All rocks types fall into one of three broad categories based on how they are formed. These three types can cycle back and form with each other depending on the changes in the external environment.
Igneous rocks contain minerals that have crystallized from magma. If the magma is situated deep in the Earth's crust, its rate of cooling will be slow, thus favouring the formation of coarse-grained rock, e.g. granite. Faster cooling at or near the Earth's surface results in finer grained rocks, such as basalt erupted from volcanoes.
Metamorphic rocks are formed by the alteration of pre-existing rocks due to the application of heat and / or pressure. One of the best-known types of metamorphic rock is marble, which is formed when limestone is subjected to high temperature and pressure.
Sedimentary rocks. Clastic sedimentary rocks are formed by the weathering, transport and deposition of debris derived from pre-existing rocks. Carbonate rocks are formed by organic or inorganic precipitation of calcium carbonate. Evaporitic rocks, such as halite (NaCI), form due to partial or
complete evaporation of seawater.
When igneous and metamorphic rocks are exposed at the Earth's surface they are subjected to an environment which is totally different to that in which they were formed. The temperature and pressure are both much lower, and the environment is oxygen rich and often wet. As a result, many silicate minerals undergo changes that may lead to their total or partial breakdown and, subsequently, to the destruction of the rocks themselves. These breakdown processes are known collectively as weathering, and it is the products of weathering that eventually accumulate to form sedimentary rocks. Sedimentary rocks themselves may be subjected to renewed cycles of weathering.
Nearly all hydrocarbon reservoir units are sedimentary rocks. After deposition, they were buried by later episodes of sedimentation. During burial, the porosity and permeability of the rocks often change as the grains become packed more tightly together and new minerals are precipitated in the pore spaces. Most hydrocarbons occur in the Earth's major sedimentary basins - regions of the crust which have subsided to accumulate thick sequences of sedimentary rocks. Oil and gas also occur in some mountain or orogenic belts.
The development of sedimentary basins is linked to past movements of huge slabs of the Earth's crust, described by the theory of plate tectonics. Before describing sedimentary basins in more detail, it is necessary to first consider the structure of the earth and the theory of plate tectonics.