Correlating biodegradation to magnetization in oil bearing sedimentary rocks
Correlating biodegradation to magnetization in oil bearing sedimentary rocks
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DOI：
10.1016/j.gca.2013.03.008
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发表时间：
2013-07
影响因子：
5
通讯作者：
S. Emmerton;A. Muxworthy;M. Sephton;M. Aldana;V. Costanzo-Álvarez;G. Bayona;W. Williams
中科院分区：
文献类型：
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作者：
S. Emmerton;A. Muxworthy;M. Sephton;M. Aldana;V. Costanzo-Álvarez;G. Bayona;W. Williams
以前曾提到碳氢化合物与其磁性特征之间的关系，但这是第一次将碳氢化合物伴生样品的大量地球化学和磁性数据结合起来的研究。我们报告了一项详细的研究，确定了来自哥伦比亚、加拿大、印度尼西亚和英国的含油沉积单元中磁性矿物学和石油生物降解之间的联系。地球化学资料表明，所有油样都来自于贫氧环境中沉积的成熟的II型干酪根。在所有样品中，生物降解在一定程度上都是明显的，并通过细菌将脂肪烃转化为极性组分而导致石油质量下降。用磁化率和磁性矿物学方法对样品的油组分含量和生物降解状态进行了比较。磁化率的显著降低与原油质量和可提取有机质的数量下降有关。进一步的磁学特征表明，优质原油以拟单区磁铁矿为主，劣质原油以较大的拟单区至多区磁铁矿和赤铁矿颗粒为主。因此，随着原油质量的下降，多域磁铁矿逐渐占优势，并出现赤铁矿。结果表明，生物降解是一个双重过程，首先，脂肪烃被脱除，从而降低了原油的品质；其次，磁性特征既产生又被破坏。这种复杂的关系可能解释了为什么争议一直困扰着之前解决磁性和碳氢化合物矿床之间联系的尝试。这些发现加强了细菌在石油系统中的重要性，并为利用磁化作用作为一种可能的勘探工具来识别地下储集层和碳氢化合物运移的新指标提供了一个平台。
A relationship between hydrocarbons and their magnetic signatures has previously been alluded to but this is the first study to combine extensive geochemical and magnetic data of hydrocarbon-associated samples. We report a detailed study that identifies a connection between magnetic mineralogy and oil biodegradation within oil-bearing sedimentary units from Colombia, Canada Indonesia and the UK. Geochemical data reveal that all the oil samples are derived from mature type-II kerogens deposited in oxygen-poor environments. Biodegradation is evident to some extent in all samples and leads to a decrease in oil quality through the bacterially mediated conversion of aliphatic hydrocarbons to polar constituents. The percentage of oil components and the biodegradation state of the samples were compared to the magnetic susceptibility and magnetic mineralogy. A distinct decrease in magnetic susceptibility is correlated to decreasing oil quality and the amount of extractable organic matter present. Further magnetic characterization revealed that the high quality oils are dominated by pseudo-single domain grains of magnetite and the lower quality oils by larger pseudo-single domain to multidomain grains of magnetite and hematite. Hence, with decreasing oil quality there is a progressive dominance of multidomain magnetite as well as the appearance of hematite. It is concluded that biodegradation is a dual process, firstly, aliphatic hydrocarbons are removed thereby reducing oil quality and secondly, magnetic signatures are both created and destroyed. This complex relationship may explain why controversy has plagued previous attempts to resolve the connection between magnetics and hydrocarbon deposits. These findings reinforce the importance of bacteria within petroleum systems as well as providing a platform for the use of magnetization as a possible exploration tool to identify subsurface reservoirs and a novel proxy of hydrocarbon migration.