Inorganic geochemistry of the oil of west Siberia: First ICP-MS data

Abstract

The evolution in innovation technologies and analytical devices has raised geochemistry to a higher level. As is known, modern petroleum geochemistry is mainly composed of organic geochemistry. The trace element composition of oils was studied usually in combustion products, whereas works on direct determination of the trace element composition of hydrocarbons are scanty and generally related to a very small number of elements and/or heavy fractions [1]. The rapid progress in the method of mass spectrometry with inductively coupled plasma (ICP-MS) has made it possible to study the trace element composition of not only rocks, but also complex organic compounds, for instance, oil and its derivatives. A series of works has recently been reported on the geochemistry of asphaltenes and bitumens, i.e., solid components of oil [2, 3]. We analyzed for the first time the trace element composition of crude oil from West Siberia by the ICP-MS method. More than 50 trace, rare earth, and other elements were determined in oils from the Shaim and Srednii Ob fields (see [4‐6] for geological data). The analyses were conducted using an ELEMENT2 highresolution mass spectrometer following the technique developed at the Laboratory of Radiogeology (Zavaritskii Institute of Geology and Geochemistry) [7, 8]. The measurement results are shown in the table. Oils from West Siberia are characterized by extremely low contents of the majority of elements. The PM-normalized trace element contents are equal to ~0.1 u for the most depleted ultramafic rocks and ~0.001 for oils. It should be noted that the PM-normalized contents of trace elements in Triassic basalts of The table shows that crude oils of the Shaim and Srednii Ob fields have relatively high contents (>1 g/t) of major (Mg, Al, Fe, Na, Ti) and transition elements (Cr, V, Ni, Cu, Zn). In the Ni‐Cu‐Cr diagram [3] for the resinous‐asphaltene fractions, these oils belong to the chromium type. Their data points fall at the continuation of the trend outlined by the authors of [3] for the West Siberian province. The contents of other trace elements in these crude oils are lower (<1 g/t), but often higher than in some rocks. The Cs, Rb, Sr, and Zr contents in the oils are comparable with those in ultramafic rocks. The U content in oils is similar to that in the basaltoids and significantly higher than that in ultramafic rocks, chondrites, and intermediate rocks [9]. Such a high U content in oils is presumably related to the reducing conditions and, correspondingly, to the presence of a geochemical barrier. These conditions could also promote the high Pb content (up to 0.3 ppm). The Ag content is also relatively high (up to 0.1 ppm), while the Au content is low (<0.002 ppm).