Origin of the unusually high dibenzothiophene concentrations in Lower Ordovician oils from the Tazhong Uplift, Tarim Basin, China

Abstract

The origin of the unusually high dibenzothiophene (DBT) concentrations in Lower Ordovician oils from the Tazhong Uplift, Tarim Basin was studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The most abundant sulfur compounds in the oils are S 1 species with doublebond equivalent (DBE) values of 1-19 and 11-48 carbon atoms. The range of the number of carbon atoms in the sulfur compounds detected by the FT-ICR MS (S 1 species with DBE=9) is about ten times larger than that for sulfur compounds detected by GC/MS (DBTs). This suggests that FT-ICR MS is a much better approach than GC/MS for characterization of DBTs in crude oils. The abundance of S 1 species with DBE=1-8 decreased with increasing thermal maturity, while the abundance of S 1 species with DBE=9 (primarily DBTs) increased. Therefore, thermal maturity is an important factor in the formation of oils with high DBT concentrations. Unusually high abundances of S 1 species with low DBE values (1-8), which include sulfide, thiophene and benzothiophene, were observed in several oils, especially the TZ83 (O 1 ) oil with high or very high thermal maturity. Thermochemical sulfate reduction (TSR) was thought to be the reason for the high abundance of these low DBE compounds in deep reservoirs, and thermochemical sulfate reduction could affect the distribution and composition of DBTs in the oils. According to the results of FT-ICR MS analysis, there are no signs that TSR is occurring or has occurred recently for most of the Lower Ordovician oils.

NSO-compounds in oil-bearing fluid inclusions revealed by FT-ICR-MS in APPI (+) and ESI (−) modes: A new method development

“…Two reasons might be responsible for this: 1) the aromatization process for the N1 compounds is faster indicating different transformation gradients for N1 versus S1 compounds or 2) the S1 compound distribution might be influenced by generation of lower DBE S1 compounds. A conceivable process for the addition of low DBE S1 compounds could be thermochemical sulfate reduction (TSR), during which low DBE organosulfur compounds are formed faster than they are destroyed, and similar cases have been described in studies reported by Li et al (2011) and Walters et al, (2015). The TSR process has been confirmed in the MX fluorite by the presence of hydrogen sulfide in fluid inclusions detected by Raman spectroscopy, and the presence of by-products from TSR e.g., calcite inclusions, solid bitumen (González-Partida et al, 2003;Tritlla et al, 2004).…”

“…compounds increased in crude oil (Li et al, 2011;Oldenburg et al, 2014). Oldenburg et al (2014) reported that low DBE sulfur compounds (lower than 5) were absent from maturity 0.68% to 1.1%.…”

“…N1 compounds, especially pyrrolic nitrogen, have been widely studied using the ESI (-) mode and relationships between N1 compound compositions and thermal maturity have been established (Hughey et al, 2004;Oldenburg et al, 2014;Poetz et al, 2014;Mahlstedt et al, 2016). The same is true for S1 compounds which have been studied in APPI (+) mode (Oldenburg et al, 2014;Walters et al, 2015) and ESI (+) mode (Li et al, 2011). In addition to the polar compounds thermal maturity is also expressed in compositional changes of aromatic hydrocarbons detected in the APPI (+) mode (Purcell et al, 2006;Rodgers and Marshall, 2007), since thermal maturation promotes aromatization of the organic matter (Tissot and Welte, 1984).…”

Geochemistry and origin of Ordovician oils in the Rewapu Block of the Halahatang Oilfield (NW China)

“…These oils cover a wide spectrum of densities, ranging from light oils to ultra-heavy oils (Table 1). Numerous studies by previous authors have summarized the geological background for the Paleozoic carbonate petroleum reservoirs in the Tabei Uplift (Zhang et al 2002;Zhang and Huang 2005;Wang et al 2008;Li et al 2011;Zhu et al 2012;Zhao et al 2015). The Tabei Uplift is one of the main structural belts and the main part of the cratonic region of the Tarim Basin (Fig.…”

Distribution and geochemical significance of phenylphenanthrenes and their isomers in selected oils and rock extracts from the Tarim Basin, NW China

“…Dibenzothiophene, dibenzofuran, and their alkylated homologues are effective molecular markers in inferring depositional environment, maturation assessment and in tracing oil charging pathways (Bao et al 1996;Wang et al 2014;Li et al 2008;Zhang and Philp. 2010;Li et al 2011Li et al , 2014. A cross-plot of alkyldibenzothiophene/alkyldibenzofuran ratio (ADBT/ADBF) versus pristane/phytane ratio (Pr/Ph) provides a powerful and convenient way to infer crude oil source rock depositional environments and lithologies (Radke et al 2000).…”