Sergo Ordzhonikidze Russian State University for Geological Prospecting, Moscow, Russia

Mustaev R. N., Associate Professor, Candidate of Geologo-Mineralogical Sciences, mustaevrn@mgri.ru

Osipov A. V., Associate Professor, Candidate of Geologo-Mineralogical Sciences

Industrial Geology of Research and Engineering Institute, Vietsovpetro JV, Ho Chi Minh, Viet Nam

Tung P. M., Deputy Head of Department, Candidate of Geologo-Mineralogical Sciences
Hai V. N., Engineer, Candidate of Geologo-Mineralogical Sciences

This study examines the origin and mechanisms of hydrocarbon accumulation in the Cuu Long Basin (offshore Vietnam), with particular emphasis on oils hosted in fractured and cavernous granitoids of the crystalline basement. A comprehensive set of molecular geochemical analyses was performed, including gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and high-performance liquID chromatography (HPLC), in accordance with ASTM and ISO standards. The distribution patterns of n-alkanes, isoprenoids, steranes, and terpanes revealed identical biomarker profiles for basement-hosted oils and oils associated with Oligocene–Miocene reservoirs. The absence of regular C₁₂ and C₁₇ isoprenoids, together with the lack of C₂₂ and C₂₇ cheilanthanes, as well as elevated cheilanthane indices and high hopane/sterane ratios, confirm the biogenic origin of these oils and indicate a significant bacterial contribution to the initial organic matter. The findings support a two-stage migration model: lateral migration within carrier beds, followed by downward migration along fault and fracture zones into the basement. It is established that hydrocarbons in the crystalline basement occur in secondary accumulations and are derived from Oligocene–Miocene source rocks. These results refine the petroleum system model of the basin and substantiate the priority of future exploration for new accumulations in basement reservoirs.

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