Under such conditions, and over long periods of time, the original organic material changed, breaking down into simpler substances called hydrocarbons - compounds of hydrogen and carbon. The result was oil, a complex mixture of hydrocarbons. Over millions of years, continents move, compressing and stretching the earth's crust. Compressing the crust creates mountain ranges. When it stretches, the sagging causes sedimentary basins where oil and gas are formed.
Oil naturally tries to flow upwards, from high to low pressure conditions.
Where possible, it makes its way to the earth's surface and escapes in seeps. Fortunately for the modern world, some of it has been trapped along the way in reservoirs. An oil reservoir, contrary to popular belief, is not an enormous underground lake. Very often, it is a seemingly solid rock, which on close inspection contains myriads of minute spaces or pores. By moving from one space to the next, and sometimes by flowing through fractures, oil migrates slowly upwards.