Paleontology in petroleum geology

The word "Paleontology" originates from Greek and refers to the  study of old life. Paleontology is the study of the history of the earth. It is simillar to a grandfather who is telling his granchildren a story about something that happened in the past. However,Paleontology does not use words to illustrate. However, it uses fossils and organisms and includes the study of fossils in order to know their evolution and relationships with each other.

Introduction

Paleontology has started in the 5th century BC and since then, it has used all possible technology to make a better explanation of the past. Modern paleontologists have a variety of tools that help them discover, examine, and describe fossils. For example, electron microscopes allow paleontologists to study the tiniest details of the smallest fossils. In addition, X-ray machines and CT scanners reveal fossils' internal structures. Recently, fossils have been analized by measuring how similar the DNA in their genomes is. However, estimating the dates of these remains is essential but difficult. Therefore, paleontologists rely on relative dating (older or younger than) or absolute dating by radiometric dating for this purpose. 

In this article, paleontology is introduced in the beginning with its subdisciplines.Then, the link between paleontology and oil industry is discussed in detail. Finally, some examples of  fossils used in petroleum exploration are discussed.  

Paleontology subdisciplines

Paleontology is a rich science that can be divided into various subdisciplines:

Micropaleontology

is the study of fossils of microscopic organisms such as protists, algae, tiny crustaceans, and pollen.

Invertebrate Paleontology

is the s tudy of invertebrate animal fossils such as mollusks, echinoderms, and others.

Vertebrate Paleontology

is the study of vertebrate fossils from primitive fishes to mammals.

Paleo botany

is the study of the history of the Plant Kingdom. It is a major branch of paleontology. Moreover, it encompasses several subdisciplines including palynology and paleo ethnobotany.

Palynology

is the study of pollen and spores, both living and fossil, produced by land plants and protists.

Human Paleontology (Paleoanthropology)

is the study of early humans, their origins, evolution, cultures, and way of life.

Taphonomy

is the study of how fossils are formed and preserved.

Ichnology

is the study of fossil tracks, trails, and footprints.

Paleoecology

is the study of the ecology and climate of the past, as revealed both by fossils and by other methods.

In oil industry, which branch is more important? The answer to this question relies on the type of fossils present in the rock. When the well is drilled ,cuttings come to the surface. Cuttings are the small pieces of rock broken up by the drill bit and brought to the surface by drilling mud. Microfossils, being small, can be recovered totally. In addition, microfossils happen to be abundant, especially in marine rocks which are the most common form of sedimentary rock in the crust of the Earth. Moreover, microfossils have many applications to petroleum geology. The two most common uses are: biostratigraphical and paleo environmental analyses. What do fossils and paleontology have to do with oil? The relationship is clarified in the most used subdiscipline of plaeontology which is micropaleontology. Therefore, micropaleontology is described in details in the following section.

Micropaleontology

Micropaleontology is the field of study dealing with small fossils that are best studied with a microscope. Microfossils include a wide variety of microscopic remains of single-celled organisms, plants, and animals.

In oil geology', paleontologists  use microfossils to determine the formation which contains specific fossils.These fossils are called index fossils and paleontologists  use them and the principle of the superposition to make a biostragraphic correlation for the drilled formations.When an oil well is drilled, small cuttings come to the surface. Sometimes, these cuttings of rock include microfossils. The further and further down you drill, the older and older these microfossils get.

Geologists and paleontologists have been able to put dates on some of these fossils. After a lot of wells were drilled, it became clear that certain microfossils were only found in certain rock layers. When you drill into a layer containing one of these index fossils, you are almost sure you know the age of that  rock. So, a certain rock layers that may contain oil can now be more easily found. For example, immagine a well is drilled through rock layers containing microfossil "x", then "y". Then, it went deeper till it encountered microfossil "z" followed by striking an oil zone. If a similar well is drilled nearby and we found microfossils "x" and "y", we can expect to see microfossil "z" next, and with it, possibly oil.

There are a great number of different types of microfossils available for use. However, three groups of them are the most important to hydrocarbon exploration: foraminifera, calcareous nannofossils, and palynomorphs'.

Foraminifera

Foraminifera or foraminifers or forams (for short) are generally very common in marine post-Palaeozoic rocks. They are quite easy to get out of soft rocks such as clays and marls, but can also be found in loose oolites, shell sands and chalks .The planktonic and benthic ones are most common. The Calcareous Benthics have one or more chambers, and live in the sediment at the bottom of the sea, on the surface of the sediment, on plants or rocks, or attached to plants. The Planktonic forams float in the sea. They are multi-chambered and tend to have quite lightweight delicate tests. Some develop keels around their edges to weigh them down a bit so they can maintain their position (depth) in the water column.

Calcareous nannofossils

Calcareous nannofossils are extremely small objects (less than 25 microns) produced by planktonic unicellular algae. As the name implies, they are made of calcium carbonates. Nannofossils have first appeared during the Mesozoic Era and have persisted and evolved through time. Calcareous nannofossils include the coccoliths and coccospheres of haptophyte algae and the associated nannoliths which are of unknown provenance. The organism which creates the coccosphere is called a coccolithophore. They are  very useful tools for biostratigraphy.

Palynomorphs.

They are organic walled fossils and include fossil pollen and spores as well as certain marine organisms such as dinoflagellates. Pollen and spores are transported by wind and water. They can travel long distances before their final deposition. Moreover, they are surprisingly resistant to decay and are common as fossils. Because of the long transport before deposition, they usually tell us little about the environment of deposition. In addition, they can be used for biostratigraphy.

Summary and Conclusions

Paleontology is the study of what fossils tell us about the the past. Therefore, fossils (index fossils) are used to make some expectations about the presence of oil in a particular place.In this article, paleontology was discussed in general. Then, an indication on how it is applied in oil industry was given. Finally, definition of the most important fossils related to hydrocarbon exploration was indicated.