Drilling test wells can cost millions of dollars. It is the job of Geologists and Petroleum Engineers to gather data about an area of interest and decide whither it would be worth the gamble to make a test well. They look at the rock formations and identify patterns that are known to commonly contain hydrocarbons.

Where are Hydrocarbons Found?

Oil is made by organic material being deposited in a layer with sediment. In time becoming sedimentary rock as it is overlaid with other material such as sand. Over time and many layers, the organic material is compressed and now much deeper in the Earth. Under all these layers and pressure it is heated from the Earth's mantel in the absence of Oxygen.

Below 175 Deg C And Oil Will Form.

If the temperatures are very high, above 250 degrees C then all carbon material will carbonize resulting in no hydrocarbons remaining. Approximately between 175 to 225 deg C, natural gas will form. Below 175 deg C and oil will form. The viscosity of the oil will be determined by the temperature. Lower temperature results in higher viscosity.

Upside Down Container Collecting The Rising Hydrocarbons.

As each hydrocarbon molecule forms, being lighter than water, it will make it's way to the Earth's surface, unless impeded in it's journey. Reservoirs being concentrations of hydrocarbons, will form where a layer of impermeable rock occurs above a hydrocarbon producing zone. This rock may commonly be dome shaped or a fault line due to tectonic Earth plate movement. This will form as an upside down container collecting the rising hydrocarbons.

Porous rock allows oil to be stored in gaps in the rock structure, similar to a sponge.

Permeable rock allows oil to flow through channels in the rock.

Rock can be porous but not permeable. There are gaps in the rock that allows storage, but the gaps do not connect to enough other gaps to allow for continuous flow.

Shale is porous but not very permeable. There are lots of holes filled with hydrocarbons. But since the holes don't meet up with other holes in other layers, there is no complete path for the hydrocarbons to flow. There is storage but no pathway.

Shale is multi layered and can have high porosity in the individual layers, but few pours between layers. So, hydrocarbons get trapped between layers within the rock.

Limestone and sandstone tend to be both porous and permeable, since the rock is made up of tiny particles that allow liquid to be both present in the particle gaps and move through the rock. Storage and flow.

With Limestone, in the presence of water, the water will dissolve the limestone creating cracks and fractures within the rock that can allow hydrocarbon molecules to flow.

Takes Millions Of Years.

All this activity takes time. The formation of the layers, the transformation of the organic material and it's slow journey upwards through rock, all takes millions of years.

About The Author
Helena Montgomery
Chewells Contributor

Helena is our longest serving contributor. She lives in an area heavily dependant upon the oil industry. She... »

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