I think separating gas and crude deposits locations is not too realistic. Usually gas and crude are near each other since both are coming from the same internal source. Maybe would be better to switch some or at least one of the harvesting spots inside the crude oil deposit sites to be gas sources? Or just add a gas spot programmatically for each crude oil deposit (ensuring no spots overlapping)?

Also, for simplicity, the natural gas should be just vanilla's Petroleum Gas.

In addition, for distinction purposes (sacrificing some realism, of course), I would humbly suggest to use some steam exhaust animation or smg like that to gas extraction spots for better visual distinction. Or at least different decoy, easily mentionable and distinctive from oil spots.

I already sent the code to the author of RSO mod to associate natural gas with crude oil, and balanced and tested it myself. I think it works well, there. I don't know how to associate resources in the base game -- I think RSO does it programmatically. I probably won't take the time to do it myself, because I really like RSO, and I highly recommend it, and we've already solved the problem that way. Should hopefully be in the next update.

Regarding the PG, note the settings in the mod. Turn off "methane" in the settings and the natural gas will just output petroleum gas. Turn off "wellheads" and you can use vanilla pumpjacks, instead. I did that part just for you! And because I think if you like it other people will too.

Regarding steam at the EXTRACTION SITE, thanks for pointing that out, and I think I know a way I can implement that.

Let me shock you with a facepalming question: what is "RSO"? ^.^
Will go checking things out...

Oh, Resource Spawner Overhaul. It's a rework mod for the resource distribution system. In vanilla, the resources are generated using perlin noise, and as far as I can see, there's no way to associate two types of resources together. So the patches will be essentially in randomish clumps.

Alrighty! So, wellheads. Is there a need for the mod setting there? Can pumpjack automatically switch to wellhead model if placed over the gas extraction site (just like drill changes over uranium)? Now I see it would be better to visually distinguish where the gas is mined if the models are different.

Also, I see some confusing point. I tried with pumpjack setting and no methane (not sure if this has any significance) and I see in the tech tree that gas boiler is unlocked by fluid handling. Doesn't make sense since pumpjack is unlocked via oil processing which is behind that tech - there would be nothing to burn then.

I have further suggestion in mind, which I kinda not sure about. It came as a result of those 2-weeks brainstorm. How about slightly messing up with vanilla oil processing? Since the devs are solid on the new change, then the refinery with basic oil processing with 1in-1out is basically the same as wellhead alone. So my proposition is this:

1) Rename Oil processing tech to "Natural gas extraction". Let it NOT unlock the refinery building at all (in that crippled pointless version as it is now). It really makes no purpose if the PG can be mined directly. The gas refining can be made in a chem plant if the setting with methane is active. The point is to introduce the refinery in a later tech in place of the advanced oil processing, so when the refinery actually appears, it will have it's full version (2-in-3out).
As a consequence, crude oil has no use before the blue science as it is based on PG only. Not sure how to deal with pumpjack then, cuz I suggest to not let the crude oil extraction sites to be accessible at that point. Maybe separating wellheads from pumpjacks does make sense then, but if it possible to lock the pumpjuck model switching and placement, I think it may be preferable. Not sure. Hope you get the idea and find an elegant way to deal with those problems.
Don't forget to unlock the gas boilers with that tech also.

2) Rename Advanced oil processing to "Crude oil processing". With this tech the crude oil extraction sites can finally be accessed via pumpjacks. The refinery comes there. No more AOP and BOP, just single Crude Oil processing equivalent to classic AOP.

So the idea is to start with PG until blue science, and introduce the refinery only in it's full version when other petroproducts come online together with the cracking tech required to balance them. I am convinced that this is a proper way to postpone "oil wall" since it has no "tear all the oil setup apart and rebuild the refineries again" flaw the dev's final version has. I think as a result, this mod will show a decent compromise between what community wanted and what devs come up with. That is how I see it. Hope you like it too.
BTW, considering things proposed, it may have sense to tweak some numbers so that the "crude oil processing" recipe outputs less PG and a bit more LO&HO, so to soften the redundancy of PG output this way, assuming that the PG production chain will most probably be already set up based on natural gas resources. Plus extra PG can be made via cracking from those two, if really needed (in case natural gas extraction wells deplete by that point causing disbalance by the time COP comes online).

The reason I stand for such changes is that the dev's approach will force new players to rebuild oil setup, which will inevitably cause the frustration for new players that have no way to know what to expect next. Not fair to punish them for not knowing the future, would you agree? And the proposed approach (setup PG chain first, then add the rest of the oil processing) does actually soften the "learning curve" by splitting the large task into two smaller, sort-of independent ones. The "oil wall" is still there, I believe, but at least it will not cause inconvenient oil setup' rebuild, which I see being one of the main design flaw in the dev's version.

Thanks for the feedback. I'll think about it! Couple of things I'll tell you about the current version, and let me know if any of this is wrong:

1)
With methane turned off, the refinery shouldn't be unlocked by the "gas processing" tech -- if it is that's a bug. But I'll think about moving natural gas processing, which should only be active with methane on, to the chem plant.
This mod lets you make fuel (with a reduced efficiency compared to making it from LO/HO/PG directly) from crude oil, since we can already make flamethrower ammo, which is essentially the same thing. I think they need to add this to the base game. Then, crude oil becomes useful, but less efficient, to directly use from the ground. I think I prefer this solution to actively changing the vanilla mechanic.
Gas-fired boilers are set to be unlocked with the "fluid handling" tech with this mod, since they can burn any fluid with a fuel_value, but if you already researched that tech, you may need to run the command "/c game.player.force.reset_technology_effects()" for it to show up.

2)
I renamed advanced oil processing to "Advanced distillation", since it also comes with an advanced natural gas processing recipe, but I moved name changes to another mod since you complained that I changed the fluid names, before. :P I could consider moving some of those name changes to this mod.

It sounds like, all-in-all, this mod isn't far from what you are saying, especially in the "no methane" mode. I'll think about your ideas. I think another way to solve the "PG overabundance" problem would be to essentially replace about 1/3 of the crude oil patches with natural gas patches. So then we're not really introducing MORE gas, but changing the ratio of what is available, and hinting at the player that there's more to access later once they move past the natural gas.

{With methane turned off, the refinery shouldn't be unlocked by the "gas processing" tech}
-I see no such tech. There is an "Oil processing" only. The description shows refinery among icons.

{This mod lets you make fuel ... from crude oil, since we can already make flamethrower ammo...}
Damn that flamethrower I never tried and forgot about! That is the only thing that gives crude oil a use before AOP. Fuck. I thought about nice separation of gas and oil like "pre-" and "post-blue science" resources, but that flamethrower sticks out as a single pre-blue tech that requires oil. Urgh!

Well, what do you think, Adamo, about pre-blue-science use of crude oil? It is kinda controversial: from one side, an abundance of crude for just a flamethrower may look senseless. So the SF from crude that you added, gives it a nice purpose, encouraging the player to get acquainted with oil before refineries come. The downside of it may be that the new player may start assuming that the whole purpose of crude resource is just that - the SF. So it may be expected that crude-based SF production get established just to be teared-off again after refining comes with better ingredients.
But from the other side, the initial goal of FF304 was to streamline blue science, therefore introducing crude oil in post-blue period without any intermediate "temporary solutions" (like SF from crude) may actually serve the purpose of splitted oil introduction with no half-measures leaving room for false assumptions and wrong builds causing rebuilding noone likes. But in this case flamethrower has to get blue flask in the tech recipe. The good thing is that it can switch to LO which is more realistic.

{Gas-fired boilers are set to be unlocked with the "fluid handling" tech with this mod, since they can burn any fluid with a fuel_value}
-Oh, I guess it is made so just for compatibility with mods that add flammable liquids that do not come from pumpjacks (Wood gasification I assume?). I just though too vanilla in this case, therefore seeing gas-boilers before pumpjacks confused me.

{I renamed advanced oil processing to "Advanced distillation", since it also comes with an advanced natural gas processing recipe}
-It's just that word "advanced" become obsolete since we cut away the "basic". How about "Petroleum distillation" or "Petroleum refining" or smg like that?

I think you have some good points and I've thought about this, too... eliminating the refinery altogether until blue science, that is, and changing the recipe names from "basic" and "advanced" to other things. I think we might be headed in this direction. Regarding the naming, A refinery is just a distillery... so with the earlier cases, we get "distilling" recipes but they are called "basic oil processing" and "basic natural gas processing", and they come in separate techs, but later, the "advanced oil processing" tech gives, with my mod, us advanced recipes for both gas and oil, you see, so thus I had to use the more general name: "advanced distilling". I like some of the ideas of where you're going, here, but I don't think I want to change any more of the vanilla stuff, yet. I think I'm going to keep with the default naming scheme for now and see where the devs are going with this. Might change that in the future, though.

Regarding the gas-fired boiler, I just thought it made the most sense to put it in fluid handling, since it

"I think another way to solve the "PG overabundance" problem would be to essentially replace about 1/3 of the crude oil patches with natural gas patches."

That separation of gas from crude as two types of resource patches is what I see as a biggest concern. Well, because I am thinking in the direction of achieving the goals the 0.17.60 was destined to, since I am totally agree with the reasoning behind the new oil changes, but strongly against the implementation of Wube's solution.
In my opinion the reason to switch just 1 extraction site for each crude oil patch to PG (instead of separating patches as it is now) is that the same outpost(s) established for PG extraction at pre-blue times can be repurposed/"upgraded" for crude oil extraction at post-blue period. IMO the pre-blue period should teach the player concepts of fluid handling and usage by letting him practicing with just one of the fluid resources, while at post-blue period the focus should be on refinery and hard balancing puzzle alone. The unnecessary distractions and side-tasks should be minimized there to let the player profit from new oil products as soon as possible. Therefore "combined" gas+crude patches will save the time for establishing new outpost for crude sourcing - just a quick upgrade of the existing outpost digging gas since pre-blue.

Therefore I recommend to find the way to programmatically switch one crude spill to gas source per EACH crude oil deposit patch instead of introducing a separate gas deposits. Intuitively, I think this should be done inside the map chunk generation event - once the black map piece get explored, if it contains crude oil deposit, one crude oil extraction site must get replaced with natural gas (or PG, dependent on setting).

Regarding the naming, A refinery is just a distillery... so with the earlier cases, we get "distilling" recipes but they are called "basic oil processing" and "basic natural gas processing", and they come in separate techs, but later, the "advanced oil processing" tech gives, with my mod, us advanced recipes for both gas and oil, you see, so thus I had to use the more general name: "advanced distilling". I like some of the ideas of where you're going, here, but I don't think I want to change any more of the vanilla stuff, yet. I think I'm going to keep with the default naming scheme for now and see where the devs are going with this. Might change that in the future, though.

To help myself synchronize my mind with yours, without playing with methane chain to see what it is and for, I scrolled your mod codes. I do like the idea, I love realism and a I am impressed by a competent way (in my opinion) the NG processing represented here, I must admit. Though I realized this mod had a different initial idea at the time of it's creation comparing to the direction we are heading (if I understand it correctly). Plus realism was surely put over the gameplay perspective. From that point, introduction of methane serves the only purpose of increased variety, which is surely no good for beginner players (sorry for being latched to the idea of developing the alternative to 0.17.60 changes, but that is the goal I am pursuing here). If I get it correctly, the only uses for it is a liquid fuel to burn or just an another option for SF (both are essentially a duplication of subset of PG uses). This way it does not bring anything conceptually new, therefore the addition of it has no beneficial impact on a gameplay, while the impact of increased variety is somewhat questionable (smg fresh for old players bored with old stuff, but another head-spinner for beginners). The only purpose for it to stay is to serve believeability of the NG processing recipe. Note that this is just my critical opinion, I am not suggesting anything here, just purely analyzing and sharing thoughts.

Shall we continue, I am afraid it would be better to branch-off a different mod, skipping the methane and startup settings to focus solely on the idea of creating the mod presenting an alternative to 0.17.60 (in regard to oil changes). I think for the "Adamo Carbon" it would be better off to be standalone then multipurposed, I think it is good as it is and by continuing unification you may have to deal with compromises which Adamo Carbon may not benefit from.

Though right now we are heading in the direction of making "0.17.60 Oil changes alternative", I may suggest a more generic name for a suggested branched mod - "Vanilla oil reworked", just to leave us some freedom in case of 0.17.60 will be just a first fiasco in more changes to come.

Let me know what you decide, I have a lot of ideas for vanilla' oil processing alternative mod, though I am pretty sure I will be unable to force my lazy ass into learning modding of Factorio, so there is no hope to ever expect "Vanilla oil reworked" made by me. :) To be clear from the start, I am signing to contributing just my creativity and critical thinking, so let me know if you feel enthusiastic towards the same mod idea, so I could contribute as much on my part as possible in those fields shall you decide to go with such mod. All up to you, boss! ;)