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Major variations in levels of [[reservoir quality]] and degrees of lateral and vertical continuity within oil and gas fields are controlled primarily by depositional factors. However, major inhomogeneities may also be produced by diagenetic alterations. These inhomogeneities in rock properties may transect or reverse trends produced by depositional controls and can significantly influence reservoir properties, including initial fluid saturations, residual saturations, waterflood sweep efficiencies, preferred directions of flow, and reactions to injected fluids. Extreme [[permeability]] stratification or the development of permeability barriers by diagenetic alteration may lead to the need to drill additional infill wells or reposition the locations of such wells, selectively perforate and inject reservoir units, manage zones on an individual basis, and revise decisions regarding suitability for thermal recovery operations.

Major variations in levels of [[reservoir quality]] and degrees of [[lateral]] and vertical continuity within oil and gas fields are controlled primarily by depositional factors. However, major inhomogeneities may also be produced by diagenetic alterations. These inhomogeneities in rock properties may transect or reverse trends produced by depositional controls and can significantly influence reservoir properties, including initial fluid saturations, residual saturations, waterflood sweep efficiencies, preferred directions of flow, and reactions to injected fluids. Extreme [[permeability]] stratification or the development of permeability barriers by diagenetic alteration may lead to the need to drill additional infill wells or reposition the locations of such wells, selectively perforate and inject reservoir units, manage zones on an individual basis, and revise decisions regarding suitability for thermal recovery operations.

A ''diagenetically complex reservoir'' is a reservoir in which the major inhomogeneities affecting fluid distribution and/or productivity are controlled primarily by diagenetic events. Diagenetic inhomogeneities are zones of reduced or enhanced [[porosity]] and/or permeability that are generated by one or a combination of the processes of cementation, compaction, replacement, dissolution, and fracturing. For a reservoir to be considered complex, the diagenetic inhomogeneities must exhibit a complex distribution that is not directly correlated with or controlled by depositional factors.

A ''diagenetically complex reservoir'' is a reservoir in which the major inhomogeneities affecting fluid distribution and/or productivity are controlled primarily by diagenetic events. Diagenetic inhomogeneities are zones of reduced or enhanced [[porosity]] and/or permeability that are generated by one or a combination of the processes of cementation, compaction, replacement, dissolution, and fracturing. For a reservoir to be considered complex, the diagenetic inhomogeneities must exhibit a complex distribution that is not directly correlated with or controlled by depositional factors.

===Stage 1. Construction of a regional geological framework===

===Stage 1. Construction of a regional geological framework===

To assist in the understanding of the depositional and diagenetic events that have created and modified the reservoir rocks, analysis of the regional geological framework can be very helpful. Elements of the regional geology most useful for this include regional thickness and lithofacies patterns, the plate tectonic history of the area, the local structural history, the history of exploration and production in the area, the burial history including major erosional and/or nondepositional events, and in particular, the thermal and pressure history of the reservoir.

To assist in the understanding of the depositional and diagenetic events that have created and modified the reservoir rocks, analysis of the regional geological framework can be very helpful. Elements of the regional geology most useful for this include regional thickness and [[lithofacies]] patterns, the plate tectonic history of the area, the local structural history, the history of exploration and production in the area, the burial history including major erosional and/or nondepositional events, and in particular, the thermal and pressure history of the reservoir.

===Stage 2. Construction of a depositional model===

===Stage 2. Construction of a depositional model===

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|   Fluorescence microscopy

|   Fluorescence microscopy

| Recognition of depositional and diagenetic components and textures in dolomitized or recrystallized limestones; porosity estimation

| Recognition of depositional and diagenetic components and textures in dolomitized or recrystallized [[limestone]]; porosity estimation

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|   Image analysis

|   Image analysis

[[file:evaluating-diagenetically-complex-reservoirs_fig3.png|thumb|300px|{{figure number|3}}[[Porosity]]-permeability semilog crosspiot with samples coded according to grain size, clay content, and dominant agent of cementation.]]

[[file:evaluating-diagenetically-complex-reservoirs_fig3.png|thumb|300px|{{figure number|3}}[[Porosity]]-permeability semilog crosspiot with samples coded according to grain size, clay content, and dominant agent of cementation.]]

Sample sites for petrographic analysis are best selected on the basis of low magnification rock descriptions generated in step 1 and through examination of semilog porosity-permeability cross plots ([[:file:evaluating-diagenetically-complex-reservoirs_fig3.png|Figure 3]]) with values keyed to major categories of [[Grain size|size]], [[Core_description#Maturity|sorting]], matrix content, cement content, or pore type, depending on their relative importance in a particular reservoir. Samples should be selected to span a wide range of porosities and permeabilities for each major type of reservoir rock (for example, sandstones that are dolomite cemented, anhydrite cemented, quartz-overgrowth cemented, or argillaceous).

Sample sites for petrographic analysis are best selected on the basis of low magnification rock descriptions generated in step 1 and through examination of semilog porosity-permeability cross plots ([[:file:evaluating-diagenetically-complex-reservoirs_fig3.png|Figure 3]]) with values keyed to major categories of [[Grain size|size]], [[Core_description#Maturity|sorting]], matrix content, cement content, or pore type, depending on their relative importance in a particular reservoir. Samples should be selected to span a wide range of porosities and permeabilities for each major type of reservoir rock (for example, sandstones that are dolomite cemented, anhydrite cemented, [[quartz]]-overgrowth cemented, or argillaceous).

Use of plug ends from homogeneous horizontal core analysis plugs for thin section, XRD, or SEM sample preparation allows for the development of quantitative relationships between data from these analyses and data from core analysis measurements. Plugs containing significant inhomogeneities, such as laminae of distinctly different grain size or degrees of cementation, should be avoided or else erroneous variance in the data set will tend to blur what otherwise might be easily recognizable clear-cut relationships.

Use of plug ends from homogeneous horizontal core analysis plugs for thin section, XRD, or SEM sample preparation allows for the development of quantitative relationships between data from these analyses and data from core analysis measurements. Plugs containing significant inhomogeneities, such as laminae of distinctly different grain size or degrees of cementation, should be avoided or else erroneous variance in the data set will tend to blur what otherwise might be easily recognizable clear-cut relationships.

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| Dissolution occurs at crest of anticline or at updip pinchout of a reservoir unit

| Dissolution occurs at crest of anticline or at updip pinchout of a reservoir unit

| CO₂ and/or organic acids generated during thermal maturation of organics seek structural or stratigraphic highs and generate acidic conditions

| CO₂ and/or organic acids generated during [[thermal maturation]] of organics seek structural or stratigraphic highs and generate acidic conditions

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| Increased cementation occurs below oil-water or gas-water contacts

| Increased cementation occurs below oil-water or gas-water contacts

===Stage 7. Model testing and revision===

===Stage 7. Model testing and revision===

Where economics dictate, it may be necessary to test the accuracy of the models developed. This can include testing by history matching of pressures, production rates, and GOR values for segments of the model or full scale testing of the complete model<ref name=pt06r155>Weber, K. J., P. H. Klootwijk, J. Knoieczek, and W. R. van der Vlugt, 1978, Simulation of water injection in a barrier-bar- type, oil-rim reservoir in Nigeria: Journal of Petroleum Technology, v. 30, p. 1555–1565, DOI: [https://www.onepetro.org/journal-paper/SPE-6702-PA 10.2118/6702-PA].</ref> (see [[Product histories]] and [[Conducting a reservoir simulation study: an overview]]). Testing can also involve drilling additional wells, conducting special engineering tests (pulse or tracer), and collecting geological data on additional samples. Revisions may also be required as additional wells, particularly infill wells, are drilled in the field.

Where [[economics]] dictate, it may be necessary to test the accuracy of the models developed. This can include testing by history matching of pressures, production rates, and GOR values for segments of the model or full scale testing of the complete model<ref name=pt06r155>Weber, K. J., P. H. Klootwijk, J. Knoieczek, and W. R. van der Vlugt, 1978, Simulation of water injection in a barrier-bar- type, oil-rim reservoir in Nigeria: Journal of Petroleum Technology, v. 30, p. 1555–1565, DOI: [https://www.onepetro.org/journal-paper/SPE-6702-PA 10.2118/6702-PA].</ref> (see [[Product histories]] and [[Conducting a reservoir simulation study: an overview]]). Testing can also involve drilling additional wells, conducting special engineering tests (pulse or tracer), and collecting geological data on additional samples. Revisions may also be required as additional wells, particularly infill wells, are drilled in the field.

==See also==

==See also==

[[Category:Geological methods]]

[[Category:Geological methods]]