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  | 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

==Critical elements of the Niobrara play==

==Critical elements of the Niobrara play==

In the fractured Upper Cretaceous Niobrara play of the central Rocky Mountain region, specific [[Using analogs to identify critical elements|critical elements]] must be present for a prospect to be successful. Since the Niobrara Formation is its own oil [[source rock]], there is no [[Drive mechanisms and recovery#Water drive|bottom water]]; [[Drive mechanisms and recovery#Gas cap drive|gas expansion]] along with [[Drive mechanisms and recovery#Gravity drainage|gravity drainage]] provide the main reservoir energy, so the structural position of a well is not a critical factor. [[Fold trap regime#Classes and definitions|Synclines, anticlines]], and any structural location in between will work. The main critical element is to find a sufficiently fractured sweet spot in which the fractures remain open during production. Careful study of numerous excellent Niobrara producing fields shows that the following critical elements must be present to achieve commercial success.

In the [[Fracture trap regime|fracture]]d Upper Cretaceous Niobrara play of the central Rocky Mountain region, specific [[Using analogs to identify critical elements|critical elements]] must be present for a prospect to be successful. Since the Niobrara Formation is its own oil [[source rock]], there is no [[Drive mechanisms and recovery#Water drive|bottom water]]; [[Drive mechanisms and recovery#Gas cap drive|gas expansion]] along with [[Drive mechanisms and recovery#Gravity drainage|gravity drainage]] provide the main reservoir energy, so the structural position of a well is not a critical factor. [[Fold trap regime#Classes and definitions|Synclines, anticlines]], and any structural location in between will work. The main critical element is to find a sufficiently fractured sweet spot in which the fractures remain open during production. Careful study of numerous excellent Niobrara producing fields shows that the following critical elements must be present to achieve commercial success.

* Maximum bed curvature

* Maximum bed curvature

* Normal fault cutting through area of maximum bed curvature

* [[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 open hole (hang a 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 underbalanced mud or air to prevent fracture damage. The Niobrara is an underpressured reservoir with petrostatic (0.33 lb/ft gradient) rather than 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.

==Applying the critical factors==

==Applying the critical factors==

==Niobrara structure==

==Niobrara structure==

[[file:developing-a-philosophy-of-exploration_fig1-2.png|thumb|{{figure number|1-2}}See text for explanation.]]

[[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.

==Niobrara maximum bed curvature==

==Niobrara maximum bed curvature==

Seismic surveys are not particularly helpful in mapping normal faults because they are 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

[[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 [[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.

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 attenuation of [[seismic data]] in fracture zones.

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

Below is a Niobrara structure cross section.

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|thumb|{{figure number|1-3}}See text for explanation.]]

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

==Niobrara open fractures==

==Niobrara open fractures==

Running samples on Niobrara wells will reveal whether open calcite crystals are present in the fractures. In addition, 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==

Surface geochemical methods—specifically, 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==

Critical elements of the Niobrara play were identified by studying known accumulations. Knowing what elements were critical allowed a focused effort that saved time and improved effectiveness. Study known examples of trap types of interest to discover critical elements, and the result will be a more effective program.

[[Using analogs to identify critical elements|Critical elements]] of the Niobrara play were identified by studying known accumulations. Knowing what elements were critical allowed a focused effort that saved time and improved effectiveness. Study known examples of trap types of interest to discover critical elements, and the result will be a more effective program.

==See also==

==See also==

[[Category:Developing a philosophy of exploration]]

[[Category:Developing a philosophy of exploration]]