What are Rocks?

To understand what rocks are, let’s break it down to basics. Atoms make up elements, elements make up minerals and minerals make up rocks. Atoms are the smallest part of any element that can exist alone and still behave like that element at any point in time. Elements are pure substances that cannot be broken down into simpler forms by any form of chemical reaction. Although, it is possible to break down elements into smaller sizes by a physical means, yet it retains its original properties no matter how small it is broken. For example, a gold bar contains the element gold (Au) and no matter how small the gold bar is broken into, each new size formed will still retain all the properties of gold. So although elements can be broken down into smaller sizes by physical means, nothing can be done to break them down chemically. In both cases, the element remains pure or retains its original properties. Let’s consider two elements common in nature, silicon and oxygen, when these two different elements combine together, we have something called silicon oxide (SiO2), this SiO2 is no longer an element but a mineral. In other words, when we combine elements together we get minerals. The SiO2 mineral is what we call sand. Now, just one grain of sand contains the SiO2 mineral. Now, imagine a plastic container containing some water, if we filled this container to the brim with sand grains and left it long enough, over time we’ll find out that the sand grains will have joined together and become one compact solid mass. This compact solid mass of SiO2 minerals is sandstone.

How Hydrocarbon Reservoirs Are Formed: The Rock Cycle

Sandstone is a rock. Armed with this understanding, we can go on ahead to define rocks. A rock is formed when mineral grains have become compacted over time into a solid mass. The minerals in a rock could be the same throughout the rock or the rock could be formed from a combination of different minerals. What is important here is that they are consolidated or compacted into one solid mass; else it is not a rock. If we broke down this sandstone into tiny bits using a hammer for instance, it returns back into sand (SiO2 mineral) and cannot be called a rock at this point.


How Rocks are Formed - The Rock Cycle

How Hydrocarbon Reservoirs Are Formed: The Rock Cycle

The process it takes to form rocks naturally on earth takes millions of years. Let’s start with igneous rocks. Inside of volcanoes are liquid minerals called magma. The heat within the earth is high enough to melt minerals into liquid (magma). On the surface the temperature is much colder compared to that found several miles in the earth. When volcanoes erupt, the magma flows upwards to the surface and empties itself on the sides of the volcano. Do not forget that the temperature at the surface is much cooler. Magma that has found its way to the surface is called larva. So due to the much colder surface temperature, the lava cools from the liquid minerals it existed as within the earth to solid minerals on the surface. Now, we already established that when minerals become compacted they form a rock. A volcanic igneous rock will be formed from this volcanic eruption when the lava cools. Sometimes however, the magma may not even get to the surface before cooling, they may flow upwards to a certain height, encounter a temperature low enough to make them solidify right before getting to the surface. This type of igneous rock called intrusive igneous rock.

Already, we know that it is possible to break down a rock back into minerals, like the sandstone example that was broken into tiny bits using a hammer. For the volcanic igneous rock, instead of hammer, natural elements like wind, water and ice can break down the rock into tiny bits and carry these bits away to other portions of the earth. Although we could finish grinding that sandstone sample within minutes, in nature, it takes millions of years to do this. The breaking down of the rocks over time is a gradual process and it is called weathering. Let’s consider how water can break down rock, when it rains the rain drop that strikes the rock could wear away small rock minerals located on the surface, if more and more rain drops continually strike the same point over time, they wear enough minerals from the rock to create a groove or a small hole in the rock. This groove will now cause even more damage to the rock as it will serve as a container to collect water. Next time it rains, some pockets of water will collect in the space now created within the solid rock mass. This water will gradually weaken the bond between minerals and cause them to easily break down and be carried away much readily. The process by which these broken rock bits are carried away is called erosion. These rock bits will be carried as far as possible until the force of the carrying agent (for example running water) weakens, at this point the rock bits will not move any further and it is said to have been deposited. These rock bits form part of what is known as sediments. When another set of sediment is brought another time, they will be deposited on the former one and thus we have one layer of sediments lying on top of another layer of sediments. This could continue until we have several layers of deposited sediments, one lying on top of the other. Over time, the pressures from the upper layers become high enough to compact the lower layers until they become one compact solid rock. The process at which downward pressure compacts sediments into rocks is called lithification and the rock formed this way is called sedimentary rock. So sedimentary rocks are formed from the weathering, erosion, deposition and lithification of fragments of other rocks. During lithification, water present in the depositional environment may also react with the rock minerals to help form cement that binds the rock grains together even more tightly.

We have seen an example of how igneous rock can become sedimentary rock due to weathering and erosion of rock fragments over time. One thing to note though is that the deeper we go down into the earth, the hotter it becomes. Now, if that sedimentary rock gets buried deep enough, the temperature and pressure at that depth could become so high that the minerals present in the sedimentary rock become altered or changes because of the great heat and pressure. The rock formed when the minerals of other rocks are altered this way is called the metamorphic rock, in other words the rock has undergone a form of metamorphosis and is now different from what the way it was before. If the metamorphic rock gets pushed even deeper, it will get to the temperature at which the minerals in the rock become liquid once more, waiting to be pushed to the surface to form igneous rock.

How Hydrocarbon Reservoirs Are Formed: The Rock Cycle


Conclusion

The oil and gas industry is most concerned with sedimentary rocks because hydrocarbons are typically found in sedimentary rocks. Looking at the way these rocks were formed, we can see that of all rock types, it is easier for organic matter to be found in sedimentary rocks than any other rock type. The organic matter can be carried with the sediments during transport and deposited. In oceanic environments, organic matter like phytoplankton can die and fall to the sea floor, then sediments will be deposited on top of them and they become trapped in the sedimentary rock. A kind of sedimentary rocks, limestone, is formed from precipitation of minerals in the sea instead of weathering and erosion of sediments. These minerals precipitate out of the water and trap dead organic matter that had fallen to the sea bed.