Granitic basement reservoirs have been the focus of increasing attention in the Asia Pacific region in recent years, following several new oil and gas discoveries in this complex reservoir type. Accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore relatively little research has gone into understanding logging tool response or pressure transient behavior in these reservoir types, and developing suitable workflows for formation evaluation. In this paper the authors propose an improved methodology for integrating various open-hole logs, production logs and well test data to better evaluate the reservoir potential of a fractured granitic formation. Since the wells are either horizontal or highly deviated, the framework also serves as one of the primary methods to asses the lateral extent of the reservoir. A case study from the region is used to illustrate the workflow. Image log interpretation, advanced acoustic measurements, nuclear logs and production logs with distributed local sensors are combined with well test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. The advantages and limitations of the proposed workflow are also discussed and the stage set for further work in this complex environment. Background Fractured granitic reservoirs differ from other types of fractured reservoirs in that they are generally considered to have no primary porosity. All the pore space in the rock is formed through fracturing and diagenetic processes. The resulting pore structure heterogeneity makes formation evaluation extremely challenging. Another complicating factor is the lithology, where the granite composition can vary and sub-vertical extrusive dykes cause abrupt lateral variations in matrix properties. This paper deals specifically with granitic oil bearing formations from southern offshore Vietnam. The typical basement reservoir consists of a faulted basement high. The majority of the porosity is thought to be associated with fractures and fault zones. High fracture density enhanced by hydrothermal alteration forms the majority of the effective pore space. Unlike basement plays in other regions the weathered zone at the top of the structure and the dykes are usually not productive, though the interface between dykes and host rock can be (Le Ngoc 2007, Nguyen 2003, Tandom 1999). In this environment the majority of new wells drilled are highly deviated or horizontal, and drilled to intersect sub-vertical fault zones. Most of the fractures are small and form the storage capacity of the reservoir. To be productive, a well should also encounter enough larger, permeable, fractures which are sufficiently connected to the storage capacity of the reservoir. The objective of the following interpretation methodology is partly to quantify as much as possible the total fracture porosity, but mainly to identify the large permeable fractures which are necessary for the well to be productive.

Summary In recent years, energy companies in the Asia Pacific region have focused increasing attention on granitic basement reservoirs, following several new oil and gas discoveries in these complex reservoirs. However, accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore, relatively little research has been conducted to understand loggingtool response or pressure-transient behavior, or to develop suitable workflows for formation evaluation in these types of reservoirs. In this paper, we propose a method for integrating various openhole logs, production logs, and well-test data to better evaluate the reservoir potential of fractured granitic formations. Because the wells are either horizontal or highly deviated, this workflow also serves as a primary method of assessing the lateral extent of a reservoir. We include a case study from the region to illustrate the workflow. Image-log interpretation, advanced acoustic measurements, nuclear logs, and production logs with distributed local sensors are combined with well-test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. We also discuss the advantages and limitations of the proposed workflow and set the stage for further work in this complex environment.