Origin of the unusually high dibenzothiophene oils in Tazhong-4 Oilfield of Tarim Basin and its implication in deep petroleum exploration |
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| Institution: | 1. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;2. Research Center of Basin and Reservoir, China University of Petroleum, Beijing 102249, China;3. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China;4. PetroChina Tarim Oilfield Company, Kuerle 841000, China;1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China;2. PetroChina Tarim Oilfield Company, Korla, Xinjiang 841000, China;1. Key Lab of Petroleum Geochemistry, CNPC, Beijing 100083, China;2. State Key Lab for Enhancing Oil Recovery, Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;3. School of Energy Resource, China University of Geosciences, Beijing 100083, China;4. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada;1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China;2. PetroChina Tarim Oilfield Company, Korla 841000, China;1. Key Lab of Petroleum Geochemistry, CNPC, Beijing 100083, China;2. State Key Lab for Enhancing Oil Recovery, Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China;3. School of Energy Resource, China University of Geosciences, Beijing 100083, China;4. Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary AB T2N 1N4, Canada;1. Petroleum Exploration & Production Research Institute, SINOPEC, Beijing 100083, China;2. Department of Geology and Geophysics, School of Environmental Sciences, University of Liverpool, Liverpool L69 3GP, UK;3. Hangzhou Branch of Research Institute of Petroleum Exploration and Development, PetroChina, Hangzhou 310023, China;4. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China |
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| Abstract: | Unusually high dibenzothiophene (DBT) concentrations are present in the oils from the Tazhong-4 Oilfield in the Tazhong Uplift, Tarim Basin. Positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used in combination with conventional geochemical approaches to unravel the enrichment mechanisms. Significant amounts of S1 species with relatively low DBE values (0–8), i.e., sulfur ethers, mercaptans, thiophenes and benzothiophenes, were detected in three Lower Ordovician oils with high thermal maturity, which were suggested to be the products of thermochemical sulfate reduction (TSR) in the reservoir. The occurrence of TSR was also supported by the coexistence of thiadiamondoids and abundant H2S in the gases associated with the oils. Obviously low concentrations of the DBE = 9 S1 species (mainly equivalent to C0–C35 DBTs) compared to its homologues were observed for the three oils which were probably altered by TSR, indicating that no positive relationship existed between TSR and DBTs in this study. The sulfur compounds in the Tazhong-4 oils are quite similar to those in the majority of Lower Ordovician oils characterized by high concentrations of DBTs and dominant DBE = 9 S1 species with only small amounts of sulfur compounds with low thermal stability (DBE = 0–8), suggesting only a small proportion of sulfur compounds were derived from TSR. It is thermal maturity rather than TSR that has caused the unusually high DBT concentrations in most of the Lower Ordovician oils. We suggest that the unusually high DBT oils in the Tazhong-4 Oilfield are caused by oil mixing from the later charged Lower Ordovician (or perhaps even deeper), with high DBT abundances from the earlier less mature oils, which was supported by our oil mixing experiments and previous relevant investigations as well as abundant authigenic pyrite of hydrothermal origin. We believe that TSR should have occurred in the Tazhong Uplift based on our FT-ICR MS results. However, normal sulfur compounds were detected in most oils and no increase of δ13C2H6–δ13C4H10 was observed for the gas hydrocarbons, suggesting only a slight alteration of the oils by TSR currently and/or recently. We suspect that the abnormal sulfur compounds in the Lower Ordovician oils might also be a result of deep oil mixing, which might imply a deeper petroliferous horizon, i.e., Cambrian, with a high petroleum potential. This study is important to further deep petroleum exploration in the area. |
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