“Pair beads” seismic anomalies: The cave features important to but missed in carbonate studies

Abstract

“Pair beads” amplitude anomalies in subsurface carbonates are the accurate seismic images on uncollapsed caves in carbonate rocks. They have been observed in carbonates worldwide, related to large mud-losses in drilling and high flowrate in production, basically missed in reservoir studies due to lack of knowledge on them. To fill the knowledge gap, the standard reference with clear geological and geophysical context is established with data from a Miocene isolated carbonate platform in Central Luconia Province. 1) In section-view with zero-phase seismic display, “pair beads” is a seismic waveform with trough above and peak below, and cave center locates along the zero-crossing between trough and peak. 2) In map/slice-view, cave distribution are usually in point, linear or dendritic pattern for cave networks. For overall understanding, worldwide available cases on “pair beads” are illustrated and explained by their geological ages and settings. 1) In Tertiary carbonates at Asia, high-quality images illustrate extensive cave distribution in platform interior. 2) In Paleozoic carbonates at Asia, America and Europe, “pair beads” have been observed in tight matrix, leading to high reservoir heterogeneity and complex fluid distribution. Lacking knowledge on “pair beads” have led to high uncertainty in drilling and production, leaving many potential unrecognized in Paleozoic plays. The result illustrates that carbonate caves are much more than expected in their wide distribution and geological settings, which reveals new knowledge gaps on carbonates, such as origin and evolution of carbonate caves and associated other problems.