Сomposition and sources of oils in the terrigenous and volcanogenic reservoirsfrom White Tiger deposit (Vietnam)

Features of hydrocarbons of crude oilscan recognize the main characteristics of source and deposition conditions of original oil source substance. Therefore, this articleinvestigated the composition of saturated and aromatic hydrocarbons of crude oils from granitoid reservoirs of the basement and fromsandy-silt sediments of Oligocene and Miocene of White Tiger deposit, which can clarify the origin of these oils. Gas chromatography-mass spectrometry method is used to study the composition of hydrocarbons of five crude oils from the basement and two fromOligocene and Miocene of White Tiger deposit. Saturated hydrocarbons (95.3-96.6% rel.) predominate in the major part of thecomposition of hydrocarbons in all the studied crude oils. They include alkanes, alkylcyclohexanes, steranes, secohopanes,sesquiterpanes, tri-, tetra- and pentacyclic terpanes. It is shown that n-alkanes dominate in all crude oils. Their composition in differentcrude oils varies slightly. The pristane to phytane ratio is more than two in all crude oils. The content of terpanes is higher than that ofsteranes. Among C27-C29 steranes C29 isomer steranes predominate (47-56 % rel.). Sesquiterpanes are C14-C16 isomers. The relativecontent of C16 sesquiterpanes reduces down the stratigraphic section. The content of secohopanes and tricyclic terpanes increases in themixture of terpanes and the content of hopanes reduces. The total content of aromatic hydrocarbons, represented by mono-, bi-, tri-,tetra- and pentacyclic structures in the mixture of hydrocarbons is small, less than 4.6% rel. Their molecular composition in theinvestigated oils does not differ significantly. The studied results of hydrocarbons of crude oils from White Tiger deposit lying inMiocene, Oligocene, and basement show the similarity of mono-, bi-, tri-, tetra- and pentacyclic aromatic hydrocarbons, alkanes, alkylandmethylalkylcyclohexanes in all crude oils. Some compositional features of crude oils (ratio of hopanes, secohopanes and tricyclicterpanes content, as well as relative content of C15-C17 n-alkanes) may be due to differences in the thermal history of this deposit. Thecomposition of isoprenoid alkanes indicates the closeness of initial conditions for accumulation of organic matter in all investigatedcrude oils. Data on the composition of steranes, tetra- and pentacyclic terpanes shows the conditions of formation of oil source organicmatter of all crude oils, which are characteristic for sedimentation of Oligocene-Lower Miocene of White Tiger deposit.

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