Hydrocarbons in the Basement of the South China Sea (Vietnam) Shelf and Structural–Tectonic Model of their Formation

Abstract

The paper considers the origin of hydrocarbon accumulations within the Pre-Cenozoic basement of the Vietnam shelf. It is shown that the formation of hydrocarbon deposits is associated with protrusive granite massifs that underwent structural–tectonic reworking at the prototectonic and posthumous (postmagmatic) tectonic stage. Together, the posthumous structure-forming processes led to changes in the viscosity properties of rocks, their tectonic and material inhomogeneity, lamination, and, as a result, vertical and lateral spatial redistribution with the formation of granite protrusions. The mechanisms by which voids and oil and gas traps formed within protrusions are considered. A conclusion on the organic nature of petroleum in the basement of the Vietnam shelf is drawn, based on the similarity of the geochemical characteristics and biomarkers of the petroleum and organic parameters in the Oligocene–Miocene sedimentary cover and basement rocks. Possible mechanisms for hydrocarbon migration and accumulation in basement rocks are considered. It is confirmed that the formation of hydrocarbon deposits occurred due to lateral and downward migration of hydrocarbons via contact zones from Oligocene and Miocene oil source rocks into crystalline massifs: into voids and increased fracture zones in the protrusion body.