Changes

  | part    =  

  | part    =  

  | chapter = Developing a philosophy of exploration

  | chapter = Developing a philosophy of exploration

  | frompg  = 1-1

  | frompg  = 1-34

  | topg    = 1-37

  | topg    = 1-36

  | author  = Edward A. Beaumont, Norman H. Foster, Richard R. Vincelette, Marlan W. Downey, James D. Robertson

  | author  = Edward A. Beaumont, Norman H. Foster, Richard R. Vincelette, Marlan W. Downey, James D. Robertson

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch01/ch01.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch01/ch01.htm

* [[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 fracture]]s

* 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 [[Geostatic and lithostatic pressure|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 [[Geostatic and lithostatic pressure|petrostatic]] (0.33 lb/ft gradient) rather than [[Normal hydrostatic pressure|hydrostatic]] (0.43 lb/ft gradient) pressure.

==Niobrara structure==

==Niobrara structure==

[[file:developing-a-philosophy-of-exploration_fig1-2.png|thumb|{{figure number|1}}An example Niobrara structure map.]]

[[file:developing-a-philosophy-of-exploration_fig1-2.png|thumb|300px|{{figure number|1}}An example Niobrara structure map.]]

In the Niobrara play, a subsurface [[Subsurface maps#Structure|structure map]] on top of the Niobrara formation is a first step to locate areas of maximum bed curvature. [[Normal fault]]s should then be mapped from subsurface well control, [[Remote sensing|photogeology]], and good old-fashioned [[The seven critical elements of fieldwork|field work]]. Cross-linears may be mapped from photogeology and [[Remote sensing|satellite imagery]]. [[:file:developing-a-philosophy-of-exploration_fig1-2.png|Figure 1]] is an example Niobrara structure map.

In the Niobrara play, a subsurface [[Subsurface maps#Structure|structure map]] on top of the Niobrara formation is a first step to locate areas of maximum bed curvature. [[Normal fault]]s should then be mapped from subsurface well control, [[Remote sensing|photogeology]], and good old-fashioned [[The seven critical elements of fieldwork|field work]]. Cross-linears may be mapped from photogeology and [[Remote sensing|satellite imagery]]. [[:file:developing-a-philosophy-of-exploration_fig1-2.png|Figure 1]] is an example Niobrara structure map.

==Niobrara maximum bed curvature==

==Niobrara maximum bed curvature==

[[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|300px|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 [[Brittleness|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

==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 and surficial geochemistry|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 signature]]s. 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==

[[Category:Value of geological fieldwork‏‎]]

[[Category:Value of geological fieldwork‏‎]]

[[Category:Traps, trap types, and the petroleum system]]

[[Category:Traps, trap types, and the petroleum system]]