Seismic Characterization and Exploration of Karst Cave in Tarim Basin

Luo, Chunshu; Xue, Fangjian

Modern karst caves are widely see and well documented in the world. They should be excellent reservoir due to their huge storage capacity and fluid flow-ability. But karst caves are not well recorded in production from karstified carbonates. They are generally considered being collapsed without room left when buried deeply. Our integrated study of the Ordovician carbonate reservoir at Tarim Basin has led to a breakthrough in understanding and drilling on the karst caves for hydrocarbon.

In legacy seismic, the strong amplitude anomalies, known as "string of beads", have been regarded as the seismic response of karst caves. Drilling on these targets has led to many high productive wells. But this understanding has also been challenged by many dry and low-productive holes, the high water-cut in production and the fact that caves drilled are much fewer than expect.

An integrated workflow has been established in this study to improve the exploration and production performance on karst carbonate at Tarim Basin. With seismic reprocessing, the reflectors of karst top become more evident and continuous and the "string of beads" has converged into "pair of beads". This improvement has made it possible to interpret and map karst caves in seismic. Attribute maps, knowledge of karst topography and software functionalities have been combined innovatively to define detailed karst topography. Integrated production analysis concluded that the cave reservoir is dominant in reserves and productions even through the fracture/vug reservoirs are dominant in log interpretation and epikarst zones has high success rate in finding hydrocarbon. Karst caves are interpreted with the combination of manual/automatic interpretation and checked in both section/ map view for accurate cave geometry and attributes. Levels of caves are defined based on connection trending of caves in seismic. Cave systems are established for cave connectivity. Cave system analysis approach is applied to predict cave fillings and fluid types with the support from paleo-topography study and seismic inversion/attribute evaluation.

As a result, hundreds of oil-bearing caves are predicted, providing the basis for reserve/potential estimation and drilling plan. New drilling has led to high productive wells. This integrated workflow and its result not only improved the exploration and production in Tarim Basin but also provide valuable references on exploring karst carbonate reservoir in the world.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013