Overpressured (abnormally high pore-pressure) zones in the subsurface are typically caused by undercompaction: rapid burial prevents pore fluids from escaping as sediment compacts, leaving the shale with anomalously high porosity and fluid content for its depth; this elevated porosity and fluid content produces a distinctive well-log signature, including higher electrical conductivity (lower resistivity, since more conductive pore fluid is retained) and longer acoustic transit time on the sonic log (since higher porosity and less compaction reduce seismic wave velocity through the rock).