Exploring Hydrocarbon Depletion
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the oldest carbon deposition is coal ,pretty stable
seenmostofit wrote:sparky wrote:.
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?
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 wrote: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?
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.
rockdoc123 wrote: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.
found this chart earlier today [img]
http://oldearthmygod.com/wp-content/upl ... events.png
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?
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 wrote: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.
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?
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?
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