# Play concept: What is a play?

Oil fields don’t just happen. Each accumulation of hydrocarbons has a unique history associated with it. For example: At some point in the Earth’s past a river deposited a thick sedimentary layer of sand. Sea  levels began to rise and burried the land in a deep sea. Fine particles settled and formed a shaley layer that will later create a seal.

Many eons pass and both layers get burried under more and more sediments. The area is slowly ripped apart by the forces of Earth’s plate tectonics. The process creates different structures. Highs and lows. The lows get burried by more and more sediments whereas the highs remain high as the rifting activity fades over time.

Many eons later an earlier deposited black shale, rich in organic content is burried deep enough to generate oil. It slowly releases the oil which then begins to rise and flow upwards towards the high.

Once it gets there it is stopped by the seal and it forms an accumulation.

This concept, which includes all the major play elements

• Reservoir
• Seal
• Source
• Trap
• HGMT (Hydrocarbon generation, Migration & Timing)

is the general idea of how oil fields are created. The unique combination of all 5 is called a play, or play concept.

Now, the reality of oilfields is way more complicated than that. In frontier basins simple concepts need to be proven. In very prolific basins like the North Sea where a lot of hydrocarbons were deposited the charge-history of a field can be very complicated. Multiple charge phases from different charge-routes etc.

Different play concepts can be broken down into just a few words like. Jurassic sandstones charged by Jurassic source rock in a rotated fault-block. A large part of  the North Sea fields work that way.

When people talk about geological plays on a very high level the terms for the individual plays often get broken down to just a simple “Jurassic Play”. It often implies the most obvious component of the play concept, the reservoir, in this case a Jurassic reservoir.

As depositional environment, thermal history, and structural elements vary strongly from basin to basin these concepts can’t be transfered 1-to-1 but often similar elements exist.

# O&G industry: Geology matters

For students in geoscience, the oil and gas industry is probably the most likely industry they will end up in. During our studies we often learn a given curriculum, have fun on fieldtrips, specialise in a certain topic, write a thesis and enroll in the next higher education level. Repeat.

The result is that fresh-out-of-college-graduates of geology and geophysics (I am one of them) often don’t know how an energy company actually operates on a day to day basis. I struggled in the beginning and I’ll try to give some insight in a series of short posts called oil and gas industry 101.

As a student you may have examined cores or logged an outcrop. These are two vastly different scales that we can observe. In an (ideal) outcrop you can walk around laterally and vertically and examine every little last detail, every little rock. You have all the data in the world. In exploration or in development you will have to judge a potential (or an existing) reservoir on the basis of surprisingly little data.

Often you only have a couple of wells, not always core data and it is usually many miles away (sometimes hundreds). So every little bit of data you can extract from it is valuable and may influence the way it is being interpreted in the end.

### What are you looking at?

Your boss is not going to ask you to write a report on how beautiful the sand-ripples are in a core and give you a pat on the back at the end of the day. The problems that need to be solved are of a much less academic nature and are always connected to one thing: economic drivers.

• How homogenous is the reservoir? Does it change if we go to bigger scales? What is the chance that it does behave differently than we think right now?
• What is the clay fraction?
• What is the extend of natural fractures? Is this a good thing or a bad thing?
• Is it’s permeability better in one direction than the other?

### How does this matter in the O&G industry?

All these questions seem to be of an academic nature and without any connection but directly influence how a field will be developed and how much money will be needed. And connecting the dots is often not easy.

• How many wells do we need? How do we need to space them? Do we need horizontal, deviated or vertical wells?
• How much sand production will there be? Do we need to install sand screens? Will this make completing the wells more expensive?
• Where do we need to perforate the well? Should we not perforate a certain interval?
• What will developing this field cost and what production profile can we expect in the future.
• Ultimately: Is this going to make money or not?

Seeing the deeper meaning of the work we do in O&G is difficult in the beginning. I certainly struggled to see why certain things where worth looking into at all. Many of my colleagues studied geology because they love rocks, they love nature and they love hiking. Their heart rate increases when they see a cool outcrop, a fossil or a rare odd granite that’s only found in that one special place. And on the job you don’t get to see these things every day. Reality sets in: work-life is not a never-ending field-trip.

If you can look past that, then the oil and gas industry offers a surprisingly vivid community of people who love their jobs. Right now the industry is in a world of pain and it will take some time to adjust and to pick up again. But in general the life of geoscientists in O&G companies is actually quite rewarding. And I don’t mean that financially. You will get lots of training opportunities and field trip throughout your carreer.

“Whoever sees the most rock wins” 🙂

Statfjord Platform – Courtesy of www.npd.no

# Spooky ghosts in seismic!

A ghost is in an unwanted part of the seismic signal. There are three different kind of ghosts, the receiver ghost and the source ghost and the combination of both ghosts. There are different methods to suppress it in the first place and to remove it from data.

As discussed earlier, the perfect seismic signal would be a peak with a white spectrum (white, as in: white sunlight contains all visible wavelengths).

Spooky ghosts in seismic. (B) Source ghost, (C) receiver ghost, (D) source/receiver ghost

Due to technical and physical limitations we can’t produce that, neither onshore nor offshore. A typical source used in offshore seismic acquisition is a device that fires air bubbles in regular intervals which produces a wave, hence an “airgun”. In an ideal world the waves would only travel downwards and illuminate the subsurface directly under us. Raypath A shows this ideal scenario. But water is isotropic and transports energy in all directions equally. Energy that travels upwards first gets reflected back down at the sea surface and effectively duplicates the signal. The targeted horizon gets illuminated twice, which is shown in raypath B and represents the source ghost. The reason for this is an almost perfect reflection coefficient of -1 at the water-to-air interface at the sea surface.

$R = {Z_1 - Z_2\over Z_2 + Z_1}^2$

Where Z is the product of acoustic velocity and density. If you put in Z2=1490*1.027 and Z1=0.001225*343 for seawater and air it gives us a reflection coefficient of 0.998902 that is almost a perfect reflector.

Physics doesn’t care where a wave comes from, it is all treated the same way in an isotropic media such as water. That means the energy that comes back and gets recorded at a receiver, continues and gets reflected again by the near-perfect water-to-air interface. It travels downwards and gets recorded a second time. This is called the receiver ghost because it happens at the receiver end, shown in raypath C. If that wasn’t complicated enough there is the combination of both effects, the source/receiver ghost combo shown in raypath D.

What does that mean? It means that in the worst case scenario, the signal you send down comes back four(!) times. We would prefer it to be only once.

There are a couple of innovative technologies out there that try to suppress the source ghost. Other technologies, mostly in data processing, focus on the receiver ghost by splitting up the wavefields with specialized receivers.

to be continued…. 🙂

Suppressing ghosts will be a new topic, coming soon.

# Sediment loads of rivers in truckloads per minute

Rivers can carry immense amounts of sediments with them. For example the Mississippi river carries a sediment load of around 150 Million tonnes annually.((www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA486343)) That is a big number. Numbers are good, numbers are solid. But often not very easy to understand, or grasp its magnitude. So lets convert it to another unit.

Introducing: TPM (Truck loads per minute)

## Truckloads per minute

A truckload is a manageable size in our heads and we can imagine the size and dimensions and how long it would take us to empty it with a shovel. Probably much longer than you’d think. Let’s use a truck like this which can carry approx. 20 tonnes on it’s dump. (please correct me if I’m wrong)

“Triaxle dump truck 2005-10-06.km”. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Triaxle_dump_truck_2005-10-06.km.jpg#mediaviewer/File:Triaxle_dump_truck_2005-10-06.km.jpg

So one truckload is 20 tonnes. With a year of 365 days, 24 hours per day and 60 minutes in each hour we arrive at the following simple formula.

$\frac{150.000.000}{365 \cdot 24 \cdot 60 \cdot 20}=14.3\text{TPM}$

So every minute the Mississippi dumps 14.3 truckloads into the Golf of Mexico.

14 Truckloads per minute. The sediment discharge of the Mississippi

Now this is just one river. There are many other major river systems in the world. http://curry.eas.gatech.edu/Courses/6140/ency/Chapter9/Ency_Oceans/River_Inputs.pdf has a comprehensive list of rivers and sediment loads.

The numbers in

If we do the same calculation for all these rivers we end up with staggering numbers (rounded).

[table width=”500px”]
River,TPM
Amazon,114
Yellow,105
Ganges/Bramaputra,101
Yantze,45
Mississippi,14
Indus,10
Orinoco,20
Copper,12
[/table]

114 Truckloads per minute. The sediment discharge of the Amazon

## Why is sediment load important?

If you look at a seismic section or a geological sketch and you often see several kilometers of sediment. It is often hard to imagine how it got there. But once you know the discharge potential of a river system it is much easier to have a feeling of how much sediments over time you are looking at.

If you let the Amazon load 114 truckloads per minute for a million years you end up with an enormous amount. The number is 60.000.000.000.000 truckloads. 60 trillion truckloads

Now the Amazon is considerably older than just one million years. It started around 11 million years ago ((http://www.livescience.com/5542-amazon-river-dated-11-million-years.html)). So since it started it loaded 660 trillion truckloads of sediment out into the basin. (Assuming constant drainage over time)

## When does a river start?

A river and it’s basin is defined by topography which is mainly controlled by tectonic events. Continents collide and form mountain ridges like the Alpes, the Himalayas, the Rocky Mountains or in the case for South America the Andes. With the rise of the mountains, erosion and the drainage of water immediately starts to tear them down again. This results in drainage patterns and individual rivers across a basin.

“Amazonrivermap” by Kmusser – Own work using Digital Chart of the World and GTOPO data.. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Amazonrivermap.png#mediaviewer/File:Amazonrivermap.png

The main part of the Andes got uplifted around 12 million years ago which led to the change of continentwide drainage patterns. One could say: “A river is as old as the mountains at its source

11 million years sounds old, but really it isn’t. On the geological timescale if falls into the mid Miocene epoch. That is basically “one week ago” for a geologist. Many of the major rivers like the Nile are thought to be several hundreds of millions of years old.

So several hundred million years times many truckloads per minute = a very high number … you get the idea 😉

Always remember magnitude of processes and never underestimate what the factor time can do.

# Exploration seismics 101 – Principle and Acquisition

This is the first part in a series of posts, called Exploration Seismics 101.

The most commonly tool in exploration for oil and gas is seismics. (Sometimes the term exploration seismology is used in the literature as well.)

It gives us an actual image of the earth’s subsurface by the same principle that is used in ultrasonic imaging at the doctor.

Subsurface image – Courtesy CGG

Seismic data can be acquired on land and on water. Although the equipment used is different, the principle stays the same. An acoustic source and a receiver are placed on the surface. The source is released and a wave of acoustic energy travels through the earth. The energy that comes back (reflected) is recorded at the receiver. The process is repeated at many different locations and the differences in traveltime are translated into an image in many complex processing steps after the end of the acquisition.

To make it more efficient, the number of receivers is increased to record the “echo” at many different places at the same time.

A reflection (“echo”) is caused by an change in the elastic properties of  the subsurface. Any change in these properties will create an acoustic contrast which causes the wave to reflect some part of its energy while the rest of the energy is transmitted further into the earth.

### Marine acquisition

Seismic acquisition on water is very efficient and cost effective compared to land acquisition. The reason for that is that the whole equipment is towed behind a ship which can move around relatively freely. Typically, several long cables filled with seismic receivers are towed behind the ship in a pattern. These streamers are typically 100m apart and many kilometers long. Industry standard nowadays is 6-8km for a typical acquisition. In some cases ultra-long cables of 12km (and up to 18km) are used.

The acoustic source is compressed air that is released by a device fittingly dubbed an “airgun”. Such an airgun is fired in regular intervals while the ship is moving.

Have a look the youtube video here  to see what it looks like under water.

It is a controversial tool because it creates a lot of noise within the water which could potentially distract or damage the hearing of marine mammals. That is why the major seismic companies employ marine mammal observers who have the authority to shut down operations if a mammal comes to close to the source.

The receivers in the streamers are evenly spaced, typically every 12.5m. The streamers are towed next to each other and are kept in position by a dynamic steering system called birds. Basically small wings that can be steered remotely to move the streamer around. An even sampling of the subsurface is desired.

Bird on a seismic streamer – Courtesy Kongsberg Maritime.

### Land acquisition

Terrestrial acquisition varies from marine acquisition in the used equipment. The overall principle is the same.

Instead of airguns, the used sources are vibrotrucks. They put out a continuous signal instead of a short burst of energy. The vibrotrucks start with a low frequency and increase it continuously. One, so-called sweep, lasts typically 10s. Since the signal is not a short burst, the recorded signals overlap and need to be separated. By cross-correlating the recorded signal with the used sweep (the so-called pilot signal)

Vibroseis trucks in the desert – Courtesy of CGG

Several trucks are typically linked together to increase the output energy. While operations, the trucks drive around from shot point to shot point.

Instead of hydrophone in streamers, land-geophones are used. They record 3 components instead of just one because this allows to record the polarization of shear waves which are transported by the hard rock.  S-Waves are filtered away by the water in marine acquisitions since water can not transport shear waves.

Putting out the geophones and recording equipment requires a lot of wiring and manual labor. This makes land acquisition typically slower and less cost-effective than marine.

On top, vegetation, streets, houses etc. are obstacles that can not be easily avoided. Months of planning and preparations have to be invested before single shot is fired in a land acquisition.

Check out the video to see them in action in the Kuwait desert.

Next up: Exploration Seismics 101 – Gathers and stacked images

Featured image (top image) courtesy PGS.

# Why is the Brent called Brent?

Brent crude is a light sweet crude from the North Sea that is an established “brand” and trading blend of many North Sea oils. It has an API of 38 and is widely used in Europe.

So why is the Brent called Brent?

The short answer is that the name comes from the Brent oilfield in the UK sector of the North Sea.

But the name of the Brent oilfield comes from somewhere else.  Indirectly it is the name of a bird. The Brent Goose. Shell and Esso which discovered the field in 1972  named all of their fields after birds. And although that is a beautiful name and many other fields are named after birds, this one is special.

The reservoir unit of the Brent field is the so called Brent-Group.

The middle Jurassic group consists of 5 formations called Broom, Rannoch, Etive, Ness and Tarbert.

Or if you spell it out differently

Broom
Rannoch
Etive
Ness
Tarbert.

The individual formations are named after glens and lochs  in Scotland.

The Brent group is probably the most important reservoir unit throughout the North Sea as it still holds billions of barrels and is an important economic pillar of the North Sea offshore industry.

### The other side of the story

The other side of the story is somewhat different. Can’t really verifiy it but that is how it is told in the industry.

As the first couple of oil fields were discovered on the UK side of the North Sea, they got identifiers. They started out with A-UK, B-UK,C-UK … and so on. Until someone realized that they would be in a bit of a pickle when they would eventually get to the letter F.

Someone thought: “hey. A-UK spells like the auk”, which is a whole family of sea birds. So the naming convention for seabirds was derived from that very first field.

They would go on with that naming convention for a long time.

Auk field

Brent field

Cormorant field

Dunlin field

Eider field

Fulmar field

and so on…

# What is API oil quality?

Sometimes we see the term API or degrees API. The API degree refers to the specific gravity of the oil. Unlike density values it is inversely defined. So a high API value means a low density and vice versa.

It goes back to the Baume scale which is a hydrometer scale to measure densities of liquids lighter than water.

The original instruments were slightly off because of errors caused by salinity and temperature. The modulus of the scales were off. 141.5 instead of 140. But too many were out in use to easily fix the problem.

The American Petroleum Institute introduced the definition of API gravity with the modulus which was used by most hydrometers to compensate. The constants used in these hydrometers are reflected in the conversion formula.

API is defined as:

$API^{\circ} = \frac{141.5}{\text{specific gravity}} - 131.5$

The lower the value, the higher its density and the higher (usually) the viscosity. Composition, density and viscosity directly translate into economics. A light oil is easier to produce than a heavy oil. The distribution of different qualities is a result of different geology in the major oil producing basins in the world and has led to different oil products that are being sold and traded differently. The most known “brands” are the North Sea Brent and and West Texas Intermediate (WTI). Since oil quality can vary locally as well the products of many oilfields are blended together to create a constant quality output for a region. Almost like blended Whisky…

E.g. the Brent blend has an API of 38 which gives it a density of approx. 0.835 g/ccm.

Heavy oils are defined as oil with an API degree of less than 20. This translates into a density of 0.934 g/ccm. Almost water.

## Worldwide distribution of oil qualities

The EIA((http://www.eia.gov/todayinenergy/detail.cfm?id=7110)) published a good overview over different oil-qualities and their regional distribution.

# Why does oil and gas rise towards the surface?

It rises if it’s lighter than its surrounding liquid and it overcomes capillary pressures, caused by the pores in the rock.

You can describe all materials by its density property, or specific gravity. Which is weight per volume. Typically measured in g/cm3 or t/m3. Since these numbers translate directly into each other I will only give the numbers without the units. Giving the density in kg/m3 is equally valid but adds some digits.
Since many rocks in this table can have heterogeneous composition it cannot be boiled down to a single number. A range is given instead. Depending on composition, mechanical compaction and chemical compaction these ranges can be surprisingly large.

[table width=”500px”]
Material,Density
Sandstone,2.2-2.7
Shale,1.8-2.8
Limestone,2.0-2.8
Salt,2.1-2.6
Chalk,1.6-2.6
Coal,1.2-1.4
[/table]

Gold has an almost 20 times higher specific gravity than water which is exploited in placer mining to separated gold from its surrounding material.

# Buoyancy

Oil and gas is typically lighter than water. Therefore, within a water environment it will rise to the surface. (I say typically lighter because there are some very heavy oils out there that are actually heavier than water)

But why? In the end it is the same answer to the question why a hot air balloon rises,( or a helium balloon).

The only difference from within a balloon to the outside is the density within. For a hot air balloon it is that hot air is slightly lighter than normal air. Helium is naturally lighter than air.

Why does one rise and the other doesn’t?

It comes down to the simple equation of hydrostatic pressure, which describes the pressure of a fluid with density RHO of a column of height h.

$P=\rho\cdot g\cdot h$

If you have different fluids in one column, the total pressure is the sum of the pressures of each corresponding subcolumn.

$P=\rho_1\cdot g\cdot h_1 + \rho_2\cdot g\cdot h_2$

Differences in pressure means the presence of a force. If all pressures are the same, there will be no active force.

Now let’s look at the pressures of points P1 and P2 in the picture. P1 and P2 are at the same height in all 3 cases. The case with an air balloon is essentially the same situation as if you had no balloon at all. P1 and P2 in those cases will be the same.

Air balloon & no balloon:
$P_2=\rho_{air}\cdot g\cdot h_1 + \rho_{air}\cdot g\cdot h_2$
$P_2=\rho_{air}\cdot g\cdot (h_1+h_2)$

Helium balloon:
$P_2=\rho_{air}\cdot g\cdot h_1 + \rho_{helium}\cdot g\cdot h_2$

Since helium has a lower density than air the resulting pressure will be less. So we have a pressure difference around the balloon’s bottom. The air surrounding the balloon therefore exerts a force on the bottom of the balloon. If the force is bigger than the actual weight of the balloon, it will rise. This is called buoyancy.

By cleverly adjusting the density, big objects like zeppelins can hover or fly.

# Oil and gas

The same principle applies to droplets of any fluid. Oil and gas is not different from that.

You know the principle from the bartender who mixes colorful drinks with many different liquors.

If a planet was completely made up of liquids and gases (and no energy source to eliminate things like wind and turbulence) you would get a separation of all different liquids and gasses by its gravity just like the perfect drink you order at a bar (but don’t! Geologists drink beer…).

Once hydrocarbons are generated by the kerogen in the source rock it begins to rise if the permeability (ability to flow) of the surrounding rock is high enough to allow it.

Permeability is mainly goverend by capillary forces. If you take a straw and put it into your drink you will see that the level of the liquid is slightly different than the level of the glas. The diameter of the straw determines the strength of the effect and how far the liquid can rise in the straw. The level can also fall. This depends on the kind of liquid used and the wetness of the surface. We say oil-wet or water-wet depending on the water saturation Sw.

Wetness boils down to the question which liquid of a mixture actually touches the surface and which one is encapsulated in the other.

Now if the buoyancy forces are bigger than the capillary forces it can flow through the rock. If not, it will be retained and the rock appears to be impermeable. Another typical misconception about oil and gas reservoirs is that a seal is some sort of magically impermeable hard boundary like someone put a steel lid over it. No, it always has residual pores that prevent migration through it by capillary forces. Another misconception is that this seal is magically strong and is all you need to keep the oil down. No. In the end, the reason why the oil stays below the seal is the overburden on top which exerts a strong pressure on the seal.

If you put a lot of gas into a reservoir the pressure will rise. A lot. Enough to actually break the seal up. This is called seal breach and can happen for example in scenarios where you uplift a whole basin. The norwegian Barents Sea is a prime example for this. So it is not enough to have this barrier. No, you need something on the other side of the barrier that pushes the barrier down. Think of if like a pot on your stove that starts to boil violently. The lid pops up and it will boil over if you just leave it like that. But if you seal it properly and push the lid down it will stay in place.

# Why is there oil in the ground and what is an oilfield? A quick guide to petroleum geology!

Why is there oil in the ground?

Oil, or more specifically hydrocarbons, is found in most major basins in the world. How did it get there? And what makes an oil field an oil field?

The shortest answer: An oil field is an “upside-down lake“. But you can’t swim in it…

This statement sounds rather strange at first, but bear with me. The biggest misconception about oil and gas fields is that they are basically cavities in the earth filled with liquid. That is not true. The liquid is stored within the pores of the rock. Take a sponge in your kitchen and soak it with olive oil and you have something more like it. The pores in the rock are much smaller than in your household sponge but the concept is the same.

It might sound strange, but the journey of an oil field starts in the mountains. Humongous mountain ranges like the Alps or the Himalayas.

Mountains rise and crumble over eons. A natural process called erosion. Water, frost and wind tear down solid rocks and transport the end-product (sediment) off to somewhere else. That somewhere else is what we call a sedimentary basin. LINK – What is a sedimentary basin?

Rivers flow over great distances and carry the sediments with them. Initial erosion and the forces of the river break down the sediments into finer and finer sediments. This ranges from coarse sand to finer silt and even finer muds. Finer sediments can be transported further by the water and deposited further away than coarser grained sands for example. What that means is that the single governing factor, what type of sediment settles down at a point in a basin, is the speed of water. In a fast flowing river only larger boulders can be deposited, whereas out in the ocean only the finest sediments are left because the coarser and heavier sediments “didn’t make it that far“.

We often speak of high- and low-energy environments.

This results in a spatial variation in grain size of the sediments. Muds get deposited far out in the basin, whereas sands end up closer to the shore. A perfect example would be a nice sandy beach we all know and love from the old family holidays. Keep those two sediments in mind – a fine grained mudstone and the sandy beach you used to build sand castles out when you were a kid.

Sedimentary basins see very different depositional environments over the course of many millions of years. Sea level rises and falls again and again, tectonic settings change (rifting apart or being pushed together), glaciers grow or shrink, land gets uplifted or “drowned”. This causes a heterogeneous distribution of different sediment types because with each different sea level position, the sediments will end up somewhere else. These different sediment types have properties like porosity and permeability. Some might act as a pretty good container to store a fluid, whereas another might act as a barrier for the same fluid.

You guessed it, the barrier is the fine mud and the container is the sandy beach. Now both sediments will get buried at some point and lie at deeper depths. The loose sediments transform into harder rock called mudstone and sandstone. The process behind this is mainly compaction from the weight of the overlying sediments, but other factors play a role as well, however, I will keep it simple.

If we deposit a fine grained mudstone directly on top of a highly porous sandstone we almost have an oil field. This would be a reservoir with a seal.

Well, right now it would still be empty (or more correct: filled with water). We need to fill it with oil for it to become an oil field.

The finer grained mudstone further out in the basin can sometimes contain high amounts of organic matter. We call it source rock. It got deposited there under anoxic condition (very little or no available oxygen). Anoxic environments typically occur in a restricted basin. A modern day example would be the black sea, high nutrient influx from surrounding landmasses and limited influx of “fresh” sea water through a bottleneck. Most of the oxygen is consumed and the rest of the organic matter gets buried with the fine grained sediments and gets therefore “trapped” in the sediments. The deeper we go down in the earth the warmer it gets. On average by about 30°C per kilometer. That means if the rock is in a depth of (typically) 2km, it gets warm enough for it to generate a simple hydrocarbons-precursor called kerogen, which then later expulses oil and gas. This is often (and not surprisingly) called the kitchen. If it gets deposited deeper, it will generate more gas and less liquids.

Now once the liquid is expulsed from the rock it will rise. Why exactly oil rises I will cover in a different article, but it does. Just like olive oil that you pour into the pot when you cook spaghetti. It wants to come to the surface. It will start moving and it will always choose the path of least resistance. This part of the process is called migration.

In our world on the surface we experience a different behavior. If you pour out a glass of water on a high mountain (and neglect any processes like evaporation and such) it will travel downwards. This glass of water will end up in the ocean. All liquids are driven there by gravity. It might take some time, but it will go there.

This is where I get back to the analogue with the upside-down lakes. If you follow a river down to the ocean and at some point in the middle you give that river a big void that it can fill up (a potential low). This will create a lake. A buffer – which is filled until it flows over and the river continues.

Lake Geneva with the river Rhone flowing through it.

The same thing happens below the earth, just the other way around. If the oil can find a potential low during its way up it will fill up that reservoir until it spills over and the oil can continue to rise like a river. In order to capture the oil in place it is not enough to just have an impermeable layer on top. We also need a trapping style. Something that defines and outlines a geometrical body made up of the reservoir rock. We call this a trap.

Sketch of a petroleum system with mirrored analogue of rivers and lakes on top.

The simplest trap we can think of is basically a hill-like structure called an anticline. In its shape not unlike a lake on the surface. It is the simplest trapping style but there are many different other types of traps that can produce an oilfield.

Now we went through most of the elements that are needed to create an oilfield. A porous reservoir (e.g. a sandstone), a trap that defines a geometric body of said reservoir, an impermeable layer on top of that structure that prevents the oil from rising further to the surface (a seal). Of course we need a source for the oil itself (a source rock) and a way to get it to the reservoir (migration). On top the reservoir, seal and trap needs to be in place before the source rock starts to expulse oil. So timing is crucial as well!

As you can see, special circumstance have to come together for Mother Nature to form an oil field. This is why we don’t find them everywhere. The process of finding these remaining fields is called exploration.

tl;dr: oilfields are highly porous rock formations in the earth that are geometrically isolated by an impermeable layer that keep the oil in place, which migrated into the reservoir over millions of years after it was generated under favorable temperature conditions in a different rock formation called source rock with high organic content.