19. Geology and Geochemistry of Shale Gas Systems

The United States is the unequivocal leader in the production of natural gas from low porosity/permeability fine-grained rocks including shales, which have previously been considered largely the source rocks for petroleum.  In fact, shale gas accumulations have become the focus of domestic gas exploration strategies, spurred on by recent successes in the Mississippian Barnett Shale (estimated mean resource of ~26 trillion cubic feet [tcf]) (Pollastro and others, 2003), Fort Worth Basin, the Devonian Woodford Shale, Anadarko Basin, the Devonian Antrim Shale (estimated mean resource of ~7.5 tcf) (Swezey and others, 2004), Michigan Basin, and the Cretaceous Lewis Shale (estimated mean resource of ~10 tcf) (Ridgley and others, 2002), San Juan Basin.

Global consumption of natural gas is projected to increase by nearly 2% annually from 100 tcf in 2004 to 163 tcf in 2030, with demand within the US estimated to increase by 0.6% per year over this time period (EIA 2007).  Moreover, future efforts to reduce carbon emissions are likely to lead to the development of plans that encourage the use of cleaner burning gas as a replacement for more carbon intensive fuels such as liquid hydrocarbons and coal.  The 2007 USGS mean estimate of the total natural gas resources of the US is ~655 tcf, which represents an increase of ~27 tcf over the predicted value for 2006 (http://energy.cr.usgs.gov/oilgas/noga/assessment_updates.html).  This increase in domestic gas resources was due exclusively to an estimated 35 tcf of gas in shales in the Permian basin that had previously been unrecognized (Schenk et al. 2007), and which more than offset a loss of ~8 tcf of conventional gas (http://energy.cr.usgs.gov/oilgas/noga/assessment_updates.html).  In fact, estimates of the shale gas resource potential within the US have consistently increased in recent years.  One recent study noted that in 1996 this was estimated to be approximately 52 tcf, by 2002 the figure had been revised to ~78 tcf, and by 2006 (using data through 2005) the number had grown to about 128 tcf (Kuuskraa, 2007).  This growth in shale gas resource potential has primarily been driven by advances in drilling and completion technologies.  Many of these techniques have just been put into practice in the past few years, and this is a rapidly evolving field.

In spite of the success of the engineering strategies for extracting the gas, the scientific understanding of the geologic processes that control the formation of these gas accumulations has not kept pace with the technical developments that have enabled their exploitation.  Fundamental questions related to shale gas accumulations still remain unanswered.  These include questions such as: How much gas did a specific shale generate?  What fraction of that total amount has been retained in the shale?  Does the gas occur as free gas in open pore spaces or as sorbed gas in the rock matrix?  What role do natural fractures play in the storage and production of gas?  The answers to these and other similar questions have significant implications for shale gas exploration and undiscovered resource assessment strategies. 

In low porosity and permeability shale gas reservoirs, the mechanism(s) by which natural gas is stored is not well-understood.  Gas may be retained either as sorbed gas (sorption onto organics and/or inorganic mineral matter) or as free gas in micropores (intergranular, fracture, or intraorganic), or a complex mixture of the two.  A more comprehensive understanding of the manner by which the gas is stored is critical for obtaining a realistic estimate of how much resource may be present in the shale reservoir as well as the ultimate recoverability of resources from these systems.  Key areas of research include establishment of the 1) geochemical characteristics of gas in shales as a function of storage mechanism (free vs sorbed gas), and 2) relative roles of organic (kerogen) and inorganic (mineralogy) phases in gas sorption as well as desorption during production.  Possible approaches that could be applied in this research include:  incremental sorption/desorption experiments, detailed microscopic rock characterization, and gas geochemical analyses of laboratory and field samples.  The successful candidate would have qualifications in any of the following disciplines:  sedimentary geology, geochemistry, and petroleum engineering.  Results from this research will enable better characterization of these parameters in shale gas systems, which in turn will facilitate future assessments of undiscovered shale gas resources.  In addition, successful completion of this avenue of research will provide a geochemical means for prediction of estimated ultimate recovery of gas in frontier areas.

References

Energy Information Administration, 2007,  International Energy Outlook 2007:  http://www.eia.doe.gov/oiaf/ieo/pdf/0484(2007).pdf

Kuuskraa, V. A., 2007,  Unconventional gas — 2: Resource potential estimates likely to change:  Oil and Gas Journal, v. 105 (35), p. 64.

Pollastro, R.M., Hill, R.J., Ahlbrandt, T.A., Charpentier, R.R., Cook, T.A., Klett, T.R., Henry, M.E., and Schenk, C.J., 2004, Assessment of undiscovered oil and gas resources of the Bend Arch-Fort Worth Basin Province of North-Central Texas and Southwestern Oklahoma, 2003: U.S. Geological Survey Fact Sheet 2004–3022, 2 p.

Ridgley, J.L, Condon, S.M.,  Dubiel, R.F.,  Charpentier, R.R., Cook, T.A., Crovelli, R.A.,  Klett, T.R., Pollastro, R.M., and Schenk, C.J., 2002, Assessment of undiscovered oil and oas resources of the San Juan Basin Province of New Mexico and Colorado, 2002:  U.S. Geological Survey Fact Sheet FS-147-02, 2 p.

Schenk, C.J., Pollastro, R.M., Cook, T.A., Pawlewicz, M.J., Klett, T.R., Charpentier, R.R., and Cook, H.E., 2008, Assessment of undiscovered oil and gas resources of the Permian Basin Province of west Texas and southeast New Mexico, 2007: U.S. Geological Survey Fact Sheet 2007–3115, 4 p.

Swezey, C.S., Hatch, J.R., Hayba, D.O., Repetski, J.E., Charpentier, R.R., Cook, T.A., Klett, T.R.,  Pollastro, R.M., and Schenk, C.J., 2005, Assessment of undiscovered oil and gas resources of the U.S. portion of the Michigan Basin, 2004: U.S. Geological Survey Fact Sheet 2005–3070, 3070, 2 p.

Proposed Duty Station: Denver, CO

Areas of Ph.D.: Sedimentary geology, geochemistry (organic and/or inorganic), or petroleum geology. The research area could include experimental, field-based, or modeling studies.

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Chemist, Research Hydrologist, Research Engineer

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Geoffrey Ellis, (303) 236-5762, gsellis@usgs.gov; Neil Fishman (303) 236 1542, nfishman@usgs.gov

Human Resources Office contact: Vanessa Chambless, (303) 236-9584, vchambless@usgs.gov

Summary of Opportunities