipation served as the predominant light carbon source for Calcites 2–3 precipitation. The onset of Upper Paleozoic gas migration-dissipation occurred during the late Early Cretaceous, which may have persisted until present. Approximately 44.66 % of the organic-derived calcite proportion provides conclusive evidence for large-scale gas migration-dissipation.