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Geology: oil and gas resevoirs and traps, porosity and permeability, secondary porosity
From Syed Tariq Hasany

For the storage of oil and gas in any reservoir rock presence of porosity and permeability is vital. However, the degree of  porosity and permeability in any rock depends upon various factors. But quality of reservoir rock, either good or poor,  rests largely of these two parameters.

Porosity is the small voids and space between the grains of the rocks and often denotes with percent. Mathematically it is volume of the voids divided by total volume of the rock. Permeability is the ability, or measurement of a rock’s ability, to transmit fluids, typically measured in darcies or millidarcies.

Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores.

There are two types of Porosity.

Primary Porosity which is the voids and spaces in between grains, particles or crystals is the initial pores of the rocks during the deposition. Such porosity reduced  due to compaction and cementation.

Secondary porosity developed after the deposition of the rock, such as fractures, vugs, shelter etc. Keep in mind that oil migration from the source to the reservoir rocks is a natural process and it is impossible to alter the conditions unless we have gained an access to the reservoir through drilling or excavation.

There is a strong link between Trap and reservoir properties (porosity and permeability)
Oil and gas sits in the pores of the reservoir rock and trap is the overall accumulation area.

There are three main types of traps

1)   Structural traps (anticlines and folds with or without faults)
2)   Stratigraphic traps: These traps  believed to developed due local to regional  lateral variations in the reservoir facies (sand transform to be shale due to change in the depositional setting ) Diagenetic traps, mineral diagenesis etc
3)   Combination traps: Combination ofthe above two type of traps. (Struct-Strat)

How effective and good the trap is depend upon the area and lateral extent of the trap; Larger the area with high porosity more reserves can be expected.

Large trap with low reservoir porosity means relatively limited capacity of the storage. Smaller areas with higher porosity mean good storage capacity.

Higher Permaeability (in darcies and many millidarcies) means better delivery of the well and large volumes of oil and gas could be obtained. This means that if permeability is high per yield production of oil could be more profitable as you do not need to drill too many wells to drain the reservoir accumulation whereas low permeable reservois need too many wells for getting economically optimum production.