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How to Know if That Circular Feature is a Karst Collapse or a Buried Static? An Example of Ray Tracing, High Density Velocity Analysis, and Offset Stacks in Fort Worth Basin, Texas

Abstract

Karst and faults are usually considered to be geohazards to be avoided by horizontal drilling programs in the Fort Worth Basin. The circular karst appear as topologically low, bowl-shaped features on 3D time migrated seismic data. While previous workers used core and preassure data to show that these features are indeed karst, there is considerable ambiguity as to whether these “bowls” are due to the gas charged fractures overburden, to structural collapse, or both. Previous workers hypothesize that the seismic disruptions in the Barnett Shale and shallower productive formations are related to deep Ellenburger karsts and not to buried statics. They based their observations on the production reports from the Bend Conglomerates from two isolated gas reservoirs separated by small scale faulting. The faults have a circular pattern related with the underlying Ellenburger Group karst collapses. To evaluate these hypotheses, we conduct 3D ray tracing analysis, coupled with analysis. We also created different offset stacked volumes to verify if the circular collapses are due to the presence of gas chimneys escaping through the karst fractures, which could represent drilling hazards for the hydrocarbon exploration operations.