A research team including colleagues at UC Davis, Lawrence Livermore National Laboratory and Shell Projects & Technology is offering a new computational study published in the Proceedings of the National Academy of Sciences reveals how hydrocarbons may be formed from methane deep in the Earth at extreme pressures and temperatures.
Hydrocarbon Formation Simulation
The research is important for understanding hydrocarbon reservoirs and fluxes within Earth’s crust. Knowledge about the thermodynamic and kinetic properties of hydrocarbons at high pressures and temperatures will assist in discovering more naturally occurring petroleum resources. This new work provides a basis for understanding the experiments that demonstrated polymerization of methane to form larger hydrocarbon molecules such as propane and crude oil and earlier methane forming reactions occurring under pressure.
Part of the research begins with the assumption that methane, the simplest petroleum hydrocarbon, is present. Biology is quite good at producing methane in prodigious amounts. Accumulating over time, the question of how larger petroleum molecules formed might be in part answered now.
Petroleum is a hydrocarbon, molecules composed of the elements hydrogen and carbon that are the main building block of crude oil and natural gas. Hydrocarbons contribute to the global carbon cycle, one of the most important cycles of the Earth that allows for carbon to be recycled and reused throughout the biosphere and all of its organisms over time.
Today the vast majority of geologists and geochemists believe that nearly all, more than 99 percent, of the hydrocarbons in commercially produced crude oil and natural gas are formed by the decomposition of the remains of living organisms, which were buried under layers of sediments in the Earth’s crust, in a region approximately 5-10 miles below the Earth’s surface.
The question for the experiments, explains UC Davis Professor Giulia Galli, a senior author on the study, center on if the formation of hydrocarbon molecules purely from a chemical deep crustal or mantle origin (known as abiogenic or non biological formed) could occur in some geologic settings, such as rifts or subduction zones.
Galli says, “Our simulation study shows that methane molecules fuse to form larger hydrocarbon molecules when exposed to the very high temperatures and pressures of the Earth’s upper mantle. We don’t say that higher hydrocarbons actually occur under the realistic ‘dirty’ Earth mantle conditions, but we say that the pressures and temperatures alone are right for it to happen.”
Galli and colleagues used the Mako computer cluster in Berkeley and computers at Lawrence Livermore to simulate the behavior of carbon and hydrogen atoms at the enormous pressures and temperatures found 40 to 95 miles deep inside the Earth. They used sophisticated techniques based on first principles and the computer software system Qbox, developed at UC Davis and LLNL.
The team found that hydrocarbons with multiple carbon atoms could form from the methane molecule with only one carbon and four hydrogen atoms. This needs temperatures greater than 1,500º K (2,240º F) and pressures 50,000 times those at the Earth’s surface; conditions found about 70 miles below the surface.
Leonardo Spanu of UC Davis, first author of the paper says, “In the simulation, interactions with metal or carbon surfaces allowed the process to occur faster — they act as ‘catalysts’. ”
The research does not address whether hydrocarbons formed deep in the Earth could migrate closer to the surface and contribute to oil or gas deposits. However, the study points to possible microscopic mechanisms of hydrocarbon formation under very high temperatures and pressures.
The debate between biological sources for the hydrogen and carbon used in petroleum formation and the abiogenic theory will continue. Subduction to 40, 70 and 95 miles all seems a little pointless. But the point is that finding the richest lodes of petroleum is a major goal for the world’s economy. Knowing how to look for reservoirs that formed from abiogenic action is worth doing.
All that needs to happen is find just one – not feeding from sedimentary deposited biological sources.
The full team presenting the paper, Stability of Hydrocarbons at Deep Earth Pressures and Temperatures in the Proceedings of the National Academy of Sciences is made up of Galli, Spanu and Davide Donadio at the Max Planck Institute in Meinz, Germany; Detlef Hohl at Shell Global Solutions, Houston; and Eric Schwegler of Lawrence Livermore National Laboratory. Shell made a funding contribution beyond the personal contribution of Mr. Hohl.
For many the research might seem to be a chase to nowhere. But factually, knowing could have immense implications. The reservoirs might not be conventional as in sedimentary rock. The upward flow over millions of years may have been simply an atmospheric dump. Perhaps some of the huge reserve of methane hydrates is a result of methane seepage into a near surface environment that simply stopped the methane from getting into the atmosphere.
There are lots of questions at this level, and the answers are going to present many more. For decades to come methane and the larger petroleum molecules are going to be needed. Better to ask now and have more ideas on how to progress and not get caught short.
By. Brian Westenhaus