Thermochemical sulfate reduction (TSR) causes increasing of sulfur content in the oils, reduction of oil quality, and produces a toxic and corrosive H 2 S which may take great adverse effect on health and equipment. Although there are several ways to identify presence of TSR in gas field, yet in the case of oil with low H 2 S content, it is still very difficult to identify TSR. 2-thiaadamantanes is one of the most effective indicators of TSR. This paper for the first time discovers 2-thiaadamantanes in the oil from Well TZ-83 in Tarim Basin. A silver nitrate silica gel chromatography was used to enrich the oil from low 2-thiaadamantanes content to the level that GC/MS and GC/MS/MS can detect. 2-thiaadamantanes were assigned by comparison of mass spectra with published data, and the fragmental ions in mass spectra were confirmed by MRM mode of GC/MS/MS. H 2 S gas produced in Tazhong area is thought to be generated from TSR since 2-thiaadamantanes were present in the oil.

Dibenzothiophene series is one of the most important compositions in crude oil, which generated under multiple geological and geochemical processes. The relationships between dibenzothiophene series and other biomarkers (C29 αα sterane 20R, and C28 triaromatic sterane 20R), combined with the research of thermochemical sulfate reduction (TSR), biodegradation, geological ages, and oil source in Paleozoic oil in Tazhong area, indicated that there are three control factors for high concentration of dibenzothiophene series. First, both middle-upper Ordovician and lower Ordovician-Cambrian source rocks are marine carbonate sedimentary rocks which could produce abundant organic sulfur compounds including dibenzothiophene series. Second, biodegradation could cause the enrichment of organic sulfur compounds. In addition, sulfate reducing bacteria was able to transfer hydrocarbons and S in SO42− in oil-bearing reservoir water into organic sulfur compound. It might be the main path to produce dibenzothiophene series. Third, sulfur compounds might have been formed by TSR, because S of SO42− in oil-bearing reservoir water could also be transferred to H2S and organic sulfur compounds under high temperature, which might result in increase of dibenzothiophene series in crude oil.

According to the present research, the cage structure makes adamantane very stable to resist cracking even at a higher temperature, so its concentration will be increased with temperature rising. But adamantane quantitative analysis of oil in Lunnan area shows a poor correlation between its concentration and geotemperature. Instead, it has a good relationship with maturation parameters such as the content of C 29 ααα 20R sterane and C 28 triaromatic steroid 20R as well as Ts/Tm. The reason is that the oil and cracked products generated in deep zone upwards charged in oil layers in low maturity and then turned into mixed oil which had higher concentration of Ts, adamantine etc. So it makes the parameters such as Ts/Tm ratio and adamantine concentration elevate. The incorporation a great amount of low molecular hydrocarbons diluted C 29 ααα 20R sterane and C 28 triaromatic steroid 20R, which resulted in the parameters of the mixed oil to indicate a higher maturation. Oils with higher adamantane concentration are mainly distributed on Lunnan fault-host zone, Sangtamu faulthost zone and Jilake structure zone. Cracked oil generated in deeper bed migrated along fractures which acted as channels upwards into the reservoir, and so adamantane content of the mixed oil increased. The source of Paleozoic oils in Tarim basin is involved only in two beds: lower Ordovician-Cambrian and middle-upper Ordovician. But according to the data in this paper, there is no obviously difference in adamantane concentration of oils from the two layers.

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