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.
Karst systems in the Tarim basin are formed by subaerial exposure and karstification during a long period of time from the Lower to the Middle Ordovician. Loucks (1999) showed that most karst reservoirs are not products of isolated collapsed passages, but instead are a product of coalesced collapsed-paleocave systems hundreds to several thousand meters across and thousands of meters long. Identifying karst related products in cores, i.e. cements and fabrics, dissolution porosity and specific features, can help to recognize and describe karst systems. The origins of coalesced, collapsed karst systems and associated product features have been studied (Kerans, 1988; Loucks and Handford, 1992; Loucks, 1999; Loucks et al 2004; McDonnell et al, 2007). Sags associated with collapse were good indicators to locate the coalesced collapsed carbonate Karsts (Lucia, 1996). Loucks et al. (2004) defined the three-dimensional, interwell-scale architecture of a Lower Ordovician Ellenburger coalesced, collapsed-paleocave system through the integration of ground-penetrating radar (GPR), shallow-core, and outcrop data. Karst systems have complex features and tend to have pronounced lateral and vertical heterogeneity, especially when multiple phases of karsting and burial are involved (Loucks and Mescher, 1997; McMechan et al., 1998; Loucks, 1999).
