In situ microanalysis of petroleum fluid inclusions by Time of Flight-Secondary Ion Mass Spectrometry as an indicator of evolving oil chemistry: a pilot study in the Bohai Basin, China

Abstract

Petroleum fluid inclusions from the Paleogene Shahejie Formation in Bohai Basin, Eastern China, were characterized usingMicro-Fourier transform-infrared (Micro-FT-IR) and fluorescence microscopy, and Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS). Primary and secondary populations of organic inclusions were recognized. The primary petroleum inclusions contain organic compounds with relatively long alkyl chains, whereas the secondary petroleum inclusions contain organic materials with relatively short alkyl chains and also some H 2S. The homogenization temperature ( T h) of the primary inclusions is lower (coeval petroleum and aqueous inclusions both mean 87 °C) than that of the secondary inclusions (coeval aqueous mean 106 °C), suggesting that the primary petroleum inclusions experienced a lower degree of thermal evolution than that of the secondary inclusions. This inference is consistent with the fluorescence characteristics of the inclusions. The Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) analysis successfully distinguished the compositions of the two populations of petroleum inclusions, and also detected traces of saline aqueous fluid present in the primary inclusions.