Abstract
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. 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. 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. The subsample-scale Rb/Sr isotope values suggest that each subsample may contain I/S particles of slightly variable degrees of illitization. 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. These results are similar to the corresponding K-Ar ages (125 Ma, 389 Ma and 234 Ma). 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. 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 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. K-Ar system) and constraining the timing of hydrocarbon charge.
Highlights
In a hydrocarbon system, knowledge of the timing of the hydrocarbon charge is crucial for understanding its evolution
The 40Ar-39Ar method has recognized practical analytical improvement compared to the K-Ar method, e.g. simultaneous analysis of radioactive and radiogenic atoms on the same aliquot, which prevents uncertainties induced by sample heterogeneity[12], the 39Ar recoil associated with the irradiation procedure may result in spurious age data for micrometer-size clay minerals[13,14,15]
The results of Rb-Sr isotope dating of the five illite samples are presented in Table 2, Figs 5 and 6
Summary
Knowledge of the timing of the hydrocarbon charge is crucial for understanding its evolution. Authigenic illitic clay commonly occurs in hydrocarbon reservoirs and its diagenesis is sensitive to fluid flow[1,5]. The isochron age is yielded through the regression of Rb-Sr isotope data obtained generally by either bulk analysis or the acid-leaching technique[10,16,17,18]. Rb-Sr analysis of different acid leaching fractions of a sample may yield an isochron age for the single sample because Sr is more leachable than Rb, generating Rb/Sr fractionation between leachate and residue[10,19,20]. Ions at leachable sites may be susceptible to geo-fluid flows and Sr isotopic heterogeneity may occur between different leaching fractions, resulting in spurious ages[7]. The aim of this study is to illustrate the feasibility of this dating method through cross-checking against K-Ar ages for the samples initially reported in ref.[13]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
