LAUSR

Abstract Cuu Long Basin, which is located in southeast offshore Viet Nam, has a number of well fractured granitic basement reservoirs. Though dozens of wells have been drilled in the pre-Tertiary basement, the actual number of wells in which hydrocarbons were discovered is approximately 30% of the total. Therefore, understanding fracture characteristics in the fractured basement is very important and plays a critical role in designing the well path. This study proposes a method to optimize well path design in the fractured granitoid basement rock. Available information, including basin tectonic evolution, basin geological characteristics, seismic interpretation, and wireline logs, especially image and sonic logs, are analyzed in order to understand the fracture characteristics and the distribution of fractures in the study area, which is referred to as Thuy Trieu Do (TTD) structure. There are two key factors related to the flow in basement of TTD structure, namely are solution-enhanced fractures and faults interpreted from seismic images. Solution-enhanced fractures from images are considered to be open fractures that contribute to flow. Within the TTD basement, the solution-enhanced fractures trend in the east-west and sub east-west directions. Faults interpreted from seismic images are complex with different fault directions, having been active in several tectonic phases. They are evaluated and classified, then the faults those are NE-SW 60–70°, E-W, and sub E-W 80–280° have potential in creation fault-related fractures of solution-enhanced fracture type. Fractures are mostly distributed in fracture zones along the fault zone. By displaying faults and fractures together both along the well path and on rose diagrams, combined with production logs analysis, it appears that surrounding faults have increased fracture density and the presence of solution-enhanced fractures in every flow zone. The above relationships were taken in analyses for the next well design in the TTD structure. In this study, for optimal well path design in the fractured basement reservoir, a systematic procedure of interpretation of various geophysical data in Cuu Long Basin has been proposed, and this approach is expected to be applicable to other fractured reservoirs in Viet Nam.