The controls on the composition of biodegraded oils in the deep subsurface – Part 4. Destruction and production of high molecular weight non-hydrocarbon species and destruction of aromatic hydrocarbons during progressive in-reservoir biodegradation

This study extends the analysis of previously well studied biodegraded crude oil case history sample sets. The analytical window is extended into the high molecular weight, aromatic hydrocarbon and non-hydrocarbon fraction of crude oils, using a 12 T ultra-high resolution mass spectrometer (FTICR-MS). Biodegradation is pervasive across compound groups and extent of degradation appears dependent on compound abundance and hence availability. Oil constituents with molecular weights up to m/z 600 (carbon number 44) are affected by in-reservoir biodegradation. Apart from special, specific compound groups possibly related to the active reservoir biomass, all hydrocarbon and single heteroatom-containing compound classes are depleted by biodegradation. Production of various highly alkylated species indicate that transformation of crude oil components often involves derivatization and preservation rather than just complete destruction of high molecular weight compounds. Whereas one case study shows good correlation between depletion of S1 species and a strong increase in SO2 species, a nitrogen enriched oil suite shows an analogous trend in the transformation of N1 species to the corresponding NO2 species. Increase in O2 species are seen in both sample sets indicating partial oxidation is a major overall process in in-situ reservoir biodegradation. These variations are important geochemically but also impact transport, interfacial and corrosion properties of oils. Nitrogen isotope systematics indicate that nitrogen-containing compounds might act as nitrogen nutrient sources or mainly as carbon sources for the microorganisms causing in-situ reservoir biodegradation depending level of biodegradation. Distributions of some heterocyclic species add a very biodegradation resistant parameter set, to the petroleum geochemists arsenal

A computational chemical study of penetration and displacement of water films near mineral surfaces

<p/> <p>A series of molecular dynamics simulations have been performed on organic–water mixtures near mineral surfaces. These simulations show that, in contrast to apolar compounds, small polar organic compounds such as phenols can penetrate through thin water films to adsorb on these mineral surfaces. Furthermore, additional simulations involving demixing of an organic–water mixture near a surfactant-covered mineral surface demonstrate that even low concentrations of adsorbed polar compounds can induce major changes in mineral surface wettability, allowing sorption of apolar molecules. This strongly supports a two-stage adsorption mechanism for organic solutes, involving initial migration of small polar organic molecules to the mineral surface followed by water film displacement due to co-adsorption of the more apolar organic compounds, thus converting an initial water-wet mineral system to an organic-covered surface. This has profound implications for studies of petroleum reservoir diagenesis and wettability changes.</p