Petroleum Geology (Part1)

We will start with a definition (according to Merriam Webster)

pe.tro.leum  (noun) : a kind of oil that comes from below the ground and that is the source of gasoline and other products.

ge.ol.o.gy (noun) : (1) a science that studies rocks, layers of soil,etc. in order to learn about the history of the earth and its life. (2) the rock, land, processes of land formation, etc of a particular area.

Geology is the science that the deals with the history and structure of the earth and its life forms and recorded in the rock. It is essential to the petroleum industry to predict where oil and gas accumulation might occur. First thing to learn : The present is the key to the past, because the processes acting on the earth today are basically the same as those that operated in the past. Therefore by using the knowledge of reconstruct the geology history of an area we can determine the formation most likely contain petroleum reservoir or not.

Petroleum deposit must be commercially valuable to have a chance making profit according to oil and gas prices, and consider of the amount recoverable petroleum, production rate and the cost drilling the well and producing the well.

Simplify rock formation with water, oil, gas and cap rock to make a oil and gas reservoir (pic. by wikipedia upload) 

A reservoir is formation of rocks, mostly solid sometimes tight rocks, with a very little tiny pores that can contain fluids. Reservoir rocks usually contains salt water along with the hydrocarbon. Because of their densities, the layer of fluids will start with the lightest, gas on the top, then oil and water the heaviest. Oil and gas companies prefer the reservoir contains all three fluids because they are valuable mineral. Gas and water often supply the force to drive the oil out, therefore inserting pumping is unnecessary.

To have accumulation commercially valuable deposit of petroleum, the reservoir have to have the right shape to contain oil and gas and some kind of seal to trap them from escaping to the surface. It is usually about 10 ft thickness with about 10% or more pore space and permeable, means the pores are connected  so all the fluids can flow to one pore to another.

The different between high porosity and high permeability (pic. by syntropolis)

As we can see from the picture above, reservoir rocks must have relatively high porosity and high permeability. Should the rock only have high porosity (left picture), the valuable oil and gas fluid can not go anywhere. They will be forever trapped in the rocks. The high permeability (right picture) will allow the valuable fluid to travel through connected pores.