Frank D. Mango

The light hydrocarbons in petroleum: a critical review

Abstract The largest petroleum fraction is between C 1 and C 9 , the so-called light hydrocarbons (LHs). They are catagenic products, formed between ∼ 75 and 140°C, but their mode of origin remains controversial. Although cracking enjoys broad support, there is little evidence, and inconsistencies argue against it. The higher hydrocarbons, for example, are too stable to source the LHs at these temperatures. Moreover, LHs do not resemble cracking products. Many are structurally like bio-precursors, but others are not and there is no dominance in natural structures consistent with cracking. Essentially all isomers are found within the alkanes, cycloalkanes and aromatics with no discernible preference for natural structures. Isomer distributions are nearly constant from oil to oil and far from thermodynamic equilibrium. It is unlikely that the LHs are formed without catalytic support. Ratios of isoalkanes are almost constant in all oils and invariant within oils from a common source. This places a powerful constraint on any theory attempting to explain their origin: Certain isoheptane ratios must remain constant throughout the course of LH generation . This is inconsistent with cracking of bio-precursors and consistent with a catalytic process in which the LHs descend from a few precursors in a process controlled by steadystate kinetics. There are two opposing views on the origin of natural gas: thermal cracking and catalysis by transition metals. Non-biogenic gas contains ∼ 85% methane, but thermal cracking consistently gives products depleted in methane. Catalysis by transition metal oxides (V, Co, Fe, and Ni), on the other hand, duplicates natural gas. Gas fractionation in migration might explain high-methane gas, and it has been offered as an alternative to catalysis, but there is little to support this idea and evidence against it. The opposing theories on the origin of LHs are critically reviewed here. I also review the early literature on composition, the distribution of LHs in sediments, and some applications, including maturity and oil correlations.

An Invariance in the Isoheptanes of Petroleum

Four isoheptanes in petroleum display a remarkable invariance in a ratio of sums of concentrations. The isoheptanes are not at thermodynamic equilibrium, nor are they fixed to some constant composition. The four isomers display coherent change in relative amounts but maintain invariance in the ratio of sums. Within sets of genetically related petroleum samples, invariance reaches levels that approach the limits of our analytical precision. The invariance is inconsistent with a chemical origin that involves the thermal fragmentation of natural products or their derivatives. It suggests a reaction process at steady state, in which relative rates of product formation are constant. A mechanism is proposed in which the four isoheptanes are formed pairwise and sequentially through two intermediates in a catalytic process that operates at steady state.

The origin of light hydrocarbons in petroleum: A kinetic test of the steady-state catalytic hypothesis

Abstract A kerogen-specific, steady-state catalytic process is proposed for the origin of light hydrocarbons in petroleum. The postulated parent-daughter scheme predicts a unique invariance in a ratio of isoheptanes and dimethylcyclopentanes. In theory this ratio should remain constant over the entire lifetime of a source-rock generating petroleum. To test this hypothesis, two very large petroleum deposits were extensively sampled giving two sample sets believed to represent large time-segments of petroleum generation. The sets display a remarkable invariance in the predicted ratio. Moreover, the ratios for the two sets are distinct and constitute the outer limits for the ratio in a large database of over 2000 samples of petroleums. Thus, the ratio of isoheptanes and dimethylcyclopentanes remains constant over the course of petroleum generation.

Transition metal catalysis in the generation of petroleum and natural gas

Abstract Certain ratios of light hydrocarbons remain virtually invariant over the course of petroleum generation, indicating steady-state catalysis rather than thermal cracking as the central feature to the mechanism of petroleum generation. Although the evidence for catalytic intervention is now compelling, the nature of the catalytic agent, its mode of activation and action are not clear. I propose that the transition metals, activated in the lipophilic domains of kerogen, are the catalytic agents in the conversion of normal paraffins into light hydrocarbons and natural gas. The process proceeds through specific catalytic steps involving 3-, 5-, and 6-carbon ring-closures and the cleavage of carbon-carbon bonds in the key steps. This hypothesis is analyzed in the context of published literature on catalysis by Ni, V, Ti, Co, and related transition metals. Activated under anaerobic conditions, these metals express extraordinary catalytic activity in each of the postulated steps. Moreover, metal-catalysis provides a reasonable kinetic pathway through which hydrogen and normal paraffins may combine to form a methane-enriched natural gas. Given the anaerobic conditions of diagenesis and a kerogenous source of hydrogen, it is concluded that the transition metals, under catagenic conditions, are potentially active catalysts in the conversion of hydrogen and paraffins into light hydrocarbons and natural gas.

The origin of light hydrocarbons in petroleum: Ring preference in the closure of carbocyclic rings

Abstract In a proposal for the generation of light hydrocarbons (LHs), n-alkane parents are catalytically transformed into daughter isoalkanes and cycloalkanes through the closure of three-, five-, and six-membered rings. Three reaction rate constants, k3: →- isoalkanes; k5: → cyclopentanes; k6: → cyclohexanes, control this catalytic process, and thus the compositions of LHs. A catalyst that preferentially promotes ring-closure of a specific carbon number is said to express ring preference (RP) in that carbon number. For example, a catalyst that preferentially generates isoalkanes over cycloalkanes would be expressing three-ring preference (3RP), meaning that k3 is greater than k5 and k6 in the catalytic process generating LHs. Oils show large compositional variations in LHs, with ratios of isoalkanes to cycloalkanes showing coefficients of variation on the order of 100%, reflecting large variations in RP. Genetically related oils (homologous oils), however, are either invariant in composition (invariant in RP) or they display systematic changes in RP. In a striking example of this latter case, RP progressively shifts to smaller rings, 6RP → 5RP → 3-RP, as parent concentrations increase. This paper addresses a curious paradox, apparently unique to the LHs: homologous oils, displaying a uniform overall geochemical composition (i.e., gravity, sulfur concentration, isotopic composition, biomarker composition, and so on), show remarkable changes in LH composition reflecting systematic changes in RP. These apparent contradictions, on the one hand a uniform overall composition reflecting a static system and on the other hand systematic changes in LH composition reflecting a dynamic system, are analyzed in the context of a steady-state catalytic hypothesis.

Transition metal catalysis in the generation of natural gas

Carbonaceous sedimentary rocks containing transition metals are known to be catalytic in the conversion of hydrogen and n-alkenes into natural gas, but the source of activity is unclear. The evidence presented here supports transition metals as the active agents. Various metal compounds in the pure state show the same levels of catalytic activity as sedimentary rocks and the products are identical. Nickel is particularly active among the early transition metals and is projected to remain catalytically robust at all stages of catagenesis. Nickel oxide promotes the formation of n-alkanes in addition to natural gas (NG), demonstrating the full scope of the hypothetical catalytic process: n−Cx=+H2→[NiO∗]NG+n−C5+n−C6+…n−Cx−1 The composition of catalytic gas duplicates the entire range of natural gas, from so-called wet gas to dry gas (60–95 + wt% methane), while gas generated thermally is consistently depleted in methane (10–60 wt% methane). These results support the view that metal catalysis is a major pathway through which natural gas is formed in the earth.

The origin of light hydrocarbons

The light hydrocarbons (LHs) are probably intermediates in the catalytic decomposition of oil to gas. Two lines of evidence support this possibility. First, the reaction was duplicated experimentally under moderate conditions. Second, natural LHs exhibit the characteristics of catalytic products, in particular a proportionality between isomers: (xy{sub i})/(x{sub i}y) = {alpha} (where x and x{sub i} are isomers; y and y{sub i} are isomers that are structurally similar to x and x{sub i}; and {alpha} is a constant). All oils exhibit this relationship with coefficients of correlation reaching 0.99. Isomer ratios change systematically with concentrations, some approaching thermodynamic equilibrium, others not. The correlations reported are the strongest yet disclosed for the LHs. Isomers are related in triads (e.g., n-hexane {leftrightarrow} 2-methylpentane {leftrightarrow} 3-methylpentane), consistent with cyclopropane precursors. The LHs obtained experimentally are indistinguishable from natural LHs in (xy{sub i})/(x{sub i}y). These relationships are not explained by physical fractionations, equilibrium control, or noncatalytic modes of origin. A catalytic origin, on the other hand, has precedence, economy and experimental support.

The origin of light cycloalkanes in petroleum

It has been suggested that the light cycloalkanes in petroleum are generated through the thermal decomposition of heavier polycyclic natural products, such as the steranes and triterpanes. However, no support could be found for the assumption that the polycycloalkanes should decompose to light cycloalkanes at typical subsurface temperatures. For example, at 150{degree}C, decahydronaphthalene-the bicyclodecyl unit fundamental to the steranes and triterpanes-has a half-life of approximately 30 billion years. At this same temperature, cyclohexane has a half-life of approximately 60 billion years. The surprising thermal stability of the cycloalkane ring can be traced to a prohibitively high activation energy for ring opening due to the steric strain associated with the {beta}-elimination step. Cholestane undergoes thermal decomposition almost exclusively by loss of the alkyl side chain. Under thermal conditions sufficiently severe to break the carbon-carbon bonds of normal alkanes (weeks, 330{degree}C), cholestane gives only insignificant amounts of light cycloalkanes. It is most doubtful, therefore, that the C{sub 5} to C{sub 9} cycloalkanes could be thermally produced from natural products like the steranes and triterpanes. An alternative hypothesis is offered in which the light cycloalkanes in petroleum are formed in a steady-state catalytic process.

The catalytic decomposition of petroleum into natural gas

Abstract Petroleum is believed to be unstable in the earth, decomposing to lighter hydrocarbons at temperatures > 150°C. Oil and gas deposits support this view: gas/oil ratios and methane concentrations tend to increase with depth above 150°C. Although oil cracking is suggested and receives wide support, laboratory pyrolysis does not give products resembling natural gas. Moreover, it is doubtful that the light hydrocarbons in wet gas (C 2 C 4 ) could decompose over geologic time to dry gas (> 95% methane) without catalytic assistance. We now report the catalytic decomposition of crude oil to a gas indistinguishable from natural gas. Like natural gas in deep basins, it becomes progressively enriched in methane: initially 80% (wet gas) to a final composition of 100% methane (dry gas). To our knowledge, the reaction is unprecedented and unexpectedly robust (conversion of oil to gas is 100% in days, 175°C) with significant implications regarding the stability of petroleum in sedimentary basins. The existence or nonexistence of oil in the deep subsurface may not depend on the thermal stability of hydrocarbons as currently thought. The critical factor could be the presence of transition metal catalysts which destabilize hydrocarbons and promote their decomposition to natural gas.

Molecular Orbital Symmetry Conservation in Transition Metal Catalysis

Publisher Summary This chapter presents the specific catalytic functions of the transition metal in the various types of transformations, and discusses the associated chemistry. Molecular orbital symmetry conservation constrains all molecular systems to specific paths of transformation. Symmetry conservation principles have proven to be powerful tools for understanding a large body of complex organic chemistry. These concepts further bear on molecular stability. A molecule in one bonding configuration transforms into other configurations primarily through allowed paths. The thermal stability enjoyed by simple olefins to a certain extent rests on orbital symmetry restraints. Both olefin cyclobutanation and double-bond isomerization (through a 1,3-hydrogen shift), involving forbidden passages, are not observed at moderate temperatures. Simple olefins are fixed in their bonding configurations and cannot interconvert through the sterically-preferred paths. The thermal interconversion of olefins is necessarily a high-temperature process involving predominantly the higher energy, allowed transformations incorporating free radical intermediates.