An Integrated Approach to Carbonate Karstic Reservoir Characterization and its Application to a Field Case in the Tarim Basin

The Lower and Middle Ordovician carbonate karstic reservoirs form an important type of reservoirs in the Tarim basin, the largest inland basin in China. Karstic feature characterization along with infill identification provides a great opportunity to delineate the distribution and connectivity of collapsed-cave systems. Based on carbonate karst concepts, this paper presents an integrated approach to characterize collapsed-cave systems and to map the geomorphology and distribution of karstic drainage components. Seismic amplitude and geometric attributes are used to identify karst features such as collapsed-cave complexes, conduits and infiltration or dissolution zones. A multivariate attribute classification technique is applied to generate karst seismic facies that highlight these features. Clustered karst features are sampled into the grid model to construct a 3D architecture model of collapsed-cave systems. This model eventually incorporates all clustered features, revealing their spatial distributions, inherent complex shapes and lateral connectivity. In the case of preserved collapsed-cave systems become disconnected and occluded as a result of infilling and roof collapsing, collapsed-cave systems need to be further calibrated in order to locate infill drilling and identify dynamic compartments. The seismic acoustic impedance attribute facilitates the identification of infill within collapsed-cave systems because chaotic breakdown breccias and cave-sediment fills have an impedance lower than that of the host limestone resulting in a significant impedance contrast. Incorporating the impedance attribute with the architecture model, the bodychecking technique is applied to searching for connected cells to extract karstic drainage components. The geomorphology and distribution of individual components can be mapped. Integrating reservoir production data and borehole imaging logs, the drainage components can be evaluated and sorted. The case study is addressed to illustrate applications of these technologies and their efficiency.