Geochemical characteristics of Tertiary saline lacustrine oils in the Western Qaidam Basin,northwest China |
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| Institution: | 1. Accumulation and Development of Unconventional Oil and Gas, State Key Laboratory Cultivation Base Jointly-constructed by Heilongjiang Province and Ministry of Science and Technology, Northeast Petroleum University, Daqing 163318, China;2. Department of Applied Geosciences and Geophysics, Montanuniversitaet Leoben, Peter-Tunner-Strasse 5, 8700 Leoben, Austria;3. PetroChina Turpan-Hami Oilfield Company, Hami 839009, China;1. Wuxi Institute of Petroleum Geology, SINOPEC Petroleum Exploration and Production Research Institute, Wuxi 214126, China;2. SINOPEC Key Laboratory of Petroleum Accumulation Mechanisms (SKL-PAM), Wuxi 214126, China;3. Energy and Mineral Resources Group (EMR), Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen University, Aachen 52056, Germany;1. College of Earth Sciences, Jilin University, Changchun 130061, China;2. Department Applied Geosciences and Geophysics, Montanuniversitaet Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria;3. Key Laboratory for Oil Shale and Paragenetic Energy Minerals, Changchun 130061, Jilin Province, China |
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| Abstract: | Based on the systematic analyses of light hydrocarbon, saturate, aromatic fractions and C isotopes of over 40 oil samples along with related Tertiary source rocks collected from the western Qaidam basin, the geochemical characteristics of the Tertiary saline lacustrine oils in this region was investigated. The oils are characterized by bimodal n-alkane distributions with odd-to-even (C11–C17) and even-to-odd (C18–C28) predominance, low Pr/Ph (mostly lower than 0.6), high concentration of gammacerane, C35 hopane and methylated MTTCs, reflecting the high salinity and anoxic setting typical of a saline lacustrine depositional environment. Mango’s K1 values in the saline oils are highly variable (0.99–1.63), and could be associated with the facies-dependent parameters such as Pr/Ph and gammacerane indexes. Compared with other Tertiary oils, the studied Tertiary saline oils are marked by enhanced C28 sterane abundance (30% or more of C27–C29 homologues), possibly derived from halophilic algae. It is noted that the geochemical parameters of the oils in various oilfields exhibit regular spatial changes, which are consistent with the depositional phase variations of the source rocks. The oils have uncommon heavy C isotopic ratios (−24‰ to −26‰) and a flat shape of the individual n-alkane isotope profile, and show isotopic characteristics similar to marine organic matter. The appearance of oleanane and high 24/(24 + 27)-norcholestane ratios (0.57–0.87) in the saline oils and source rocks confirm a Tertiary organic source. |
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