Metathesis in the generation of low-temperature gas in marine shales
© Mango and Jarvie; licensee BioMed Central Ltd. 2010
Received: 28 September 2009
Accepted: 20 January 2010
Published: 20 January 2010
The recent report of low-temperature catalytic gas from marine shales took on additional significance with the subsequent disclosure of natural gas and low-temperature gas at or near thermodynamic equilibrium in methane, ethane, and propane. It is important because thermal cracking, the presumed source of natural gas, cannot generate these hydrocarbons at equilibrium nor can it bring them to equilibrium over geologic time. The source of equilibrium and the source of natural gas are either the same (generation under equilibrium control) or closely associated. Here we report the catalytic interconversion of hydrocarbons (metathesis) as the source of equilibrium in experiments with Cretaceous Mowry shale at 100°C. Focus was on two metathetic equilibria: methane, ethane, and propane, reported earlier, Q (K = [(C1)*(C3)]/[(C2)2]), and between these hydrocarbons and n-butane, Q* (K = [(C1)*(n-C4)]/[(C2)*(C3)]), reported here for the first time. Two observations stand out. Initial hydrocarbon products are near equilibrium and have maximum average molecular weights (AMW). Over time, products fall from equilibrium and AMW in concert. It is consistent with metathesis splitting olefin intermediates [Cn] to smaller intermediates (fission) as gas generation creates open catalytic sites ([ ]): [Cn] + [ ] → [Cn-m] + [Cm]. Fission rates increasing exponentially with olefin molecular weight could contribute to these effects. AMW would fall over time, and selective fission of [C3] and [n-C4] would draw Q and Q* from equilibrium. The results support metathesis as the source of thermodynamic equilibrium in natural gas.
Thermal cracking has been accepted as the source of natural gas for decades [1, 2]. Although alternatives have been proposed and deficiencies in thermal cracking theory cited , it has retained extraordinary allegiance over time. This was in spite of the fact that laboratory simulations had consistently failed to generate gas resembling natural gas [4–10]. Natural gas (C1-C4) contains about 80% wt methane while experimentally generated gas from thermal cracking was always depleted in methane, and remained so over prolonged periods of cracking . Higher methane concentrations had been generated, but only at extraordinary temperatures (> 400°C) where ethane and propane decompose . It has been argued that natural gas is generated depleted in methane, and becomes enriched in methane after generation by some unspecified fractionation [12, 13]. But, there is no sign of the hypothetical heavy fraction in conventional reservoirs and no plausible explanation for its disappearance . Thermal cracking has nevertheless been embraced as the primary source of natural gas and alternatives essentially dismissed as possible contributors [4–10].
This changed with the recent disclosure of gas generation at temperatures 300° below thermal cracking temperatures . It was catalytic gas generated from marine shales under anoxic conditions, natural catalysis carried from the subsurface requiring no artificial stimulation. Shales generated gas in aperiodic episodes at ambient temperatures under inert gas flow. When gas was retained in closed reactors, it reached metathetic equilibrium in methane, ethane, and propane, and became enriched in methane over time . Natural gas was also shown to be constrained to equilibrium in molecular and isotopic compositions. Other reports have shown counter-intuitive effects in low-temperature gas generation over time . Shales released increasing concentrations of lighter hydrocarbons over time, the exact opposite to desorption or other simple first-order processes.
Here we address metathesis as a possible source of equilibrium in our experiments. The results and proposed mechanism are presented in reverse order, with the proposed kinetic scheme presented first as context for the experimental results that follow.
Results and Discussion
Proposed Kinetic Scheme
Intermediates are symbolized by [Cn], where [ ] denotes an active open site (a metal) and Cn an unsaturated hydrocarbon CnH2n bonded to it. [Cn] in the following reactions does not infer a specific structure. It can be any one of the three structures in Figure 1, for example. Metathesis splits unsaturated hydrocarbons into two unsaturated hydrocarbons reversibly. For example, the metallocyclobutane in Figure 1, C4H8 is split into C1H2 + C3H6 or 2 C2H4. It should not be confused with thermal cracking. These are unsaturated hydrocarbons bonded to transition metals. The interconversions are therefore low-energy interconversions that can proceed with facility at low temperatures.
Thus, [C4] in Figure 1 can transfer either ethylene or propylene to an adjacent [ ], generating [C2] + [C2] or [C1] + [C3], respectively.
Metathetic reactions occur between adjacent sites in this scheme. High concentrations of [ ] will promote fission through rx. 3 and high concentrations of [Cn] will promote equilibrium through rx. 4. Both are metathesis reactions. Concentrations of [Cn] and [ ] will control which of the two processes dominate and how they will effect product average molecular weight (AMW). Fission lowers AMW while equilibrium has little effect on it.
Under initial conditions, before gas generation, intermediates should be near equilibrium. The system would be saturated in [Cn] and only reactions like 4 could proceed. Saturated hydrocarbons concentrated over catalyst surfaces (active surfaces) could interconvert with intermediates (reactions 1 & 2) bringing saturated hydrocarbons to equilibrium over time. The onset of gas generation with heating should therefore release hydrocarbons near equilibrium. It would also increase concentrations of [ ] promoting fission and products with lower MW over time.
This hypothetical scheme best fits our experimental results and is used below as context for data presentation.
It is instructive at this point to consider a conventional model for the release of gas from carbonaceous shale under gas flow at low temperatures. We assume the shale is inert and gas is released by desorption. The source of gas is in-place saturated hydrocarbons: Cn = CnH2n+2. [Cn] denotes CnH2n+2 dissolved in kerogen and bitumen and adsorbed on inorganic surfaces. There are no pathways like rx. 3 to lower MW because thermal cracking rates at 100°C are essentially zero . Heating will drive the lighter hydrocarbons from the shale before the heavier hydrocarbons. Thus, the gas desorbed from the shale will increase in MW over time, but the increase is a fractionation with no net change in MW. The decrease in MW in our scheme is due to fission with a net change in MW.
Product Molecular Weights over Time
The distribution of C1-C5 hydrocarbons generated from Mowry shale, 100°C.
Other factors could contribute as well including the preferential adsorption of higher hydrocarbons to active surfaces. The possibility of thermal anomalies must also be considered. Since generation is probably exothermic , larger amounts of heat released early in the reaction could dislodge disproportionate amounts of saturated higher hydrocarbons from active surfaces. Early episodes of thermal desorption would inflate early hydrocarbon yields thus contributing to an artificial decline in yields over the first hours of reaction.
Metathetic Equilibria over Time
Coefficients of correlation (R2) between five reaction variables, Mowry shale, 11 hours at 100°C.
Gas generated from Mowry shale at 100°C is catalytic gas generated through metathesis. A number of correlations (Table 2) support this view. Product AMW, equilibrium quotients, yields, and reaction times all correlate. A kinetic scheme is proposed in which gas generation promotes lower MW products over time with a fall from equilibrium. The experimental results fit this scheme remarkably well. Metathesis catalyzed by low-valent transition metals is very likely the source of gas in these experiments. It provides an attractive alternative to thermal cracking as the source of natural gas. It is in many respects superior. Metathesis explains three properties of natural gas that are not easily explained otherwise: thermodynamic equilibrium , high methane concentrations , and evolution of wet gas to dry gas over geologic time .
The Cretaceous Mowry shale is whole core (2500 m) from an unknown well in Colorado. Rock-Eval: S1 = 2.61 mg hydrocarbon/g rock; S2 = 9.33 mg hydrocarbon/g rock; S3 = 0.15 mg CO2/g rock; Tmax = 439°C. Total Organic Carbon (Leco) = 2.5%. The experimental procedures for sample preparation and product analysis are described elsewhere . Closed experiments were carried out in 5 ml glass vials fitted with PTFE/SIL septa purchased from Cobert Ass. Samples ground to 60 mesh under argon (~1 g) were placed in vials under argon, and sealed with open screw caps fitted with septa. Caps were secured to the vials with plastic electrical tape to prevent leakage under heating. Vials were heated in a convection oven at 100°C (± 5°) for 11 hours under argon. Aliquots of product gas were removed hourly by inserting two needles into the vial through the septum; one withdrew 2 ml of gas into a gas-tight syringe and the other injected 2 ml argon into the vial to replace the gas withdrawn. Gas was withdrawn and injected simultaneously (the injecting needle was under moderate argon pressure) with the injecting needle near the septum and the withdrawing needle near the shale. The gas samples were analyzed and discarded. The results of duplicate experiments are in Table 1. Duplicate experiments did not use aliquots of 60 mesh shale. Different samples from the same source were subjected to the same experimental procedures: grinding in argon, sieving, and so forth. The variations in yield and product compositions shown in Table 1 therefore reflect heterogeneity in sample, variance in sample preparation, and product analysis.
We thank Petroleum Habitats and Worldwide Geochemistry for their generous support of this research.
- Tossot BP, Welte DH: Petroleum Formation and Occurrence. 1984, Springer, New YorkView ArticleGoogle Scholar
- Hunt JM: Petroleum Geochemistry and Geology. 1995, Freeman, New YorkGoogle Scholar
- Mango FD: Methane concentrations in natural gas: the genetic implications. Organic Geochemistry. 2001, 32: 1283-1287. 10.1016/S0146-6380(01)00099-7.View ArticleGoogle Scholar
- Winters JC, Williams JA, Lewan MD: A laboratory study of petroleum generation by hydrous pyrolysis. Advances in Organic Geochemistry. Edited by: Bjorøy M, et al. 1983, Chichester: Wiley, 524-533.Google Scholar
- Burnham AK, Braun RL: General kinetic model of oil shale pyrolysis. In Situ. 1985, 9: 1-23.Google Scholar
- Castelli A, Chiaramonte MA, Beltrame PL, Carniti P, Del Bianco A, Stroppa F: Thermal degradation of kerogen by hydrous pyrolysis. A kinetic study. Advances in Organic Geochemistry. Edited by: Durand B, Behar F. 1989, Pergamon Press, Oxford, 1077-1101.Google Scholar
- Tannenbaum E, Huizinga BJ, Kaplan IR: Role of minerals in the thermal alteration of organic matter - I. Generation of gases and condensate. Geochimica et Cosmochimica Acta. 1985, 49: 2589-2604. 10.1016/0016-7037(85)90128-0.View ArticleGoogle Scholar
- Lewan MD: Evaluation of petroleum generation by hydrous pyrolysis. Philosophical Transactions Royal Society, London. 1985, 315: 123-134.View ArticleGoogle Scholar
- Ungerer P, Pelet R: Extrapolation of oil and gas formation kinetics from laboratory experiments to sedimentary basins. Nature. 1987, 327: 52-54. 10.1038/327052a0.View ArticleGoogle Scholar
- Horsfield B, Disko V, Leistner F: The micro-scale simulation of maturation: outline of a new technique and its potential applications. Geologische Rundschau. 1989, 78: 361-374. 10.1007/BF01988370.View ArticleGoogle Scholar
- Erdman M, Horsfield B: Enhanced late gas generation potential of petroleum source rocks via recombination reactions: Evidence from the Norwegian North Sea. Geochimica et Cosmochimica Acta. 2006, 70: 3943-3956. 10.1016/j.gca.2006.04.003.View ArticleGoogle Scholar
- Price LC, Schoell M: Constraints on the origins of hydrocarbon gas from compositions of gases at their site of origin. Nature. 1995, 378: 368-371. 10.1038/378368a0.View ArticleGoogle Scholar
- Snowdon LR: Natural gas composition in a geological environment and implications for the processes of generation and preservation. Organic Geochemistry. 2001, 32: 913-931. 10.1016/S0146-6380(01)00051-1.View ArticleGoogle Scholar
- Mango FD, Jarvie DM: Low-temperature gas generation from marine shales. Geochemical Transactions. 2009, 10: 3-10.1186/1467-4866-10-3.View ArticleGoogle Scholar
- Mango FD, Jarvie DM, Herriman E: Natural gas at thermodynamic equilibrium: Implications for the origin of natural gas. Geochemical Transactions. 2009, 10: 6-10.1186/1467-4866-10-6.View ArticleGoogle Scholar
- Mango FD, Jarvie DM: The low-temperature catalytic path to natural gas: Wet gas to dry gas over experimental time. Geochemical Transactions. 2009, 10: 10-10.1186/1467-4866-10-10.View ArticleGoogle Scholar
- Gault FG: Mechanisms of skeletal isomerization of hydrocarbons on metals. Advances in Catalysis. 1981, Academic Press, New York, 30: 1-90. full_text.Google Scholar
- Pines H: The Chemistry of Catalytic Hydrocarbon Conversions. 1981, Academic Press, New York, 264-275. 248View ArticleGoogle Scholar
- Herrison JL, Chauvin Y: Catalyse de transformation des oléfines par les complexes du tungstène. II. Télomérisation des oléfines cycliques en présence d'oléfines acycliques. Makrom Chem. 1971, 141: 161-10.1002/macp.1971.021410112.View ArticleGoogle Scholar
- Banks RL, Bailey GC: Industrial & Engineering Chemistry, Product Research & Development. 1964, 3: 170-Google Scholar
- Waples DW: The kinetics of in-reservoir oil destruction and gas formation: constraints from experimental and empirical data, and from thermodynamics. Organic Geochemistry. 2000, 31: 553-575. 10.1016/S0146-6380(00)00023-1.View ArticleGoogle Scholar
- Stull DR, Westrum EF, Sinke GC: The Chemical Thermodynamics of Organic Compounds. 1969, John Wiley & Sons, New YorkGoogle Scholar
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