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The age yielded by bulk analysis represents an average estimate of a suite of cogenetic samples, instead of a single sample, thus may be unfavorable for precious samples (e.g. 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, thus generating Rb/Sr fractionation between leachate and residue, it becomes achievable to determine the Rb-Sr isotopic composition for illitic clays in small quantities (3–4 mg).

The red dashed line is regressed without subsample B-4; (C) KQ1.

The red dashed line is regressed without subsample C-5; (D) Q1.

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.

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 regression of the remaining four subsamples yields an isochron age of 111 ± 36 Ma (2σ, Fig. There is no obvious relation between H6: Regression of the isotope data yields an isochron age of 148 ± 68 Ma (2σ, Fig. One subsample (B-4) slightly deviates from the isochron, and the regression without this point yields an identical age within uncertainty (141 ± 61 Ma, 2σ, Fig. Initial , as the negative relation between IR values and K contents shows in Fig. Because Rb has a geochemical behavior similar to K (Fig.

8C), Rb is also introduced to I/S during the illitization process (Fig. The broad positive correlation between the Sr contents and IR values of I/S indicates that the variation in Sr contents by illitization may be insignificant (Fig. Therefore, the Rb/Sr fractionation of clay samples is mainly controlled by the addition of Rb during the illitization of I/S.

In a hydrocarbon system, knowledge of the timing of the hydrocarbon charge is crucial for understanding its evolution.

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