REGIONAL GEOLOGY OVERVIEW

Blocks 102 and 106, with an area of about 14,000-km2 are located in the Song Hong Basin in the western part of the Gulf of Bac Bo (Gulf of Tonkin). The offshore blocks are near the northern coast of Vietnam and are shown on Figure 1. The blocks are approximately 120-km southeast of Hanoi and 40 km offshore from the Nam Dinh province in 30 m of shallow offshore water. The Song Hong Basin, or Bac Bo Basin as it is sometimes called, is one of the largest basins in Southeast Asia. It was formed during late Cretaceous to early Tertiary time. The Song Hong basin contains a large northwest-southeast trending rift basin containing as much as 10,000 m of Tertiary sediments in its deep central area. The Tertiary sequence contains Eocene, Oligocene, Miocene and Pliocene plastic sediments deposited mostly in delta plain to coastal and upper shore face environments. The underlying basement is believed to be composed of Triassic granite rocks, Devonian to Permian limestone and metamorphic rocks of various ages.

The basin was essentially stable in Late Cretaceous as evidenced by the occurrence of evaporite, shale, and a small amount of sandstone. During late Cretaceous to early Tertiary, the south China plate moved southeastward along the Red River Suture Zone. Consequently, rifting activities were initiated along the suture and drove the Indochina plate to the southeast. The Song Hong basin is characterized as a group of rifted grabens and half grabens, which were rapidly filled by fluvial and lacustrine sediments of Eocene and Oligocene age. Alternating strike slip movement of the Indochina and China plates caused structural inversion. The first phase began at Middle Miocene and continued to the end of late Miocene. The fault system, such as the Red River, Song Lo, Vinh Linh and Tien Lang faults, mainly trend in the northwest-southeast direction. The Beibu Wan basin, a successfully explored rift basin in Chinese waters to the northeast, trends ENE-WSW and intersects the Song Hong basin in the south central part of Block 106. Two NW-SE trending basement high complexes have been delineated by seismic and magnetic data. One is in the southwestern part of Block 102. The other extends from the northern part of Block 102 to the southwestern part of Block 106.