The Story of Petroleum

Oil: The Geopolitical and Geological Story of Venezuela, Saudi Arabia, and Yemen

Why Does Venezuela Float on Oil, While Yemen Does Not?

The first time I went to Thailand, I saw a Yemeni waiter in a Bengali restaurant in Bangkok. When people from Bangladesh go to Saudi Arabia in search of work, people from Yemen—Saudi Arabia’s neighboring country—go abroad in search of work. Venezuela has the largest proven oil reserves in the world, yet its neighboring countries like Guatemala and Mexico do not. Similarly, in the Middle East, Saudi Arabia is wealthy due to its abundant oil resources and its people live luxurious lives—while their neighbor Yemen struggles so much with lack of resources that many people cannot even eat properly.

When we talk about “oil-rich countries,” we usually picture Saudi Arabia, Venezuela, Iraq, Iran—the vast deserts of Arabia and the wide plains of Latin America. But we rarely ask: How was this oil actually formed? Why do only a few regions of the world have such massive oil reserves, while neighboring countries do not?

In this article, we will explore step by step –

• How is petroleum (crude oil) actually formed?
• Why did Venezuela become one of the world’s top oil reservoirs?
• What is the geology of the Orinoco Belt and the chemical characteristics of heavy crude?
• Why does Saudi Arabia have so much oil, while neighboring Yemen has so little?

This story is not just about geology – it is a combined drama of time, oceans, life forms, and tectonic plates over hundreds of millions of years. In the final act of this drama, we see today’s global oil map.

How Is Petroleum Formed? A Story of Hundreds of Millions of Years of Geo-Biological Evolution

Petroleum is not a “magical” liquid; it is primarily formed from the dead remains of algae, plankton, tiny marine organisms, and plants that once lived in ancient seas, lakes, and wetlands. Over millions of years, these organic materials were transformed into oil and gas through pressure, heat, and chemical reactions.

1) Accumulation of Organic Matter: Birth of the Source Rock

Millions of years ago, ancient seas and lakes were full of algae (phytoplankton) and tiny organisms (zooplankton). They lived, grew, and died. Most of their remains decomposed, but a portion settled on the oxygen-poor (anoxic) seafloor, mixing with mud and sand.

This is how organic-rich rock formed, which geologists call the Source Rock.
It is usually shale or calcareous mudstone, containing
Total Organic Carbon (TOC)—the amount of organic carbon—ranging from 2% to 10%.

2) Transformation Under Heat and Pressure: From Kerogen to Oil and Gas

Over time, more sediment accumulated above, pushing the rock layers deeper. As depth increased:

• Temperature increased (about 25–30°C per kilometer)
• Pressure increased

Under these conditions, the organic matter in the Source Rock first transformed into Kerogen—a solid, complex organic substance.
When temperatures reached about 60–120°C, Kerogen slowly broke down into liquid hydrocarbons (oil). This stage is called the
Oil Window.

At higher temperatures, 120–200°C, Kerogen and oil further cracked into natural gas. This stage is called the
Gas Window.

3) Migration of Oil: From Source Rock to Reservoir Rock

The oil and gas formed in the Source Rock do not remain there for long because:

• Oil and gas are lighter than rock
• The Source Rock is usually low in porosity and permeability

So oil and gas move upward and sideways through fractures and pores.
This process is called Primary Migration, followed by Secondary Migration when they enter
Reservoir Rock—such as sandstone or limestone, which have higher porosity and permeability.

4) Getting Trapped: Traps and Cap Rock

If oil could move freely, it would often reach the surface and escape like gas or oxidize and disappear. But in reality,
impermeable rock (Cap Rock) often blocks its path.

Examples include:

• Shale
• Evaporite/Anhydrite (salt-rich rock)

Under these cap rocks, oil accumulates in folded or faulted structures. These are called:

• Anticline trap – oil accumulates at the crest of folded rock layers
• Fault trap – oil is trapped when a fault juxtaposes reservoir rock against cap rock
• Stratigraphic trap – oil is trapped due to changes in rock layers and their geometry

These traps are where modern Oil Fields are formed.

Why Do Neighboring Countries Not All Have Oil?

Venezuela is currently one of the countries with the largest proven oil reserves in the world — especially because of the Orinoco Oil Belt. The question is, why are other countries in the same region not as oil-rich? The answer lies in geology, sedimentation, and tectonic history.

The Orinoco Belt of Venezuela is essentially a type of Foreland Basin — formed in front of a large mountain range.
Due to the uplift of the Andes Mountains, this part of Venezuela gradually subsided and turned into a large basin where millions of years of sediment accumulated.

1) Orinoco Belt: A Super-Giant Foreland Basin

Characteristics of this basin:

• Length: more than 600 kilometers
• Width: 70–100 kilometers
• Age of sediments: mainly Paleogene (about 30–40 million years)
• Environment: shallow sea, delta, fluvial (river-driven), and wetlands

This region was once part of an ancient sea, where abundant algae and marine organisms lived. Their dead remains mixed with mud and sand and formed
organic-rich Source Rock.

2) Abundance of Organic Matter and Favorable Conditions

In the Orinoco region:

• The ancient sea had abundant plankton and algae
• The seafloor had low oxygen, so organic matter did not decompose quickly into gas
• Sedimentation rates were high, so organic matter was rapidly buried and preserved

Because of all these factors, the quality of the Source Rock became extremely high and gained the ability to generate massive amounts of hydrocarbons.

3) Reservoir and Cap Rock of the Orinoco Belt

The oil in the Orinoco Belt is mainly stored within sandstone, which is relatively loose and porous. Above it lies:

• Thick shale layers that block the upward movement of oil

As a result, the oil could not rise fully to the surface; instead, it spread within the lower layers and formed massive oil reservoirs.

4) Why Do Neighboring Countries Have Less Oil?

Some of Venezuela’s neighboring countries do have oil, but not massive reserves like Venezuela. There are several reasons for this:

• No similar foreland basin — meaning such a large and deep sedimentary basin did not form
• Less accumulation of organic matter — ancient seas and deltas were positioned differently
• Different tectonic stability — some areas have more faulting, some lack continuous Source Rock

So even though the geography is close, the geological and sedimentary history made Venezuela an “exceptionally rich” oil province.

Heavy Crude: Chemical and Physical Properties of Orinoco Oil

The oil from Venezuela’s Orinoco Belt is known as Heavy oil or Extra-heavy oil.
Unlike conventional light crude, these oils are thick, sticky, and more expensive to process.

1) What Does API Gravity Mean?

API Gravity is a measure of how “light” or “heavy” a crude oil is.

• Light crude: usually API > 30°
• Medium crude: API 22–30°
• Heavy crude: API 10–22°
• Extra-heavy crude: API < 10°

The API gravity of Orinoco Belt oil is generally 8–16° — meaning it falls into the Heavy to Extra-heavy category.

2) Chemical Composition: Asphaltene, Sulfur, and Metals

Heavy crude contains:

• High Asphaltene content — complex, high–molecular-weight hydrocarbons that make the oil thick and dark
• High Sulfur content — can be 2–5%, causing environmental and refining challenges
• Metals (Nickel, Vanadium) — can damage catalysts and complicate refining processes

Heavy crude is typically sticky, with high viscosity — often behaving like thick molasses at room temperature.

3) Why Is Heavy Crude Expensive to Extract?

Unlike light crude, heavy crude does not always flow easily just by drilling a well. Because:

• Its viscosity is high, so pumping the oil is difficult
• Often heat must be applied in the field — steam injection, thermal recovery

As a result, even though Orinoco’s oil reserves are massive, both technology and capital are required in large amounts for extraction and processing.

Saudi Arabia vs. Yemen: Different Oil Fortunes on the Same Peninsula

Now let’s come to the Arabian Peninsula. The question is very natural:
Why does Saudi Arabia have such massive oil reserves, while neighboring Yemen has comparatively so little?

1) Arabian Plate and the Tethys Sea: Saudi Arabia’s Geological Fortune

Saudi Arabia sits on what is known as the Arabian Plate. This plate was once part of the seafloor of the ancient
Tethys Sea.
In this sea:

• There was extremely high primary biological productivity (algae, plankton)
• Many regions of the seafloor had low oxygen
• Organic matter mixed with sediments and accumulated into shale and calcareous mud

These rocks later became the primary Source Rock for Saudi Arabia’s modern oil fields.

Another major feature of Saudi Arabia’s oil reservoirs is:

• High-quality Reservoir Rock: such as the Arab-D limestone, which has excellent porosity and permeability
• Strong Cap Rock: thick evaporite/anhydrite layers that block the upward movement of oil
• Extensive Structural Traps: large anticline structures (such as the Ghawar field)

In other words, from the generation of oil to its trapping, Saudi Arabia’s entire petroleum system is “almost ideal.”

2) Yemen’s Geology: Rift, Volcanism, and an Unstable Basin

Even though they share the same peninsula, Yemen’s geology is very different. In Yemen:

• Rift and volcanic activity are high — rock layers are fractured and uplifted
• Sedimentary cover is thin — meaning Source Rock and Reservoir Rock are less continuous
• Faulting is extensive — even if oil forms, traps can break, causing oil to escape or disperse

Yemen does have some oil fields (such as in the Marib and Shabwa regions), but they are small and scattered.
There are no super-giant fields like those in Saudi Arabia, because the full set of Petroleum System elements (Source, Reservoir, Trap, Seal) rarely align perfectly in one place.

3) “Two Neighbors, Two Fates”: A Result of Geology, Not Geopolitics

To us, Saudi Arabia and Yemen appear politically different and economically worlds apart today. But if we look for the root cause of this difference,

we find that this inequality is primarily the result of ancient seas, plate tectonics, and sedimentation processes.

One region (Saudi Arabia) received the “perfect recipe” for generating and trapping oil; the other region (Yemen) inherited fractured, unstable geology where oil may form but cannot be preserved in large quantities.

The Oil Map Is Actually a Geological Map

We often see oil as “wealth,” “resource,” or “economic blessing.” But behind a country being oil-rich or oil-poor, there is no divine blessing, luck, or political leadership’s “achievement.” Instead, there are:

• The history of ancient seas and lakes
• Accumulation and preservation of organic matter
• Plate tectonic collisions, rifting, and basin formation
• The quality and continuity of sedimentary rocks
• The presence of cap rocks and traps

Venezuela, Saudi Arabia, Iraq — all sit on geological regions where millions of years ago nature created a “Perfect Geological Factory” for oil formation.

On the other hand, countries like Yemen did not get that opportunity due to earthquakes, rifting, and a history of volcanic instability. So even while sharing the same peninsula, their economic realities today are vastly different.

Therefore, when we talk about oil in the context of politics, economics, war, or development, we must remember — behind all of this lies a silent geological story written deep within the Earth over hundreds of millions of years.