Why Is (Most) Of The Oil Offshore?

I realize that most of the on-shore oilfields have been exhausted, simply because they have been pumped for so long…but why are most of the new oilfields offshore?
Take Brazil-after 50+ years of on-shore exploration, Brazil had very limited oil production-until they began to drill offshore. The result: giant oilfields that may be as big as Saudi Arabia.
However, they are in VERY (7,000-10,000 feet) deep water.
Are most new oil finds likely to be offshore? Have drillers largely given up on drilling on land?

Most of everything is offshore. This planet is mostly covered by water, you know. That, plus what you said about the easily-accessible ones (i.e., on dry land) being depleted.

The Earth is 70% covered by oceans so its not surprising that much of the oil strata is under water.

You’ve answered your own question, really. We’ve found, and exploited, most of the oil fields that are in accessible, easily-drilled areas. The remaining fields are going to be in more remote areas (polar, tropical forest, offshore areas) which are more difficult to drill.

Actually, most (I think all) of the undersea finds are in continental shelf-continental slope areas, which are much smaller in total area than the continents themselves. Really deep sea areas don’t seem to be conducive to the accumulation of the type of sediments that yield fossil fuels.

Isn’t it also true that we didn’t have the ability to reliably dig in deep waters until relatively recently? We’ve been inching out into deeper and deeper waters. Immediately after WWII drilling in 20 feet of water was considered state-of-the-art. The North Sea was exploitable in the 60s because its average depth is less than 300 feet. There are only 200 or so rigs in the world capable of drilling at more than 5000 feet.

We didn’t because we couldn’t. Now we can, so we will.

Oil concentrates in anticlines, areas where layers of sedimentary rock are folded ‘upwards’, in an A shape as opposed to a V. This limits the areas in which it can be found to where sedimentary rock is folded into anticlines. For example, there’s no oil under the Canadian Shield, or most of the land area of Scandinavia – they’re igneous/metamorphic shields. Not all anticlines have oil deposits, but nearly all if not all oil deposits are in anticlines.

Having gotten that into place, what everybody else has been saying then makes sense – we’ve exploited most major easily accessible on-land anticdlines that have crude oil. The ones left are in places like the AWNR, boreal Siberia, interior Borneo, that are difficult of access. Hence continental shelf and continental slope deposits are now being targeted.

It reduces to an economics/environment question – where can we find oil that can be drilled for and exported to where it can be refined with the least expense and without wrecking a relatively fragile ecosystem?

Cite for the part I bolded? Saudi Arabia has 267 billion barrels of proven reserves. I hadn’t seen anything suggesting the Brazilian finds are likely to be within an order of magnitude of that.

The oil off of Brazil is nothing new, they have been drilling and capping wells that can produce there since the 70’s.

There have been some large finds in Brazilian waters in the last 5 years. It’s just that large here means around 10bn barrels, not 100+bn like the Ghawar Field in Saudi Arabia.

The Bakken and Three Forks in the Williston Basin in North Dakota and Wyoming have had estimates of over 20 billion barrels of recoverable reserves recently. That would be the largest discovery in decades, and it is onshore.

There is a tremendous amount of activity in the oil window of the Eagle Ford in Texas. Also, higher liquids component windows of the Woodford and Marcellus are being heavily drilled.

Tertiary enhanced oil recovery via CO2 is becoming a bigger play with Denbury’s activity in the Gulf Coast regions of Lousiana and Texas. No doubt this will grow much larger with more success as it also has the potential to be joined by carbon capture and sequestration from refineries.

Of course there is also just plain conventional oil drilling going on in other areas of the U.S. The U.S. has a lot of onshore activity for both oil and gas. I can’t really speak to other country’s activities, but nobody has given up on onshore here.

It’s not as much a factor but a lot of land belongs to regular people. It’s not always the property of the national government.

If you’re dealing with a town or city, you might be able to drill, but you’re almost certainly going to have to deal with potentially several individual landowners, their mineral rights, and any restrictions on operations they care to impose.

Like I said, it’s not the biggest factor, but it does come up from time to time.

The dinosaurs that became WD-40 washed into the sea, ha ha.

Seriously, the ancient animal life that eventually became crude oil came from accumulations where organic detrius washed down from he higher ground, then eventually settled and was covered with sediment. The pressure of this sediment created oil. The sediment covering prevented it from reaching the surface, generally. As someone with real geological knowledge points out, if the rocks fold in an A pattern, you get underground domes where the oild and gas rise to the peak. (I assume the Tar Sands and Shale Oils of ALberta and Wyoming are because the geology did not contain the oil in a dome)

So major oil fields tend to be from sediment accumulations at the outflows of rivers, especially large river systems from the “good old days”. Note that until grass evolved (after the dinosaurs) erosion was probably a lot stronger than today. Saudi Arabia, Texas and the Gulf of Mexico, the Arctic around the McKenzie delta, etc. and now the Amazon are all erosion sediments. The oil found up and down the west side of the Rocky mountains, from Texas to the Arctic circle, are from the days a hundred million years ago and more when a shallow sea covered what is now the prairies adn the material washed down from the nearby mountains.

The Canadian Shield does not have oil because it is a rock formation from 3 billion years ago, long before the burst of life that turned to oil. It’s rather logical that oil-bearing sediments would accumulate over the eons in areas where the river currents stop (the ocean) and the sediments get deposited.

Once we’ve exploited the land-based oil, all that’s left is off-shore. Depends how desperate we are for oil…

Following on, one of the critical things about oil and gas deposits is that there is a seal over the deposit. No seal, and the hydrocarbons will have long since escaped. Good seals are made from sedimentary layers, salt layers, or volcanic flows. Salt is deposited by drying out seas. A major sedimentary seal is limestone (or generically carbonates). So, two of the three seals really involve seas, either current or historic. The Gulf of Mexico for instance has dried out many times over the eons, and there are significant salt deposits capping worthwhile deposits.

The other problem with hydrocarbons is that you need them cooked just right. You need a good recipe (trying to avoid ingredients containing sulfur is nice), but you mustn’t overcook them. The world has lots of overcooked fossil energy deposits; we call them coal. Luckily coal won’t escape if there isn’t a good seal. But neither will it flow up a pipe.

Then there is the middle ground. Stuff that sort of flows, but doesn’t escape easily. We call that tar. Given you can make oil from tar sands, the OP’s initial question is arguably working from a false premise. If you count the tar sands deposits, the really big deposits are on land. Arguably.