利用伊利石40Ar/39Ar法多模式年龄确定油气成藏期

Diagenetic period is dated by 40Ar/39Ar method of authigenetic illites, which filled the pores of clastic rock during diagenetic period, and has being used to determine the ages of oil and gas accumulation. The difficulty of dating comes from influence of clastic illites and nucleic recoil. This application aims to date authigenetic clay minerals with 40Ar/39Ar method and determine the key period of diagenesis and hydrocarbon accumulation with improved pretreatment methods and integrated analysis of multi-model. The rapid freezing-thawing technique frees of non-clay minerals and prevents clay minerals from alteration during extraction. A series of techniques is performed to avoid sample spraying during encapsulating and crushing in vacuo. Three statistical models are figured out: (1) The regression analysis of the plateau ages with grain sizes within a same sample shows an age of smallest grain size; (2) The regression analysis of the plateau ages with clastic contents within a same grain size shows an age free of clastic minerals; (3) The plateau ages corrected with recoil gasses provide an recoil-corrected age. The ages of authigenetic illites are concluded from comprehensive analysis of three models and comparison of K-Ar method after the influence of the recoil loss is evaluated, which are used to infer an age of the key period of diagenesis and hydrocarbon accumulation. The main progress is to date authigenetic illites of multi-genetic, multi-grain minerals fine grains by improved preprocess techniques and multi-model of statistics.

自生伊利石是碎屑岩在成岩期形成的胶结物，利用40Ar/39Ar法定年确定主成岩期，以此推断油气充注时间。定年的难点来自碎屑伊利石和核反冲的干扰。本项目拟在改进前处理技术的基础上，提出多模式综合定年方法，来确定碎屑岩的主成岩期和成藏期。拟采用快速冷冻-解冻循环碎样技术来防止非粘土矿物的混入和粘土矿物的蚀变，在真空封样-解封中采取措施以克服真空包裹的喷样问题。拟构建三种统计模式：(1)同一样品不同粒径的坪年龄作粒径回归，获得粒度趋零的年龄；(2)相同粒径不同样品的坪年龄作碎屑含量回归，获得碎屑含量趋零的年龄；(3)对坪年龄作反冲校正，取得反冲校正平均年龄。通过三种模式的综合分析及与K-Ar法年龄的对比研究推断自生伊利石的年龄，评估核照射反冲的影响，从而确定主成岩期和成藏期。主要创新点是改进前处理技术以解决提纯和核反冲问题，再结合多种统计模型来解决多成因、多颗粒和细颗粒矿物的定年问题。

（研究内容）本项目对鄂尔多斯盆地苏里格气田中二叠统储集层中的伊利石开展40Ar-39Ar法定年研究。本项目研究了粒度变化和碎屑含量变化对平均年龄（相当于坪年龄）、等时线年龄和总气体年龄的影响，比较分析了反冲气体对坪年龄的影响。（重要结果）研究结果表明，伊利石和蒙脱石单矿物分级可以获得较精确的粒度区间，多矿物的分级得到了相对稳定的粒度区间；通过平均年龄、等时线、总气体三种年龄的包络线分析、趋势分析和回归分析，都获得了相应分析的极小值年龄，认为趋势分析和回归分析是细颗粒粘土矿物混合样品的重要分析手段，其中平均年龄的分析结果最可信，总气体年龄的分析结果与之相当，等时线年龄的分析结果偏小；通过粘土成分、岩相、年龄变化及趋势分析，成岩作用东强西弱，热事件和油气充注时间160.1Ma-160.3Ma。其结论与前期分析的成藏期一致，比K-Ar法（141Ma-145Ma）更早。核反冲对于趋势分析和回归分析的影响可以忽略。（科学意义）本项目研究提出了一种多模式极值年龄，各种方法相互印证，提高了极值年龄的可信度；提出了区域趋势分析和回归分析的分析思路，通过空间信息和统计学相结合的方法，分析统一热场下极值年龄的空间变化，建立了热事件及其相伴的油气成藏模式；本项目认为统计分析方法是处理细颗粒混合多矿物定年数据的有效手段，其中趋势分析和回归分析是最重要手段，还可以避开核反冲影响的处置。