[dukey]

just a quick question: if an oil field pumps mostly light sweet and output peaks etc and most of the field is depleted. what is left ? is it more light sweet or heavy sour. some fields are just heavy sour anyway ?

[Novus]

I think most fields are a mixture of Light and Heavy crude. The light stuff at the top is pumped out first and then the some heavy stuff is pumped out. In the past when wells started pumping heavy crude the wells were capped and the drillers moved on. Now we have no choice but to pump and use the heavy stuff.

[RonMN]

To my understanding, most oil wells have the light sweet crude floating on the top...it's easy to pump (like sucking water thru a straw), and it contains the most energy.

at about the half-way point you've gotten almost all the light sweet & now you'll only have the heavy sour crude...it's harder to pump (like sucking a think milk shake thru a straw), and it only has 60% of the energy that light sweet has.

At a point, it becomes so thick & difficult to pump that it is no longer worth the cost of trying to pump it out of the ground.

I don't know if there are any wells that are all heavy sour from top to bottom...

[NTBKtrader]

Does anyone know the US's capacity to refine heavy crude??

[rockdoc123]

non, non et non. There is no such thing as a typical oil field. Some fields are composed of nothing but light sweet crude.....some are composed of nothing but heavy crude.....some composed of nothing but light sour crude.....some composed of condensate and some composed of waxy crude and some composed of mixtures.
As a primer, and I'll try to keep it as simple as possible... as source rock matures it begins to generate first biogenic gas, followed by a small amount of heavy ends followed by liquid petroleum. The make up of the source rock usually determines what trace elements such as sulphur or vanadium might be present and also the wax content. Time and temperature determine how mature the petroleum generated will be. Hotter and longer means lighter ends in the oil and if buried deep enough the source rock can generate condensate, wet gas and dry gas. The petroleum which is generated from the source rock migrates to the reservoir. While in the reservoir other changes can happen. The reservoir can be buried deeper and with increased temperature the oil can be cracked to condensate and/or gas. Another change that happens to oil in the reservoir is it can become heavier....either from biodegradation (aerobic or anerobic bugs eat the light ends) or fresh water washing.
In terms of pools which have gravity segregation (lighter ends over heavy ends) there are generally a few reasons for this. Oil pools often have free gas caps either because gas was added into the reservoir after oil and the oil is saturated or the pressure in the reservoir is at or close to bubble point. In this case because gas is less dense it sits on top. Heavy oil pools are also often segregated but they can be either lighter on top and heavier below or the other way around. In the former case fresh water washing might be the answer whereas in the latter it could be biodegradation (bugs don't like higher temperatures so the higher up zones would see more activity, hence heavier oil).
Sorry if that sounds a bit complicated but in fact it is complicated. In short no two oil fields are exactly the same....each has it's own story to tell.

[Tanada]

Rocdoc123, I have always heard that 90% of petroleum source rocks are Jurrasic or Cretaceous in age. Recently however I keep seeing references to Triassic source rock. Is this a 1/3rd increase in the total quantity of source rocks? Can you explain the ages when source rocks were formed in lay terms? I know some of the oldest coal beds predate all other fossil fuels, or so my reading has lead me to believe. Is this true?

[rockdoc123]

The main Tertiary source rocks present are in offshore areas where there has been substantial rapid burial under conditions of somewhat elevated heat flow. Good examples would be the GOM and the Nile Delta and most recently the new discoveries off East Africa. In these cases a tremendous amount of sediment was dumped onto transitional crust (change from continental to oceanic crust). Continental crust is much thicker and hence the heat flow is lower whereas oceanic crust is thinner and has higher heat flow, transitional crust being between the two. What this means is that source beds in thick young sediments deposited on oceanic crust have a better chance of reaching hydrocarbon maturity than similar sediments deposited on continental crust.
The preponderance of Jurassic and Cretaceous source rocks is due to a combination of situations. The seas which were present during this time period were subject to tremendous up-welling which resulted in substantial organic material being present. The Jurassic source rocks formed in many areas at the start of rift phases (examples being parts of west Africa, the North Sea, some of the older grabens related to the Central African rift system) and as a consequence end up with very high total organic carbon as the related seas and lakes were isolated. The Cretaceous seaways are very extensive and very important along the continental margins of Africa and South America having been deposited in the narrow expanding seaways as Pangea rifted apart. Because of the extensiveness of the seaways and subsequent uplift you find a lot of onshore areas with Cretaceous age source rocks (eg. Western Canada, North Africa) but the requirement in these cases is extensive burial. That is accomplished in Western Canada as a result of Laramide uplift (the rockies) which resulted in a tremendous amount of rock being eroded and deposited during the Cenozoic. Indeed this burial resulted in another less common source rock in the Devonian becoming quite important and it is likely responsible for most of the oil ever found in Western Canada, including the oil sands.
But there are source rocks of other ages, less extensive and common mainly because they required special circumstances. An example would be the PreCambrian source rocks present in central Australia.
The effectiveness of the source rock is also a factor. There are basically 3 main kerogens simply termed Type I, II and III. Type I kerogens tend to be formed in lacustrine environments and can produce a lot of oil for a given mass, Type II kerogens tend to be formed in oceanic environments and also are oily but have more gas present. Type III kerogens are mainly from terrestrial plant material and tend to produce gas. Each kerogen has a different activation energy associated with the petroleum generating process, meaning that Type I kerogens might take more heat/burial to start generating hydrocarbons than say Type II kerogens. That being said there are many places in the world where extremely rich source rocks seem to have not produced a lot of hydrocarbon, a good example might be the Mississippian aged Chainman shales in Utah which are incredibly rich but do not seem to have produced much in the way of hydrocarbon.
So to summarize, and I apologize for the detail, you need an organic rich source rock, it has to be buried to some depth in order to get increased temperatures (time of burial is also a factor) and the thermal gradient active where it is buried is also a factor. These factors combine well for the Cretaceous source rocks worldwide but much less so for the Tertiary and other age source rocks. There are of course important exceptions (eg: Devonian in Western Canada).
To get an understanding of the extent of the Cretaceous aged source rocks you could have a look at Chris Scotese's Paleomap project where he has created reconstructions of plate boundaries and seaways throughout the Paleozoic.
http://www.scotese.com

[sparky]

.
Thanks rockdoc that was brilliant ,

another requirement is that no massive geologic disturbances occur ,
fracturing is not only man made ,
the result is the same hydrocarbon migrate to the surface

the oldest carbon deposition is coal ,pretty stable
one could mention the past climate variations such as the Arctic flowering
massive vegetal matter deposit generating the present Alaskan and Siberian oil fields

[seenmostofit]

$this->bbcode_second_pass_quote('sparky', '.')

the oldest carbon deposition is coal ,pretty stable

[Tanada]

$this->bbcode_second_pass_quote('seenmostofit', '')$this->bbcode_second_pass_quote('sparky', '.')

the oldest carbon deposition is coal ,pretty stable

[seenmostofit]

So both gas and coal can be Devonian in age, even if most North American coal is younger than the large Devonian shale gas deposits in the US. Is there anything in your reference which says that coal must be formed prior to natural gas? Both are basically the result of organic material being compressed and heated, and coal can certainly generate natural gas, but if they both rely on the same original organic matter, why can't they both have been formed at the same general geologic era?

[Tanada]

$this->bbcode_second_pass_quote('seenmostofit', 'S')o both gas and coal can be Devonian in age, even if most North American coal is younger than the large Devonian shale gas deposits in the US. Is there anything in your reference which says that coal must be formed prior to natural gas? Both are basically the result of organic material being compressed and heated, and coal can certainly generate natural gas, but if they both rely on the same original organic matter, why can't they both have been formed at the same general geologic era?

[ralfy]

What do you think of Manifa?

"Manifa Oil: Malodorous, But Really Not That Bad"

http://www.theoildrum.com/node/9056

[rockdoc123]

$this->bbcode_second_pass_quote('', 'B')y the same token most petroleum seems to have come from algae like organic precursors which have been around even longer, but like natural gas petroleum has a chance to escape every time there is an earth quake or a deposit is eroded into. That is why I asked about Triassic petroleum, I had been under the impression that the formations older than the Jurassic had mostly escaped in the intervening time until now.

[Tanada]

$this->bbcode_second_pass_quote('rockdoc123', '')$this->bbcode_second_pass_quote('', 'B')y the same token most petroleum seems to have come from algae like organic precursors which have been around even longer, but like natural gas petroleum has a chance to escape every time there is an earth quake or a deposit is eroded into. That is why I asked about Triassic petroleum, I had been under the impression that the formations older than the Jurassic had mostly escaped in the intervening time until now.

[rockdoc123]

$this->bbcode_second_pass_quote('', 'f')ound this chart earlier today [img]
http://oldearthmygod.com/wp-content/upl ... events.png
[/img]
showing eight major anoxic petroleum forming events in the geological record. Why is it that only the three most recent events are major oil sources today? Is it likely the earlier events will become greater sources in the future as the more recently formed reservoirs are depleted? Can you point to specific basins where these different epoch petroleum sources are being or have been recovered in commercial quantities?

[sparky]

.
In Australia , an excedingly old continental chunk , we have those depleted rocks ,
there still is some possible production in the Moomba bassin
but some deposits are pretty much depleted

[rockdoc123]

$this->bbcode_second_pass_quote('', 'I')n Australia , an excedingly old continental chunk , we have those depleted rocks ,
there still is some possible production in the Moomba bassin
but some deposits are pretty much depleted

[Tanada]

$this->bbcode_second_pass_quote('Rockdoc123', 'O')K first lets address the timing of anoxic events. They are from youngest to oldest:
Tertiary Period, Eocene Epoch
Mesozoic Era, Cretaceous Period
Mesozoic Era, Jurassic Period
Mesozoic Era, Triassic Period
Paleozoic Era, Devonian Period
Paleozoic Era, Silurian Period
Paleozoic Era, Ordovician Period
Paleozoic Era, Cambrian Period

As the graph demonstrates the two most important source rocks in terms of how much has been generated are the Jurassic and Cretaceous. The Jurassic source rock dominance is no doubt biased by the Hanifa shales and its equivalents throughout the Middle East, these source rocks being responsible for Ghawar and most of the very large reservoir accumulations throughout Saudi Arabia, Qatar, Kuwait, Iraq, Iran and the UAE. There are other areas where Jurassic source rocks are responsible for hydrocarbon accumulations such as the North Sea and Gulf of Suez.
The Cretaceous source rocks are strongly biased to the major accumulations of hydrocarbons along the Atlantic margins of Africa and South America but there are also major contributions in the Middle East, India, SE Asia, Gulf of Mexico and the North American foreland basin.

The Tertiary source rocks are largely confined to the areas of very rapid recent subsidence which means large deltas such as in the Gulf of Mexico, Nigeria, offshore Egypt, Ghana, India.

[rockdoc123]

$this->bbcode_second_pass_quote('', 'o') the recently drilled sub salt deposits off of Brazil that were in the news so much in 2010 and 2011, what age are those source rocks?