Application of sulfur and carbon isotopes to oil-source rock correlation: A case study from the Tazhong area, Tarim Basin, China

Up until now, it has been assumed that oil in the Palaeozoic reservoirs of the Tazhong Uplift was derived from Upper Ordovician source rocks. Oils recently produced from the Middle and Lower Cambrian in wells ZS1 and ZS5 provide clues concerning the source rocks of the oils in the Tazhong Uplift, Tarim Basin, China. For this study, molecular composition, bulk and individual n-alkane delta C-13 and individual alkyl-dibenzothiophene delta S-34 values were determined for the potential source rocks and for oils from Cambrian and Ordovician reservoirs to determine the sources of the oils and to address whether delta C-13 and delta S-34 values can be used effectively for oil-source rock correlation purposes. The ZS1 and ZS5 Cambrian oils, and six other oils from Ordovician reservoirs, were not significantly altered by TSR. The ZS1 oils and most of the other oils, have a V'' shape in the distribution of C-27-C-29 steranes, bulk and individual n-alkane delta C-13 values predominantly between -31% to -35% VPDB, and bulk and individual alkyldibenzothiophene delta S-34 values between 15% to 23% VCDT. These characteristics are similar to those for some Cambrian source rocks with kerogen delta C-13 values between -34.1% and -35.3% and delta S-34 values between 10.4% and 21.6%. The oil produced from the Lower Ordovician in well YM2 has similar features to the ZS1 Cambrian oils. These new lines of evidence indicate that most of the oils in the Tazhong Uplift, contrary to previous interpretations, were probably derived from the Cambrian source rocks, and not from the Upper Ordovician. Conversely, the delta C-13 and delta S-34 values of ZS1C Cambrian oils have been shown to shift to more positive values due to thermochemical sulfate reduction (TSR). Thus, delta C-13 and delta S-34 values can be used as effective tools to demonstrate oil-source rock correlation, but only because there has been little or no TSR in this part of the section. (C) 2015 Elsevier Ltd. All rights reserved.