There are two commonly accepted theories to explain origination of petroleum – organic and inorganic.
The inorganic theory assumes that carbon and hydrogen combined deep underground due to high pressure and temperature and thus generated oil and gas which then found passages in porous rocks to migrate to natural traps in subsurface formations.
More commonly known is the organic theory, according to which most part of hydrocarbons generated of remains of plants and animals that lived in ancient seas or rivers. For millions of years rivers carried masses of mud, slime and sand. Products of developing flora and fauna along with slime and mud participated in circulation of substances. During different geological ages which lasted for many hundred thousand years, mountains were being destructed by erosion, their fragments were being carried to seas, and masses of land were absorbed by the ocean. Mountains generated during one geological age were covered by mountains of another geological age.
The organic matter carried by waters was settling on sea bottom. The amount of organic matter which accumulated into one mass depended upon conditions of each geological age. Massive thicknesses of water and prolonged periods of continuous conditions meant that accumulation of organic matter would be significant. Short periods of continuous conditions meant that accumulation of organic matter would be small. Today it is oil and gas which we produce, and thickness of pay zone is thickness of original amount of organic matter.
Maybe millions years later, when conditions changed, other materials were carried to the top of organic matter accumulation. They could be a new type of flora or fauna, sand carried with water, or dust and mud. Thickness of a new layer of depositions fully depended upon duration of specific conditions of the period.
Oil and gas migration from the area of their generation to areas where they are located today, was both vertical and horizontal. That was associated with oil and gas tendency to migrate upwards through water thickness, which filled free spaces of formation depositions during their original generation.
As long as gas is lighter than oil, and oil is lighter than water, oil and gas tend to migrate upwards. This migration continues until they come to tight non-porous rock, which does not let oil and gas to migrate further. Oil and gas come into a trap and generate reservoirs from which we produce oil and gas today.