Using observations from the extensional basin setting in Vietnam, conceptual models were developed to simulate and analyse fracture systems typical of crystalline basement in such structural settings. Information from field observations, seismic surveys and three-dimensional (3-D) structural modelling were integrated and used to build geologically realistic 3-D fracture networks. A major advantage of the method used in this study is that it allows a better understanding of the apparently chaotic fracture networks characteristic of such rocks, and of the processes responsible for fracturing. Several fracture generating processes are discussed and modelled, with emphasis on tectonic fractures and the relation to structural modelling. An example is presented highlighting the differences in fracturing in the hanging wall and footwall during lithospheric extension superimposed on a primary (igneous) cooling fracture network. Results suggest that during flexural uplift, the hanging wall is significantly more deformed than the footwall, implying the former is more prone to fracturing than the footwall for both kinematic and flexural isostatic processes.Fractured basement has proved to be a successful petroleum play in the extensional offshore setting of SW Vietnam ( Fig. 1 and Areshev et al. 1992).The unconventional nature of this play demands different exploration and production strategies compared with sedimentary plays. Typically a comprehensive exploration strategy is ill defined. Cartoon style concepts have been defined (e.g. Areshev et al. 1992;Tandom et al. 1997), but few attempts have been made to link the concepts in a quantitative way to the available data, and to test these concepts in a predictive manner.The structurally high horst blocks form the usual target (Fig. 2). The seal on the fractured reservoir is formed by shale deposits overlying the basement. The basement is most probably charged with hydrocarbons from the side and above from the earliest clastic lacustrine deposits (Sladen 1997). The presence of an extensive connected fracture network in the basement then defines whether a fault block is a potential viable prospect.In this paper we reassess some of the structural concepts that define the petroleum play. Modern methods of appraising fractured reservoirs rely on a number of data sets including those from cores, FMI, wireline, video and field observations. More often than not, the prediction of regional fracture networks requires a full understanding of these diverse data sets. Here we present a novel technique that incorporates all available data into a comprehensive model describing the range of fracture systems found in extensional settings in crystalline rocks. In the absence of primary porosity and permeability, the basement reservoir potential is entirely defined by fractures forming the secondary porosity and permeability. Fracture appraisal is therefore of prime importance for the successful development of these fields. Three basic questions arise from this, namely: (1) where are the frac...