未熟-低熟油中金刚烷烃类化合物的成因及其潜在意义研究

In the present literatures for diamondoid hydrocarbons study in petroleum geochemistry field at home and abroad, much more attention have been paid to determining the thermal maturity of organic matter in high or over-mature source rocks and the extent of oil cracking in the reserviors. Generally speaking, based on the known results in literatures, there is a close relationship between the distribution, the relative composition and the concentration variation of diamondoid hydrocarbons and the highly thermal maturation in the source rocks and crude oils. Therfore, they have usually been used as the indicators for highly thermal evolution of organic matter. However, it has been found that diamondoid hydrocarbons are very common in the immature to lower source rocks and the crude oils with immaturity and lower maturity in Subei Basin and Qaidam Basin, and could be identified easily even by full scan of GS-MS analysis. In addition, their relative compositions such as methyl adamantane index(MAI) and methyl diamantane index(MDI) in those geological samples are obviously higher compared to their thermal maturity and could not be explained by the present known viewpoints. This phenomenon may imply that the generation of diamondoid hydrocarbons in geological samples occurred during the early stage of thermal evolution of organic matter in the source rocks, and it could not be true that their relative compositions and concentration variation could be controlled completely by thermal maturation. Therfore, it is theorectically necessary and practically significant that the distribution, the relative compositions and the concentration variation of diamondoid hydrocarbons would be studied in the source rocks and the crude oils with lower thermal evolution in this program. At the same time, this research is very useful to the recognization of the geological-geochemical conditions for the generation of diamondoid hydrocarbons and the boundary condition for their geochemical application.

现阶段国内外对金刚烷类化合物的研究主要集中在确定高、过成熟烃源岩中有机质的成熟度和原油在高演化阶段遭受热裂解程度的判识上，一般认为这类化合物的分布与组成特征及浓度变化均与高演化作用密切相关，因而把它作为高、过成熟作用的标志。但是，在我们的研究中发现在有机质成熟度偏低的烃源岩和未、低成熟油中用全扫描就能容易地检测出金刚烷类化合物，而且其相对组成如甲基金刚烷指数（MAI）和甲基双金刚烷指数（MDI）却明显偏高。依据现有的观点这显然与其所处的演化阶段或所经历的热成熟作用是不相符的，这一现象揭示出地质样品中的金刚烷类化合物形成于有机质热演化的早期阶段，而且其相对组成特征也不一定完全受控于热成熟作用。因此，开展低演化烃源岩和未、低成熟油中金刚烷类化合物的分布、组成与浓度变化的研究对揭示这类特殊化合物的分布规律、形成的地质-地球化学条件和认识其地球化学应用的边界条件不但具有理论意义，同时具有实用价值。

淡水湖沼环境和咸水-半咸水环境烃源岩中金刚烷类烃的分析结果表明，只有在Ro值大于0.80%和1.0%的成熟和高成熟烃源岩中才检测到完整的C0-4烷基单金刚烷系列和C0-3烷基双金刚烷系列，且单金刚烷系列先于双金刚烷系列形成，两者在高演化阶段并存。未成熟烃源岩中则不存在此类化合物，低成熟烃源岩常可检测到三甲基或四甲基单金刚烷，但C0-2取代物因杂峰干扰严重而难以辨认。这一现象表明地质样品中金刚烷类烃是热演化作用的产物，而且它们可能是从多甲基取代的化合物开始形成，其形成过程基本不受沉积环境、有机质来源和烃源岩岩性的影响，因而它们可以作为衡量有机质热演化程度的可靠标志。.在一些未成熟和低成熟烃源岩（Ro值=0.45%-0.70%间）中，也可检测到浓度较低的完整C0-4烷基单金刚烷系列，尤其是在断裂特别发育的苏北盆地，且其甲基单金刚烷指数MAI与成熟烃源岩中相近，这与其成熟度是不相称的。这与成熟烃源岩所生油气在向上运移过程中浸染了上覆低演化烃源岩有关，研究时需加以甄别。.柴达木盆地和库车坳陷淡水湖沼相烃源岩所产原油均已成熟，但有差异。在成熟原油中仅检测到烷基单金刚烷系列，如冷湖3和4号构造上的原油，而在高成熟油和凝析油中不但检测到烷基单金刚烷系列，同时还检测到烷基双金刚烷系列，且它们的浓度正相关于其成熟度，相关参数是指示原油成熟度的可靠指标。.苏北盆地所产原油有低熟油和成熟油之分，且均能检测到完整的C0-4烷基单金刚烷系列，但都缺乏双金刚烷系列，表明它们的演化程度均没有达到高成熟阶段。对比发现，单金刚烷系列的分布与组成和浓度变化与原油成熟度之间不存在相关性，且低熟油和成熟油中MAI值相近，据此判断这些低熟油中的单金刚烷系列应该与成熟油气的贡献有关，它们应该属于低熟油和成熟油的混源油。甾萜烷生物标志物的分布与组成可以区分原油的成因类型，但不同成因类型原油中金刚烷类化合物的分布面貌几乎完全一致，这一现象表明它们是一类不受沉积环境性质和有机质来源影响的特殊化合物。