Oil and gas reservoirs may be divided into two great groups, (i) those caused by local deformation with or without rupture of strata; and (2) those due to the varying porosity of rock. In the first group the container rock is uniform in lithology and continuous over wide areas. In the second group the container rock is characterized by variable lithology and by discontinuity.
All oil and gas reservoirs are closed. The open or terrace type of structure nowhere constitutes an oil trap, nor does it cause accumulation of oil in commercial quantities. It is not uncommon for more than one type of reservoir to be present in a single field, but this fact can be recognized only by detailed knowledge of subsurface conditions.