Unraveling the origin and charge of Halahatang oils, Tarim Basin

Abstract

Uncertainty about the source of the oils from the Halahatang region of the Tabei Uplift, Tarim Basin (NW China) presents an ongoing challenge to exploration. Previous analyses of several Halahatang oils showed isotopic (δ¹³C < –32 ‰) and aliphatic hydrocarbon distributions (e.g., V-shaped C₂₇-C₂₉ steranes) more typical of regional Ordovician source rocks than the Cambrian rocks that are the predominant source of petroleum of the Tarim Basin, supporting prospects for a second major regional oil source. In pursuit of a more definitive source assignment of Halahatang oils and further insight into their depositional environment and charge history, a detailed molecular appraisal of the aromatic hydrocarbon composition of 27 marine oils and complementary stable sulfur and carbon isotopic analyses were conducted. The oils were from different Halahatang wells resolved into three separate block groups, with variations to the extent molecular and isotopic data was influenced by secondary alteration (i.e., thermal maturity, biodegradation and, possibly, minor thermochemical sulfate reduction) evident among the groups. The major aromatic products of all oils were alkylated naphthalenes, phenanthrenes, dibenzothiophenes and trace levels of thiadiamondoids were detected in a few Group II and III oils. Aryl isoprenoids, typical biomarkers of an euxinic depositional environment, were also conspicuous in lower maturity Group I and II oils (Rc < 0.9 %), but absent in higher maturity oils (Rc up to 1.02 %). Molecular evidence of severe biodegradation (e.g., unresolved complex matter, 25-norterpenoids) was evident in some Group I oils, although some of these also showed coincident non-biodegradation molecular features (e.g., low MW n-alkanes) implying a mixing of biodegraded and non-biodegraded charges. The δ³⁴S values of the bulk oil and their dibenzothiophene and alkyldibenzothiophene products were generally in the range +17 to +23 ‰,although some oils impacted by biodegradation showed slightly heavier δ³⁴S values (>+26 ‰). The Group II oils were not significantly impacted by secondary processes and their aromatic signature (e.g., aryl isoprenoids), δ³⁴S data and whole oil δ¹³C values were closely correlated with regional Lower Cambrian source rocks and are atypical of Upper Ordovician source rocks. These results identify the Halahatang oils as a further representation of Lower Cambrian-derived oils of the Tabei Uplift.