P. T. Dien1 and N. Q. Thang2
1Vietnam Petroleum Institute
The BachHo oil field in the CuuLong Basin, offshore South-East Vietnam with granitoid basement reservoir and rich lacustrine Oligocene source rock is famous not only in Vietnam but also in the whole South-East Asia. Most of the previous researchers considered that the porosity of this reservoir was the result of various processes such as tectonic, weathering, hydrothermal, volumetric shrinkage of cristalised magma (Tran Le Dong et al., 1996, 1999; Ngo Xuan Vinh, 1999). But most porosities of these fractures and cavities are filled with terrigenous materials when the granitoid massif was in contact with the depositional enviroment to become a basement of the sedimentary basin. On the basis of analyzing the formation, structure and evolution history of BachHo structure it is shown that effective porosity type of the granitoid basement reservoir is simultaneous result of many factors as follows: -The volume shrinkage due to diagenesis of the Paleogene wedge-like sedimentary sequences accumulated in the downthrown block of the reverse fault on the western slope of the basement massif; -The rigid, brittle, non-flexuring characteristics of the granitoid massif on the hanging wall of the thrust fault making the basement vulnerable to partial collapse; -The continuous piling up of sedimentary sequence from the beginning of Miocene to present which has increased continuously the load on the top of the upthrown granitoid block on the hanging wall.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado