oa Use of the Ratio of 1,4-Dimethyl- to 1,3,4-Trimethyl-Adamantane as a Source-Related Proxy in Petroleum Geochemistry: Two Case Studies

Diamondoid hydrocarbons play a crucial role in petroleum evaluation. Various diamondoid-based ratios help determine the source, maturity, and biodegradability of oil. Because of their high thermal stability and resistance to biodegradation, diamondoid-based parameters are particularly useful for characterizing highly mature and/or biodegraded oils, which often lack traditional biomarkers. In particular, the ratio of dimethyl- to trimethyl-adamantanes, is the focus of this study. Specifically, marine-derived oils tend to have higher methyl/trimethyladamantane and dimethyl/trimethyladamantane ratios than terrigenous oils. Lorenzo and colleagues in 2024 used the ratio of 1,4-dimethyladamantane (DMA) to 1,3,4-trimethyladamantane (TMA) as an indicator of source rock characteristics. Interestingly, their research found no significant differences in the behavior of this parameter across different levels of thermal maturity when comparing marine and terrestrial sources. Authors present bulk geochemical data for the oil samples from the Guico Field and two crude oils (Mch 3-9X and Mch 8-4X) from the Orinoco Oil Belt, along with their corresponding DMA/TMA ratios. Bulk geochemical characteristics align with DMA/TMA ratios < 0.7 for Mch-9X and GV-009, around 0.85 for SGV-25, and > 1.0 for the remaining samples. Results confirm that the 1,4-dimethyladamantane to 1,3,4-trimethyladamantane (DMA/TMA) ratio is a reliable source-related proxy for assessing highly biodegraded oils.