1040: "Lakes and Oceans"

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Re: 1040: "Lakes and Oceans"

Postby TheSoberPirate » Fri Jul 13, 2012 11:48 pm UTC

Pfhorrest wrote:Something just occurred to me.

If oil is decomposed organic material, how does it get that deep? Shouldn't it be up near the surface around where we find fossils? Or is the organic material that became oil way, way older than I thought it was, and has been that thoroughly buried?

I'm not a petroleum geologist but I think part of the reason is that we don't look for fossils that deep underground. We're only drilling that deep for oil because we've already used up most of the oil from shallow, easy to extract deposits. Improving technology and high demand make it profitable to keep drilling increasingly deeper wells to access progressively deeper and harder to get to deposits. Presumably we'd find fossils just as deep as we find any fossil fuels, it just isn't worth drilling down 5,000 m to get them.

A quick check of Wikipedia says some oil deposits are from the middle Cambrian, so you've got 500 million years or so of deposition in some cases.

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Re: 1040: "Lakes and Oceans"

Postby J Thomas » Sat Jul 14, 2012 12:50 am UTC

Pfhorrest wrote:
J Thomas wrote:If the minority view is right, we will never completely run out of methane because more is being produced all the time. The rate that it's produced is unknown but probably not real large. Still it provides hope. If we can get many millions of tons of methane a year from down in the mantle or something, we'll never run out and there will never be a fossil fuel shortage.

Presumably even if that minority view is right, we will eventually run out of methane anyway because there's still a limited amount of carbon in the rock, unless it's being cycled back into rock in some comparable timely fashion (and not just turning into mulch and CO2). Is it just that "eventually" is negligibly short of "never" in that case?

There's probably a whole lot of limestone getting subducted at plate boundaries. It's unknown how much carbonate rock gets subject to just the right conditions to create methane. It's unknown how much methane escapes before it is converted to something we can't use. It's unknown how fast the escaping methane reaches us.

There's no quantitative evidence for any of it. But our current fuel needs amount to about 12 billion tons of oil equivalent a year. About 82 billion barrels. Around 500 trillion cubic feet of natural gas. Unless I've missed a decimal point or gotten a conversion wrong, that's all the natural gas we'd need to collect and ship to where we need it, to provide our current energy needs. The earth might produce that much new natural gas fresh each year. I really don't know. It seems strange to think that could have been going on for the last hundred years while none of our scientists noticed. But every now and then things do sneak up on us If we've been getting that much new carbon every year, the world must also be sequestering it faster than we think.

My own guess is that if it happens at all, it's at a much lower rate. The hypothesized chemical reactions have never been observed in nature. There could be other reactions which consume methane, which the original hypothesizers did not think of because their goal was to discover unlimited fossil fuels. But I don't know. It might be a good start to get an estimate how much carbonate rock gets subducted each year, and that would be your maximum. Of course, not all the carbon can turn to methane.... It's potentially a very complicated problem and the computer models are not all that good yet.
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Re: 1040: "Lakes and Oceans"

Postby Pfhorrest » Sat Jul 14, 2012 12:59 am UTC

TheSoberPirate wrote:A quick check of Wikipedia says some oil deposits are from the middle Cambrian, so you've got 500 million years or so of deposition in some cases.

Mostly what caught me as surprising about it is the idea of things deeper in the continental crust than the deepest oceans reach, and if there was that much deposition over that much time it should have filled the oceans in, when to my knowledge that's not how oceans disappear. Of course just below you JT hints at the obvious answer that eluded me: oceans disappear when their floors slide underneath continents, and lots of biosludge is all over the bottoms of the oceans, which easily explains how you'd get lots of oil that deep under a continent. It's not deposition burying it, it's tectonic plates shifting.
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Re: 1040: "Lakes and Oceans"

Postby Bill_The_Pony » Wed Feb 15, 2017 10:14 pm UTC

I just saw this comic as a poster on the xkcd Store site, and was intrigued. But I noticed one goof: The surface elevation of Crater Lake, OR is actually 1,883m (6,178ft) yet is shown with a surface elevation lower than Lake Superior. Similarly, the surface elevation of Lake Baikal, Siberia is 455.5m (1,494ft), yet also shown with a surface at sea level.

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Re: 1040: "Lakes and Oceans"

Postby Copper Bezel » Thu Feb 16, 2017 4:12 pm UTC

Best he really could have done is to place notes next to each of them. The image has a top and things. But it is a little misleading when some of the lakes are placed to represent their surface elevation.

I'd mostly forgotten this strip existed, so I kinda want to thank you for bumping it. Comics like this tend to be my favorites in xkcd.
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Re: 1040: "Lakes and Oceans"

Postby hargikas » Thu Mar 22, 2018 6:07 am UTC

I think that the depth of the oil well of the deep water horizon is wrong.

It was drilling a deep exploratory well, 18,360 feet (5,600 m) below sea level, in approximately 5,100 feet (1,600 m) of water.

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Re: 1040: "Lakes and Oceans"

Postby airdrik » Wed Mar 28, 2018 7:51 pm UTC

The depth of the hole where the disaster occurred was at 5,600 m below the surface, but the record set by that rig (mentioned on this page) was 10,685 m below the surface in 1,259 m of water. So only off by a few hundred meters (should be the 11 km line instead of below it)

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