深层有机酸成因及稳定性研究

The low molecular organic acids occur in substantial concentrations in many aqueous geologic fluids and are thought to take part in a variety of geochemical processes, such as the dissolution of minerals, the formation of natural gas and the transport of metals in ore-forming fluids. Previous thermal simulation experiments showed that the degradation of kerogen and the oxidation of alkanes are both contributed to the formation of low molecular organic acids. However, there still remain substantial controversies and lack of systematic studies in our knowledge of the distribution, cause and stability about the organic acids in deep strata. In this study, the low molecular organic acids in deep source rock and reservoir were extracted by deionized water to analyze the distribution, quantity and isotopic characteristics of organic acids, so as to explore the distribution characteristics and the possible origins of low molecular weight organic acids in deep source rocks and reservoir. The deep oil samples will be selected to study the formation mechanism of low molecular organic acids under high temperature and pressure in semi open system. This study focuses on alkanes oxidation in the formation of organic acids and establishment of models for organic acid formation in different pathways and the basis for identification; the chemical kinetics experiments were also conducted to study the decomposition of low molecular dicarboxylic organic acids in the presence of Iron ores, the products were quantitatively analyzed to obtaining the decomposition rate and kinetic parameters of different organic acid. This study can deepen the understanding of organic acid fluid behavior in deep strata, and it is of great significance to dissect the development zone of the deep - quality reservoir

地质体中低分子量有机酸分布广泛，参与了矿物溶蚀、天然气形成、成矿元素运移搬运等多重地质作用。热模拟实验显示，干酪根降解和烷烃类化合物氧化都可以形成低分子量有机酸，但深层有机酸分布、成因和稳定性方面存在较大争议，缺乏系统研究。本项目拟对深层源岩和储集岩中低分子量有机酸开展去离子水萃取，分析其中有机酸分布特征、数量、单分子有机酸同位素特征，探索深层源岩和储集岩低分子有机酸分布特征及可能成因；选取凝析油样品开展半开放型高温高压模拟条件下低分子量有机酸形成机理方面的研究，重点探讨烷烃氧化形成有机酸地球化学特征及形成过程，建立不同路径有机酸形成的模型及判识依据；开展有含铁矿物存在条件下低分子量二元有机酸分解动力模拟实验，对反应产物进行定量分析，获取不同有机酸分解的动力学参数及分解速率。这一研究可以深化深层有机酸流体行为的认识，对于深层优质储层发育带解剖具有十分重要的意义。

深层-超深层有机酸分布特征及形成机制尚不明确，本项目选择我国不同含油气盆地的沉积岩和地层水样品，开展了有机酸分布特征、不同类型有机质热解生酸实验和不同类型有机酸稳定性模拟实验研究，发现沉积岩和深层-超深层地层水中均含有一元羧酸（甲酸、乙酸、丙酸和丁酸）和二元羧酸（乙二酸、丙二酸和丁二酸），一元酸含量明显高于二元酸，一元酸以甲酸和乙酸为主，首次在地层水中发现高浓度的甲酸。沉积岩中有机酸分布受TOC含量、埋深、储集层含气性、热成熟度和有机质类型等多因素共同控制，不同类型有机质中有机酸的分布特征明显不同，II型有机质（页岩）相对富集乙酸，而III型有机质（煤岩）相对富集甲酸；有机质热解生酸实验表明，有机质的生酸产率受控于有机质的成熟度、含氧量、热解温度和含水量，水能明显提高有机质热解生酸产率，且热解温度越高，水的贡献越显著，水在高温条件下形成的氢氧自由基（HO·），可氧化有机组分形成有机酸，溶蚀不稳定矿物导致孔隙度增加。基于有机酸的分布特征及形成机制，结合有机质热解生酸结果，建立了III型有机质有机酸地质演化模型，提出了有机酸“双峰式”演化模型。高温高压水-岩模拟实验揭示出乙二酸比乙酸更容易分解，具有较低活化能；反应体系中高pH值和钾长石的存在能提高乙酸和乙二酸分解反应的反应速率；高的流体压力和静岩压力均会抑制乙酸和乙二酸的分解，因此，我国西北低地温场的沉积盆地有利于有机酸保存，有利于深层-超深层优质储层的形成，这些研究对深层油气勘探具有重要的参考意义。