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Sunday, May 12, 2013

V.C. Kelessidis

The general saying among oil well drillers is that oil is where you find it, meaning that oil has been found in traditional and non-traditional places. What of course is considered traditional is that oil is found in sedimentary rocks, very close to the surface in the beginning of the century, while nowadays it may be found at considerable depths, now reaching almost 9000 m from the surface. There are of course finds in fractured basement rocks (metamorphic or igneous rocks) from where they are produced (Sircar, 2004). Batchelor and Gutmanis (2005) have compiled an extensive list of fields producing hydrocarbons from basement rocks, although most petroleum geologists dismiss them as being of non-commercial value.

However, White Tiger, the oil field in Vietnam may prove them wrong because it is an excellent example of production from basement rock. The field currently produces 350.000 barrel oil per day, expecting to produce overall 600 million barrels (47 years of production at this place). The granitic basement rock is highly fissured with apparent permeabilities ranging from a few mD to up to 464 mD (Chan et al., 2006). The oil that is produced, however, has been characterized of biogenic origin (Nemchenko et al., 2007) with migration from underlying sedimentary rocks.

Of course, we find oil ‘where it is’, where it has remained for ages, but how was it formed? Current belief is that oil is of biotic origin, through accumulation of organic matter (plankton, single cell organisms that floated on ocean surface) and sedimentation followed by burial. For large periods organic material has been under very high pressures and temperatures, in the range of 130-150 °C , in a ‘cooking pot’ and gradually transformed to petroleum. Because of its lower density, it has migrated upwards and some surfaced and was lost, while some has hit non-permeable layers (the seal) and accumulated in the porous sedimentary rocks creating the world’s oil and gas fields.

There is, however, another school of thought, not very well known until recent years, which is gaining, though, momentum. It is the theory of abiotic (or abiogenic) origin of petroleum, that hydrocarbons have been formed in the depths of Earth by reduction of CO2 and H2 gases in the presence of metal catalysts (Gold and Soter, 1980; Kenney, 1994; Krayushkin et al., 1994; Glasby, 2006; Wikipedia, 2009). The consequences of course of such a theory, if true, could be extraordinary, as Earth’s mantle becomes the inexhaustible provider of the cheapest energy source on Earth, by today’s standards, and shattering not only the oil-depletion myth but also pointing out to oil-rich regions in places devoid as prolific as before, because of belief of biogenic origin. Nikolai Alexandrovich Kudryavtsev (Kudryavtsev, 1951) was the first to start the theory of abiotic generation of hydrocarbons, in what has become the modern Russian-Ukrainian theory of abyssal, abiotic petroleum (Kropotkin, 1986; Kenney et al., 2002). However, Abbas (1996) starts the history as early as 1877 by Mendeleev and provides a good overview as well as pros and cons about the two points of view.

In principle, the abiotic theory states that under high pressures (less than 5000 bar) and high temperatures (between 500 and 1500 °C ) methane could be formed from reduced carbon resulted from calcite. The process has been supported theoretically, via thermodynamic analysis, and experimentally (Kenney et al., 2002). Methane may also be formed from volatile rich fluids resulting from partial melting of rocks within Earth’s interior (National Academy Press, 2007). Thermodynamics indicate that at 1300 K, CO2 and CO should be the predominant carbon rich gases, while at lower temperatures CH4 should be predominant (Eugster and Skippen, 1967), with Symmonds et al., (1994) supporting the first argument by measurements.

Strong support for this hypothesis is the fact that methane and hydrocarbons are abundant in the outer solar system (Gold, 1979, 1984, 1985, 1993). There is reported evidence of abiotic formation of complex organics from methane in Saturn’s satellite Titan’s atmosphere (National Academy Press, 2007), although it is stated that there may be no connection to primitive Earth, because at the low surface temperature of Titan (at 46 K) all water is turned into ice. Methane, ethane and acetylene have also been discovered in Comet C/1996 B2 Hyakutake (Mumma et al., 1996). The finding of very deep gas reservoirs, down to almost 10000 m, with extremely high success rates of more than 55%, has also been reported as evidence of abiotic generation of hydrocarbons (Corsi, 2005). Very recent works (Cathcart, 2007; Paropkari, 2008) have been suggesting that we should be rethinking about oil exploration strategies in view of the substantial evidence about abiotic hydrocarbon origin.

Kenney et al. (2002) analyzed theoretically, via thermodynamic computations, the possibilities for hydrocarbon generation at high pressures and temperatures and showed that it is possible. They went on and performed successful experiments, using a specially built high pressure apparatus (Nikolaev and Shalimov, 1999) at pressures of 50 kbar, temperatures to 1500 °C . Using only as reagents solid iron oxide and 99.9% pure marble, wet with triple distilled water, they were able to generate methane. They reported that at pressures lower than 10 kbar only methane was formed while at pressures greater than 30 kbar a multi-component hydrocarbon mixture was formed including methane, ethane, propane, n-alkanes as well as alkenes, in distributions characteristic of natural petroleum.

Scott et al. (2004) have also reported in situ observations of hydrocarbon generation via carbonate reduction at upper mantle temperatures and pressures, forming methane from FeO, CaCO3-calcite and water at temperatures ranging between 500 and 1500 °C and pressures between 50 and 110 kbar. The authors were confident of the abiogenic theory of hydrocarbon generation thus concluding that Earth’s hydrocarbon budget is much larger than it is currently thought.

Petroleum generation under hydrothermal conditions, with certain metals or alloys used as catalysts, has been amply demonstrated at lower temperatures and pressures. For e.g. Horita and Bernt (1999) used a nickel-iron alloy, similar to what could be found within Earth’s crust, to catalyze the slow, under other conditions, reaction of methane generation from dissolved bicarbonate, under hydrothermal conditions at 200 and 400 °C and 500 bar. Without the catalyst, no methane was formed, concluding that abiogenic methane may be more widespread than originally thought.

Proskurowski et al. (2008) suggested, through analysis of components in hydrothermal oceanic vents that abiotic synthesis in nature of hydrocarbon fluids may occur in the presence of ultramafic rocks (which comprise mostly Earth’s mantle), water and moderate amounts of heat. On the other hand, Konn et al. (2008) analyzing data from same and other vents did not find conclusive evidence of the fact. He noted that, although amounts of hydrocarbons attributed to abiogenic origin were found, their signature has been difficult to characterize owing to the abundance of biogenic material. This is not far from the findings of Robinson (1963) who had noted at the time that the observed petroleum composition cannot really be attributed to biological origin, suggesting a primordial mixture to which bioproducts have been added. Ji et al. (2008) also presented results of generating a range of alcanes up to pentane, not only methane, from CO2 and H2 in hydrothermal conditions with cobalt as catalyst at 300 °C and pressures as low as 300 bar.

Szatmari (1989) suggested the hypothesis of petroleum formation by Fischer-Tropsch synthesis, which is distinct from the organic and the inorganic coming from degassing theory of Gold. Foustoukos and Seyfried (2004) also demonstrated the acceleration of hydrocarbon production from CO2 and H2 with the Fischer-Tropsch reaction, using chromium and iron bearing minerals as catalysts, at 390 °C and 450 bars. Recent reports (Sherwood-Lollard et al., 2002) have identified traces of abiotically derived hydrocarbons in Kidd Creek hard rock mines. In the laboratory, abiotic synthesis of more complex organic compounds has been reported in aqueous media (McCollom et al., 1999).

Glasby (2006) gives a historical overview on the origin of hydrocarbons. He dismisses both the Russian-Ukrainian theory and the theory of gas degassing by Gold, as being non thermodynamically sound. He does not discuss, however, the Fischer-Tropsch type of reactions, pointed out above. Hence, his work serves as a very good reference, but to the author’s opinion, the final arguments are not as strong as they should have been. Interesting to note that he dismisses the Ukrainian theory on the basis of better evidence for the origin of higher hydrocarbons from organic matter, using better techniques, and noting that the theory is even forgotten in Ukraine, which is not true, as it has been recently demonstrated (Kutcherov, 2007; Kitchka, 2007).

V.C. Kelessidis. Challenges for very deep oil and gas drilling - will there ever be a depth limit? 3rd AMIREG International Conference (2009): Assessing the Footprint of 220 Resource Utilization and Hazardous Waste Management, Athens, Greece

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