One Direct Indicator for Reservoir in Fractured Carbonate

Chen, Guangjun¹, Mohamed Redal¹, Jianguo Zhu², Wei Meng² (1) Schlumberger, Beijing, China (2) Northwest Bureau, SinoPec,

The reservoirs in the Tahe oilfield are fractured carbonates with extremely severe anisotropy, typically karsted. The rock is a very tight limestone, with matrix porosity of < 2%. Vugs and caves connected by natural fractures are the storage. Reservoirs are developed along with a weathering crust. Some wells produce oil at a relative high rate, while others have a very quick breakthrough or produce water initially. Accurate characterization of the reservoirs to enable increased oil and decreased water production is a challenge.

In an attempt to directly characterize the reservoirs seismic variance was used, which calculates the direct measurement of dissimilarity of seismic traces and produces much sharper and more distinct terminations than those observed in normal amplitude data. Calibrated with core, log, and high-resolution resitivity data, fractures, caves, and vugs are all shown to result in high variance values, indicating that a reservoir can be developed only where variance shows high values. Variance results are displayed in 3D or in cross-sections, and are used to identify the geological features in the volume. Based on the variance indicators, caves and vugs can be directly identified to predict the carbonate reservoirs, which significantly aids properties modeling and production management.

While seismic variance does not provide a perfect definition of the reservoir, it has proven to be a reliable direct indicator for reservoir facies in the fractured carbonates of the Tahe field.