![]() ![]() Several long-lived radiometric isotope systems, including potassium-argon (K-Ar) and rubidium-strontium (Rb-Sr), are hosted in illitic clay, and these isotope systems can document the timing of clay diagenesis and constrain fluid flow history 3, 6, 7, 8, 9, 10. Authigenic illitic clay commonly occurs in hydrocarbon reservoirs and its diagenesis is sensitive to fluid flow 1, 5. Emplacement of hydrocarbon in a porous reservoir alters its chemical condition and affects mineral diagenesis 1, 2, 3, 4, 5. In a hydrocarbon system, knowledge of the timing of the hydrocarbon charge is crucial for understanding its evolution. K-Ar system) and constraining the timing of hydrocarbon charge. The outcomes of this study show the robust potential of Rb-Sr clay subsample geochronology for cross-checking isotopic ages yielded by other systems (e.g. The Rb-Sr isotope analyses of the other two samples (YM35-1 and Q1) that did not yield isochron ages suggest the conditions for producing isochrons were not satisfied, which may be caused by disturbance of the isotope system by a post-charge hydrothermal event. The isotopic ages are consistent with the timing of hydrocarbon charge which varies in different drillholes as constrained by basin modelling, indicating that a closed-system behavior is attained by the hydrocarbon charge that inhibits the illitization of I/S. These results are similar to the corresponding K-Ar ages (125 Ma, 389 Ma and 234 Ma). Three of the analyzed samples (H6, KQ1 and TZ67) generated Rb-Sr isochron ages of 141 ± 61 Ma, 332 ± 32 Ma and 235 ± 8 Ma (errors quoted at 2σ), respectively. The subsample-scale Rb/Sr isotope values suggest that each subsample may contain I/S particles of slightly variable degrees of illitization. The 87Rb/ 86Sr ratio of clays is fractionated mainly by the addition of Rb during the illitization of mixed-layer illite/smectite (I/S), which is the dominant clay species in the Tarim Basin samples. This study applied the isotope-dilution thermal ionization mass spectrometry technique for small samples (3–4 mg) to conduct illite Rb-Sr isotope dating of five illitic clay samples from the Silurian bituminous sandstone (SBS) intersected by five drillholes in the Tarim Basin, NW China. Illitic clay is ubiquitous in clastic hydrocarbon reservoirs, and the host for several radiometric isotopes such as the potassium-argon (K-Ar) and rubidium-strontium (Rb-Sr) systems. ![]()
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