Thank you for your most informative explanations. Another question if you don’t mind: have all the world’s continents truly been well explored for potential oil & gas deposits? if ocean deposits are being found 23,000 - 35,000 below the surface of the sea might there not be a lot of land deposits/oil and gas fields at similar depths below solid ground? Have all the promising places on planet earth already been tested to such depths, or is there a way to know in geological terms that there are no more major finds to be had on land?
Onshore:
Parts of the Canadian, and Russian Arctic and Alaska [mostly ANWR but not solely ANWR.]
Parts of Eastern Siberia [the data is lacking — the Russians may have already have tested this possibility.]
Parts of onshore Africa.
A small area in Eastern Iraq. Western Iraq is probably pie in the sky as their known giant oil fields fall in a fairly narrow band which does not include Western Iraq.
The same is also likely in Saudi Arabia which has been more thoroughly explored than the Saudis are stating publicly. The best chance for largish totally new finds in Saudi Arabia may actually be in the Red Sea rather than the Persian Gulf or “Empty Quarter” areas. Time will tell. [Saudi Arabia does have a couple of very large undeveloped fields that have been know about for a long while. These have had problems either with finding appropriate production technology or with bad oil quality / metal pollution problems. These are going into production in the next couple of years. After that?]
Offshore the list of hopeful areas is longer [Arctic Ocean, Deep Water Mexican Gulf of Mexico, more to come in Brazil, Angola maybe Nigeria or its neighbors — and admittedly a number of other places] but would we [Brazil and similarly Jack II in the U.S. portion of the Gulf of Mexico] be drilling 16,000 foot wells in 7,000 feet of water, 180 miles from shore if the easy stuff had not already been tested? The best news about this new Brazilian find as well as Jack II is the apparent emergence of techniques for exploring under salt deposits which are almost opaque to normal seismic techniques. Once again, I am by no means and expert in this area, but I don’t think they were blindly punching holes through the salt to find out what lays below.
Offshore Florida and the Atlantic Coast don’t look that encouraging, but should be opened for testing. Southern California offshore has some potential, but as I understand it, little chance for giants ... and good luck on getting CA, FL or the East Coast open in the near future.
One other thing about offshore. The Chinese claim to have discovered a very large field in shallow water [Bohia Bay?] fairly recently so I am somewhat hopeful that the best estimates from what I still consider to be the realists are low.
To return to my main point, we need a lot of success and we need it soon or we are fast approaching the peak in oil production.
Sorry about the lack of specificity, but I hope this helped.
Under the standard theory, oil came into existence from sediments rich in algae which bloomed in shallow seas. The algae died ... sank to the bottom ... the organic rich mud built up to depth and was covered with other sediments becoming shale ... the thermal gradient [the tendency for the temperatures to increase at depth] kicked in and over the course of geologic time the result was to cook the organic matter in the shale into oil. This oil being lighter than the salty water which also was present floated upward until it found a place where it could go no further [”a trap”] bounded by impermeable rocks and usually including a high spot in the formation, a fault, or maybe just surrounding parts of the formation where the porosity or permeable dropped below a certain point where the oil could no longer migrate.
If the trap itself has sufficient porosity and sufficient permeability, is large enough in terms of area and volume, and if enough oil had migrated to that trap, you have a prospective oil field.
Oil shale is theorized to be a deposit that was never buried deeply enough to become oil by cooking out of the shale. It remains trapped in the shale as “kerogen.” The process for tar sands is a little different [I must confess ignorance on the particulars] but the end result is “bitumen” which is a lot like asphalt. It lacks enough hydrogen molecules to make it flow at ambient temperatures.
Now [finally!] to your point. Natural gas may or may not come from the same source as oil. There is clearly a lot of methane [most of what is referred to be natural gas] that is not organic in nature. In any event, although oil seems to exist only at relatively shallow depths with a limit of about 15 to maybe 17 thousand feet based on rock temperatures, natural gas and some associated liquids such as propane, butane and ethane can be found in commerical quantities much greater depth [well beyond 20 thousand feet -— IIRC 25,000 feet is not out of the question.]
Now [really] the answer to your question: If is the depth within the rock, and the rate at which the rock temperatures increases as depth increases the defines the limits of the oil window. Normally 15 to 17 thousand feet is about the maximum depth for finding crude oil. Presumably because the rock was a little cooler than normal, both this find off Brazil and for Jack II the 17,000 foot level is still within the oil window.
There is an opposing theory that oil is abiotic i.e. not the result of the decay of plant matter. I see no particular merit to this theory, but in any event the observations from gas wells seems to bear out the belief that you can go very deep searching for gas, but in doing so you won’t find oil.