Abstract

The petroleum system of the Cuu Long Basin, offshore Vietnam, has become well known, not only in Vietnam but throughout SE Asia, because of its rich lacustrine source rock and unusual fractured and hydrothermally altered granite basement reservoir. Basin analysis has lead to the recognition of many different petroleum plays, not only in Tertiary sediments, but also in the pre-Tertiary plutonic-volcanic basement, and this has lead to new exploration objectives.

The consolidated basement of the Cuu Long Basin was formed by the continental collision that lead to the disappearance of the Tethys Ocean. The Mesozoic plutonic-volcanic basement complex was affected by several phases of normal faulting during the Early Cenozoic, causing it to become a fractured, oil bearing reservoir. Large differential movements of fault blocks during the rift stage in the basin caused different levels of subsidence and the formation of fractured horst blocks and structural noses, leading to the formation of basement high traps.

The Cuu Long sedimentary basin is characterized by graben-depression structures caused by Oligocene rifting and Miocene sagging. Oligocene sediments in the central grabens are composed of lacustrine and tidal-lagoonal fine grained clastics; some of these are good reservoir targets. Other sequences are characterized by tidal-lagoonal shales, which are both source rocks and effective caprocks. Miocene sediments are widely distributed throughout the basin. A Lower Miocene play involves sandstones laid down in a predominantly tidally influenced environment. The Middle Miocene marine-lagoonal shale sequences are regionally extensive and form effective top seals. The association of Oligocene source rocks and Oligo-Miocene seals, with both igneous basement and Oligo-Miocene siliclastic reservoirs, is common throughout the Cuu Long Basin.

There is a thick Late Miocene and Pliocene-Recent sequence that provides the overburden necessary for source maturation and seal development. The Cuu Long Basin is considered one of the most prospective in SE Asia.