Changes

===Unconformities and subcrops===

===Unconformities and subcrops===

[[Mapping unconformities|Surfaces of unconformity]] can be especially useful [[Marker|marker horizons]] for structure contour mapping ([[:file:subsurface-maps_fig2.png|Figure 2]]). In many fields, unconformities are the location of sealing [[shale]]s and/or source rocks above reservoir pay. [[Subcrop map]]s, traces of productive zones, barriers, or marker horizons mapped on the unconformity surface are invaluable for planning well placement and for [[Field development|reservoir development]].

[[Mapping unconformities|Surfaces of unconformity]] can be especially useful [[Marker|marker horizons]] for structure contour mapping ([[:file:subsurface-maps_fig2.png|Figure 2]]). In many fields, [[Unconformity|unconformities]] are the location of sealing [[shale]]s and/or source rocks above reservoir pay. [[Subcrop map]]s, traces of productive zones, barriers, or marker horizons mapped on the unconformity surface are invaluable for planning well placement and for [[Field development|reservoir development]].

===Pressure===

===Pressure===

===Isopach===

===Isopach===

A contour map of equal values of true stratigraphic thickness is an ''isopach map'' ([[:file:subsurface-maps_fig4.png|Figure 4]]). Except for vertical wells in horizontal beds, corrections for [[Wellbore trajectory|wellbore deviation]] and formation [[dip]] are needed to make isopach maps.

A [[contour]] map of equal values of true stratigraphic thickness is an ''isopach map'' ([[:file:subsurface-maps_fig4.png|Figure 4]]). Except for vertical wells in horizontal beds, corrections for [[Wellbore trajectory|wellbore deviation]] and formation [[dip]] are needed to make isopach maps.

===Isochore===

===Isochore===

===Isochron===

===Isochron===

An ''[[Mapping with two-dimensional seismic data#Time interval maps|isochron map]]'' is a contour map of equal values of seismic traveltime between selected events.<ref name=pt06r141 /> Isochron maps are the seismic analog of isochore maps and, as such, are intended to derive thickness information from seismic data. Isochroning between events above and below a pay horizon, for example, would estimate pay thickness. Renick and Gunn<ref name=pt06r109>Renick, H. Jr., Gunn, R. D., 1989, Triangle Ranch Headquarters field development using shallow core holes and high-resolution seismic data: Geophysics, v. 54, n. 11, p. 1384–1396, DOI: [http://library.seg.org/doi/abs/10.1190/1.1442602 10.1190/1.1442602].</ref> present a good case history of using isochron and time-structure maps to generate “isopach” and elevation-structure maps. Their isochron-isopach approach delineated reef trends for further development drilling and used well penetrations through a shallow horizon for depth control on a deeper horizon. Phipps<ref name=pt06r99>Phipps, G. G., 1989, Exploring for dolomitized Slave Point carbonates in northeastern British Columbia: Geophysics, v. 54, n. 7, p. 806–814, DOI: [http://library.seg.org/doi/abs/10.1190/1.1442709 10.1190/1.1442709].</ref> documents the pros and cons of using isochron thins and structural highs as exploration drilling criteria for dolomitized Devonian limestones.

An ''[[Mapping with two-dimensional seismic data#Time interval maps|isochron map]]'' is a contour map of equal values of seismic traveltime between selected events.<ref name=pt06r141 /> [[Isochron]] maps are the seismic analog of isochore maps and, as such, are intended to derive thickness information from seismic data. Isochroning between events above and below a pay horizon, for example, would estimate pay thickness. Renick and Gunn<ref name=pt06r109>Renick, H. Jr., Gunn, R. D., 1989, Triangle Ranch Headquarters field development using shallow core holes and high-resolution seismic data: Geophysics, v. 54, n. 11, p. 1384–1396, DOI: [http://library.seg.org/doi/abs/10.1190/1.1442602 10.1190/1.1442602].</ref> present a good case history of using isochron and time-structure maps to generate “isopach” and elevation-structure maps. Their isochron-isopach approach delineated reef trends for further development drilling and used well penetrations through a shallow horizon for depth control on a deeper horizon. Phipps<ref name=pt06r99>Phipps, G. G., 1989, Exploring for dolomitized Slave Point carbonates in northeastern British Columbia: Geophysics, v. 54, n. 7, p. 806–814, DOI: [http://library.seg.org/doi/abs/10.1190/1.1442709 10.1190/1.1442709].</ref> documents the pros and cons of using isochron thins and structural highs as exploration drilling criteria for dolomitized Devonian limestones.

==Mapping to calculate reserves==

==Mapping to calculate reserves==

The product ''A'' × ''H'' is the reservoir bulk volume, and the product ''A'' × ''H'' × ϕ is the reservoir pore volume. The general determination of bulk reservoir volume involves mapping reservoir area in plan view and mapping net pay in terms of true vertical thickness to provide a common presentation of dipping beds or deviated wells. An [[Subsurface maps#Isochore|isochore]] map of net pay should be contoured using well control points and interpolated or extrapolated using available seismic and well test data and the geologist's interpretation of [[Depositional environments|depositional]] and [[Diagenesis|diagenetic]] history.

The product ''A'' × ''H'' is the reservoir bulk volume, and the product ''A'' × ''H'' × ϕ is the reservoir pore volume. The general determination of bulk reservoir volume involves mapping reservoir area in plan view and mapping net pay in terms of true vertical thickness to provide a common presentation of dipping beds or deviated wells. An [[Subsurface maps#Isochore|isochore]] map of net pay should be contoured using well control points and interpolated or extrapolated using available seismic and well test data and the geologist's interpretation of [[Depositional environments|depositional]] and [[Diagenesis|diagenetic]] history.

“Net” pay (see [[Effective pay determination]]) implies that some formation thickness has been excluded from consideration by either (1) occurring below an [[Fluid contacts|oil-water contact]] (or above a gas-water contact), or (2) having porosity and/or [[permeability]] values below a “cutoff” limit for productivity. Not all net pay is necessarily productive at a given well spacing. Discontinuous productive horizons between wells might be described, for example, by the concept of net pay to net connected pay ratio.<ref name=pt06r103>Poston, S. W., 1987, [https://www.onepetro.org/book/peh/spe-1987-36-peh Development plan for oil and gas reservoirs], in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 36-1–36-11.</ref>

“Net” pay (see [[Effective pay determination]]) implies that some formation thickness has been excluded from consideration by either (1) occurring below an [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water+contact oil-water contact] (or above a [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-water+contact gas-water contact]), or (2) having porosity and/or [[permeability]] values below a “cutoff” limit for productivity. Not all net pay is necessarily productive at a given well spacing. Discontinuous productive horizons between wells might be described, for example, by the concept of net pay to net connected pay ratio.<ref name=pt06r103>Poston, S. W., 1987, [https://www.onepetro.org/book/peh/spe-1987-36-peh Development plan for oil and gas reservoirs], in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 36-1–36-11.</ref>

===Porosity===

===Porosity===

===Permeability===

===Permeability===

[[Permeability]] (''k'') can also be mapped and contoured (see [[Core-log transformations and porosity-permeability relationships]]). As for [[Water saturation|saturation]] values, some care must be exercised in mapping permeability because values must be derived from indirect measurements. Typically, permeabilities are derived from wireline log [[Porosity|porosities]] transformed on the basis of core permeability versus porosity cross plots. Permeabilities can be reported at ambient laboratory conditions of pressure or adjusted to reservoir conditions of [[confining pressure]]. Similarly, permeabilities can be [[Absolute permeability|absolute permeabilities]] to air (nitrogen) or liquid or [[Effective permeability|effective permeabilities]] to oil in the presence of [[irreducible water]]. Permeability values in an individual well are thickness weighted and typically [[Averaging|averaged harmonically, arithmetically, or geometrically]], depending on [[flow geometry]]. Alternatively, [[flow capacity]] (''kH'') values derived from [[pressure transient testing]] can be divided by net pay thickness (''H'') to yield a liquid permeability value for a well.

[[Permeability]] (''k'') can also be mapped and contoured (see [[Core-log transformations and porosity-permeability relationships]]). As for [[Water saturation|saturation]] values, some care must be exercised in mapping permeability because values must be derived from indirect measurements. Typically, permeabilities are derived from wireline log [[Porosity|porosities]] transformed on the basis of core permeability versus porosity cross plots. Permeabilities can be reported at ambient laboratory conditions of pressure or adjusted to reservoir conditions of [[confining pressure]]. Similarly, permeabilities can be absolute permeabilities to air (nitrogen) or liquid or effective permeabilities]] to oil in the presence of [http://petrowiki.org/Glossary%3AIrreducible_water_saturation irreducible water]. Permeability values in an individual well are thickness weighted and typically [[Averaging|averaged harmonically, arithmetically, or geometrically]], depending on [[flow geometry]]. Alternatively, [[flow capacity]] (''kH'') values derived from [[pressure transient testing]] can be divided by net pay thickness (''H'') to yield a liquid permeability value for a well.

===Porosity thickness===

===Porosity thickness===

==Other maps==

==Other maps==

A variety of other maps can come into play during the development of a specific reservoir. Maps of facies, facies architecture(for more information, see [[Lithofacies and environmental analysis of clastic depositional systems#Clastic depositional lithofacies and environments|Clastic lithofacies]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization#Carbonate sediments and environments|Carbonate lithofacies]]), paleoenvironment, and isolithology might be particularly important in selecting [[stepout well location]]s and planning reservoir development strategy. Other reservoir properties such as [[Reservoir temperature|temperature]] can have value for specific reservoir engineering applications, particularly where potentially temperature-sensitive chemical [[stimulation]], production, or recovery technology might be involved.

A variety of other maps can come into play during the development of a specific reservoir. Maps of facies, facies architecture for more information, see [[Lithofacies and environmental analysis of clastic depositional systems]] and [[Carbonate reservoir models: facies, diagenesis, and flow characterization]]), paleoenvironment, and isolithology might be particularly important in selecting [[stepout well location]]s and planning reservoir development strategy. Other reservoir properties such as [[Wikipedia:Temperature|temperature]] can have value for specific reservoir engineering applications, particularly where potentially temperature-sensitive chemical [[stimulation]], production, or recovery technology might be involved.

 

 

==See also==

==See also==

[[Category:Geological methods]]

[[Category:Geological methods]]