沥青Re-Os同位素年龄地质意义解读

Interpretation of geological significance of Re-Os isotopic age of bitumen

Re、Os亲有机质的特性使得Re-Os同位素在富有机质地质样品中得到了广泛应用;沥青由大量重组分有机质组成,对Re、Os具有较强的富集能力,并且沥青样品中Re-Os同位素封闭性较好,因此沥青Re-Os同位素已经成为当今国际上研究的一个热点。然而,沥青Re-Os同位素年龄究竟是代表烃源岩的沉积时代、油气成藏时代、还是油气藏破坏后油气运移时代,至今还没有明确的定论。本文对新疆乌尔禾沥青矿、四川矿山梁沥青矿样品进行了Re-Os同位素分析,结果显示,乌尔禾沥青矿Re-Os同位素年龄为269±9Ma,老于周围白垩系砂岩地层沉积时代,Os同位素初始比值^(187) Os/^(188 )Os(Os i)为0.367±0.028,小于二叠纪海水的Os i同位素比值,表明得到的乌尔禾沥青Re-Os同位素年龄反映的是乌尔禾油气藏的成藏时代。矿山梁沥青矿Re-Os同位素年龄为537.3±5.2Ma,老于围岩(寒武系、奥陶系、志留系地层)沉积时代,Os同位素初始比值为0.716±0.028,与早寒武世海水的Os_(i)同位素比值一致,表明得到的矿山梁沥青Re-Os同位素年龄代表烃源岩的沉积时代。结合前人发表的沥青Re-Os同位素数据,沥青Re-Os同位素等时线年龄可能反映烃源岩的沉积时代,也可能代表油气藏的形成时代,这主要取决于油气成藏过程中,具有不同Re-Os同位素特征的原油是否能够混合均匀。沥青Re-Os同位素年龄能否反映油气藏遭受破坏而发生运移的时代,主要取决于沥青的热演化程度以及油气藏破坏过程中原油的混合程度。沥青Re-Os同位素研究不仅能够为油气成藏时代的厘定提供了一种有力的工具,而且对于油气烃源岩的判别以及油气成因研究都具有十分重要的意义。

Re-Os isotopes have been widely used in organic-rich geological samples due to their organophilic characteristics.Asphalt is composed of a large amount of reconstituted organic matter,which has a certain enrichment ability for Re and Os.Re-Os isotopes in bitumen have good enrichment properties.Therefore,the study of Re-Os isotope of bitumen has become a hot topic in the world.However,it is not clear whether the Re-Os isotopic age of bitumen represents the sedimentary age of source rocks,the age of hydrocarbon accumulation,or the age of hydrocarbon migration after hydrocarbon reservoir destruction.In this study,Re-Os isotopic analysis of the Wuerhe bituminous from Xinjiang and Kueshanliang in Sichuan is carried out.The results show that the Re-Os isotopic age of Wuerhe bituminous is 269±9Ma,which is older than the sedimentary age of the surrounding Cretaceous sandstone strata.The initial Os isotope ratio of ^(187) Os/^(188 )Os(Os_(i))is 0.367±0.028,which is lower than the Os i isotope ratio of Permian seawater,indicating that the Re-Os isotopic age of Urhe bitumen reflects the accumulation age of Urhe reservoir.The Re-Os isotopic age of Kuoshanliang bituminous is 537.3±5.2Ma,which is older than the sedimentary age of surrounding rock(Cambrian,Ordovician and Silurian strata).The initial Os isotope ratio is 0.716±0.028,which is consistent with the Os i isotope ratio of the Early Cambrian seawater.It is indicated that the Re-Os isotopic age of the bitumen is the sedimentary age of the source rock.The isochron age of bitumen Re-Os isotope may reflect the sedimentary age of source rocks or the formation age of oil and gas reservoirs,which mainly depends on whether the crude oils with different Re-Os isotope characteristics can be evenly mixed in the process of hydrocarbon accumulation.At present,there is no exact evidence that the Re-Os isotopic age of bitumen can reflect the age of oil and gas reservoir migration caused by damage.The interpretation of bitumen Re-Os isotopic age provides a powerful tool for the determination of hydrocarbon accumulation age,and is of great significance for the identification of hydrocarbon source rocks and the study of hydrocarbon origin.