Tag Archives: geology

Clastic Sedimentary Rocks

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Clastic deposition covers about 75% of the Earth’s surface and clastic sedimentary rocks can be categorized into 3 groups based on grain size. Table 1 demonstrates grain size and type of sedimentary rocks.

Table 1 – Grain Size and Sedimentary Rock

 (Ref: http://www.takenote.it/blog/wp-content/uploads/2012/01/table13.jpg)

Fine Grained Sedimentary Rocks – Mudstone

Mudstone is a fine grain sedimentary rock and this is the most abundant type of sediments. Its grain size is less than 1//16 mm, so people cannot differentiate it with normal eyesight. Typical mudstones are siltstone, claystone and shale and the most common minerals are quartz, feldspar, calcite and clay. The fine grained rock can show the least about its formation because of a very small grain size. Mudstone is defined as sediment with a large component of clay sized material. Mudstone occurs due to slow settling from slow currents. This fine grain sediment is typically formed on blanketing ridges, continental shaves, seafloor, abyssal plain and in trenches. Continue reading →

Clastic Sedimentary and Its Environments

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Clastic sediment rocks are rocks which are formed from broken pieces of other pre-existing rocks by physical weathering. Then, rock particles are transported to lower lying areas. Mechanically eroded small pieces of rocks are usually formed in an angular shape because of a natural fracture point when erosion takes place initially.

When rock particles are transported, their shapes become rounded because of abrasion. Figure 1 demonstrates the shape of clastic particles. Furthermore, rock particles will be sorted due to conditions; for example, the flow rate of water transporting particles, size and weight of rocks, and the hardness of each rock. Eroded rock particles will be more rounded and well sorted as time progresses.  Figure 2 shows the definition of clastic particle sorting.

shape of clastic particle

Figure 1 – Shape of Clastic Particles

Figure 2 – Sorting of clastic sedimentary rocks

(Ref Image: http://www.tankonyvtar.hu/hu/tartalom/tamop425/0038_foldrajz_mineralogy_Da/images/Fig_2_7.jpg) Continue reading →

Basic Sedimentation

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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

(Ref Image: http://www.eschooltoday.com/rocks/images/sedimentary-rocks-formation-process.png)

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

(Ref Image – http://shorncliffe-rocks-bada.weebly.com/uploads/2/9/9/7/29976129/7806393_orig.jpg)

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

( Ref Image: http://f.tqn.com/y/geology/1/S/r/y/flintnodin.jpg)

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

(Ref Image: http://www.chemistryexplained.com/photos/coal-3347.jpg)

References 

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

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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

(Ref Image:  http://www.geolsoc.org.uk/ks3/webdav/site/GSL/shared/images/education_and_careers/RockCycle/Rock%20Cycle%20all%20labels.jpg)

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.

(Ref Image:  https://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Mineralogy_igneous_rocks_EN.svg/640px-Mineralogy_igneous_rocks_EN.svg.png) Continue reading →

Plate Margins in Oil and Gas Industry

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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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