川西矿山梁下寒武统沥青脉油气生成时间的厘定——来自于固体沥青Re—Os同位素等时线年龄的证据

川西龙门山北段沥青脉与油苗广泛分布，尤其是下寒武统沥青脉，说明在龙门山褶皱带北段东缘具有寻找震旦系—寒武系油藏的良好前景。尽管有关该区下寒武统沥青脉的来源研究取得了一些进展，但具体来源于那套烃源岩还存在较大的分歧，同时对固体沥青脉的油气生成时间还缺乏系统研究。铼_锇(Re—Os)同位素测年法作为一种创新性的油气成藏定年方法，可对川西龙门山北段矿山梁下寒武统沥青矿的油气生成时间及其来源进行探索研究。通过系统的条件实验，建立了固体沥青、原油中沥青质提取、溶样、Re—Os纯化富集及分离等前处理技术及Re—Os同位素测年方法，主要步骤依次为样品制备、样品酸化学消解、Re和Os纯化富集与化学分离、Re和Os含量与同位素比值分析、Isoplot软件处理等。川西广元地区下寒武统沥青脉Re_Os同位素年代学及其油源分析研究表明，下寒武统固体沥青热成熟度低，油气生成时间早，约在572～559Ma之间，对应于新元古代晚震旦世，反映其来源于时代古老的烃源岩；从固体沥青Re—Os等时线年龄、初始¹⁸⁷Os/¹⁸⁸Os比值、碳同位素组成以及生物标志物等综合研究表明下寒武统沥青脉来源于震旦系陡山沱组优质烃源岩，该烃源岩于572～599Ma期间在低成熟阶段生成的稠油，进入灯影组地层中形成古稠油藏，后期由于构造抬升，震旦系灯影组古油藏破坏形成了现今的沥青脉。

Definition of petroleum generating time for Lower Cambrian bitumen of the Kuangshanliang in the west Sichuan Basin,China:Evidence from Re-Os isotopic isochron age

Bitumen veins and oil seepages,especially Lower Cambrian bitumen veins,were widely distributed in the north part of Longmenshan Mountain in western Sichuan Basin,which indicates that the prospect for Sinian and Cambrian oil reservoir is fine in the area.Although some progress has been made in the origin of bitumen veins in Lower Cambrian,there are still big differences in the source rocks.At the same time,petroleum producing time of bitumen vein is lack of systematically researches.Rhenium(Re)-Osmium(Os)isotopic chronology is a new method to apply for geochronology of hydrocarbon accumulation,and can directly date the age of source rock,oil,oil sand and bitumen.The pre-treatment means and experimental instrument on extraction and dissolution of asphaltenes,purification,enrichment and separation of Re and Os were established by a series conditional tests.Re-Os isotopic chronology consists of sample preparation,dissolution,Re and Os purification and chemical separation,Re and Os isotope mass spectrometry measurements,and data treatment of by Isoplot software.The research for Re-Os isotopic isochron age and source rock of bitumen indicates that the asphalt maturity is low,the generating time of predecessor hydrocarbon is between 572Ma and 559Ma of Late Sinian in Neoproterozoic era,and it came from the very old source rocks in the Lower Cambrian in the Kuangshanliang structure.By means of Re-Os isochron ages,initial187Os/188Os ratios,carbon isotope compositions and biomarkers,the Lower Cambrian bitumen in Guangyuan area is originated from the fine source rocks of the Doushantuo Formation in Sinian,and the hydrocarbon source rocks were of low maturity and generated a certain amount of thick oil from 572Ma to 559Ma.Subsequently,thick oil entered the Dengying layer and formed paleo-oil reservoir.Because of the late uplift,paleo-oil reservoir in the Sinian Dengying Formation was constructed and oil dispersed upward and formed into present asphalt vein.