The fractured granite basement in the Nam Con Son basin of southern offshore Vietnam has become a new exploration target in recent years. However, there are two major challenges in the early days of exploration. First of all, it is not always easy to locate a drilling target, for example, a faulted zone or a better fractured area on seismic due to the often degraded data quality within basement. Secondly, it can be difficult to drill the target with better well trajectory due to often poor understanding of the potential productive fracture systems. Those two challenges are also very common for most of the fractured basement reservoirs. In this study, available regional geology data including structural geology information and outcrops have been collected to help geoscientists to gain a basic understanding of the local geology related to the basement and fracture development. A full suite of well data from an existing wildcat well in the same block including conventional open hole logs, advanced well logs (i.e. borehole images and SonicScanner), mud logs as well as PLT (production logs) and DST data was carefully studied to extract all possible information on natural fracture and fault. This resulted in two main outputs - the fracture density log along the wellbore and the defined potential fracture/fault system with dip and strike. Then the fracture output from the existing well was used to fully calibrate the seismic attribute analysis for the whole block, which mapped out the fracture density in 3-D volume. A new exploration well with high deviation was designed on the basis of fracture density volume data as well as the understood potential productive fracture/fault attitude (dip & orientation) and drilled smoothly. A commercial hydrocarbon discovery was made in this new basin first time in the fractured basement. Introduction Hydrocarbon has been discovered in the Miocene and Oligocene clastics as well as carbonates in the Nam Con Son basin, southern offshore Vietnam since 1990's. However, drilling activity for the Pre-Tertiary granite basement has been still very limited without any commercial discovery until late 2010. This paper intends to summarize the fracture characterization for the studied field where the 1st commercial oil discovery in the basement was made in the Nam Con Son basin as well as the learned experience on how to define a drilling target based on the limited existing well data and seismic data in the exploration stage for this unconventional fractured tight reservoir.

No abstract