Possible Diagnostic Explanations to the ¹³C Enriched Crude Oils Sourced from the Cambrian-Lower Ordovician Rocks in the Tarim Basin, NW China

Zewen Liao, Hu Liu, Bin Cheng, Yankuan Tian, and Ping’an Peng
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China

The oils, recently found sourced from the Cambrian-Lower Ordovician strata rocks in the Tarim Basin, NW China, were characterized by enrichment in ¹³C, with the values of bulk oils around -28‰, which were typically heavier than those oils sourced from the Middle-Upper Ordovician strata by 3-6‰. The reversal stable carbon isotopic patterns were found between the ¹³C enriched oils and their kerogens.

Thus the carbonate and organic carbon isotope profiles of Nanhua (Cryogenian) to Cambrian strata section in Kuluketage Mountains, northeastern Tarim Basin, NW China, were studied to probe their possible implications to the ¹³C enriched crude oils. Along the section the soluble organics were generally more enriched in ¹³C than their corresponding kerogens (δ¹³C_Ker and δ¹³C_A in Fig.1), which seems as a possible evidence for the reversal stable carbon isotopic patterns between the ¹³C enriched oils and their kerogens. However the very low concentration of solubles (typically below 0.01 wt% based on the samples) cannot preclude the contribution from exogenous sources, and diagenesis (e.g., biodegradation) and/or thermal maturation are other susceptible influence factors to these soluble organics.

Anyway, covariances between different parameters in Fig.1 make some more sense. With respect to the fairly higher TOC concentration of the Nanhua strata, along with their enriched δ¹³C_Ker and δ¹³C_A, the possible relationship between Nanhua source rocks and the ¹³C enriched oils in Tarim Basin should be probed, in addition to the possible explanation from the reversal stable carbon isotopic patterns.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013