油藏中生物降解和热蚀变的叠加次生蚀变过程的模拟实验研究

Quite a few oil reservoirs in typical superimposed basins of China have superimposed two kinds of secondary alterations, including biodegradation at the early stage and thermal alteration at the late stage. Such superimposed secondary alterations can lead to significant variations in the characteristics of both bitumen (oil) composition and biomarker distribution. Hydrocarbon gas generation potential and thermal stability of crude oil may also be altered during biodegradation. Therefore, superimposed secondary alterations may lead to deviations in bitumen-source correlation, hydrocarbon gas generation potential evaluation, and hydrocarbon gas generation stage judgement. In this project, a sequence of crude oils (bitumen sand) of identical origin and various biodegradation stages will be used in both routine closed-system pyrolysis simulation experiments and gold tube closed-system kinetic simulation experiments. Catalytic hydropyrolysis will be used to release covalently bound biomarkers from thermal altered bitumen residues. The heteroatom-containing polar compounds, hydrocarbons, free biomarkers in thermally altered biodegraded oils and the covaltently bound biomarkers released from bitumen residues, will be analyzed by FT-ICR MS and GC-MS, respectively. The effect of superimposed secondary alterations on oil compositions, together with the distribution of free biomarkers and bound biomarkers, will be explored. Kinetic parameters will be calculated based on the results from gold tube closed-system kinetic simulation experiment. The effect of biodegradation on both hydrocarbon generation potential and hydrocarbon generation behavior will be explored. The achievements of this project will benefit the understanding of the compositional variations in crude oil during superimposed alterations in reservoirs, benefit the exploration in effective bitumen-source correlation methods for reservoirs that had experienced superimposed secondary alterations, and also benefit the understanding of hydrocarbon generation behavior of biodegraded bitumens in superimposed basins of China.

我国典型叠合盆地一些油藏先后经历了包括早期生物降解作用和晚期热蚀变在内的叠加次生蚀变作用的改造，导致储层沥青（原油）的化学组成和生物标志物特征发生改变；生物降解沥青的生烃潜力和热稳定性也可能发生变化，因而可能导致进行烃源对比、计算储层沥青的生烃量和判断生烃时机过程中发生偏差。本项目拟通过对同源、不同生物降解程度的储层沥青进行封闭体系下的常规热模拟实验和生烃动力学热模拟实验，并结合催化加氢热解、FT ICR-MS和GC-MS等方法手段，以揭示叠加次生蚀变作用对沥青的化学组成、游离态生物标志物特征和从固体沥青（或沥青质）释放的键合态生物标志物特征的影响及其机理；并探讨生物降解作用对沥青生烃潜力和生烃行为的影响。研究成果对于揭示我国叠合盆地油藏的叠加次生蚀变过程、对于探索适合于叠加次生蚀变改造油藏的烃源对比方法和指标、对于揭示生物降解储层沥青的生烃过程都有重要的理论和实际意义。

在国际SCI刊物Organic Geochemistry发表文章首次提出了叠加次生蚀变的概念。为了模拟叠合盆地油藏原油的叠加次生蚀变过程，以母源相同、生物降解程度不同的原油样品为实验对象，进行了黄金管封闭体系热模拟实验以及生烃动力学数值模拟。结果表明，生物降解作用会导致原油生气潜力的明显降低和产焦率的升高。中度-严重生物降解原油的产焦率在Easy%Ro为1.6–2.5范围内是正常原油的2-4倍，最终生气潜力比正常原油低约20%，因此在利用储层固体沥青的量反推原油裂解气资源量时需考虑可能的生物降解的影响。. 在生油窗范围内，不同生物降解程度原油的化学组成、生标参数都会发生变化。比如C27–29规则甾烷的分布会在中度生物降解阶段失效，而TT23/H30和H29/H30的比值会在严重生物降解阶段失效。同时，这些参数又都会被Easy%Ro≥0.91的热蚀变改造而失去指示油源的意义。相比之下，三环萜比值TT23/(TT23+TT24)既抗严重生物降解又耐热蚀变。常用的成熟度指标也会受到早期生物降解的影响，但这些影响导致的差异会在热蚀变过程中逐渐减小而趋同，即热蚀变会逐步消除掉早期生物降解的影响。例如，生物降解原油中的甲基菲指数MPI-R在Easy%Ro为0.91-2.11的范围内与正常原油差别不大。. 叠加次生蚀变作用对原油中极性化合物组成的影响非常明显，会导致有机酸的相对富集和中性氮化合物的相对减少，而热蚀变则恰好相反。脂肪酸比环烷酸更容易受到生物降解作用的影响，但热稳定性要高于环烷酸。基于不同DBE值有机酸含量的主成分分析可以快速区分样品间相对的生物降解作用和热蚀变的强度。此外，对这些原油沥青质的在线瞬时热解（Py-GC-MS）表明，沥青质结构中的键合态生物标志物在后期的热改造、生物降解这两种次生改造过程中几乎不受影响，能够很大程度保留有机质输入源的原始信息。. 总之，本项目深化了对油藏的叠加次生蚀变过程的认识，揭示了其对油气资源评价、油气源对比的影响及其机理，为可能发生了叠加次生蚀变的油气藏的精准资源评价和成藏研究提供了理论依据，因此研究成果对于叠合盆地的油气勘探具有重要的理论和实际意义。