The Possibility of Applying Geophysical Methods to Study Fractured Basement Reservoir in the Continental Shelf of Vietnam
Authors: Tan Mai Thanh , Thap Nguyen Quoc 
Published: October 2008

The discovery of hydrocarbon in fractured basement rocks opened a new trend for oil and gas prospect in Vietnam. This type of reservoir is different from others in the world and has complicated characteristics.

The paper presents the characteristics of the fractured basement reservoir and shows the possibility of applying 3-D seismic, well logging, etc. to study them.The highly fractured nature of basement reservoirs is created during the process of tectonic deformation, cooling, hydrothermal and weathering.With more effective of 3D seismic methods, the geological models of this reservoir are built more accurate. Since these type of reservoirs is not similar to others in bedded sedimentary environment, the use of diffraction waves beside reflection ones is necessary for seismic data processing.The porosity and other petrophysical indicator of fractured basement reservoir can be estimated by not only conventional logs (CNL, LDL, DT, NGT) but also resistivity logs (LLD, LLS), aperture analysis (FMS/FMI) and other methods.

The results of geophysical researches contribute the enhancement oil and gas exploration in Vietnam, but for a better understanding of this type of reservoirs, more investments in research and development are required.

Characteristics of Pre-Cenozoic basement reservoir in the Continental Shelf of Vietnam

In the Continental Shelf of Vietnam, oil and gas has been produced not only from Eocene, Oligocene, Miocene formations but also from Pre-Cenozoic fractured granite basement rocks. The first oil and gas reservoir in fractured basement was discovered in 1986. After that, several oil and gas fields have been found from the fractured basement. Experiment and production data have confirmed that a large amount of pore space exists in this huge mass of fractured granite rock, and the pore space can contains oil. Therefore, it has opened a new play fairway and new exploration objective in Vietnam.

The structure of Pre-Cenozoic basement in the Continental Shelf of Vietnam is very complex. The structure is basically composed of magmatic rocks, and formed at different stages. The main lithology of the basement consists of granites, granodiorites, weathered granitoides, etc. The age of these basement rocks is of Late Jurassic - Early Cretaceous (97-178 million years)(Dong et al, 1997; Tan et al, 1996). Discovery and field distribution in the Southern Vietnam's Offshore is shown in Figure 1. A geological cross-section is shown in Figure 2 (San et. al, 1997).

Effective porosity is the most important parameter in the fractured granite basement reservoirs. It is related to, and controlled by fracturing, cooling, hydrothermal and weathering processes.

Tectonic deformation factor plays a key role in the formation of fracture network in basement. Tectono - magmatic activities occurred in Jurassic - Cretaceous and ended in Middle Miocene. Together with rifting and faulting, fracture zones were also formed. The combined effects of tectonic deformation and subsequent diagenetic overprint have produced a fabric that is a mixture of fractures, vugs and channels. This is a favorable condition for hydrocarbon from the Oligocene - Miocene source rocks to migrate and accumulate into the basement highs.