Why is oil common offshore? Or is it?

Howdy. I see that the Interior department declared oil-drilling off-limits within ~100 miles of the FL shore. http://dailynews.yahoo.com/h/nm/20010702/pl/energy_oil_leasing_dc_1.html

This got me thinking: there’s quite a lot of drilling (or possible drilling in FL’s case) for oil just offshore of FL, along the entire Gulf coast, in the Pacific offshore of southern CA, in the North Sea near the British Isles, and so forth.

Question: Is oil more common, or easier to get to, over continental shelves or other shallow seas than under the mainland?

(Or perhaps oil drilling is easier there because there aren’t all those pesky houses and such in the way…?)

I am not a geologist (IANAG?), but I used to work in a company that designs offshore oil drilling equipment.

Lemme tell ya, offshore oil drilling is WAY more expensive than onshore. A few years ago, it cost over five billion (yes, nine zeros) Canadian dollars to put ONE oil rig off the coast of Newfoundland. That amount of money would drill an awful lot of holes in Texas.

Nowadays, drilling can be directed underground: a hole can start out vertical and can be driven off-vertical - even horizontal. Drill strings can be miles long, so things like houses are not usually a problem. That one rig off Newfoundland will be able to drill about fifty holes fanning out for miles around, efficiently exploiting the oilfield.

Offshore drilling used to be done from piers build out from the shore. They only reached a few hundred yards from shore. Now oil derricks are mounted on barges, platforms standing on the sea bottom, ships and on purpose-built semi-submersible drill rigs. These can drill almost anywhere, but water depth is restricted to less than a thousand feet or so (that was a wild-assed guess). Most of the ocean floor is far too deep to drill.

It’s not that oil is necessarily more abundant in shallow coastal areas, it’s that that’s the frontier of oil drilling technology. Most onshore areas are already developed, and deep water areas are inaccessible - for now.

So I think what I’m hearing is:

Offshore oil is more common now, because onshore oil is less common now, because we’ve already pumped a lot of the onshore oil, which is why there is so much interest in offshore drilling even though its fairly expensive.

Is that a decent summary (as well as a fantastic run-on sentence)?

Great run-on sentence, and yeah, basically you got it. Oil companies will only dig up oil that they can make a profit on. There’s still lots of oil left onshore (in tapped-out holes and weird formations like oil sands, for example, and let’s not forget forbidden areas like the ANWR in northern Alaska), it’s just not currently profitable to get it. Heck, it’ not currently possible to get at a lot of it. But wait 'till oil hits $100 per barrel…

Hydrocarbons are primarily sourced by microbial marine life (and plants), so part of the answer is that we tend to look for oil and gas in places where there were oceans. A lot of those places are still oceans. That’s very simplistic; nevertheless, marine paleoenvironments are our primary hunting grounds.

Let me interject here that your run-on sentence, squeegee, was a pretty decent summation of the other part of the answer. The onshore locations were drilled first and, as such, the easier to identify targets there were exploited long before offshore drilling became economically feasible. So, for a long period when the offshore opened up, onshore exploration was more difficult; but still very much less expensive. The late '70s and '80s saw the development of the “easy” offshore properties.

But there’s still more to be drilled offshore, as we go further out, the data improves, new ideas (both geological and technological) develop and demand remains unceasing.

I work exclusively onshore these days. Our targets are generally smaller than they were ten years ago, but advances in techniques have allowed us to pursue quarry that were economically unpursuable (I think that may be the first time I’ve ever used that word) just a few years ago.

Seismic data is the primary data source for hunting, and, while offshore drilling is very much more expensive than onshore (which isn’t cheap - land rig day rates are almost 2.5 times what they were 2 years ago), marine data is much cheaper to acquire, and is usually (sigh) of much better quality.

So, yes, there’s a reason to look in the oceans and part of it is that the onshore U.S. reserves were the first to be exploited.

Hope this helps.

(Kamandi, I’m guessing you’re thinking of tar sands or oil shales.)

So beatle, when you’re hunting for oil, do you get to wear a safari hat? Or is it all done by looking at computer-generated maps in an office cubicle with a Dilbert calendar on the wall?

Nah, Arnold, that’d be old hat. Nowadays, when I con the workstation I wear a helmet with a flame paint job while the safety harness holds me in the chair (got a dead man’s switch on the mouse). :wink:

I’ve yet to spot Dilbert in the neighborhood.

Thanks, beatle, great information.

A question:

Why is the offshore data cheaper to acquire and of higher quality?

My WAGs would be:

  • again, no intervening populations under the water who might object to some seismic thumping;
  • other groups than oil geologists may also be acquiring data for other (scientific?) purposes, and the data is made available to the public, including the oil folk;
  • There’s still low-hanging fruit in the offshore area for data gathering, where on-shore areas that haven’t yet been analyzed are less accessible.

Are any of these guesses correct, or am I wildly off-target?
Thanks again for your insights.

This may be getting off-topic, but what the heck, we’re already talking about geology and oil:

And of the places that are no longer oceans, some are now on or somewhat under continents, and some former oceans areas have been subducted back into the mantle.

Question: the land above/around a rich oil field is subducted: what happens to the oil? Does it stay with the continental plate (to return someday as what? Diamonds? Coal? Roof sealant?) or burble up to the surface somehow and stick around? Is a plate subducted in it’s entirety or are volatiles such as oil and gases released during the subducting ?

Sorry if this hijacking my own thread, but I’m curious…

You’re generally on the right beam. There’s a few reasons that include (for the U.S.):

• No surface damages to pay (makes a big difference).

  • Onshore shooting (industry jargon for acquiring seismic data) often involves drilling holes for explosives or 'dozing senderos for vibrators, and you pay the landowners recompense; lumber companies make you buy the trees you have to cut down, but you don’t actually get the lumber, farmers you pay for their crops;
    • Offshore permitting is not a problem while land permitting often is - that is to say that a land seismic acquisition program can be held up by recalcitrant surface or mineral owners while you can generally shoot anywhere you want in the water (which is all federal or state owned - no lumber companies or farmers to deal with);
    • Offshore acquisition has far less culture to deal with - you might have to gap the occasional drilling rig or production platform, but cultural impediments to shooting are far less frequent than the onshore plethora of shopping malls, highways, farmhouses (oh, did I mention cities?)
    • Attaining denser source and receiver coverage is cheaper offshore.
  • The more inputs and samples you have, the better the data;
  • The preferred source onshore would be explosives, and each shothole loaded with dynamite, pentolite, C5, whatever, adds significant cost to your program; offshore you just fire that airgun array again;
  • Each receiver for an onshore survey must be physically planted in the ground by a paid human being; moving that array of receivers requires much human labor; offshore that array of receivers, once out, just follows your boat around and is on station whenever you fire the source;
    So, improving data quality by increasing the number of source points and receiver points has a miniscule marginal cost offshore (typically an airgun source) compared to that of onshore.
    • For a long time the primary targets offshore were shallower Pliocene and Miocene targets that were shallower than a lot of the onshore targets and, with the higher density of data as well as an often higher frequency content in the recovered data, it was easier to discern details in an offshore remote image than in onshore such.
    • The higher density of data samples allows iterative processes, such as velocity analyses, a better chance of getting closer to the solution.

Good question. I’ve never worked active subduction zones and I don’t know the answer, but I’ll hazard a stab at it.

One of the hardest issues to grasp, I’ve found, for someone who doesn’t have some background in geology, is just how much time we’re talking about. The mechanical movements my colleagues and I discuss every day in whodunit scenarios took millions of years to come about. And some truly significant events took place in the small part of a day. A jagged timeline. You get used to it.

Well, the subduction of oceanic crust proceeds at a geologic (I think the common term is glacial) pace. And as that happens, whatever existing reservoir trapping geometry that might have existed will be breached as oil or gas sands are bent, broken, rubbed up against each other, etc. during the subduction process. While I’ve lived with the slow processes of geology, let me be the first to tell you that when given an opportunity, a methane molecule that’s been parked on a dead-end street for 65 million years can still get up and go. And will. Now. Today.

So I suspect late migration of hydrocarbons would occur around a subduction zone, charging whatever younger potential reservoir rocks are in the feed zone. As for the hydrocarbons that don’t manage to escape, well, I don’t know. Let me think about it.