Trap কী?

Example 18.9 (A trap in an anticline)

A seismic survey maps the top of a sandstone sealed by mudstone. It forms an arch whose crest lies $1800\,\mathrm{m}$ below the surface. Around the crest, the deepest contour line that still closes all the way round lies at $2050\,\mathrm{m}$; below it, the sandstone opens to the side. A well at the crest finds gas down to $1880\,\mathrm{m}$. How tall a column of oil and gas can the trap hold, and how thick is the oil?

Solution. Oil and gas fill the arch from the crest downwards; any that sinks below the last closed contour spills out to the side, at the spill point, and migrates on. The column is at most $2050 - 1800 = 250\,\mathrm{m}$ tall. If the trap is full, gas fills the top $1880 - 1800 = 80\,\mathrm{m}$ and oil the next $2050 - 1880 = 170\,\mathrm{m}$, with water below.