Week 10

This week’s lectures started with a recap of previously learnt material, such as the origins of petroleum and the details of the petroleum system.

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The petroleum system is made up of several components and processes, which are:

Components:

- The source rock

- The reservoir rock

- The seal rock

- Trap

Sandstone makes for a great reservoir rock for its high porosity and permeability.

Another important type of rock is siliciclastic rock, which are clastic noncarbonate sedimentary rocks. They are made of silica mostly, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits that are subsequently lithified. They are sandstone based rocks accounting for about 50 - 60% of the world oil and gas exploration.

Another sedimentary structure of note is tafoni. Tafoni are small cave-like features found in granular rock such as sandstone. Tafoni have also been called fretting, stonelace, stone lattice, honeycomb weathering, and alveolar weathering.

Traps are often a result of faults. There are two types of faults, normal faults and reverse ones.

Seals are very fine grain sediment, which prevent the hydrocarbons from migrating to the surface.

Processes:

- Generation

- Migration

- Accumulation

- Preservation

As for the origin of natural gas and oil, it is due to the decay process. Organic matter that existed a long time ago decomposed and was buried underneath sediments and eroded matter, due to transportation and deposition. Under conditions of high pressure and temperature, this matter is then lithified and turns into source rock.

Although oil and gas are produced in a similar manner, there are certain factors that make them end up having very different characteristics.

Organic matter like algae, bacteria, and land plant (such as Exinite, Cutinite, Sporonite, and Resinite) turn into Sapropelic Kerogen, which turns into mainly oil.

On the other hand, organic matter like land plant (non-fluorescent vitrinite) changes into Humic Kerogen which turns into mainly gas.

Kerogen is basically a mixture of organic compounds.

There are 5 types of kerogen, which are:

- Type I: Hydrogen to carbon ratio > 1.25, Oxygen to carbon ratio < 0.15, produces liquid hydrocarbons.

- Type II: Hydrogen to carbon ratio < 1.25, Oxygen to carbon ratio 0.03 to 0.18, produces a mix of gas and oil.

- Type III: Similar to Type II but high in sulfur.

- Type IV: Hydrogen to carbon ratio < 1, Oxygen to carbon ratio 0.03 to 0.3, produces a mixture of coal and gas.

- Type V: Hydrogen to carbon < 0.5, Contains organic matter in the form of polycyclic aromatic hydrocarbons, no potential to produce hydrocarbons.

As for the migration process, there are 3 types of migration: primary, secondary and tertiary.

Primary migration is when the hydrocarbon leaves the source rock. Secondary is any movement within the reservoir rock. Tertiary migration is when the hydrocarbon moves to the surface.

The factors that lead to migration:

a) Burial

b) Compaction

c) Separation of constituents of source rock

d) Increase in volume

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