Unconventional Resources

Buried hydrocarbons: A resource for biogenic methane generation

Just 3% of Mother Nature’s vast buried coal, carbonaceous shale and residual oil could be treated by biogenesis to produce 1,000 Tcf of methane.

Mark Finkelstein, Roland P. DeBruyn, Jeffrey L. Weber and James B. Dodson, Luca Technologies

Buried hydrocarbons, especially those that cannot be easily and economically recovered, represent a universal opportunity to meet future natural gas needs. We have come to accept landfill gas microbially generated from municipal wastes as a viable means of methane generation. However, we fail to recognize massive coal, shale and residual oil deposits as vast supplies of old biomass that nature has anomalously concentrated for us, just as we concentrate municipal waste in landfills.

These residual oils, coal and carbonaceous shales are abundant in nature, and the results outlined below demonstrate that these resources can be readily transformed to methane in the lab. Because of the prodigious amount of substrate available for conversion to methane, only a small portion of this resource needs to actually be converted in the field to have a large economic impact. The indigenous microbial communities responsible for this conversion perform the necessary metabolic reactions in an environmentally benign fashion in real time.

Luca Technologies (Luca) has obtained and characterized samples from over 25 geologically distinct locations in North America. Many of these are – or have the potential to become – Geobioreactors.* The company is now finalizing plans to begin in situ field trials to link favorable lab findings with field implementation. This effort can be carried out using the incremental economics provided by coalbed methane (CBM), shale gas, and oil fields that have already been drilled, based on primary recovery economics. Extending the lives of such wells gives us more use from existing infrastructure, while limiting further surface impacts.

INTRODUCTION

As unconventional natural gas production becomes an ever-increasing component of natural gas supply, and therefore more “conventional,” we must look toward the application of new technologies and innovations to these unconventional resources to help us keep pace with dwindling supplies and rising demand. While constituting less than 10% of the US’s overall natural gas production, biogenic methane is a fast – growing segment of our natural gas supply. The increasing number of biogenic gas plays such as the Powder River basin (PRB) of Wyoming, the Antrim Shale in Michigan, and the emerging CBM projects in Alberta, suggest that we are at the beginning of a large growth curve.

An understanding of how this methane is generated and the parameters that affect its rate of generation has been the focus of an intense research effort by a number of institutions. Lab studies and field results indicate that a large untapped resource base exists that could substantially increase and extend the world’s natural gas supplies.

Only 25-40% of the oil in a typical reservoir can be cost-effectively brought to the surface using current waterflood and enhanced oil recovery technologies. Similarly, most coal cannot be mined economically due to depth constraints; and the mining of organic-rich shales to produce oil has so far proven to be uneconomic. This trapped oil, buried coal and carbonaceous shale represent a vast energy resource that is largely unrecoverable with existing technologies.

The microbial creation (“biogenesis”) of methane was first recorded over 225 years ago.¹ It has since come to be recognized as a common, natural process in the biosphere and geosphere. Due to the presence of huge resources of hydrogen and carbon in buried organic materials, the potential exists for the generation of immense volumes of methane if biogenesis can be “domesticated.”

Microbial conversion of residual oil, coal and kerogen to methane (natural gas), in real time, has the potential to create a substantial source of natural gas. Presented here is experimental evidence that indigenous microbial communities in some oil reservoirs and coal seams can be stimulated to produce economically significant volumes of methane.

THE SIGNIFICANCE OF BIOGENESIS

A previous USGS study estimated that biogenic methane comprised 22% of the methane known to the petroleum industry.² When the methane estimated to exist in gas hydrates is added, greater than 50% of world methane resources is thought to be biogenic in origin.³ While estimates of the total hydrocarbon reserves in the US vary widely, a conservative estimate of the substrate in place (SIP) for possible conversion to methane is shown in Table 1. This table uses the ultimate analysis, or chemical composition, of each of these substrates to determine the amount of sequestered hydrogen they contain and, therefore, the absolute upper limit of methane which could be produced from them microbially.

The kerogen, or carbonaceous portion of the shales, corresponds to about 2,000 billion short tons (BSTs), whereas over 3,000 BSTs of coal exist in the lower 48 states, and about twice that amount is thought to exist in Alaska. Residual oil in the US that does not lend itself to recovery with known technologies is estimated to total about 262 Bbbl. While all of this substrate cannot possibly be converted to methane, the amount of hydrocarbon resource that can be managed for the production of microbial methane is enormous. Conversion of just 3% of these hydrocarbon substrates to methane translates to 1,000 Tcf of natural gas, over 40 years’ worth of US natural gas supply, at the present usage rate of 23 Tcf per annum.

Equally significant are the rates of methane generation. While the conventionally recognized thermogenic process requires millions of years to bury and transform organic material into oil and gas, biogenesis can be observed as occurring in real time in anaerobic landfills, lab experiments, and occasional coal samples undergoing desorption testing. The rates can be striking. For example, a large landfill in Canada has produced more than 26 MMcfd of low-btu gas.⁴

Microcosm studies of PRB coal samples in Luca’s lab, using formation coal and water samples without chemical additives, have produced at rates which would equal 66 Mcf/day if they could be replicated over 40 acres in a typical 50-ft-thick PRB coal seam. Hydrogen and carbon resources in this same 50-ft- thick coal over 40 acres would support this rate for almost 250 years, if 25% of the coal’s hydrogen were liberated and incorporated into methane, or about 1,000 years if 100% conversion could be achieved. Even greater rates are obtained if appropriate nutritional and chemical amendments are added.

DOCUMENTED US BIOGENIC METHANE OCCURRENCES

Biogenic methane has several recognizable characteristics that distinguish it from thermogenic methane:

• Fractionation of isotopic carbon, resulting in “light” gas. In nature, about 99% of carbon atoms are the familiar ¹²C isotope, with nuclei containing six protons and six neutrons. The remaining carbon atoms are the ¹³C and (rarely) ¹⁴C isotopes, which hold one or two additional neutrons. Because microbial processes preferentially utilize ¹²C, biogenic methane contains less ¹³C and ¹⁴C isotopes, which are, as a result, slightly concentrated in dissolved inorganic carbon (DIC) remaining in the formation waters. The isotopic character of the methane and the DIC are both important in recognizing biogenesis.
• Gas “dryness.” Biogenesis breaks down complex organic material and, through a series of microbial processes, creates increasingly smaller molecules and ultimately methane. Through heat and pressure, thermogenesis shatters the chemical structure of the complex organic material, yielding hydrocarbon molecules ranging from methane to crude oil, with a wide array of molecular sizes. Unless it is mixed with thermogenic fluids, it is characteristic of biogenic methane that it is rarely associated with significant concentrations of other hydrocarbon gases or liquids.

In the US, biogenesis is recognized in a number of basins:

• Powder River basin CBM^5,6
• Antrim shale in Michigan⁷
• Siltstones and tight sandstones of the Northern Plains⁸
• Monument Butte oil field of the Unita basin, Utah⁹
• Coals of the San Juan basin¹⁰
• Forest City basin coals¹¹
• Black Warrior basin coals¹²
• Reservoirs in the Gulf of Mexico and Cook Inlet, Alaska^2,13
• Others.³

In addition to the above, study of the in situ microbial degradation of crude oil indicates that greater than 50% of all oil reservoirs experience some biogenic conversion of oil to methane.¹⁴

Real-time biogenesis of methane has been observed in the following substrates:

• Powder River basin coals, ⁶ gas desorption canisters,⁵ others
• Michigan basin Antrim shale⁷
• Forest City basin coal: lab studies,¹¹ gas desorption canisters (others)
• Cook Inlet coal¹³
• Uinta basin, Monument Butte oil field: lab studies⁹
• Powder River basin, Bell Creek oil field: lab studies (this article)
• Appalachian basin coals: lab studies (Luca, unpublished). 

The above data raise the distinct possibility that vast reservoirs of buried hydrocarbons found in the US, and the rest of the world, can serve as Geobioreactors* for the generation of large quantities of methane. A Geobioreactor is a combination of a hydrogen/ carbon bearing geologic formation with naturally occurring or introduced microorganisms capable of making methane in real time. While all hydrogen/ carbon geologic formations are not active Geobioreactors, Luca has been able to demonstrate that samples taken from some inactive formations are capable of being activated in the lab with either nutritional amendments or cross-inoculations with specific microbial consortia.

CREATION OF HYDROCARBONS

Plants use the power of sunlight to combine carbon dioxide, water and nutrients into complex sugars, carbohydrates, waxes and lignins. Aquatic phytoplankton, while smaller in mass but more productive, grow on similar constituents. The majority of our hydrocarbon resources were initially made from these photosynthetically-derived materials.

Most organic carbon is returned to the atmosphere through the carbon cycle as aerobic (oxidative) decomposed organic matter. However, a small portion (usually less than 1%) of this biomass is preserved in sediments where it typically supports the growth of a diverse community of microorganisms. While only a modest share of this biomass is buried below an oxidative level in any given year, over geologic time, this buried biomass aggregates into vast hydrocarbon deposits. The world’s oil, coal and gas reservoirs have been formed from only a fraction of this anaerobically preserved material.

LABORATORY BIOGENESIS STUDIES

To understand and enhance in situ biogenic activity, it is necessary to re-create formation conditions in the lab. Great care is taken to avoid chemical and microbial contamination during extraction of samples from the ground, in transport and in the lab. For example:

• Samples are acquired in the field with minimized exposure to air. Solid-phase samples such as core (or, rarely, cuttings) are immersed in an oxygen-free environment as quickly as possible, and then stored and transported in sealed containers.
• Lab experiments are prepared using field samples (substrate, water, microbes) wherever possible. Sealed containers are opened and samples are prepared in humid, anaerobic glove bags to avoid oxidizing or drying. Inside the glove bag, outer (potentially oxidized) surfaces of cores are cut away and discarded.
• Macroscopic geological observations are recorded and photographs are taken of the core. Samples are prepared for microscopic visualization and testing methane generation potential.
• Experiments are set up in small glass serum bottles with butyl rubber stoppers, both impervious to gas diffusion. 
• Samples removed from the bottles for analysis are taken using sterilized syringes.
• All experiments, often including numerous chemical and microbial amendments, are set up with multiple replicate bottles of each condition. Having replicates allows the averaging of small variations in sample volume and condition, and provides vision of possible contamination of individual sample bottles.

Samples were also prepared for various types of microscopy. In several studies, scanning electron microscopy (SEM) was used to view the three-dimensional structure of a sub-bituminous coal. As the magnification increases to 550X from 17X in panels A-C in Fig. 1, it is apparent that the micro-fracture apertures found within the coal are not only sufficient to allow water and gas to pass through them, but also wide enough for the average microbial cell to move through or congregate within the fracture. This is evident as a colony of microbes is seen at 7,000X in panel D, Fig. 1. Typical fracture apertures range upward from 2 to 10 micrometers and can easily accommodate a wide range of microbes.

Fig. 1. Scanning electron micrographs of Powder River basin (PRB) coal samples at increasing magnification.

Luca has used atomic force microscopy (AFM) to look at subtle perturbations on the face of sub-bituminous coal surfaces. Rather than a smooth, glassy surface, the high resolution AFM phase image of sub-bituminous coal depicted in Fig. 2 indicates a symmetric, “scale-like” surface, showing the geopolymer molecules which are the building blocks of the coal matrix. The intricate channels between the large geopolymer molecules denote pores through which gas and water molecules (but not microbes) pass. These pore diameters range up to 2 to 50 nanometers (a thousand times smaller than the fractures). A diagram depicting the fractures, meso and micro-pores, methane, water and microbes is shown in Fig. 3. The internal structure of this low-rank coal allows the microbes to travel along and exist in the fractures while water (and nutrients) as well as gases (including methane) can rapidly be transported through pores as well as the fractures.

Fig. 2. Atomic force micrographs of the surface of PRB coal.

Fig. 3. Void-space architecture of sub-bituminous coal, blue symbolizing a water molecule, red a methane molecule.

Microbes were also observed growing at the oil-water interface of the Monument Butte experimental oil samples. These microbes were detected using a phase-contrast fluorescent microscope. A number of different dyes were used to differentiate live cells from dead cells. Most of the microbes are rod-shaped with some elongated paired rods evident. While it is likely that a variety of microbes are present, it was not possible to visually identify these particular microbes as to genus and species.

Luca’s other types of analyses focus on the identification of key members of the microbial population (consortium), and how the microbes respond to changes in their environment.

MICROBIAL METHANE GENERATION

In typical experiments, direct measurements of methane concentration are taken over the course of a 3 to 10-month period from aliquots removed from the experimental bottles. The bottles contain substrate (coal or oil), formation water and, in some instances, either stimulatory or inhibitory amendments. In Fig. 4, methane generation is monitored over a 6-month period from samples obtained from the Dietz coals of the PRB. Sample bottles containing coal and formation water (unamended) showed a steady rise of methane in the headspace, peaking at slightly more than 3% at 135 days. When a nutritional enhancer is added (Amendment 1), an immediate and significant boost in methane generation is observed, reading almost 9% methane after 135 days. It should be noted that while the headspace contains only 3% to 9% methane, this methane represents almost 100% of the newly generated gas.

Fig. 4. Comparison of methanogenic activities observed in Dietz PRB coal samples under varying conditions.

A similar response has been observed in over a dozen coal samples taken from a variety of different coal seams. Control experiments in which the bottle contents are either sterilized, exposed to oxygen, or mixed with a known methanogen inhibitor, bromo-ethanesulfonic acid (BESA),¹⁵ all demonstrated little or no methane generation. Oxygen is a known inhibitor of methanogenic bacteria, which are considered among the most oxygen-sensitive of all the anaerobes. Short exposures have been shown to efficiently kill several species.¹⁶ Therefore, it is not surprising that sterilization, BESA treatments, or exposure to oxygen will inhibit and perhaps destroy biogenesis.

In parallel experiments, a small sample of conventional crude oil recovered from the Bell Creek oil field of the PRB is used in place of coal as substrate for methane generation. Fig. 5 is similar to Fig. 4 in that the sterile control sample generated negligible methane, the unamended sample generates an intermediate level of the methane, and the amended sample generates a substantial amount of methane. Oil from the Monument Butte region of Utah is waxy and contains a high proportion of straight-chain alkanes. This is quite different than the cyclic aliphatic and aromatic molecules typically found in Bell Creek oil.^17,18Nevertheless, a very similar methane generation profile was observed in experimental bottles (data not shown). This would indicate that, in addition to areas containing conventional crude oil, areas with large accumulations of waxy oil could prove to be important sites for the bioconversion of residual oil to methane. For example, Daqing field in northeast China is thought to contain 25 Bbbl of waxy residual oil.^19,20 This residual oil, left in place after successful waterflood operations, represents a potential biogenic gas substrate of about 80 Tcf at 100% conversion efficiency, compared to total reported Chinese natural gas reserves of between 46 and 53 Tcf.²¹

Fig. 5. Comparison of methanogenic activities observed in Bell Creek oil samples under varying conditions.

Microbial communities. The microbes residing in Geobioreactors make up a community called a consortium. This interdependent community is largely unique for each location, with at least three different members responsible for separate metabolic reactions within each Geobioreactor. These reactions include the initial breakdown of the large hydrocarbon molecule, the hydrolysis of this molecule to simple acids and alcohols, and then the conversion of these smaller chemicals to methane. Any interruption in this chain of events will short-circuit the entire process and prevent methane formation.

Interestingly, Luca has observed deficiencies in this microbial progression in samples from some Geobioreactors and has been able to ameliorate the problem with addition of microbes from a different Geobioreactor.

RAMIFICATIONS OF EXPERIMENTAL METHANE GENERATION

The volumes of methane created during experiments can be extrapolated to scaled-up field dimensions using gas percentage composition measurements, SIPs, and the Ideal Gas Law. For example, the upper section of Table 2 summarizes data from oil wells drilled on a 40-acre spacing to produce from a 20-ft-thick sand with 12% porosity and 40% residual oil saturation, after waterflood, based on data obtained from Monument Butte oil after 87 and 297 days. The calculated Mcf/day/well of methane, 132 and 50 Mcf/d, respectively, is similar to ongoing production rates of these wells.

In the same table, extrapolated rates of methane generation from coal yield values of 66 Mcf/day/well in unamended samples, and 217 Mcf/day/well in amended coal samples over 159 days, over 40 acres. Using these numbers, a 640-acre plot of land containing a 50-ft seam of coal could potentially yield 393 Bcf of methane. This is considerably more than the 2 Bcf expected from a field of this size in a conventional accumulation.

The above results are encouraging when extrapolated to in situ field conditions. While the lab conditions have not yet been optimized, field results could surpass or fall short of those obtained in the lab. Nutritional and chemical amendment packages have not been extensively explored, and the same amendments are observed to have different impacts on different substrates.

Each Geobioreactor environment is a unique combination of substrate chemistry, water chemistry and microbial community. The ideal set of conditions for stimulating and sustaining methanogenesis will most likely be dissimilar for each substrate, and perhaps vary between different deposits of a similar substrate. Fortunately, the potential volumes of methane available are so large that the expense of defining the most optimal conditions is modest compared to the reward. 

LITERATURE CITED

  ¹  Volta, Alessandro, 1776, In Paoloni, C., Storia del Metano, Tipografia SAPIL, Milano, 1976.

  ²  Rice, D.D. and G.E. Claypool, “Generation, accumulation, and resource potential of biogenic gas,” AAPG Bulletin, Vol. 65, 1981, pp.5-25.

  ³  Claypool, G., Personal communications, 2004.

  ⁴  Gidda, T., “Landfill gas and renewable energy in developing countries: Links to Kyoto,” AAPG Annual Convention, Calgary, Alberta, June 2005.

  ⁵  Law, B.E., D.D. Rice and R.M. Flores, “Coalbed gas accumulations in the Paleocene Fort Union formation, Powder River Basin, Wyoming:In Schwochow, S.D.” S.D. Murray and M.F. Fahy, eds., Coalbed methane of Western North America, RMAG, Denver, Colorado, 1991, pp. 179-190.

  ⁶  Luca Technologies, LLC, “Active biogenesis of methane in Wyoming’s Powder River Basin,” 2004, http://www.lucatechnologies.com/resources/files/Web-Files/PRB%20Report%20112004%20version.pdf.

  ⁷  Martini, A.M., J.M. Budai, L.M. Walter and M. Schoell, “Microbial generation of economic accumulations of methane within a shallow organic-rich shale,” Nature 383, 1996, pp. 155-158.

  ⁸  Rice, D.D., “Relation of natural gas composition to thermal maturity and source rock type in San Juan basin, northwestern New Mexico and southwestern Colorado,” AAPG Bulletin, Vol. 67, 1983, pp. 1119-1218.

  ⁹  Luca Technologies, LLC, “Residual oil deposits as a substrate for methane Geobioreactors*,” 2005, http://www.lucatechnologies.com/resources/files/Web-Files/Luca-Articles/Reports/Residual Oil Deposits Report Version 040805.pdf.

¹⁰  Scott, A.R., W.R. Kaiser and W.B Ayers, Jr., “Thermogenic and secondary biogenic gases, San Juan basin, Colorado and New Mexico – Implications for coalbed gas producibility,” AAPG Bulletin, Vol. 78, 1994, pp. 1186-1209.

¹¹  Martini, A.M., K. Nusslein and S.T. Petsch, “Enhancing microbial gas from unconventional reservoirs,” GTI GasTIPS, Spring 2005, pp. 3-6.

¹²  Pashin, J.C. et al., “Geologic screening criteria for sequestration of CO₂ in coal: Quantifying potential of the Black Warrior coalbed methane fairway, Alabama,” Report of January 2004 to DOE/NETL under contract DE-FC26-00NT40927.

¹³  Barker, C.E. and T.A. Dallegge, “Secondary gas emissions during coal desorption, Marathon Grassim Oskolkoff-1 Well, Cook Inlet Basin, Alaska,” Natural gas from coal and shale section, AAPG Annual Convention, Calgary, June 2005.

¹⁴  Head, I.M, D.M. Jones and S.R. Larter, “Biological activity in the deep subsurface and the origin of heavy oil,” Nature 426, 2003, pp. 344-352.

¹⁵  Muller, V., M. Blaut and G. Gottschalk, “Bioenergetics of methanogenesis,” In: J.G. Ferry (ed.) Methanogenesis: Ecology, physiology, biochemistry & genetics,” Chapman & Hall Microbiology Series, 1993, New York.

¹⁶  Zinder, S.H., “Physiological ecology of methanogens,” In: J.G. Ferry (ed.), “Methanogenesis: Ecology, physiology, biochemistry & genetics,” Chapman & Hall Microbiology Series, 1993, New York.

¹⁷  Altken, C.M., D.M. Jones and S.R. Larter, “Anaerobic hydrocarbon biodegradation in deep subsurface oil reservoirs,” Nature 431, 2004, pp. 291-294.

¹⁸  Hou, B.K., L.B.M. Ellis and L.P Wackett, “Encoding microbial metabolic logic: Predicting biodegradation,” Journal of Industrial Microbiology and Biotechnology, 31, 2004, pp. 261-272.

¹⁹  Central News Agency, “Daqing oil field cuts oil production by seven percent,” March 29, 2004.

²⁰  Yang, W., “Daqing oil field Peoples’ Republic of China: A giant field with oil of non-marine origin,” AAPG Bulletin, 69, 1985, pp. 110-111.

²¹  Energy Information Agency, “Annual Energy Review,” 2002, http://tonto.eia.doe.gov/FTPROOT/multifuel/ 038402.pdf.

THE AUTHORS
	Mark Finkelstein, Vice President of Biosciences, Luca Technologies, LLC, obtained his PhD in molecular biology from SUNY at Buffalo in 1976. His duties include coordination of scientific research, interface with outside labs and universities, support of the IP effort, and government grant writing and reporting. He held several management positions at the National Renewable Energy Laboratory, including director of the Biotechnology Center for Fuels and Chemicals. Dr. Finkelstein has over 25 years of work in the biotech industry coordinating and funding complex research efforts.
	Roland P. DeBruyn, Vice President – Engineering Sciences and founder for Luca Technologies, received a BS degree in chemical engineering from the University of Calgary in 1976 and an MBA from the University of Phoenix in 1986. His responsibilities at Luca include the synthesis and integration of advanced petroleum reservoir science with the company’s biologic research. He has over 25 years of engineering experience working complex oil and gas reservoirs for a variety of small and large oil/gas companies. Mr. DeBruyn is a member of the SPE.
	Jeffrey L. Weber, Vice President – Geosciences and founder, Luca Technologies, received his BS in geophysics from Texas A&M in 1971. His responsibilities at Luca include synthesis and integration of advanced geology, hydrology and geophysical sciences with the company’s biologic research. During his 30 years in the oil/gas industry, he has served in a senior geoscience management capacity with various organizations within the oil/gas industry. Mr. Weber is a certified professional geologist, a member of AAPG, the AIPG and the SEG.
	James B. Dodson, Vice President of Business Development and Legal & Intellectual Property, Luca Technologies, received a Juris Doctorate degree from the University of Denver in 1987 and a BS in geology from Michigan State University in 1984. His responsibilities at Luca include management of the company’s intellectual property rights and patents. He has been employed in several senior management positions within the oil/gas industry, including responsibilities for business development. Mr. Dodson is a member of the AAPG, the Rocky Mountain Association of Geologists, and the Colorado and Denver Bar Associations.