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  = 21-62

  | topg    = 21-68

  | topg    = 21-63

  | 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

}}

}}

It is important early on in a play if diagenetic changes play a major role in trap modification. Test, production, geochemical, and show data provide the best criteria to recognize diagenetic modifications.

It is important early on in a play to consider if [[Diagenesis|diagenetic]] changes play a major role in [[trap]] modification. [[Production testing|Test]], production, geochemical, and [[Mudlogging:_drill_cuttings_analysis#Basic_sample_analysis|show]] data provide the best criteria to recognize diagenetic modifications.

==General geologic indications==

==General geologic indications==

* Geologic setting conducive to development of paleotopographic relief

* Geologic setting conducive to development of paleotopographic relief

* Unconformities present

* [[Unconformity|Unconformities]] present

* No relationship between present-day depth and hydrocarbon production for a given zone

* No relationship between present-day depth and hydrocarbon production for a given zone

* Hydrocarbon accumulations in the absence of structural closure and not following closures created by primary facies changes

* Hydrocarbon [[accumulation]]s in the absence of structural [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=closure closure] and not following closures created by primary facies changes

==Petrographic indicators==

==Petrographic indicators==

Following are petrographic indicators of the potential presence of diagenetic traps:

Following are petrographic indicators of the potential presence of diagenetic traps:

* Early, pervasive, prehydrocarbon cements present updip from known production within a given facies (updip seal vs. downdip reservoir)

* Early, pervasive, prehydrocarbon cements present [[Dip|updip]] from known production within a given facies (updip [[seal]] vs. downdip [[reservoir]])

* Late, deep-burial cements or fabrics present, such as saddle or nonplanar dolomite, ferroan poikilotopic calcite, and anhydrite

* Late, deep-burial cements or fabrics present, such as [http://www.minersoc.org/pages/Archive-MM/Volume_53/53-373-547.pdf saddle]] or nonplanar dolomite, ferroan [http://encyclopedia2.thefreedictionary.com/poikilotopic poikilotopic] calcite, and [[anhydrite]]

* Abrupt vertical change in the amount of deep-burial cements present but not coincident with a change in depositional fabric (paleo oil–water contacts)

* Abrupt vertical change in the amount of deep-burial cements present but not coincident with a change in depositional fabric ([[Fluid contacts|paleo oil–water contacts]])

* Secondary [[porosity]] present but not related to subaerial exposure (subsurface deep burial dissolution)

* Secondary [[porosity]] present but not related to subaerial exposure (subsurface deep burial dissolution)

* Zones of secondary porosity interbedded with tight rock in a depositionally homogeneous facies

* Zones of secondary porosity interbedded with tight rock in a depositionally homogeneous facies

* Field boundaries within a given formation not coincident with structural closure or facies boundaries

* Field boundaries within a given formation not coincident with structural closure or facies boundaries

* Tilted oil–water contacts present

* Tilted oil–water contacts present

* Adjacent structures not in pressure communication

* Adjacent structures not in [[Pressure compartments|pressure communication]]

* Pressures in oil-charged reservoirs unusually high

* Pressures in oil-charged reservoirs unusually high

* Most of the wells characterized by high initial potential followed by rapid, sharp decline in flow rates; water cut typically low

* Most of the wells characterized by high initial potential followed by rapid, sharp decline in flow rates; water cut typically low

==Example: diagenetic trap==

==Example: diagenetic trap==

Weyburn field, Alberta, is an example of a giant diagenetic trap (1 billion BOE). Primary trap geometries appear to be along the updip termination of the Midale dolomite above the potential bottom seal of the Frobisher anhydrite and beneath the top seal of the Mesozoic section. If only these trapping geometries were used to locate the trap, sequence stratigraphic mapping initially would not have located the trap. However, microporous dolomites are present near the Mesozoic sequence boundary in the updip portion of the Midale dolomite. These Mesoporous dolomites downdip form the reservoir facies.

Weyburn field, Alberta, is an example of a giant diagenetic trap (1 billion BOE). Primary trap geometries appear to be along the [[Dip|updip]] termination of the Midale [[dolomite]] above the potential bottom seal of the Frobisher [[anhydrite]] and beneath the top seal of the Mesozoic section. If only these trapping geometries were used to locate the trap, [[Sequence stratigraphy|sequence stratigraphic]] mapping initially would not have located the trap. However, [[Wikipedia:Microporous material|microporous]] dolomites are present near the Mesozoic [[Definitions_of_depositional_system_elements#Sequence_boundaries|sequence boundary]] in the updip portion of the Midale dolomite. These [[Wikipedia:Mesoporous material|mesoporous]] dolomites downdip form the reservoir facies.

The microporous strata form the lateral seal. The sinuous updip edge is a large waste zone that contains live oil shows in microporous strata, indicating the accumulation is downdip.

The microporous strata form the [[lateral]] seal. The sinuous updip edge is a large waste zone that contains live oil shows in microporous strata, indicating the [[accumulation]] is downdip.

[[:file:exploring-for-stratigraphic-traps_fig21-41.png|Figure 1]] contains a [[cross section]], map, and summary of the field.

[[:file:exploring-for-stratigraphic-traps_fig21-41.png|Figure 1]] contains a [[cross section]], map, and summary of the field.

==See also==

==See also==

 

* [[Types of diagenetic traps]]

* [[Types of diagenetic traps]]

* [[Petrological information about diagenesis]]

* [[Diagenesis: information from petrology]]

==External links==

==External links==

[[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]]