Oil fields: heavy crude or light crude?

[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]

.

the oldest carbon deposition is coal ,pretty stable

The Pennsylvanian aged coal beds of North America are approximately 300 million years old. The natural gas contained in such copious amounts in the Marcellus shale of Pennsylvania is Devonian in age, and perhaps 100 million years older. What do you mean that the oldest carbons deposits are coals?

[Tanada]

.

the oldest carbon deposition is coal ,pretty stable

The Pennsylvanian aged coal beds of North America are approximately 300 million years old. The natural gas contained in such copious amounts in the Marcellus shale of Pennsylvania is Devonian in age, and perhaps 100 million years older. What do you mean that the oldest carbons deposits are coals?

Devonian coal beds and even older precambrian beds are also in existance, most of them either have been mined out or are not yet considered commercially viable. See http://www.wsgs.uwyo.edu/coalweb/librar ... intro.aspx

Although coal has been discovered in rocks as old as the Precambrian Era, most coal dates from the Devonian Period (some 400 million years ago (mya)). At this time, land-based plants with woody tissue became abundant thereby making it possible for peat deposits to accumulate to a size that would eventually make mineable coal seams. As the geologic time scale to the left shows, the two major world-wide coal forming periods were the Pennsylvanian (320 to 286 mya) and the Paleocene to Early Eocene (66 to 52 mya). In the United States, eastern coals are of Pennsylvanian age whereas western coals are Paleocene to Early Eocene in age.

[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]

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?

As far back as you can find a swampy region and organic material natural gas will have formed, the problem is over geologic time the older a gas deposit is the more chances for it to have escaped into the air through any available crack. Coal beds being made up of solids pretty much stay in the ground until the deposit is eroded away or someone digs them up.

By 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.

[ralfy]

What do you think of Manifa?

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

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

[rockdoc123]

By 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.

sorry, I misunderstood what you were getting at, obviously reading too quickly.
The Triassic is a different story. If you have a look at Scotese's site and the reconstruction for the Triassic you will see there are few areas where you have the right shallow emergent seaway situation happening such as seen in the Cretaceous and Jurassic which means as a global percentage Triassic source rocks would not contribute as much as either the Cretaceous or Jurassic. There are examples of Triassic shales/marls that act as source rocks in the Barents Sea, southern Europe, Iraq and Western Canada but they appear to be local and are not well understood.
As to preservation you are right but it isn't so much about the time that the pre-Jurassic rocks were sitting in the gas window that controls this but rather what tectonics they have been subjected to over time. As an example in northern Australia the Permian Pre-Cambrian acts as a good source rock and there is still a lot of hydrocarbon around. The reason for this is the Australian craton has been extremely stable for a very long time, having not been subject to any uplift events. On the other hand in many parts of the world sediments older than Jurassic were buried deep enough to generate hydrocarbons prior to subsequent uplift events in the Late Mesozoic and Cenozoic. Once uplifted the maturation process is shut-off and hydrocarbons which had migrated are often re-migrated or lost to surface as traps formed prior to the uplifting are breached. Burying these source rocks to depth again will result in further maturation but by that time they may be too lean to generate much in the way of additional hydrocarbons.

So yes preservation of trap is an important point for all source rocks to reservoir petroleum systems.
Somewhere in my mess of various papers I have a chart that shows the % of source rocks globally by age, when I find it I'll post it.

[Tanada]

By 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.

sorry, I misunderstood what you were getting at, obviously reading too quickly.
The Triassic is a different story. If you have a look at Scotese's site and the reconstruction for the Triassic you will see there are few areas where you have the right shallow emergent seaway situation happening such as seen in the Cretaceous and Jurassic which means as a global percentage Triassic source rocks would not contribute as much as either the Cretaceous or Jurassic. There are examples of Triassic shales/marls that act as source rocks in the Barents Sea, southern Europe, Iraq and Western Canada but they appear to be local and are not well understood.
As to preservation you are right but it isn't so much about the time that the pre-Jurassic rocks were sitting in the gas window that controls this but rather what tectonics they have been subjected to over time. As an example in northern Australia the Permian Pre-Cambrian acts as a good source rock and there is still a lot of hydrocarbon around. The reason for this is the Australian craton has been extremely stable for a very long time, having not been subject to any uplift events. On the other hand in many parts of the world sediments older than Jurassic were buried deep enough to generate hydrocarbons prior to subsequent uplift events in the Late Mesozoic and Cenozoic. Once uplifted the maturation process is shut-off and hydrocarbons which had migrated are often re-migrated or lost to surface as traps formed prior to the uplifting are breached. Burying these source rocks to depth again will result in further maturation but by that time they may be too lean to generate much in the way of additional hydrocarbons.

So yes preservation of trap is an important point for all source rocks to reservoir petroleum systems.
Somewhere in my mess of various papers I have a chart that shows the % of source rocks globally by age, when I find it I'll post it.

I found 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?

[rockdoc123]

found 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?

OK 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.

The problem with answering your question regarding earlier events becoming more important is that the chart you pointed to refers to the importance of the anoxic events in terms of how much hydrocarbons associated with source rocks deposited in those time periods has been discovered, not how much has actually been generated nor how much has been preserved, nor how much may yet be discovered. Why does this matter? The older source rocks may actually have generated a considerable amount of hydrocarbons that have been lost through time due to uplift and erosion or seal breach and surface weathering, water washing etc. There may indeed not be a lot of organic material left in those source rocks that can generate hydrocarbons. That is certainly the case in areas of North Africa where the Silurian source rocks are quite important. In the very few place where those rocks did not suffer normal burial they have extremely high Total Organic Content (a measure of how much oil could be generated) but most samples collected throughout the oil fields show quite low TOC indicating most of the generating capacity has already been realized. Also depending on the date that this graph was produced it might not have taken into account the tar sands of Alberta and Venezuela nor the oil shales in North America. This could change the picture considerably. There would be some important controversy here, however, as I believe there is still some argument as to whether or not the Alberta oil sands have a Devonian or Cretaceous source.

A better way of looking at this would be the total amount of hydrocarbons actually generated during those periods, a number that requires some modeling using methods developed decades ago by the Russians. The old Robertsons Research group out of Wales did a world wide source rock study and produced such numbers and maps. Unfortunately I no longer have access to that material.

[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]

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

my understanding is that some of the shale oil/gas being chased is actually in Proterozoic rocks (PreCambrian age and older) sitting north of the Cooper Eromanga basins (can't remember the name of the actual basin). In most of the world rocks of this age were buried so deep that they have been cooked long beyond the end of the gas window and essentially metamorphosed. Because of its isolated nature during plate tectonic evolution Australia was not exposed to a lot of sediment influx from uplift events which preserved a lot of these older rocks.

[Tanada]

OK 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.

Thanks for taking the time to answer in detail! I appreciate it and I am sure others do as well.

So 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?

Also I have seen several places speculate on major deposits in the deep Arctic basin's from the Azola event just as the PETM was declining, do you think those are realistic expectations?

[rockdoc123]

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?

I believe that they are mainly the mid-Cretaceous source rock that is so prolific as a source along both Atlantic margins but there is a possibility that Late Jurassic sources come into play. I haven't run across a paper that talks about oil typing of the sub-salt (not something I've followed too closely due to the high cost of the wells and tight control by Petrobras), I'm pretty sure it is out there and I'll just have to look.

Also I have seen several places speculate on major deposits in the deep Arctic basin's from the Azola event just as the PETM was declining, do you think those are realistic expectations?

Very rich organic material but it would have had to have been buried quite deep under elevated heat flow conditions given it's age. Areas where there has been strong outflow of post Eocene sediments (deltas) would be the most likely spots for this to work. I haven't come across any geochem modeling in my normal meanderings throught the literature but will have a looksee as to what I can find.