塔里木盆地轮南油田奥陶系和三叠系原油碳同位素偏移的古环境解释

UNUSUAL DISTRIBUTION OF STABLE CARBON ISOTOPE COMPOSITIONS FOR N-ALKANES OF ORDOVICIAN AND TRIASSIC CRUDE OILS FROM LUNNAN OILFIELD, TARIM BASIN, NW CHINA: PALEOENVIRONMENTAL IMPLICATIONS

通过对轮南14井正构烷烃和类异戊二烯烃的单体联同位素分析,证实该井奥陶系和石炭系油藏具有相同或相似的油源,均属奥陶系源岩产物;而三叠系油藏与奥陶系、石炭系油藏具有较大的碳同位素差异,反映其油源不同;无论是正构烷烃还是类异戊二烯烃的单体碳同位素值,均呈现出三叠纪油藏较轻、奥陶纪原油较重的分布,这与地史上原油碳同位素的长期变化规律相吻合。对于奥陶纪和三叠纪之间如此大的时间跨度,不宜于以常规的简单的母源、成熟度和沉积环境方面的差异来解释上述“异常”分布,而应该结合大气CO_2浓度及生物生长速率等环境变化因素来解释。资料表明,地史中全球范围内的古环境(大气CO_2浓度变化和藻类生长速率)演变共同造成了奥陶纪的正向变化和三叠纪的负向变化。

Compound specific carbon isotope analysis was conducted for crude oils collected from a drilling well and the entire oilfield at Lunnan, Tarim Basin, NW China, and the results demonstrated that there was an unusual distribution of δ13C showing that carbon isotope was enriched in Ordovician oils and depleted in Upper Triassic oils. This unusual distribution is consistent with the distribution and evolution of carbon isotope of crude oils from all over the world in geological history, and cannot be explained by simply utilizing conventional models based on the differences in organic facies or sources, maturity, deposi-tional environment, or migration fractionation, etc. . in consideration of the great span in geologic times-cale between Ordovician and Triassic periods. It is illustrated from a survey of geochemical literature that the unusual distribution of stable carbon isotope of crude oils was associated with the global paleoenviron-mental changes such as in atmospheric CO2 and algae growth rates, etc. , and was indeed resulted from the combination of the positive excursion of δ13C values in Ordovician period and the negative excursion in Triassic period. Crude oils, as a product of sedimentation and a kind of fossil fuels, recorded the paleoenvir-onmental and paleoclimatical changes in geological history, their stable carbon isotope is also controlled by these changes.