Oil and Natural Gas - How do they form and accumulate?

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• published: 14 Oct 2014
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Starting out from the source rock where they are formed, hydrocarbon molecules, which are lighter than the water impregnating the upper part of the earth's crust, set off on an upward journey through the rocky layers towards the earth's surface. When they reach porous and permeable rocks, which characterize oil or gas reserves, they can accumulate in great quantities. However, a hydrocarbon deposit will only form if the reservoir rock is capped by a layer of impermeable rocks, preventing oil and gas from rising vertically towards the surface, and if it forms a closed space, preventing oil and gas from rising up laterally. In the source rock, hydrocarbons are present in higher volumes and are under greater pressure than the initial kerogen. Little by little, they are released into the rocky layers impregnated with water located adjacent to the source rock. Because hydrocarbons are lighter than water, gas and oil tend to rise towards the earth’s surface by circulating between the mineral particles of the rocks. This slow, constant movement is called migration. Some hydrocarbons are prevented from moving upwards: • Either because they are dissolved in the water contained in the rocks they traverse (this affects gas much more often than liquid hydrocarbons). • Or because they stick to the rock grain. This phenomenon is known as migration losses. These losses can be significant, especially if the oil and gas have a long way to travel. This is why some source rock hydrocarbons will never be suitable for drilling. In order to form a hydrocarbon deposit, there must first be a reservoir rock. These are sedimentary basins, which are made up of sedimentary rocks that enable the migration of oil and gas. They are formed from grains, solid particles deposited in seas, oceans, lakes or lagoons. The rocks differ according to the size of the grains from which they are formed. These grains come into contact with each other but there are also empty spaces between them. These spaces define the porosity of each rock: the higher the percentage of these spaces within the rock, the more porous the rock. These porous spaces are more or less interconnected. This connectivity is defined by their permeability, which characterizes the ability of liquids (water, oil or gas) to circulate within the rock through the porous network. These empty spaces between porous rocks, normally occupied by water, may also contain hydrocarbons. Oil exploration engineers seek to prospect reservoir rocks – or just reservoirs – which combine good porosity (large quantities of hydrocarbons) and good permeability (the ability to extract these hydrocarbons thanks to the ease at which they can move around inside the rock). These two qualities are not always present within the same rock: clay is porous because it is made up of particles with large empty spaces between them and impermeable because these particles are in the form of flat sheets piled tightly on top of each other, which impedes the circulation of fluids. Rocks that are fractured or cracked make good reservoirs because the hydrocarbons can move around more freely. However, the migration of hydrocarbons through a reservoir rock is not enough to form an oil or gas deposit. Unless the hydrocarbon molecules are prevented from rising, they will only pass through the reservoir rock instead of accumulating within it. To trap the hydrocarbons, an impermeable rock – called a seal or cap rock – is needed to form a barrier above the reservoir rock. While crystallized clay and salt layers (evaporites) form the best cap rocks, any rock that is sufficiently impermeable – such as highly compact carbonates – can serve as a cap rock. Accumulations (or pools) of oil and gas, like those of gold, silver and many other minerals, are rare. And those that have commercial significance are even more so. The geology of oil and gas deposits is complicated and cannot be fully told herein. Pools form over millions of years when a certain set of geological conditions exists or are fulfilled. Among others, important factors are: 1) source rocks, primarily marine, wherein oil and gas are formed from organic materials such as microscopic plant and animal remains; 2) porous and, permeable rocks to provide a route for oil and gas migration from source rock to a place of entrapment or accumulation; 3) a suitable trap containing sufficient porosity and permeability to allow migration and accumulation or storage, and an impervious cap rock to prevent escape; 4) an energy source such as gas or water under pressure sufficient to move the fluids from pore space to pore space and out into the well bore once the trap, or reservoir, has been penetrated by the drill.