Energy Minerals Division
Gas Shales Tricky to Understand
The success of technological plays such as the Barnett Shale and other shales has proven the potential of shale-gas resources -- and if the recent number of technical sessions, short courses and workshops on gas shales is any indication of its significance, gas shales will be an important component of the world gas supply in the future.
Shale traditionally has been regarded as a hydrocarbon source rock or seal. Following applied research and experimentation by government, academia and industry over the past few decades, shales currently are recognized as complex gas reservoirs that require unconventional thinking to produce gas.
It has taken many decades to reach the current understanding of how gas is stored in coal beds and how to produce the gas (coalbed methane). In many ways, shales are even more complex than coals, and our knowledge of shale-gas production is still at the beginning of the learning curve.
Shale as a rock is defined as a “fine-grained detrital sedimentary rock,” but can vary in mineralogy (e.g., clay, silicate and carbonate minerals), texture and fabric. Shale as a rock formation (e.g., Caney Shale) contains a mixture of grain sizes and lithologies (e.g., black and gray shales, siltstone, limestone). Gas shales are thought of in the lithostratigraphic sense.
Gas shales are self-contained petroleum systems (hydrocarbon source, migration pathway, reservoir and seal). Low-permeability gas-shale plays are recognized as technological plays -- and advances in horizontal drilling, fracture stimulation, micro-seismic fracture mapping and the application of 3-D seismic data have contributed to their success.
Excluding the petroleum engineering completion issues (e.g., slickwater and cross-linked gel fracture stimulation), the geologic approach to gas shales is to evaluate the source rock and reservoir properties. Recent articles and presentations have described important variables and have classified gas shales based on the presence of biogenic, thermogenic or mixed methane.
Some important variables include:
- Depth, thickness and lateral extent of the shale (to define the play boundaries).
- Type (oil or gas generative), quantity (total organic carbon), thermal maturity and adsorptive capacity of organic matter.
- Type (e.g., biogenic, thermogenic, or mixed methane), amount (gas content), composition (e.g., methane, carbon dioxide, nitrogen) and Btu content of gas.
- Pore structure and distribution.
- Mineralogy of the shale (important in designing the fracture stimulation).
Some variables may be either beneficial or detrimental to gas production. For example, even though it is well known that micro-fractures are essential for shale-gas production, natural fractures are beneficial in some settings (e.g., the gas cap in an anticline) but may be detrimental if they extend out of the reservoir zone and either leak gas or connect with sources of water. Thermogenic methane may be associated with oil from oil-generative organic matter in the oil window where the oil may decrease the permeability and impede the movement of gas.
All shales are not alike, even those containing the same type and amount of organic matter at the same thermal maturity.
Mineralogy is very important for a successful well completion. Gas production is dependent on the ability to create fractures, the presence of natural fractures or the occurrence of interbedded permeable lithofacies.
Some words of wisdom that I received from a gas-shale operator are to treat gas shales as fine-grained tight-sand reservoirs. Silica-rich shales behave better during fracture stimulation than clay-rich shales.
Many questions remain to be resolved in evaluating gas shales. For example:
- What is the importance of faults and natural fractures?
- What are the contributions of free gas and sorbed gas?
- What is the drainage area?
- What is the minimum thermal maturity needed for shales containing oil-generative organic matter to be an economic gas shale?
- How does gas migrate by diffusion in shales?
- Does amorphous organic matter have a role in gas diffusion?
A large amount of data must be compiled, modeled and evaluated before venturing into a gas-shale play. You will never have all of the answers. Operators have told me that sometimes the best thing to do is to go with the information that you have and drill a well.
To learn more about gas shales, I encourage you to become an EMD member and access the Gas Shale Committee area of the EMD members-only Web site (http://emd.aapg.org/members_only/gas_shales/) for articles, presentations, reference lists on gas shale and shale, Web links and a calendar of gas shale meetings, short courses and workshops.