Cave System Analysis - An Effective Approach to Predict Hydrocarbons in Cavernous Carbonate Reservoir

Paleo krast caves, even deeply buried over 5000 meters, are important exploration targets in Tarim Basin west China. In seismic, large caves could be well observed as strong amplitude anomalies and are characterized as “string of beads” in legacy seismic data. They have dominated the production and reservers of the Ordovician carbonate reservoirs. Successful drilling on these targets quite depends on reliable prediction of fluid types in these karst caves. But seismic hydrocarbon detection has a high uncertainty due to the high variation in cave geometry and the low seismic resolution in deeply-buried carbonate. To improve drilling success rate, cave-system analysis approach was developed in our study as an effective approach to predict hydrocarbons in cavernous carbonate reservoir. Hundreds of paleo-caves have been seismically interpreted. Based on extending tendency of individual cave interpretation, the connections are delineated between caves forming multi-level and multi-branch cave systems through study area. These complex cave systems could be analog to the Mammoth Cave. This analog suggests that most caves in study area were connected during their development time. They are now much less connected due to sediment infilling and collapsing. Within the cave systems, the cave entrances, exits, collapses and relative high points of the cave systems are identified to provide the underpinnings for predicting cave fillings and fluid types, with the support from detailed structural mapping, paleo-topography study, production performance diagnosis, and seismic inversion/ attribute evaluation. Several hundreds of oil-bearing caves are predicted within the study area, providing the main basis for drilling plan. New drilling has led to several high productive wells after two year’s unsuccessful drilling in study area.