Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Exploring for stratigraphic traps

  | chapter = Exploring for stratigraphic traps

  | frompg  = 21-1

  | frompg  = 21-53

  | topg    = 21-68

  | topg    = 21-55

  | author  = John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks

  | author  = John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Mapping unconformities (sequence boundaries) is part of an effective exploration effort. Facies, [[porosity]] systems, and hydrocarbon shows are evaluated in context with an unconformity surface to predict trap location. Traps can be located above (onlap traps) or below (truncation traps) the unconformity. Seals, reservoir-quality rocks, and shows should be evaluated in terms of their relationship to the unconformity (see .<ref name=ch21r12>DolsonMuller, D., S., 1994, Stratigraphic evolution of the Lower Cretaceous Dakota Group, Western Interior, U., S., A., in Caputo, M., V., Peterson, J., A., Franczyk, K., J., eds., Mesozoic Systems of the Rocky Mountain Region, U., S., A.: SEPM Rocky Mountain Section, p. 441–456.</ref>

Mapping [[Unconformity|unconformities]] (sequence boundaries) is part of an effective exploration effort. Facies, [[porosity]] systems, and hydrocarbon shows are evaluated in context with an unconformity surface to predict trap location. Traps can be located above (onlap traps) or below (truncation traps) the unconformity. Seals, reservoir-quality rocks, and shows should be evaluated in terms of their relationship to the unconformity.<ref name=ch21r12>Dolson, J. C., and D. S. Muller, 1994, Stratigraphic evolution of the Lower Cretaceous Dakota Group, Western Interior, USA, in M. V. Caputo, J. A. Peterson, and K. J. Franczyk, eds., Mesozoic Systems of the Rocky Mountain Region, USA.: SEPM Rocky Mountain Section, p. 441–456.</ref>

==Procedure: mapping unconformities==

==Procedure: mapping unconformities==

Follow these suggested steps to map unconformities for prospects.

Follow these suggested steps to map unconformities for prospects.

# Map subcrop and supercrop lithology and formations.

# Make an isopach map from the [[unconformity]] to a flat datum above or below the unconformity to define paleotopography or paleostructure (see [[:file:exploring-for-stratigraphic-traps_fig21-35.png|Figure 1]]).

# Identify the best locations for truncation or onlap traps on the basis of the location seal and reservoir-quality rocks.

| Map subcrop and supercrop lithology and formations.

| Make an isopach map from the unconformity to a flat datum above or below the unconformity to define paleotopography or paleostructure (see Figure 21-35).

| Identify the best locations for truncation or onlap traps on the basis of the location seal and reservoir-quality rocks.

==Making isopachs==

==Making isopachs==

[[file:exploring-for-stratigraphic-traps_fig21-35.png|left|thumb|{{figure number|1}}See text for explanation.]]

[[file:exploring-for-stratigraphic-traps_fig21-35.png|300px|thumb|{{figure number|1}}diagrammatic cross section showing how to isopach above and below an unconformity.]]

The diagrammatic cross section in [[:file:exploring-for-stratigraphic-traps_fig21-35.png|Figure 1]] shows how to isopach above and below an unconformity. For more explanation, see Busch.<ref name=ch21r8>Busch, D., A., 1974, Stratigraphic Traps in Sandstone—Exploration Techniques: [http://store.aapg.org/detail.aspx?id=131 AAPG Memoir 21], 174 p.</ref>

The diagrammatic [[cross section]] in [[:file:exploring-for-stratigraphic-traps_fig21-35.png|Figure 1]] shows how to isopach above and below an [[unconformity]]. For more explanation, see Busch.<ref name=ch21r8>Busch, D. A., 1974, Stratigraphic Traps in Sandstone—Exploration Techniques: [http://store.aapg.org/detail.aspx?id=131 AAPG Memoir 21], 174 p.</ref>

[[file:exploring-for-stratigraphic-traps_fig21-36.png|thumb|{{figure number|2}}.<ref name=ch21r19 /> Copyright: Mountain Geologist.]]

==Subcrop and facies mapping example==

==Subcrop and facies mapping example==

[[file:exploring-for-stratigraphic-traps_fig21-36.png|300px|thumb|{{figure number|2}}Subcrop isopach and formation/lithology maps and a cross section whose location is shown on the isopach map.<ref name=ch21r19 /> Copyright: Mountain Geologist.]]

The Lower Cretaceous Cutbank Sandstone is the largest valley-fill trap in the Rocky Mountains (180 million BOE recoverable).<ref name=ch21r19>Gries, R., Dolson, J., C., Reynolds, R., G., H., 1993, Structural and stratigraphic evolution and hydrocarbon distribution, Rocky Mountain Foreland, in Macqueen, R., W., Leckie, D., A., eds., Foreland Basins and Fold Belts: AAPG Memoir 55, p. 395–425.</ref> The trap (illustrated below) is a combination valley wall and fossil oil-water contact trap. In this example, a flat datum within the Jurassic Sawtooth Formation was picked as the lowermost datum. Where the Sawtooth Formation is absent by onlap, the erosional top of the Mississippian was used, introducing some error on a local scale. “Thicks” are generally paleohills, and “thins” are incised valleys, although the “thick” to the west of the field is also caused by syndepositional westward thickening of the Jurassic section.

 

The Lower Cretaceous Cutbank Sandstone is the largest valley-fill trap in the Rocky Mountains (180 million BOE recoverable).<ref name=ch21r19>Gries, R., J. C. Dolson, and R. G. H. Reynolds, 1993, [http://archives.datapages.com/data/specpubs/basinar3/data/a136/a136/0001/0350/0395.htm Structural and stratigraphic evolution and hydrocarbon distribution, Rocky Mountain Foreland], in R. W. Macqueen, and D. A. Leckie, eds., Foreland Basins and Fold Belts: [http://store.aapg.org/detail.aspx?id=143 AAPG Memoir 55], p. 395–425.</ref> The trap (illustrated below) is a combination valley wall and fossil [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water%20contact oil-water contact] trap. In this example, a flat datum within the Jurassic Sawtooth Formation was picked as the lowermost datum. Where the Sawtooth Formation is absent by onlap, the erosional top of the Mississippian was used, introducing some error on a local scale. “Thicks” are generally paleohills, and “thins” are incised valleys, although the “thick” to the west of the field is also caused by syndepositional westward thickening of the Jurassic section.

Below are subcrop isopach and formation/lithology maps and a cross section whose location is shown on the isopach map ([[:file:exploring-for-stratigraphic-traps_fig21-36.png|Figure 2]]). Dashed and hachured areas are shale; stippled areas are sandstone. The arrows on cross section A-A′ show the subcrop isopach interval. The north-south trending isopach thin and shaded area on the maps shows the location of the giant Cutbank field.

[[:file:exploring-for-stratigraphic-traps_fig21-36.png|Figure 2]] contains subcrop isopach and formation/lithology maps and a cross section whose location is shown on the isopach map. Dashed and hachured areas are shale; stippled areas are sandstone. The arrows on cross section A-A′ show the subcrop isopach interval. The north-south trending isopach thin and shaded area on the maps shows the location of the giant Cutbank field.

==Using subcrop maps to analyze seals==

==Using subcrop maps to analyze seals==

The subcrop lithology and formation map in Figure 21-36 illustrates where regional bottom and lateral seals are located. The trap is found where the porous Cutbank Sandstone abuts impermeable Jurassic shales. Where the Cutbank Sandstone abuts the permeable Swift Sandstone, [[leakage]] occurs updip. Oil migrates into the Cutbank where the valley bevels northward into a migration path at the top of the Mississippian strata.

The subcrop lithology and formation map in [[:file:exploring-for-stratigraphic-traps_fig21-36.png|Figure 2]] illustrates where regional bottom and [[lateral]] seals are located. The trap is found where the porous Cutbank Sandstone abuts impermeable Jurassic shales. Where the Cutbank Sandstone abuts the permeable Swift Sandstone, [[leakage]] occurs updip. Oil migrates into the Cutbank where the valley bevels northward into a migration path at the top of the Mississippian strata.

==Using subcrop maps to predict valley fill==

==Using subcrop maps to predict valley fill==

Subcrop maps also help us predict lithologic content of the supercropping valley networks. In the example above, fine-grained sandstone and shale entered the Cutbank system from the east and south from eroding hills comprised of fine-grained Morrison and Swift strata. Coarse-grained gravels are confined exclusively to the main alluvial-fan system that eroded Paleozoic strata to the west.

Subcrop maps also help us predict lithologic content of the supercropping valley networks. In the example above, fine-grained sandstone and shale entered the Cutbank system from the east and south from eroding hills comprised of fine-grained Morrison and Swift strata. Coarse-grained gravels are confined exclusively to the main [[alluvial]]-fan system that eroded Paleozoic strata to the west.

In the Muddy Formation of the Western Interior of the United States, many shale-filled valleys occur where local tributaries have only subcropping Skull Creek Shale for a provenance. These valleys can form seals for subcropping reservoirs.<ref name=ch21r10>Dolson, J., C., ed., 1991, Unconformity Related Hydrocarbon Exploitation and Accumulation in Clastic and Carbonate Settings: Rocky Mountain Assoc. of Geologists and Exploration Geoscience Institute (Colorado School of Mines) Core Workshop, Golden, Colorado, 297 p.</ref><ref name=ch21r12 />). Within deep basins, where the location of coarse gravels may be a primary reason to retain or create porosity, accurate regional reconstructions of erosional networks and their provenance areas are a must (<ref name=ch21r35>Putnam, P., E., Oliver, T., A., 1980, Stratigraphic traps in channel sandstones in the Upper Mannville (Albian) of east-central Alberta: Canadian Society of Petroleum Geologists Bulletin, vol. 28, p. 489–508.</ref>

In the Muddy Formation of the Western Interior of the United States, many shale-filled valleys occur where local tributaries have only subcropping Skull Creek Shale for a provenance. These valleys can form seals for subcropping reservoirs.<ref name=ch21r10>Dolson, J. C., ed., 1991, Unconformity Related Hydrocarbon Exploitation and Accumulation in Clastic and Carbonate Settings: Rocky Mountain Assoc. of Geologists and Exploration Geoscience Institute (Colorado School of Mines) Core Workshop, Golden, Colorado, 297 p.</ref><ref name=ch21r12 /> Within deep basins, where the location of coarse gravels may be a primary reason to retain or create porosity, accurate regional reconstructions of erosional networks and their provenance areas are a must.<ref name=ch21r35>Putnam, P. E., and T. A. Oliver, 1980, Stratigraphic traps in channel sandstones in the Upper Mannville (Albian) of east-central Alberta: Canadian Society of Petroleum Geologists Bulletin, vol. 28, p. 489–508.</ref>

If the sequences are thick enough to image, chaotic seismic signatures within the valley may also suggest reservoir fill; smooth, parallel reflectors suggest shale fill.

If the sequences are thick enough to image, chaotic seismic signatures within the valley may also suggest reservoir fill; smooth, parallel reflectors suggest shale fill.

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Exploring for stratigraphic traps]]

[[Category:Exploring for stratigraphic traps]]