Sequence stratigraphy and petrographic characteristics of Oligocene sediments in the West trough of the Cuu Long Basin, Vietnam

Abstract

The Cuu Long Basin is the major source of Vietnam's oil and gas production. In clastic sediments, the Oligocene and Lower Miocene formations are the main objects of oil and gas exploration and exploitation activities. Oligocene sediments are both source and reservoir rocks; therefore, studying the petroleum system of these formations is of great interest. This paper discusses detailed stratigraphy and lithological properties to clarify the Oligocene sedimentary characteristics and their significance in the petroleum system. Study results show that Oligocene sediments are divided into three sequences, including eight parasequences and are dominated by the terrestrial sediments. Sequence E, mainly coarse-grained, contains low palynomorphs and rich organic matter with mainly palynomaceral 1 and is deposited in freshwater fluvial settings under high-energy conditions, showing sediments not so far from the original areas. The high level of detrital matrix materials is the main cause of poor or negligible porosity of the rock. The parasequences of Sequence D contain abundant palynomorph assemblages, characterized by swamp and freshwater alga taxa. They are deposited mainly in the shallow lake with a decreasing energy level pointing upward. The volume of organic matter in sediments is very rich, with sapropel organic matter (SOM) rates reaching up to 80%, indicating that the hydrocarbon generation potential is mainly oil-prone and some oil-gas mixtures. The porosity is poor and reservoir quality is affected by detrital matrix and authigenic minerals. The parasequences of Sequence C are characterized by the extreme abundance of freshwater algae, deposited mainly in the shallow-to-deep lake under moderate energy conditions, and rich sapropel organic matter, suggesting mainly oil-prone in hydrocarbon generation potentials. The porosity ranges from fair to good and the reservoir quality is estimated as the best in Oligocene sediments. A new finding of marker assemblages provides evidence to pinpoint the stratigraphic boundary between the Lower Miocene and Upper Oligocene. This finding helps to complete the stratigraphic framework of the area and to correlate the stratigraphy to adjacent sedimentary basins. This study provides information on depositional environmental conditions, organic matter content, hydrocarbon generation potential, and reservoir quality that relate to source rock, reservoir, and seal; these are the elements of a petroleum system. A complete petroleum system assessment requires a holistic view of all the petroleum system elements.