Organic Matter to Form Source Rock

Oil and gas that we are drilling today comes from a biogenic origin and it is formed with proper time and temperature. Organic matter is one of the most important parts of hydrocarbon generation. This topic will give you an overview of how organic matter will be transformed to hydrocarbon.


Starting with plants and algae, take carbon (CO2) from the atmosphere and process it to form glucose and this starting process is called photosynthesis. Glucose is transformed into more complex organic compounds. Trees, for example will grow bigger because they use photosynthesis to convert into energy. When animals and trees die, the organic matter is typically oxidized and this will create CO2 and put water back into environment again.  However, in some situations when organic matter is buried quickly in areas where there is no oxygen, the organic matter may be preserved. If the organic matters are buried in proper conditions, petroleum may be formed. Continue reading

Geological Time Scales

Geological time scales help us to know the age of formations and three types of time scales are relative, absolute time and magnetic polarity scales. Relative time scale relates to an order in which a specific rock sequence occurs, but absolute time is an actual time that is derived from the chemical half-life of minerals in rocks. Magnetic polarity uses a concept of magnetic sequences to age the rocks.

Continue reading

Clastic Sedimentary Rocks

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


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

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: Continue reading

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

(Ref Image:

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 –

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:

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:


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.