Changes

==Terminology==

==Terminology==

To develop a working classification of shale tectonics, recognition that two primary classes of features occur is critical: (1) those associated with extrusion of fluids and material that does not involve grain-to-grain contact,<ref name=Battanietal_2010 /><ref name=Delisleetal_2010 /> and (2) those associated with larger scale [[deformation]] of apparent highly overpressured mud or shale substrates involving grain-to-grain plastic flow.<ref name=Elsleyandtieman_2010 /><ref name=Wieneretal_2010>Wiener, R. W., M. G. Mann, M. T. Angelich, and J. B. Molyneux, 2010, [http://archives.datapages.com/data/specpubs/memoir93/CHAPTER09/CHAPTER09.HTM Mobile shale in the Niger Delta: Characteristics, structure, and evolution], ''in'' L. Wood, ed., Shale tectonics: [http://store.aapg.org/detail.aspx?id=1023 AAPG Memoir 93], p. 145–160.</ref> In addition, several varieties of highly fractured seep features exist.<ref name=Warrenetal_2010 /> These features allow fluid leakage from overpressured beds, but they do not always involve plastic or fluid-mud extrusion. Shales form a variety of nonextrusive structures, many of which resemble features generated through mobile salt movement, such as domes, welds, and walls. In contrast, muds form a variety of extrusive features, such as volcanoes, ponds, and flows and can erupt explosively.

To develop a working classification of shale tectonics, recognition that two primary classes of features occur is critical: (1) those associated with extrusion of fluids and material that does not involve grain-to-grain contact,<ref name=Battanietal_2010 /><ref name=Delisleetal_2010 /> and (2) those associated with larger scale [[deformation]] of apparent highly overpressured mud or shale substrates involving grain-to-grain plastic flow.<ref name=Elsleyandtieman_2010 /><ref name=Wieneretal_2010>Wiener, R. W., M. G. Mann, M. T. Angelich, and J. B. Molyneux, 2010, [http://archives.datapages.com/data/specpubs/memoir93/CHAPTER09/CHAPTER09.HTM Mobile shale in the Niger Delta: Characteristics, structure, and evolution], ''in'' L. Wood, ed., Shale tectonics: [http://store.aapg.org/detail.aspx?id=1023 AAPG Memoir 93], p. 145–160.</ref> In addition, several varieties of highly [[fracture]]d seep features exist.<ref name=Warrenetal_2010 /> These features allow fluid leakage from overpressured beds, but they do not always involve plastic or fluid-mud extrusion. Shales form a variety of nonextrusive structures, many of which resemble features generated through mobile salt movement, such as domes, welds, and walls. In contrast, muds form a variety of extrusive features, such as volcanoes, ponds, and flows and can erupt explosively.

Several terms are used to describe the processes and features of shale mobility. These include shale tectonics, mud diapirism, shale diapirism and diapirs, mobile shales, mud volcanoes, mud diatremes, mud flows, and finally, from the Greek, argillokinesis and pellitokinesis. Although many scientists continue to argue against the existence of mobile shale, the term is well entrenched in the lexicon and unlikely to go away. A [http://www.americangeosciences.org/georef/georef-information-services GeoRef] search on the topic of mobile shale resulted in 34 instances of the combined term present in the peer-reviewed literature. Mobile shale and mud are the primary subject of a 2003 publication from the Geological Society of London, Subsurface Sediment Mobilization.<ref name=Vanrensbergenetal_2003>Van Rensbergen, O., R. R. Hillis, A. J. Maltman, and C. K. Morley, 2003, Subsurface sediment mobilization: Introduction, ''in'' P. Van Rensbergen, R. R. Hillis, A. J. Maltman, and C. K. Morley, eds., [http://sp.lyellcollection.org/content/216/1 Subsurface sediment mobilization]: Geological Society (London) Special Publication 216, p. 1–8.</ref> This single publication accounts for 36 additional articles on the subject, quadrupling the previous literature offering.  

Several terms are used to describe the processes and features of shale mobility. These include shale tectonics, mud diapirism, shale diapirism and diapirs, mobile shales, mud volcanoes, mud diatremes, mud flows, and finally, from the Greek, argillokinesis and pellitokinesis. Although many scientists continue to argue against the existence of mobile shale, the term is well entrenched in the lexicon and unlikely to go away. A [http://www.americangeosciences.org/georef/georef-information-services GeoRef] search on the topic of mobile shale resulted in 34 instances of the combined term present in the peer-reviewed literature. Mobile shale and mud are the primary subject of a 2003 publication from the Geological Society of London, Subsurface Sediment Mobilization.<ref name=Vanrensbergenetal_2003>Van Rensbergen, O., R. R. Hillis, A. J. Maltman, and C. K. Morley, 2003, Subsurface sediment mobilization: Introduction, ''in'' P. Van Rensbergen, R. R. Hillis, A. J. Maltman, and C. K. Morley, eds., [http://sp.lyellcollection.org/content/216/1 Subsurface sediment mobilization]: Geological Society (London) Special Publication 216, p. 1–8.</ref> This single publication accounts for 36 additional articles on the subject, quadrupling the previous literature offering.  

Argillokinesis is a broadly applied, all-encompassing term used to describe the dynamics of uncompacted flexible clays. With broad consideration of previous literature, as well as numerous conversations with the scientific community of subsurface researchers, we propose herein to use the term shale tectonics to define the structuring within a basin associated with shale or mudstone plasticity or mobility, either as the cause of such mobility or as a result of such mobility. Mobile shales are defined as any manifestation of clay constituents (indurated or not) that show evidence of microscopic-scale fluid or plastic movement. We acknowledge that such mobility is currently poorly understood and may be some manifestation of microscopic shearing or as a reconstitution of partly indurated muds through diagenetic alteration.

Argillokinesis is a broadly applied, all-encompassing term used to describe the dynamics of uncompacted flexible clays. With broad consideration of previous literature, as well as numerous conversations with the scientific community of subsurface researchers, we propose herein to use the term shale tectonics to define the structuring within a basin associated with shale or [[mudstone]] plasticity or mobility, either as the cause of such mobility or as a result of such mobility. Mobile shales are defined as any manifestation of clay constituents (indurated or not) that show evidence of microscopic-scale fluid or plastic movement. We acknowledge that such mobility is currently poorly understood and may be some manifestation of microscopic shearing or as a reconstitution of partly indurated muds through diagenetic alteration.

==Identification of subsurface mobile shale==

==Identification of subsurface mobile shale==

* Carrier beds for migrating fluids and gases.

* Carrier beds for migrating fluids and gases.

In AAPG Memoir 8, Diapirs and Diapirism, Musgrave and Hicks<ref name=Musgraveandhicks_1988>Musgrave, A. W., and W. G. Hicks, 1968, [http://archives.datapages.com/data/specpubs/structu1/data/a153/a153/0001/0100/0122.htm Outlining shale masses by geophysical methods], ''in'' E. Braunstein and G. D. O'Brien, eds., Diapirism and diapirs: AAPG Memoir 8, p. 122–136.</ref> provided a set of characteristics for what appear to be displaced shale masses in the Gulf of Mexico: (1) low-velocity sound transmission, in the range of 6500–8500 ft/s (1981.2–2590.8 m/s) with very little increase in velocity with depth; (2) low density, estimated to be in the range of 2.1–2.3 g/cm³; (3) low resistivity, approximately 0.5 ohm m; and (4) high fluid pressure, about 90% of the overburden pressure. Other authors (Henry et al.<ref name=Henryetal_2010 />) have documented sonic velocities in near surface (~0–3500 ft [~0–1070 m] below surface) mobile muds of onshore Trinidad to be lower than that of water! Authors from a range of disciplines have attributed these behaviors to high fluid pressure within the shales. At times, these densities are reduced to the point that the shales will rise as a mass in a diapiric fashion that is similar to that of salt. However, such behavior is not widely documented.

In AAPG Memoir 8, Diapirs and Diapirism, Musgrave and Hicks<ref name=Musgraveandhicks_1988>Musgrave, A. W., and W. G. Hicks, 1968, [http://archives.datapages.com/data/specpubs/structu1/data/a153/a153/0001/0100/0122.htm Outlining shale masses by geophysical methods], ''in'' E. Braunstein and G. D. O'Brien, eds., Diapirism and diapirs: AAPG Memoir 8, p. 122–136.</ref> provided a set of characteristics for what appear to be displaced shale masses in the [[Gulf of Mexico]]: (1) low-velocity sound transmission, in the range of 6500–8500 ft/s (1981.2–2590.8 m/s) with very little increase in velocity with depth; (2) low density, estimated to be in the range of 2.1–2.3 g/cm³; (3) low resistivity, approximately 0.5 ohm m; and (4) high fluid pressure, about 90% of the overburden pressure. Other authors (Henry et al.<ref name=Henryetal_2010 />) have documented sonic velocities in near surface (~0–3500 ft [~0–1070 m] below surface) mobile muds of onshore Trinidad to be lower than that of water! Authors from a range of disciplines have attributed these behaviors to high fluid pressure within the shales. At times, these densities are reduced to the point that the shales will rise as a mass in a diapiric fashion that is similar to that of salt. However, such behavior is not widely documented.

Improved seismic technologies have allowed a step change forward in the interpretation of shale tectonics in the subsurface with many previously interpreted mobile masses now much better defined. In some instances, improved imaging techniques have shown features previously interpreted as shale diapirs to be tightly folded anticlinal cores (Elsley and Tieman<ref name=Elsleyandtieman_2010 />). However, other instances exist in which shale substrates do appear to show inflation and upward mobility (Wiener et al.<ref name=Wieneretal_2010 />). Because criteria for interpreting mobile shales are not well documented in literature, many of the characteristics that were developed for interpreting mobile salts have been applied to shale basins, albeit with mixed success. In addition, in basins where both salt and shale occur, geoscientists commonly fail to differentiate mobile shales from mobile salts in seismic images. To rectify this deficiency, the following criteria for differentiating salt and shale are provided.

Improved seismic technologies have allowed a step change forward in the interpretation of shale tectonics in the subsurface with many previously interpreted mobile masses now much better defined. In some instances, improved imaging techniques have shown features previously interpreted as shale diapirs to be tightly folded anticlinal cores (Elsley and Tieman<ref name=Elsleyandtieman_2010 />). However, other instances exist in which shale substrates do appear to show inflation and upward mobility (Wiener et al.<ref name=Wieneretal_2010 />). Because criteria for interpreting mobile shales are not well documented in literature, many of the characteristics that were developed for interpreting mobile salts have been applied to shale basins, albeit with mixed success. In addition, in basins where both salt and shale occur, geoscientists commonly fail to differentiate mobile shales from mobile salts in seismic images. To rectify this deficiency, the following criteria for differentiating salt and shale are provided.

* Shale welds are less prevalent in shale systems than salt welds are in salt systems.

* Shale welds are less prevalent in shale systems than salt welds are in salt systems.

* Shales have a much slower velocity than salt. Salt will be underlain by seismic pull-up versus no pull-up beneath shale features.

* Shales have a much slower velocity than salt. Salt will be underlain by seismic pull-up versus no pull-up beneath shale features.

* Salt requires a pipe several kilometers wide to facilitate vertical migration. In contrast, fluids migrating through shales will crack and hydraulically fracture a zone through which they will migrate upward.

* Salt requires a pipe several kilometers wide to facilitate vertical migration. In contrast, fluids migrating through shales will crack and hydraulically [[fracture]] a zone through which they will migrate upward.

* Shale may flow laterally but will develop overhangs of less than 6 km (3.7 mi). In contrast, salt overhangs may be on the order of tens of kilometers.

* Shale may flow laterally but will develop overhangs of less than 6 km (3.7 mi). In contrast, salt overhangs may be on the order of tens of kilometers.

* Ductile behavior of shales is unlikely above 80&deg;C (176&deg;F), and brittle behavior is more likely. Therefore, this temperature will provide a plasticity [[basement]] within a basin to constrain interpretation of tectonically active shales.

* Ductile behavior of shales is unlikely above 80&deg;C (176&deg;F), and brittle behavior is more likely. Therefore, this temperature will provide a plasticity [[basement]] within a basin to constrain interpretation of tectonically active shales.