Changes

An Appendix following Part 2 of this chapter provides maps with tabular legends referencing various worldwide shale resource plays, both gas and oil, that currently have wells drilled, in progress, or planned. Some speculative shale resource plays are included, and some prospective shale resource plays on this map will necessarily require updating based on drilling results. Certainly, other known source rocks are likely prospective as shale resource system plays, particularly marine shales, but also lacustrine and fluvial-deltaic systems.

An Appendix following Part 2 of this chapter provides maps with tabular legends referencing various worldwide shale resource plays, both gas and oil, that currently have wells drilled, in progress, or planned. Some speculative shale resource plays are included, and some prospective shale resource plays on this map will necessarily require updating based on drilling results. Certainly, other known source rocks are likely prospective as shale resource system plays, particularly marine shales, but also lacustrine and fluvial-deltaic systems.

Producible natural gas shale resource systems in the United States provide a means of energy independence in natural gas for the foreseeable future. This may be for the next decade or for decades to come, depending on the economic, environmental, and political conditions for shale-gas production. This energy independence is created by the remarkable success achieved by the development of unconventional shale-gas resources. United States independent exploration and development companies have found and produced a huge surplus of natural gas, thereby making it a very inexpensive carbon-based energy source with a large remaining development potential.

Producible natural gas shale resource systems in the United States provide a means of energy independence in natural gas for the foreseeable future. This may be for the next decade or for decades to come, depending on the economic, environmental, and political conditions for shale-gas production. This energy independence is created by the remarkable success achieved by the development of unconventional shale-gas resources. United States independent exploration and development companies have found and produced a huge surplus of natural gas, thereby making it a very inexpensive carbon-based energy source with a large remaining development potential.

The tenacity of George Mitchell, his compatriots at MEDC, and Devon and Southwestern Energy's successful drilling program in shale that led to the evolution of this play type cannot be overstated. Eventually, their successes brought the potential of shale-gas resource systems to national and, ultimately, global levels.

The tenacity of George Mitchell, his compatriots at MEDC, and Devon and Southwestern Energy's successful drilling program in shale that led to the evolution of this play type cannot be overstated. Eventually, their successes brought the potential of shale-gas resource systems to national and, ultimately, global levels.

CHARACTERISTICS OF SHALE-GAS RESOURCE SYSTEMS

==Characteristics of shale-gas resource systems==

What are the characteristics of these shale resource plays that caused them to be either overlooked or ignored? It was certainly not their source rock characteristics because most are organic-rich source rocks at varying levels of thermal maturity that have sourced conventional oil and gas fields in virtually every basin where they have been exploited. Although their petroleum source potential is well known, their rock properties were very unattractive for reservoir potential amplified by their recognition as not only source rocks, but also as seal or cap rocks, certifying their nonreservoir properties. However, their retention and storage capacity for petroleum was largely ignored and mud gas log responses noted with the somewhat derogatory shale-gas moniker. Because these shale resource plays were a combination of source rocks and seals, the retention of hydrocarbons is a factor that was overlooked. Diffusion, albeit a slow process, suggested that oil and especially gas were mostly lost from such rocks over geologic time. For example, modeling petroleum generation in the Barnett Shale indicates that maximum generation may have been reached 250 Ma (Jarvie et al., 2005a). Because of a complex burial and uplift history, maximum generation could have been reached about 25 Ma, but nonetheless, retention of generated hydrocarbons to the present day was not perceived as likely or certainly not to a commercial extent. As such, even in a good seal rock, diffusion should have resulted in a substantial loss of gas, thereby limiting the resource potential of the system. The presence of fractures, although healed, and the presence of conventional oil and gas reservoirs in the Fort Worth Basin, suggested that expulsion and diffusion had possibly drained the shale. In addition, gas contents measured on the MEDC 1-Sims well, 1991, were not very encouraging, suggesting non-commercial amounts of gas (Steward, 2007).

What are the characteristics of these shale resource plays that caused them to be either overlooked or ignored? It was certainly not their source rock characteristics because most are organic-rich source rocks at varying levels of thermal maturity that have sourced conventional oil and gas fields in virtually every basin where they have been exploited. Although their petroleum source potential is well known, their rock properties were very unattractive for reservoir potential amplified by their recognition as not only source rocks, but also as seal or cap rocks, certifying their nonreservoir properties. However, their retention and storage capacity for petroleum was largely ignored and mud gas log responses noted with the somewhat derogatory shale-gas moniker. Because these shale resource plays were a combination of source rocks and seals, the retention of hydrocarbons is a factor that was overlooked. Diffusion, albeit a slow process, suggested that oil and especially gas were mostly lost from such rocks over geologic time. For example, modeling petroleum generation in the Barnett Shale indicates that maximum generation may have been reached 250 Ma (Jarvie et al., 2005a). Because of a complex burial and uplift history, maximum generation could have been reached about 25 Ma, but nonetheless, retention of generated hydrocarbons to the present day was not perceived as likely or certainly not to a commercial extent. As such, even in a good seal rock, diffusion should have resulted in a substantial loss of gas, thereby limiting the resource potential of the system. The presence of fractures, although healed, and the presence of conventional oil and gas reservoirs in the Fort Worth Basin, suggested that expulsion and diffusion had possibly drained the shale. In addition, gas contents measured on the MEDC 1-Sims well, 1991, were not very encouraging, suggesting non-commercial amounts of gas (Steward, 2007).

FIGURE 2. Comparison of Montney Shale and Doig Phosphate in terms of total organic carbon (TOC) and porosity. The Montney Shale shows poor and inverse correlation to TOC, whereas the Doig Phosphate shows good and positive correlation indicative of organically derived porosity. The positive y-(porosity) intercept for the Doig indicates about 2% matrix porosity. The inverse correlation of the Montney Shale is suggestive of a hybrid system where porosity is derived primarily from matrix as opposed to organic porosity. BV = bulk volume; R2 = linear correlation coefficient.

FIGURE 2. Comparison of Montney Shale and Doig Phosphate in terms of total organic carbon (TOC) and porosity. The Montney Shale shows poor and inverse correlation to TOC, whereas the Doig Phosphate shows good and positive correlation indicative of organically derived porosity. The positive y-(porosity) intercept for the Doig indicates about 2% matrix porosity. The inverse correlation of the Montney Shale is suggestive of a hybrid system where porosity is derived primarily from matrix as opposed to organic porosity. BV = bulk volume; R2 = linear correlation coefficient.

ORGANIC RICHNESS: TOTAL ORGANIC CARBON ASSESSMENT

==Organic richness: total organic carbon assessment==

One of the first and basic screening analyses for any source rock is organic richness, as measured by total organic carbon (TOC). The TOC is a measure of organic carbon present in a sediment sample, but it is not a measure of its generation potential alone, as that requires an assessment of hydrogen content or organic maceral percentages from chemical or visual kerogen assessments. As TOC values vary throughout a source rock because of organofacies differences and thermal maturity, and even depending on sample type, there has been a lengthy debate on what actual TOC values are needed to have a commercial source rock. All organic matter preserved in sediments will decompose into petroleum with sufficient temperature exposure; for EampP companies, it is a matter of the producibility and commerciality of such generation. In addition, the expulsion and retention of generated petroleum must be considered. However, original quantity (TOC) as well as source rock quality (type) of the source rock must be considered in combination to assess its petroleum generation potential.

One of the first and basic screening analyses for any source rock is organic richness, as measured by total organic carbon (TOC). The TOC is a measure of organic carbon present in a sediment sample, but it is not a measure of its generation potential alone, as that requires an assessment of hydrogen content or organic maceral percentages from chemical or visual kerogen assessments. As TOC values vary throughout a source rock because of organofacies differences and thermal maturity, and even depending on sample type, there has been a lengthy debate on what actual TOC values are needed to have a commercial source rock. All organic matter preserved in sediments will decompose into petroleum with sufficient temperature exposure; for EampP companies, it is a matter of the producibility and commerciality of such generation. In addition, the expulsion and retention of generated petroleum must be considered. However, original quantity (TOC) as well as source rock quality (type) of the source rock must be considered in combination to assess its petroleum generation potential.

The computed GOC values from these TOCo values are variable, ranging from about 25 (Bossier 2) to 62% (Haynesville 1). As previously suggested, such variation is good reason to segregate various organofacies of source rocks into percentages based on thickness instead of using a single average value. Differences in the Bossier and Haynesville shales have also been reported in the core-producing area of northwestern Louisiana on highly mature cuttings and core samples, although four facies were identified in the Bossier (Novosel et al., 2010). A dramatic difference in the amount of GOC exists between the two formations and within the Tithonian itself. These organofacies differences in the Tithonian may explain the dramatic difference in TOC values reported for the Bossier Shale in Freestone County, Texas (Rushing et al., 2004), and an unidentified location by Emme and Stancil (2002). Available data for the Tithonian Bossier Shale suggest an about 1% TOC value on average in central Texas, with a value nearer 4% in easternmost Texas and in Louisiana.

The computed GOC values from these TOCo values are variable, ranging from about 25 (Bossier 2) to 62% (Haynesville 1). As previously suggested, such variation is good reason to segregate various organofacies of source rocks into percentages based on thickness instead of using a single average value. Differences in the Bossier and Haynesville shales have also been reported in the core-producing area of northwestern Louisiana on highly mature cuttings and core samples, although four facies were identified in the Bossier (Novosel et al., 2010). A dramatic difference in the amount of GOC exists between the two formations and within the Tithonian itself. These organofacies differences in the Tithonian may explain the dramatic difference in TOC values reported for the Bossier Shale in Freestone County, Texas (Rushing et al., 2004), and an unidentified location by Emme and Stancil (2002). Available data for the Tithonian Bossier Shale suggest an about 1% TOC value on average in central Texas, with a value nearer 4% in easternmost Texas and in Louisiana.

TOP 10 NORTH AMERICAN SHALE-GAS PLAYS

==Top 10 North American shale-gas plays==

Based on available data, HIo values were derived or taken from immature sample populations for each of these source rocks (Table 4). These data show that most of these source rocks have HIo values near P50 (475 mg/g), although the Haynesville Shale is higher than the P10 value. The values of TOCpd with minimum, maximum, and standard deviation and the TOCo from HIo and P50 HIo values for these top 10 shale-gas resource plays are also shown.

Based on available data, HIo values were derived or taken from immature sample populations for each of these source rocks (Table 4). These data show that most of these source rocks have HIo values near P50 (475 mg/g), although the Haynesville Shale is higher than the P10 value. The values of TOCpd with minimum, maximum, and standard deviation and the TOCo from HIo and P50 HIo values for these top 10 shale-gas resource plays are also shown.

The available characteristics of these top 10 shale-gas resource systems are summarized in Table 5 for all available data or calculations.

The available characteristics of these top 10 shale-gas resource systems are summarized in Table 5 for all available data or calculations.

WORLDWIDE ACTIVITY IN SHALE-GAS RESOURCE SYSTEMS

==Worldwide activity in shale-gas resource systems==

Although shale-gas resource plays were slow to spread from the Barnett Shale into other United States and Canadian plays, a worldwide surge in interest has occurred since about 2006. The primary activity outside North America has been in Europe, where several companies including major oil and gas companies have secured land deals and have started drilling and testing of these plays. Key countries in this pursuit are Germany, Sweden, and Poland.

Although shale-gas resource plays were slow to spread from the Barnett Shale into other United States and Canadian plays, a worldwide surge in interest has occurred since about 2006. The primary activity outside North America has been in Europe, where several companies including major oil and gas companies have secured land deals and have started drilling and testing of these plays. Key countries in this pursuit are Germany, Sweden, and Poland.

The lower Saxony Basin of Germany has been studied extensively over the years. In the 1980s, the research organization KFA in Julich, Germany, was funded to drill shallow core holes into the lower Jurassic Posidonia Shale. In the Hils syncline area of the lower Saxony Basin, thermal maturity ranges from 0.49% to about 1.3% Ro (Rullkotter et al., 1988; Horsfield et al., 2010). These cores and their published data provide a wealth of information on this Lower Jurassic source rock and potential resource play. The TOCo values average about 10.5%, with GOC values averaging 56% of the TOCo. Given the high oil saturations reported in the Posidonia Shale (Rullkotter et al., 1988), there may be potential for shale-oil resource plays in the oil window parts of the basin.

The lower Saxony Basin of Germany has been studied extensively over the years. In the 1980s, the research organization KFA in Julich, Germany, was funded to drill shallow core holes into the lower Jurassic Posidonia Shale. In the Hils syncline area of the lower Saxony Basin, thermal maturity ranges from 0.49% to about 1.3% Ro (Rullkotter et al., 1988; Horsfield et al., 2010). These cores and their published data provide a wealth of information on this Lower Jurassic source rock and potential resource play. The TOCo values average about 10.5%, with GOC values averaging 56% of the TOCo. Given the high oil saturations reported in the Posidonia Shale (Rullkotter et al., 1988), there may be potential for shale-oil resource plays in the oil window parts of the basin.

The worldwide exploration effort for shale-gas resource plays will continue for years to come and will likely impact global energy resources in a very positive way. Although recent concerns over groundwater contamination are of extreme importance to all of us both outside and within industry, it should be noted that more than 40,000 shale-gas wells have been hydraulically stimulated and more than one million conventional wells (Montgomery and Smith, 2010). Hype often takes precedence over facts as indicated, for example, by recent cases involving groundwater wells in the Fort Worth Basin. The United States Environmetal Protection Agency cited recent drilling operations in the Barnett Shale as the cause of these groundwater wells, but this was not substantiated. Geochemical data conclusively proved that although gas existed in these water wells, the gas migrated from shallow gas-bearing reservoirs in the basin and not from drilling operations in the Barnett Shale itself (Railroad Commission of Texas, 2011). Although there is and should be concern over any groundwater contamination issues, most of which are the result of ongoing geologic processes, the track record from drilling all the shale-gas wells and such evidence as cited in the Railroad Commission of Texas (2011) hearing provide support for the safe drilling record of industry. Accidents will occur in all industries, and human endeavors and regulations assist in minimizing such unwanted results by all parties, including companies doing the exploration and production, because their livelihoods also depend on positive impact. Perhaps the biggest concern should be the rapid expansion of shale-gas drilling that has stressed the need for availability of a qualified and knowledgeable workforce. As such, management and the supervision of work and drilling crews become perhaps of equal importance as regulatory efforts to improve environmental safety based on new geologic and chemical information.

The worldwide exploration effort for shale-gas resource plays will continue for years to come and will likely impact global energy resources in a very positive way. Although recent concerns over groundwater contamination are of extreme importance to all of us both outside and within industry, it should be noted that more than 40,000 shale-gas wells have been hydraulically stimulated and more than one million conventional wells (Montgomery and Smith, 2010). Hype often takes precedence over facts as indicated, for example, by recent cases involving groundwater wells in the Fort Worth Basin. The United States Environmetal Protection Agency cited recent drilling operations in the Barnett Shale as the cause of these groundwater wells, but this was not substantiated. Geochemical data conclusively proved that although gas existed in these water wells, the gas migrated from shallow gas-bearing reservoirs in the basin and not from drilling operations in the Barnett Shale itself (Railroad Commission of Texas, 2011). Although there is and should be concern over any groundwater contamination issues, most of which are the result of ongoing geologic processes, the track record from drilling all the shale-gas wells and such evidence as cited in the Railroad Commission of Texas (2011) hearing provide support for the safe drilling record of industry. Accidents will occur in all industries, and human endeavors and regulations assist in minimizing such unwanted results by all parties, including companies doing the exploration and production, because their livelihoods also depend on positive impact. Perhaps the biggest concern should be the rapid expansion of shale-gas drilling that has stressed the need for availability of a qualified and knowledgeable workforce. As such, management and the supervision of work and drilling crews become perhaps of equal importance as regulatory efforts to improve environmental safety based on new geologic and chemical information.

The TOCpd values for high-thermal-maturity shales are typically only indicative of the nongenerative part of the TOC. The TOCo and its GOC content provide the key to hydrocarbon generation and organic storage (porosity) development. When HIo is known, %GOC can readily be determined by dividing the HIo by 1177. The TOCpd does provide an indication of adsorptive capacity of a shale.

The TOCpd values for high-thermal-maturity shales are typically only indicative of the nongenerative part of the TOC. The TOCo and its GOC content provide the key to hydrocarbon generation and organic storage (porosity) development. When HIo is known, %GOC can readily be determined by dividing the HIo by 1177. The TOCpd does provide an indication of adsorptive capacity of a shale.

Shale-gas resource systems provide an ample source of carbon-based energy that can be used to mitigate the environmental impact, while powering worldwide consumption needs and reducing conflicts over the access to such energy. The result is a better world for us that bridges the gap to an environmentally neutral energy future.

Shale-gas resource systems provide an ample source of carbon-based energy that can be used to mitigate the environmental impact, while powering worldwide consumption needs and reducing conflicts over the access to such energy. The result is a better world for us that bridges the gap to an environmentally neutral energy future.

* Alexander, R., D. Dawson, K. Pierce, and A. Murray, 2009, Carbon catalyzed hydrogen exchange in petroleum source rocks: Organic Geochemistry, v. 40, p. 951–955, doi:10.1016/j.orggeochem.2009.06.003.

* Alexander, R., D. Dawson, K. Pierce, and A. Murray, 2009, Carbon catalyzed hydrogen exchange in petroleum source rocks: Organic Geochemistry, v. 40, p. 951–955, doi:10.1016/j.orggeochem.2009.06.003.

* Bharati, S., R. L. Patience, S. R. Larter, G. Standen, and I. J. F. Poplett, 1995, Elucidation of the Alum Shale kerogen structure using a multidisciplinary approach: Organic Geochemistry, v. 23, no. 11–12, p. 1043–1058, doi:10.1016/0146-6380(95)00089-5.

* Bharati, S., R. L. Patience, S. R. Larter, G. Standen, and I. J. F. Poplett, 1995, Elucidation of the Alum Shale kerogen structure using a multidisciplinary approach: Organic Geochemistry, v. 23, no. 11–12, p. 1043–1058, doi:10.1016/0146-6380(95)00089-5.