Distinguishing Cambrian from Upper Ordovician source rocks: Evidence from sulfur isotopes and biomarkers in the Tarim Basin

摘要

The reported source rocks for the abundant petroleum in the Tarim Basin, China range from Cambrian to Lower Ordovician and/or Upper Ordovician in age. However, the difference between the two groups of source rocks is not well characterized. In this study, pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of CrCl2 reduction and grinding. The kerogen and coexisting pyrite samples were then analyzed for delta S-34 values. Results show that the kerogen samples from the Cambrian have delta S-34 values between +10.4 parts per thousand and +19.4 parts per thousand. The values are significantly higher than those from the Lower Ordovician kerogen (delta S-34 of between +6.7 parts per thousand and +8.7 parts per thousand), which in turn are generally higher than from the Upper Ordovician kerogen samples (delta S-34 of between -15.3 and +6.8 parts per thousand). The associated pyrite shows a similar trend but with much lower delta S-34 values. This stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil delta S-34 values from other basins, suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite delta S-34 values. The relatively high delta S-34 values in the Cambrian to Lower Ordovician source rocks are associated with abundant aryl isoprenoids, gammacerane and C-35 homohopanes in the extractable organic matter, indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification. Compared with the Upper Ordovician, the Cambrian to Lower Ordovician source rocks show abundance in C-28 20R sterane, C-23 tricyclic terpanes, 4,23,24-trimethyl triaromatic dinosteroids and depletion in C-24 tetracyclic terpane, C-29 hopane. Thus, delta S-34 values and biomarkers of source rock organic matter can be used for distinguishing the Cambrian and Upper Ordovician source rocks in the Tarim Basin.