Abstract:
Analysis of solid bitumens from the Tengiz Field, Kazakhstan by X-ray photoelectron spectroscopy provides evidence for a new reservoir alteration process – reactive polar precipitation via the formation of ether cross-linkages. Solid bitumens, which may occupy >30% of the pore space, were analyzed in situ and as isolated concentrates of the material in vein/fracture-fills or encapsulated within micropores. Their compositions were found to vary greatly, particularly in the amounts of aromatic carbon and in the organic oxygen mostly present in C–O single bond environments. While most solid bitumen in Tengiz reservoir rocks arose from asphaltene precipitates, surfaces are coated with a relatively aliphatic- and oxygen-rich material. Organic sulfur occurs exclusively in aromatic sulfur environments in most samples, while nitrogen is in pyrrolic and pyridinic environments. To account for the average organic composition of solid bitumen, we propose that the coatings are formed by condensation of low-molecular weight polar compounds that are normally soluble in most oils. A likely reaction mechanism involves free radicals, produced by thermally labile non-aromatic sulfur (–SH) forms, reacting with hydroxyls. This reaction results in a release of H2S and the formation of compounds with at least one ether-linkage and multiple heteroatoms within the aromatic cores. Similar reactions are believed to convert the reactive polar precipitate and previously precipitated asphaltenes into toluene-insoluble bitumens. This hypothesis explains the chemical nature of the solid bitumens, accounts for previously reported anomalies in reflectance, and is fully consistent with current geohistory models. Whether this reservoir alteration process occurs outside of the North Caspian Basin is not known, but we suspect that the process occurs elsewhere. Thin coatings of polar precipitates, which may be undetectable by conventional optical microscopy, could have significant impact on reservoir properties, such as surface wetability or mineral diagenesis.
