Changes

* [[Fault trap regime|Normal fault]] cutting through area of maximum bed curvature

* [[Fault trap regime|Normal fault]] cutting through area of maximum bed curvature

* Presence of a cross-lineation

* Presence of a cross-lineation

* Open calcite crystals lining the fractures

* [[Open calcite crystals]] lining the fractures

* Well must penetrate above critical elements in a more fracturable (more calcareous) bench within the Niobrara

* Well must penetrate above critical elements in a more fracturable (more calcareous) bench within the Niobrara

* Completion must be [[Well completions#Open hole completions|open hole]] (hang a [[Well completions#Liner completions|slotted liner]]) with cement-block fractures

* Completion must be [[Well completions#Open hole completions|open hole]] (hang a [[Well completions#Liner completions|slotted liner]]) with [[cement-block fractures]]

* Drilling must be conducted with [[Analysis of mud weights|underbalanced mud]] or air to prevent fracture damage. The Niobrara is an [[Causes of underpressure|underpressured reservoir]] with petrostatic (0.33 lb/ft gradient) rather than [[Normal hydrostatic pressure|hydrostatic]] (0.43 lb/ft gradient) pressure.

* Drilling must be conducted with [[Analysis of mud weights|underbalanced mud]] or air to prevent fracture damage. The Niobrara is an [[Causes of underpressure|underpressured reservoir]] with [[petrostatic]] (0.33 lb/ft gradient) rather than [[Normal hydrostatic pressure|hydrostatic]] (0.43 lb/ft gradient) pressure.

==Applying the critical factors==

==Applying the critical factors==

[[file:developing-a-philosophy-of-exploration_fig1-3.png|thumb|{{figure number|2}}A Niobrara structure cross section.]]

[[file:developing-a-philosophy-of-exploration_fig1-3.png|thumb|{{figure number|2}}A Niobrara structure cross section.]]

Higher resistivity on electrical logs shows the brittle, more fracturable benches within the Niobrara. Constructing a detailed [[Geological cross sections|cross section]], showing fracturable benches, maximum bed curvature, and any surface and subsurface [[Fault trap regime|normal fault]]ing, allows the explorer to visualize and accurately plot the angle at which the well bore must be drilled to penetrate the critical elements.

Higher [[resistivity]] on electrical logs shows the brittle, more fracturable benches within the Niobrara. Constructing a detailed [[Geological cross sections|cross section]], showing fracturable benches, maximum bed curvature, and any surface and subsurface [[Fault trap regime|normal fault]]ing, allows the explorer to visualize and accurately plot the angle at which the well bore must be drilled to penetrate the critical elements.

Seismic surveys are not particularly helpful in mapping normal faults because they are [[Fault trap regime|listric]] with about 100-300 ft of throw at the surface and about 30-80 ft of throw in the Niobrara. The faults are usually not present below the Niobrara. Therefore, although the Mesa Verde provides good seismic marker beds, the underlying Mancos and Niobrara

Seismic surveys are not particularly helpful in mapping normal faults because they are [[Fault trap regime|listric]] with about 100-300 ft of throw at the surface and about 30-80 ft of throw in the Niobrara. The faults are usually not present below the Niobrara. Therefore, although the Mesa Verde provides good seismic marker beds, the underlying Mancos and Niobrara

Formations usually do not have them. Occasionally, upward-lying normal faults will produce. [[Seismic data]] are useful in delineating these faults because good marker beds are usually present below the Niobrara. The fault can then be projected upward and is sometimes associated with a dim spot due to [[Vertical and lateral seismic resolution and attenuation#Attenuation|attenuation]] of [[seismic data]] in fracture zones.

Formations usually do not have them. Occasionally, upward-lying normal faults will produce. [[Seismic data]] are useful in delineating these faults because good marker beds are usually present below the Niobrara. The fault can then be projected upward and is sometimes associated with a [[dim spot]] due to [[Vertical and lateral seismic resolution and attenuation#Attenuation|attenuation]] of [[seismic data]] in fracture zones.

[[:file:developing-a-philosophy-of-exploration_fig1-3.png|Figure 2]] is a Niobrara structure cross section.

[[:file:developing-a-philosophy-of-exploration_fig1-3.png|Figure 2]] is a Niobrara structure cross section.

==Niobrara open fractures==

==Niobrara open fractures==

[[Overview of routine core analysis|Running samples]] on Niobrara wells will reveal whether open calcite crystals are present in the fractures. In addition, [[The seven critical elements of fieldwork|field work]] has shown that the same kind of calcite present in subsurface fractures within the Niobrara also occurs in fractures associated with brittle beds at the surface, such as the Mesa Verde group.

[[Overview of routine core analysis|Running samples]] on Niobrara wells will reveal whether [[open calcite crystals]] are present in the fractures. In addition, [[The seven critical elements of fieldwork|field work]] has shown that the same kind of calcite present in subsurface fractures within the Niobrara also occurs in fractures associated with brittle beds at the surface, such as the Mesa Verde group.

==Surface geochemistry applied to Niobrara play==

==Surface geochemistry applied to Niobrara play==

[[Principles of surface geochemical exploration|Surface geochemical methods]]—specifically, [[Seepage activity|soil gas]] surveys—have proven useful in exploring for these types of traps. The computer compares hundreds of soil-gas ratios very quickly. Also, very sensitive chromatographs have improved the detection of vertical microseepages of hydrocarbons above these fractured reservoirs. The main method of exploration with this technique is to conduct surveys over a number of known commercial accumulations to establish productive signatures. Then a survey over the prospect may provide useful information that can be integrated with the other exploration techniques to help locate a drillsite.

[[Principles of surface geochemical exploration|Surface geochemical methods]]—specifically, [[Seepage activity|soil gas]] surveys—have proven useful in exploring for these types of traps. The computer compares hundreds of soil-gas ratios very quickly. Also, very sensitive [[chromatographs]] have improved the detection of vertical microseepages of hydrocarbons above these fractured reservoirs. The main method of exploration with this technique is to conduct surveys over a number of known commercial accumulations to establish [[productive signatures]]. Then a survey over the prospect may provide useful information that can be integrated with the other exploration techniques to help locate a drillsite.

==Conclusion==

==Conclusion==