Bit Tooth Energy blog takes a fascinating look at petroleum creation and deposition at various depth, using the Eagle Ford shale play as an example.
As a rough rule of thumb down to 15,000 ft the hydrocarbon is more likely to be oil, (which is thus referred to as the Oil Window) and below that it is more likely to be gas. That is only a rough rule of thumb, and one must remember that over time there has been a lot of uplifting and eroding, so that 15,000 ft isn’t necessarily what it used to be.
And the Eagle Ford shale is a fairly good example of this. If we use the EIA map of the play you can see that in the North, where the reservoir is about 6,000 ft deep the hydrocarbon is oil, while further South, where the deposit is down at around 14,000 ft then the hydrocarbon is dry gas. And in between it is what is known as a wet gas.
...You may note that the condensate from the wells in the wet gas zone have produced around 2.3 million barrels, while there has only been about 1.6 million barrels of crude produced. It is also worth noting that while the natural gas coming from the formation has been twice the equivalent volume of oil, the market for natural gas, at the moment is still down at around $4.6 per kcf, which using the Apache conversion, would give it a price of around $27.60 a barrel of oil equivalent. On the other hand the condensate is a light high quality product, and West Texas Intermediate crude is running at the moment at around $88.30 a barrel. _BitToothEnergy
The Bit Tooth article above talks about petroleum as coming from 500 million years of algal deposition in sediment, and conversion at depth -- heat and pressure -- to petroleum. The deeper the sediment, the more likely to be converted to gas, according to the source.
Now, if you look at the timelines provided above, you should note that photosynthetic organisms have been converting massive quantities of CO2 into organic carbon for over 3 billion years. The graphic below provides the mirror image of that process -- the production of atmospheric O2 juxtaposed by the evolutionary time of origin for various photosynthetic organisms.
Oxygen is a waste product of photosynthesis, and organic carbon is the main product. Rapid production of oxygen should correspond to a rapid production of organic carbon -- which was then buried in sediments. Whether this organic carbon became coal, kerogen, bitumen, petroleum, or other hydrocarbon resource, would depend upon where it ended up in the Earth's crust, or perhaps, mantle.
Recent science suggests that short chain hydrocarbons in the mantle are capable of surviving for significant periods of time at high pressure and temperature, perhaps later to migrate into the crust. These mantle hydrocarbons may have been subducted into the mantle via oceanic crust, or may be generated abiotically within the mantle itself.
The important thing to understand, is that massive quantities of organic carbons have been buried in the Earth for over 3 billion years. They no doubt exist in a wide variety of forms, from minimally altered to partially transformed to fully transformed, etc. Some of this carbon was no doubt oxidised to CO2 in the mantle and released into the atmosphere for recycling. Some of it no doubt seeped into the biosphere and was metabolised by microbes.
It is highly unlikely that humans have even begun to account for most of this missing organic carbon. Most of it will not be economically recoverable in any form. But we will not actually know that until we look for it, and find it.
By. Al Fin