Rocks are made up of minerals. These rocks make up the crush of the earth.
There are three basic types of rocks:
- Igneous rocks are formed when melted rock cools and solidifies. Melted rock may come in the form of magma, when it is found underneath the Earth’s surface. It can also come in the form of lava, when it is released unto the Earth’s surface during a volcanic eruption. Some examples of igneous rocks are granite, scoria, pumice, and obsidian. Pumice, for instance, is formed when lava made up of melted rock, water, and trapped gas is ejected from a volcano during a violent eruption. As the ejected material undergoes very rapid cooling and depressurization, some of the trapped gas escape, leaving holes and gas bubbles on the solidified material.
- Sedimentary rocks start forming when soil and other materials on the Earth’s surface are eroded and finally settle down, forming one layer of sediments. As time passes, more and more materials get eroded and settle on the older layers. Thus, layer upon layer is formed. The lower layers undergo intense pressure due to the weight of the upper layers, eventually evolving into rocks. Some examples of sedimentary rocks are sandstone, limestone, shale, conglomerate, and gypsum. Sandstone, for instance, is a result of depositions of sand from beaches and rivers. You can find them mostly in deltas, since this is where the rivers flow into the ocean.
- Metamorphic rocks are actually products of rocks that have undergone changes. Thus, a metamorphic rock may have originally been an igneous, sedimentary, or even another metamorphic rock. The changes occur when the original rocks are subjected to extreme heat and pressure beneath the Earth’s surface. They may also occur when the original rocks are caught in the middle of two colliding tectonic boundaries.
Only sedimentary rocks are considered to have potential as reservoir rocks for oil and gas, because it has empty spaces within them for oil and gas. Some rocks are known as source rocks (where plant and animal life contained in the rock actually decomposed under high pressure and temperature and where oil and gas actually formed) and others are known as reservoir rocks (where oil and gas move and rest within the spaces in the rocks). Reservoir rocks has different properties such as porosity, permeability and fluid saturation.
The word “petroleum” is sometimes used interchangeably with “hydrocarbon”. Petroleum can exist in solid form such as tar or pitch, or in liquid form such as crude oil which is what pumped up the ground before sent to refineries. Natural gas is a lighter form of hydrocarbon free in the formation pores, dissolved in liquid, or attached to rocks.