The issue isn't getting nearly the attention it deserves, but many who watch our oil supply think we crossed the threshold of what is known as "peak oil" recently, with November 2005 being the likely month for the peak.
This has some amazingly serious implications for our civilization, but I'm not going to dig into that just yet. I set out to write a top five list, but ran into trouble with data points after the first four due to a variety of reasons, so this ended up being a top four and some commentary list.
Ghawar
Saudi Arabian oil is produced by Aramco, which is privately held and they don't talk all that much about their fields and facilities, but the giant field for the world, Ghawar, is closely watched. This 18 by 168 mile field produces five million barrels of oil a day - about 6.25% of the global total production.
When you look at a cross section of an oil field like Ghawar it looks like a mountain range, only underground. This means Ghawar is the type of field where they can inject salt water and then collect the oil that floats on the surface. Healthy fields produce oil and gas without this pressure raising strategy. The following article contains anecdotal evidence that Ghawar's oil may contain as high as 55% water. Ghawar is about to "water out".
http://www.energybulletin.net/...
Cantarell
The asteroid strike in the Yucatan that wiped out the dinosaurs sixty five million years ago created the massive Cantarell field, one of a number found in Campeche Bay. This field produces about half of what Ghawar does, or roughly 3% of the global supply. Petroleos Mexicanos, generally known as PEMEX, has stated that the field is declining roughly 15% annually from its 2.1 million barrel per day peak in 2005.
Cantarell has a different shape than the underground mountain range topography of Ghawar. The worlds largest nitrogen gas production plant is hooked directly to the field and the gas, under pressure, is used to drive the oil from the rock so that it can be pumped to the surface. The article below has an excellent picture of Cantarell in cross section and excellent information on geology but mistakenly identifies it as the third largest rather than second largest producer.
http://home.entouch.net/...
Burgan
Kuwait is home to Burgan, the current third largest oil field in the world by production volume. This field was number two for a while before the nitrogen injection in Cantarell, which may explain the discrepancies found in some articles. This one is about 1.5% of the global supply.
Burgan has produced as much as 2.0 million barrels a day but is currently running at 1.7 million barrels per day. The underlying structures are complex and the geology is such that the oil is partitioned into parcels of various size. Burgan will produce for many years to come but in ever declining amounts as the largest parcels are drained first.
http://www.gregcroft.com/...
DaQing
China's domestic market gets about a million barrels a day from the Da Qing field. Numbers are, of course, tightly controlled by the Chinese government, but they've stated that they're managing the reservoir so that production declines by 7% annually in order to extend its life. There is much wizardry in reservoir management but the outsider's consensus is that they're being pushed along at this 7% annual decline rate by forces out of their control. This is roughly the same size as Burgan or 1.5% of the global market.
I've Googled a good bit this evening and this skimpy Wikipedia article pretty much sums up the information available about the field without a subscription to an oil industry trade journal.
http://en.wikipedia.org/...
And now the waters get a bit muddy.
Kirkuk and Rumailia North
The two Iraqi fields were producing 900k and 700k barrels daily, respectively, before George Bush decided armed robbery was a good foundation principle for foreign policy. I discovered them in this very good paper on giant oil fields. They don't get discussed much and I would say given their age and the fact that their peaks were 1.5Mbbl and 1.2Mbbl respectively they're likely in terminal decline like these other fields.
This PDF contains an excellent paper on giant oil fields and its only a little dated with an apparent publication date in 2001 or 2002.
http://www.nps.edu/...
U.S. supergiant oil fields
Doesn't the U.S. have its own supergiant fields? Well, no, not really. Prudhoe Bay produced two million barrels a day at its peak in 1988 but this has slid to about 450k barrels daily. This puts it in the top twenty but its sinking fast.
http://www.washingtonpost.com/...
Once upon a time the East Texas Field was the source for oil in the states, peaking at X barrels per day. The decline of this field and United States domestic production in general was predicted by M. King Hubbert in a 1956 paper and actually came to pass in 1970 or 1971.
http://en.wikipedia.org/...
What are the implications?
Marion King Hubbert, in addition to correctly predicting the peak of U.S. oil production around 1970 in the paper published in 1956 also predicted a global peak "in about half a century". That would be now.
The four fields for which I actually got information, Ghawar, Cantarell, Burgan, and Da Qing represent one eighth of the global supply and all have recently gone into decline. If you take the time to dig further you will find two cases - either the information is a closely guarded state secret, or the large field is in decline.
There has been a recent large field discovery in Kazakhstan. If developed it may replace the output from Burgan. The Alaska National Wildlife Refuge field could produce as much as Cantarell does today, or around 1.5Mbbl/day. The ANWR oil is easily accessible but this is not a deep formation, containing only about five hundred days worth of U.S. usage or ten billion barrels. Oil fields have a depth of supply aspect as well as how much may be extracted daily and all of the broad, deep supplies have already been found and exploited.
So ... not a good scene for our civilization. What does it all mean? This diary is the second in a series of three which I'm writing that cover Burgundy's Triumvirate of Collapse: Economy, Ecosystem, Energy. Once I've got all three areas covered I'll start drawing some conclusions.
(UPDATE: silly me! If you want to really know whats going on you should look at The Oil Drum, which is where I got schooled on this stuff)