Basic Sedimentation

After we’ve learned about the three types of rocks in the Earth, we will focus on sedimentary rocks because these are rocks which can be reservoir rocks. In this article, we will start with sedimentation and learn how this process creates sedimentary rock.

A process starting with erosion and transportation of eroded material to a deposition area is called sedimentation. Eroded particles settle out of suspension and are deposited into a layer form. With time, more layers form on top of the lower layers and press down onto the lower sedimentary layers. The compaction force pushes water out from the layers. Salt crystals glue sediment particles together and finally the sediments are lithified into sedimentary rocks – this is a part of diagenesis process. Figure 1 show the process of creating sedimentary rock. Stratification results from sedimentary particles arrangement in rock layers. For each rock stratum, there is a distinct layer of sediment and this is called “bedding.”

Figure 1 – Process of Sedimentary Rock

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Diagenesis is a process of the modification of sediments into sedimentary rocks. Mineralogy and the texture of sediment are altered due to chemical and physical changes which convert unconsolidated sediment into rock.

The diagenesis process includes:

  • Physical compaction by extreme pressure – this step expels water from sedimentary layers
  • Growth of new diagenetic minerals
  • Dissolution of soluble elements of clastic rocks
  • Recrystallization and remineralization

Sedimentary rocks can be classified into 3 categories, which are Clastic, Chemical and Organic.

Clastic Sedimentary Rocks

Clastic sedimentary rocks are formed by mechanical weathering of existing rocks and some examples of clastic sedimentary rocks are breccia, conglomerate, sandstone, siltstone and shale.

Figure 2 – Conglomerate

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Chemical Sedimentary Rocks

Chemical sedimentary rocks are formed by a chemical process and some examples of chemical sedimentary rocks are salt, iron ore, chert, flint, some dolomites, and some types of limestone.

Figure 3 – Chert

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Organic Sedimentary Rocks

Organic sedimentary rocks are formed by an accumulation of animals and plants and some examples of organic sedimentary rocks are coal, some dolomites, and some types of limestone.

Figure 4 – Coal

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Richard C. Selley, 2014. Elements of Petroleum Geology, Third Edition. 3 Edition. Academic Press.

Norman J. Hyne, 2012. Nontechnical Guide to Petroleum Geology, Exploration, Drilling & Production, 3rd Ed.. 3 Edition. PennWell Corp.

Richard C. Selley, 1997. Elements of Petroleum Geology, Second Edition. 2 Edition. Academic Press.

Basic Rock Types

Understanding basic rock types gives you more ideas on how each type of rock is formed and this is a good basic for understanding geology.

Rocks can be classified into three main types which are igneous rocks, sedimentary rocks and metamorphic rocks. Figure 1 shows the relationship between the three main rock types.


Figure 1 – Rock Cycle

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Igneous Rocks

Igneous rocks are the most abundant type of rock on the Earth because it makes up about 70% of all rocks. Minerals such as quartz, plagioclase feldspar, pyroxene and olivine are important types of igneous rocks. Figure 3 illustrates minerals found in common igneous rocks.

Figure 2  – Minerals in Common Igneous Rocks.

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Plate Margins in Oil and Gas Industry

After learning about the Earth’s structure and Plate Tectonics, it is known that the Earth’s plates move relative to nearby plates; therefore this will result in high geological activities such as volcanic eruptions and earthquakes at the plate margins. It is also possible to have geological activities in the middle of plates, but it is quite uncommon. There are three type of plate margins; convergent, divergent and transform.

Divergent Margins

This happens when the Earth’s plates are moving apart and a new lithosphere is created (Figure 1).  In the oceans, the divergent process has produced the mid-ocean ridge system, which can also be described as a global range of underwater mountains. There are several ocean ridges such as the Juan de Fuca Ridge, the Mid-Atlantic Ridge, the Galapagos Rise and the East Pacific Rise. On land, the divergent margins create rift valleys such as the Red Sea and East African Rifts (Figure 2).

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Earth Structure and Plate Tectonic

The basic concept of the Earth structure and Plate Tectonic is good to know for drilling oil and gas wells.

The image below (Figure 1) shows the Earth structure. There are tree compositional layers which are curst, mantel and core. At the center of the Earth, the center core is a solid iron core which is surrounded by a liquid iron core. Core has a range between 2,900 – 6,370 km from surface. Mantel which has its range between 100 – 2,900 km consists of upper and lower mantel. Lower mantle (Meshosphere) is hot but strong due to high pressure however upper mantle is weak, hot and molten. Crust is a surface of the Earth and a majority of the Earth crust is made up of iron, silicon, oxygen and magnesium.

Figure 1 – Earth Structure

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