Abstract

Lower Oligocene formation, E member at the HSD-4X borehole was determined to be situated at the depth of 3387 to 3727 m with a thickness of 340 m and was divided into 3 beds.

-      Bed 1 as lowest part situated at the depth of 3727-3553 m, with a thickness of 174 m, is composed mainly of rocks with minor portion of clastic sediments.

-      Bed 2 as middle part situated at the depth of 3553-3450 m, with a thickness of 103 m, is constituted by equal portion of effusives and clastic sediments.

-      Bed 3 as upper part of 3450-3387 m, with a thickness of 63 m, is dominated by clastic beds with minor portion of effusives.

This Lower Oligocene Formation, E member is constituted by clastic sediments  and  volcano-sedimentary beds whose lithology is completely different from that of Trà Cú Formation stratotype which was established at the Cửu Long-1 borehole, Trà Vinh Province, where the stratotype consists typically of molassic, coarse-grained sediments.

The study of lithological composition, distribution and depositional environment of E member correlated with Trà Cú Formation is essential task for reservoir characterization and hydrocarbon potential evaluation for Lower Oligocene Formation, E member in the study area (northeast of Cửu Long Basin).