The Experiment Is The Whole Environment

Petroleum

Petroleum is a naturally occurring mixture composed principally of hydrocarbons found within Earth. The word comes from Latin terms meaning rock and oil, literally, rock oil. Petroleum is not one chemical substance. Crude oil contains many different hydrocarbon molecules together with smaller quantities of compounds containing sulfur, nitrogen, oxygen, metals and other constituents.

Hydrocarbon

A hydrocarbon is a chemical compound made entirely of hydrogen and carbon. Methane, CH₄, is the simplest hydrocarbon. Larger hydrocarbons contain increasingly complex arrangements of carbon and hydrogen atoms. Depending upon molecular structure and surrounding conditions, hydrocarbons may occur as gases, liquids, waxes, heavy oils or solid material.

Generation

Generation, in petroleum geology, means the chemical production of petroleum hydrocarbons from precursor material. It must be distinguished from migration, the subsequent movement of petroleum through rock and accumulation, the concentration of petroleum within a reservoir or trap. The age and history of a petroleum reservoir therefore do not automatically measure the duration of the chemical reactions that generated the petroleum.

Organic Matter, Kerogen and Bitumen

Organic matter is carbon-containing material derived principally from organisms and their remains. Petroleum contains molecular biomarkers that preserve evidence of biological precursors, including compounds related to molecules produced by microorganisms, algae and land plants. Much petroleum source material therefore had an earlier biological history in Earth’s surface or near-surface environment before burial transferred it into a subterranean system.

Kerogen is solid, complex organic material dispersed through sedimentary rock that is relatively insoluble in ordinary organic solvents. It forms from preserved and chemically altered organic matter and can serve as a precursor from which petroleum hydrocarbons are generated.

Bitumen, in the experimental petroleum context used here, is organic material soluble in organic solvents that can form as an intermediate between insoluble kerogen and hydrocarbon-rich expelled oil. Laboratory experiments reveal a broadly observable sequence in which kerogen decomposes toward bitumen and bitumen subsequently yields increasingly hydrocarbon-rich petroleum.

Cracking and Maturation

Cracking means breaking large carbon-containing molecular structures into smaller molecules. Thermal cracking refers to reactions in which elevated temperature contributes energy necessary for those molecular changes.

Thermal maturation describes the progressive chemical alteration of organic matter as it experiences a thermal history. Maturity is the resulting chemical state. Maturation time is the interval during which the transforming conditions operated. Those terms must not be treated as identical measurements.

The Oil Window

The oil window is the range of subsurface conditions, especially temperature, within which organic matter is considered favorable for generation of liquid petroleum. It is not one universal depth or temperature because source-rock chemistry, geothermal conditions, burial history, fluid state and other variables differ among sedimentary basins. At still higher thermal maturity, liquid hydrocarbons can be increasingly cracked toward lighter hydrocarbons and gas.

Laboratory One

Hydrous Pyrolysis

Pyrolysis means chemical decomposition produced through heating. Hydrous means involving water. Hydrous pyrolysis therefore means heating organic material in the presence of water under controlled conditions.

In petroleum experiments, natural immature source rock or kerogen-bearing material can be placed in a sealed vessel with water and subjected to controlled temperature, pressure, confinement and reaction time. These experiments provide known starting material, known boundary conditions, recoverable products, and a directly measured laboratory clock.

Natural kerogen-bearing source rocks have produced petroleum hydrocarbons experimentally on clocks measured in hours and days. Hydrous-pyrolysis programs commonly use reaction intervals around 72 hours while varying temperature. Experiments using natural source rocks at approximately 300 to 350 degrees Celsius have produced expelled hydrocarbon-rich oil, with increasing conversion as the thermal conditions become more favorable.

Kerogen to Bitumen to Oil

Experiments using natural Type-I kerogen from Green River Formation oil shale provide a particularly clear sequence. Under hydrous conditions, heating around 330 degrees Celsius for 72 hours drove substantial kerogen decomposition toward bitumen. At approximately 350 degrees Celsius for another controlled 72-hour experimental condition, bitumen decomposition proceeded toward hydrocarbon-rich oil.

Other experiments using natural source rocks have varied temperature and duration over ranges from roughly 12 to more than 100 hours and observed overlapping stages of kerogen decomposition, bitumen production and hydrocarbon-rich oil generation. Petroleum generation is therefore experimentally observable as a chemical transformation on a human clock when sufficiently strong conditions are supplied.

Water is Part of the Experiment

Water is not merely incidental to these experiments. Comparison of wet and dry thermal treatments shows that liquid water can materially alter the reaction pathway. In hydrous systems, cracking and hydrocarbon production can be favored, while comparable dry conditions can favor cross-linking and formation of more insoluble organic material.

Isotopic experiments further demonstrate that hydrogen derived from water can become incorporated into organic material during thermal maturation. The water-bearing environment therefore participates chemically as well as physically in the transformation.

Pressure and Confinement

Pressure is an essential part of the physical environment, but its function cannot be reduced to the statement that more pressure always produces petroleum faster. Experiments varying fluid pressure show that increasing pressure can retard hydrocarbon generation, maturation and thermal destruction under particular conditions.

This is not evidence that pressure is unimportant. It demonstrates that pressure functions within a coupled system. Pressure affects fluid state, density, phase behavior, confinement, pore-fluid conditions, transport and the ability of water and volatile products to remain engaged with the reacting material. Its effect depends upon the rest of the environment.

Confinement is especially important experimentally. At elevated temperature, an open system permits water and volatile reaction products to escape. A sealed and pressurized system can retain them. The laboratory therefore does not merely heat organic matter. It constructs a controlled physical environment in which the relevant materials remain engaged.

The Experiment is the Whole Environment

The petroleum experiment cannot be reduced to one master variable. Source material, preservation state, water, chemistry, confinement, pressure, temperature, mineral surfaces, sequence and duration between transitions operate together.

Remove water and the reaction pathway can change. Alter temperature and the rate and products change. Change pressure and fluid behavior changes. Open the system and volatile components may escape. Alter the starting organic material and the resulting petroleum-generating behavior changes.

The experiment therefore demonstrates an ordered physical system rather than isolated variables. Pressure does not replace temperature. Temperature does not replace water. Water does not replace chemistry. Confinement does not replace any of them. The material responds to the whole state of the system.

A proposed natural history cannot separate experimentally coupled conditions by an arbitrary interval without accounting for what happens to the material during that interval. Oxidation, microbial activity, chemical alteration, diffusion, evaporation, cross-linking and other reactions do not stop merely because the next stage of a proposed history has not yet begun. Sequence and intermediate preservation therefore become part of the physical problem.

Laboratory Two

The Sedimentary Basin

Earth supplies a natural counterpart in sedimentary basins. A sedimentary basin is a large region in which sediments accumulate and may subsequently undergo substantial burial and subsidence. Organic-rich sedimentary rocks within such basins can become petroleum source rocks.

A source rock is a sedimentary rock containing sufficient organic matter or kerogen to generate petroleum under suitable conditions. With burial, the source rock can experience increasing confinement, overburden, temperature, pore-fluid pressure and changes in water-rock chemistry.

Lithostatic pressure means pressure associated with the weight of the overlying rock column. Pore-fluid pressure means pressure carried by fluid occupying microscopic pores and fractures within the rock. These pressures are distinct but interact mechanically within buried sedimentary systems.

Where Earth Supplies the Conditions

Petroleum-generating source rocks commonly occur kilometers beneath the present surface. The exact depth of an oil-generating interval varies with geothermal gradient, source-rock chemistry, basin structure and thermal history, but depths of roughly 2 to 5 kilometers are common in petroleum systems.

The Green River Formation of the Uinta Basin provides a useful scale comparison. Organic-rich source rocks containing abundant Type-I kerogen occur in deeply buried portions of the basin, with petroleum-generating intervals at depths on the order of 3 kilometers, approximately 10,000 feet, and with substantial pore-fluid pressure. The system therefore contains organic-rich rock, kerogen, water-bearing pore space, confinement, elevated temperature, lithostatic loading and pore-fluid pressure, the same classes of variables deliberately assembled in Laboratory One.

Other sedimentary basins display vertical thermal progressions from immature organic matter through oil-generating conditions and, at still greater thermal maturity, toward gas generation. Earth therefore demonstrates naturally that different portions of a buried organic system occupy different physical and chemical regimes.

Burial, Confinement and the Surface Origin

Petroleum biomarkers establish that much of the carbon-bearing source material had a prior biological history before entering the deeply buried petroleum system. Source rocks now kilometers beneath Earth’s surface therefore preserve a physical contrast between an earlier depositional environment and their present subterranean position.

Burial may include episodes of rapid sediment accumulation, subsidence, tectonic movement and other large-scale changes. Rapid burial where independently demonstrated can quickly remove organic material from open surface conditions and establish new chemical boundary conditions. Massive burial and confinement are therefore legitimate physical variables to investigate rather than merely chronological descriptions.

Burial itself does not manufacture petroleum. It constructs and changes the environment in which preservation, kerogen formation, pore-fluid behavior, pressure, temperature and later hydrocarbon-generating reactions can occur.

Preservation Before Generation

Domain 5 established that biological material enters an immediate competition between destruction and preservation. Domain 6 begins with the material that survives that competition sufficiently to become petroleum precursor material.

The conventional petroleum sequence does not require finished crude oil to remain exposed while waiting for deep burial. Original biological material is altered during early burial, and a portion becomes resistant organic matter represented by kerogen. The physical question then becomes whether that precursor remains chemically capable of later petroleum generation throughout the proposed burial history.

Kerogen provides a physically plausible preserved precursor. But the duration of its residence in an immature state is a separate question from the experimentally measured duration of its conversion once petroleum-generating conditions are supplied.

Kinetics

Kinetics is the study of how rapidly a physical or chemical process occurs and what conditions control its rate. In petroleum research, kinetics asks how rapidly a particular source material transforms under specified temperature, pressure, water, chemistry and boundary conditions.

A measured experiment might establish that a source rock produces oil after 72 hours at a particular elevated temperature.

Predicting how long a comparable reaction would require at a much lower temperature is a different operation: kinetic extrapolation.

Activation Energy and Frequency Factor

Activation energy is the energy barrier that must be overcome for a particular chemical reaction to proceed. The frequency factor, also called the pre-exponential factor, is a parameter in the Arrhenius relationship associated with the frequency and configuration of molecular reaction opportunities.

Experimental reaction rates at elevated temperatures can be used to estimate these kinetic parameters. The resulting mathematical relationships are then used to calculate expected reaction rates at substantially lower temperatures.

The Arrhenius Bridge

The Arrhenius relationship describes the strong dependence of many chemical reaction rates on temperature. In simplified terms, increasing temperature generally accelerates a reaction, while decreasing temperature can slow it dramatically.

Petroleum laboratories cannot observe a ten-million-year reaction directly. Instead, researchers measure transformations at elevated temperatures over hours, days, or longer laboratory intervals, derive kinetic parameters and extrapolate those relationships toward the lower temperatures and heating rates reconstructed for sedimentary basins.

This distinction is fundamental. The high-temperature laboratory transformation is directly observed. The long low-temperature generation duration is calculated from kinetic models combined with reconstructed burial and thermal histories.

The Extrapolation is Not the Measurement

The Arrhenius relationship is well established in chemistry, and kinetic extrapolation is scientifically useful. But petroleum studies themselves demonstrate that the extrapolation carries uncertainty. Different experimental configurations applied to the same source rocks can produce different kinetic parameters and therefore different predictions of the timing and extent of petroleum generation.

Source rocks classified within the same broad kerogen type can also exhibit different kinetic behavior. Estimated activation energies and frequency factors depend upon experimental method, material composition and mathematical treatment.

The proper conclusion is therefore not that geological petroleum-generation durations are invented, nor that rapid high-temperature generation proves rapid natural generation at lower temperature. The conclusion is that directly observed laboratory duration and kinetically reconstructed geological duration are different categories of evidence and must remain identified as such.

Four Different Clocks

Domain 6 requires several chronological questions to remain separate. The first concerns the biological history of the original carbon-bearing material. The second concerns preservation and transformation of that material into kerogen. The third concerns conversion of kerogen through bitumen into petroleum hydrocarbons. The fourth concerns migration, accumulation, alteration and residence of the petroleum after generation.

A geological age assigned to a source rock or basin does not automatically measure the duration of the kerogen-to-petroleum reaction. Likewise, laboratory petroleum generation in 72 hours does not establish that a natural petroleum reservoir formed in 72 hours. Each clock must be tied to the physical event it actually measures.

Domain 6 Finding

Petroleum generation is a condition-dependent chemical transformation of preserved carbon-bearing material. Laboratory experiments using natural kerogen-bearing source rocks demonstrate that substantial kerogen-to-bitumen and bitumen-to-hydrocarbon-rich-oil transformations can occur on directly measured clocks of hours and days when the required water-bearing, thermal, chemical, pressure and confinement conditions are supplied.

Earth contains the same classes of ingredients and physical conditions naturally within sedimentary basins. Biological source material becomes buried and preserved, kerogen occurs within organic-rich source rocks, water occupies pore systems, overburden supplies lithostatic loading, fluids carry pore pressure and increasing burial can supply elevated temperature and confinement. Petroleum occurs within and beyond these systems as the completed natural product.

The experiment is the whole environment. No single variable substitutes for the others. Pressure, temperature, water, chemistry, confinement, source material, sequence and intermediate preservation interact to determine what transformation occurs.

The long generation durations commonly assigned to lower-temperature natural petroleum systems are not direct observations of reactions running for millions of years. They are kinetic reconstructions derived from experimentally measured reaction behavior, Arrhenius relationships, source-rock properties and reconstructed burial and thermal histories. Those reconstructions may constrain natural history, but calculated formation time and directly measured formation time must not be treated as the same measurement.

Laboratory One demonstrates what preserved organic matter can do when the necessary environment is assembled. Laboratory Two demonstrates that Earth supplies those materials and conditions at planetary scale. The remaining chronological question belongs to the natural burial and thermal history and must be established independently rather than inferred merely from the existence of petroleum.

Research Record

Key experimental and natural comparisons used in this investigation include hydrous-pyrolysis experiments on natural immature source rocks and kerogen over clocks measured in hours and days; Green River Formation Type-I kerogen experiments demonstrating kerogen-to-bitumen and bitumen-to-hydrocarbon-rich-oil transformation under water-bearing elevated-temperature conditions; wet-versus-dry experimental controls demonstrating the chemical importance of water; pressure experiments demonstrating that pressure can alter and in some conditions retard generation and thermal destruction; kinetic studies comparing hydrous and open-system pyrolysis; Arrhenius extrapolation from elevated-temperature laboratory measurements to lower-temperature basin conditions; and deeply buried natural petroleum source systems containing kerogen, pore water, confinement, elevated temperature, lithostatic loading and pore-fluid pressure.

Produced by The Lilborn Equation Team:

Michael Lilborn-Williams

Thomas Jackson Barnard

Audrey Williams