The oil business is very complex and organizations struggle to uncover hidden oil resources beneath the earth’s surface so that they can be profitably exploited for business purposes. The primary challenge for these organizations is to understand the key characteristics of mother earth so as to identify where the wealth of oil and gas reserves are accumulated. This sound understanding is essential for the geological engineers who help the organizations in their mission to extract oil from the earth. Let us briefly see the earth matters such as, what are the formations, where the oil reserves are present and different types of rocks, that are relevant for the understanding of the oil reserves presence beneath the earth’s surface.

Earth – a giant big onion

The earth consists of different layers such as the crust, upper mantle, mantle, outer core and inner core. The crust is the top most part of the earth and it is the thinnest layer under seabed. It consists of solid rocks mostly with silicon and oxygen. The upper mantle and mantle are the thickest parts of the earth and together they form the middle layer of the earth. Though they are the heaviest, they are composed of hot soft rocks containing iron and magnesium. As the depth increases, the density of this layer also increases. The final portion of the earth consists of the outer core and inner core with the heaviest layer and dense substance containing iron and nickel. From an oil and gas perspective, only the top most layer of the earth, which is the crust, is important and relevant for finding oil reserves.

Oil as a nuisance value

Oil and gas are resources naturally present on the earth. In the initial days, human beings did not recognize these resources to be useful. There were instances when petroleum seeping out on the surface in some areas and they were considered as spoiled areas as petroleum polluted soil and water. This was a big challenge for the inhabitants to handle and they contained the seepages by mobilizing huge quantities of sand and other materials over the place. Hence, there were times when petroleum was considered as a nuisance by the society.

This changed dramatically when people discovered how useful petroleum was – first as fuel for lamps to bring light to homes and cities and later for energy generation. It became even more important after the industry developed more and more products that were based on oil. Soon, oil and gas turned into resources that are intensely sought after. More exploration activities commenced and oil reserves started depleting fast. Only in very rare cases do oil and gas appear naturally at the surface these days. Most of the times oil and gas is usually hidden deep below thick layers of rock. To discover the resources, engineers must have a profound knowledge of the earth and put in a lot of effort in research.

Oil play – where are the oil reserves hidden?

As petroleum resources have become a premium content, more efforts and expert knowledge have led to a clear understanding of where the oil reserves are exactly located. As stated earlier, oil reserves are found in the top most layer of the earth called crust. Only up to 10 kilometers of the upper crust area is suitable for the oil to be present in a stable condition for a long period of time. Beyond that point, the atmosphere becomes too hot for oil to sustain and it starts to decay and it is not suitable for use or extraction. It is interesting to know the processes that work together for the generation of the oil and gas and the prerequisites for a reservoir where oil and gas can accumulate to an amount that is feasible to exploit. It should make economic sense to recover the oil from the reserves as it entails huge investments in the process.

To understand the petroleum play concept, we need to gain full comprehension of a few key terms such as cap rock, source rock, reservoir rock, trap and migration. Oil and gas reserves are very mobile as they are very light as compared to other formulations of the rock. Hence, they start moving up striving to reach the surface of the earth. If the content is in gaseous form and this movement is not halted, it reaches the surface and escapes into the atmosphere. In case of liquid substance, it will stay on the ground waiting for bio-degradation. If stopped during the movement by a rock formation due its lack of permeability, then the oil reserves start moving laterally to a point where they find a spot through which they can travel upward. This impermeable rock formation is called cap rock.

A source rock is the origin where hydrocarbons are produced or generated. They are basically organic shale layers and due to high pressures and temperatures, hydrocarbons are emitted from these rocks. The nature of these source rocks is highly porous and permeable. The source rock formations are interconnected and hence the petroleum reserves can easily travel through them.

The rock formations sandwiched between the cap rock and source rocks are called reservoir rocks, and this is where large quantities of hydrocarbons are trapped. After expulsions from the source rocks, hydrocarbons flow into the reservoir rocks and this process is called migration. A trap is a favorable situation where the hydrocarbons are stored between the source and cap rocks. It is interesting to note that if any of these factors does not exist, then the accumulation of hydrocarbon reserves does not happen. Eventually, the oil reserves would reach the earth’s surface only to vanish in thin air or degrade themselves to be buried into the earth to start the fresh cycle of hydrocarbon formation.

Complex area which requires expertise

What we discussed in this article is the tip of the iceberg regarding the characteristics of earth formulations that lead to accumulation of the petroleum reserves. Specialized knowledge and experience is required to gauge the earth’s surface that would lead to potential hydrocarbon presence. Please note that, as discussed before, the reservoir rock may be far away from the traces of hydrocarbons seen on the earth surface. The idea is to get to the core of the reservoir rocks where the treasure of black gold lies for which highly specialized skill, technology and quintessential wisdom of geology are necessary prerequisites.