Hydrocarbon Generation

Hydrocarbon Generation – Mike Arthur – Penn State

Hydrocarbons take a long, long time to develop. How does that happen?

It all starts in a special kind of body of water. This water body may be an inland sea or a large lake. The key characteristic of this kind of water body is that it is density-stratified into layers by large temperature or salinity differences, and these layers of water are not disturbed or mixed for long periods of time.

Lakes with this type of non-mixing stratification are called meromictic lakes. The bottom layer of a meromictic body of water is typically anoxic, meaning it contains virtually no dissolved oxygen, or it may even be euxinic – having high dissolved sulfide content. Either of these conditions (or both) prevents most life forms from living in that lower layer, while the upper layer may support abundant life.

In fact, the surface layers of such lakes or inland seas often encourage the growth of tiny biological organisms. As these prolific plankton and algae fall to the lake or sea floor, they accumulate and do not decompose due to the lack of oxygen and the presence of toxic sulfide. Since so few organisms exist in this lower-level environment to break down the organic matter, it grows in thickness over years, decades and centuries.

This may continue for thousands to even millions of years as the tectonic plates move, the underlying basin floor subsides, and mountains begin to rise from former plains. Rivers draining these mountains continuously bring nutrients to the sea, stimulating biologic productivity.

As the tectonic movements further uplift the mountain belt, old rivers change course and new rivers begin to flow on rising landforms and sediments – sand, silt and clay – are transported into the formerly stable and stratified water body.

These sediments bury the organic matter, sealing it from decomposition, so that even after the anoxic water goes away, that layer is protected.

That layer of organic matter that started at the bottom of the anoxic sea might now be buried under thousands of feet of new sediments.

Now… that layer that we’ve been watching begins to undergo an interesting chemical process.

Thousands of feet of rock strata exert a great deal of pressure on this organic matter and begin to heat it. When just the right combination of heat and pressure are applied to just the right kind of organic matter (particularly plankton and algae) over millions of years, that organic matter is converted into oil and natural gas and other hydrocarbons.

Oil and Gas Windows – Mike Arthur – Penn State

I want to take a closer look at how heat and pressure convert organic matter over time into oil, gas, and other hydrocarbons. Geologists often refer to this process as “cooking” the organic matter. Like cooking anything, if you make it too hot or you cook it for too long, it doesn’t come out in very good condition.

First, I need to explain that the pressure of rock pushing down, also called lithostatic pressure, compacts sediments, while deeper and deeper burial makes things heat up. It gets hotter the further down you go in a predictable way.

For example, let’s assume that the thermal gradient with depth has an approximate value of 23 degrees Celsius per kilometer of depth for the Appalachian Basin today. This calculation assumes 23 degrees Celsius as a minimum value for the gradient. Temperature gradients vary from basin to basin depending on the basin type and when it was formed. We know it gets hotter as you get deeper because we can measure it. Stick a thermometer down a hole and see how hot it is at various depths. If you were to go down far enough, such as into the mantle, it gets so hot that the rock is partially molten magma.

So deep in the ground, we have pressure and we have heat. Now, bring in the organic matter. The amount of heat applied to that organic matter and the amount of time over which it is heated are crucial in determining what that organic matter becomes next.

If you have good source materials heated to a temperature of about 60 degrees Celsius, or the top of the “oil window,” as it’s called, that organic matter begins to liquefy into oil. Peak oil generation occurs at about 120 Celsius and drops back off as things get up to around 150 Celsius. If you use that (23) degrees per kilometer of depth calculation I mentioned, the top of the oil window would occur at about 2000 meters depth.

If it gets hotter, from about 150 to 260 Celsius, as the result of further burial, then the organic matter and any oil remaining in the source rock transform into gas. That temperature range is the so-called gas window. If it gets even hotter than that, hotter than 260 degrees Celsius, then everything will eventually burn off (pause) and, in time, there will be no gas or oil left in the rock at all. In fact, all that’s left in rocks that have been heated that much is a burnt residue of black gunk.

I’m going to use the Marcellus Shale to illustrate how these oil and gas windows worked in a real basin. The beginning of this story might sound familiar.

390 million years ago, the region of what is now North America contained an inland sea. The living organisms in that sea deposited a thick layer of organic matter over millions of years. As continents shifted, the basin subsided and the organic matter got buried. Over forty or fifty million years, that organic matter got to a depth of 6000 feet, putting it in the oil window, and, appropriately, it started generating oil. Much of that oil migrated out of the source rock heading upwards. Some of that oil got caught in traps. Some seeped out. As the millions of years tick by, the layer now known as the Marcellus continued subsiding, eventually making it down to 20,000 feet or 6000 meters, bringing the temperature up to 160 degrees Celsius about 250 million years ago, putting it in the gas window. As it descended from 6,000 to 20,000 feet, the oil that remained in the source rock either migrated out of the rock or was converted to gas and the as yet untransformed kerogen that hadn’t been turned into oil began to be transformed into gas.

Then, conditions changed and over the next 250 million years, the basin was uplifted, overlying strata were eroded, and the Marcellus Shale horizon rose to, let’s say, about 8000 feet, which is where we find it today.

This rise allowed it to cool back down into the oil window, but we’re not going to get any more oil out of it because all the kerogen has been cooked into gas already during the earlier, deeper burial. So, as it rose, the lowering of the temperature allowed the gas to remain in good condition all the way to today, when we find it as high quality dry gas. When we say dry gas, we mean there isn’t a lot of liquid oil or anything else coming up the pipe during production. There’s no liquid oil because it all got cooked out to gas.

That’s how the oil and gas windows work.

This brings up two questions for me that I think are worth exploring: If we know how much heat, pressure, and time it takes to make this stuff, can we make it in the lab? How about at scale?

Second, how is it that we just happen to be alive at the time oil and gas is in such good condition for us to use? How did we hit this time window so perfectly?

Let’s answer those.

First, the lab question. If all it takes to make hydrocarbons is cooking some plant matter under pressure, can we do that in the lab? The short answer is yes, we can! The big difference is the earth does this over huge geographic areas, like thousands of square miles, over very long time scales, and under extreme pressures. We can recreate the temperatures and pressures in the lab. That’s the easy part. We don’t have millions of years to wait, so for us to do this quickly enough for it to be usable, by us, we have to spend a great deal of energy to speed up the process which ends up making it not cost effective. We’re spending nearly as much energy producing it as we can get out of it, or more. On top of that, we can only make it in tiny batches compared to what the earth has done. So in short, so far, no one has been able to make enough artificial hydrocarbons cheaply enough for it to be economically feasible. But it has been done.

If there comes a day when producing it artificially is cheaper than producing it from existing natural reserves, it’s quite likely that we will do that, unless we’ve moved on to a completely different type of fuel by then. At the moment, that day appears to be a long way off.

How about that question of humans hitting the time window just right?

Any hydrocarbon source that we are exploiting now is only one of many possible sources. Other older and deeper source rocks have already been completely spent and shallower potential source rocks are immature, which is to say they are not ready yet. Basically, no matter when one is placed on Earth over the last 500 million years, there would likely be similar distributions of high-quality hydrocarbon resources available. Just different ones. Many younger strata removed by erosion may have reservoired lots of hydrocarbons which were lost by oxidation. An example of a resource that’s in the process of being lost is the Canadian “oil sands” which have been uplifted and nearly exhumed and are now being oxidized and water-washed near the surface, altering their original chemical composition and making them more difficult to extract. If we had been here a few million years ago, they would have been a prolific conventional oil field. And had we arrived a million years in the future, they’d be gone – eroded, oxidized and redistributed – lost to the ravages of time.