Changes

A number of surfaces are typically mapped during [[Field development|reservoir development]] to show closure and other limits to reservoir production. Maps of top of pay and bottom of pay can also be “subtracted” to determine pay thickness.

A number of surfaces are typically mapped during [[Field development|reservoir development]] to show closure and other limits to reservoir production. Maps of top of pay and bottom of pay can also be “subtracted” to determine pay thickness.

[[file:subsurface-maps_fig1.png|left|thumb|{{figure number|1}}Structure map of the top of the T5 marker, Frlo Formation, Brazoria County, Texas. (After <ref name=pt06r12>Bebout, D. G., Loucks, R. G., Gregory, A. R., 1978, Frio Sandstone reservoirs in the deep subsurface along the Texas Gulf coast—their potential for production of geopressured geothermal energy: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 91, 93 p.</ref>.)]]

[[file:subsurface-maps_fig1.png|left|thumb|{{figure number|1}}Structure map of the top of the T5 marker, Frlo Formation, Brazoria County, Texas. (After Bebout el al.)<ref name=pt06r12>Bebout, D. G., Loucks, R. G., Gregory, A. R., 1978, Frio Sandstone reservoirs in the deep subsurface along the Texas Gulf coast—their potential for production of geopressured geothermal energy: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 91, 93 p.</ref>]]

===Structure===

===Structure===

===Fault planes===

===Fault planes===

[[Faults]] are special surfaces whose traces will show on structure contour maps ([[:file:subsurface-maps_fig1.png|Figures 1]] and [[:file:subsurface-maps_fig2.png|2]]). Faults form bounding surfaces for some reservoirs, and sufficient well control might exist to contour map the fault surface itself. Projections of subsurface data into the plane of the fault are also useful “maps” for reservoir development, but are more appropriately described as [[Geological cross sections|cross sections]]. (For details of construction of fault plane maps, see the chapter on “Conversion of Well Log Data to Subsurface Stratigraphic and Structural Information”.)

[[Faults]] are special surfaces whose traces will show on structure contour maps ([[:file:subsurface-maps_fig1.png|Figures 1]] and [[:file:subsurface-maps_fig2.png|2]]). Faults form bounding surfaces for some reservoirs, and sufficient well control might exist to contour map the fault surface itself. Projections of subsurface data into the plane of the fault are also useful “maps” for reservoir development, but are more appropriately described as [[Geological cross sections|cross sections]]. (For details of construction of fault plane maps, see [[Conversion of well log data to subsurface stratigraphic and structural information]].)

[[file:subsurface-maps_fig3.png|thumb|left|{{figure number|3}}Map of pressure response to pattern flood, Judy Creek field, western Canada, 1974 and 1975. Contour interval is 2750 kPa. (After <ref name=pt06r58>Jardine, D., Wilshart, J. W., 1987, Carbonate reservoir description, in Tillman, R. W., Weber, K. J., eds. Reservoir Sedimentology: SEPM Special Publication 40, p. 129–152.</ref>.)]]

[[file:subsurface-maps_fig3.png|thumb|left|{{figure number|3}}Map of pressure response to pattern flood, Judy Creek field, western Canada, 1974 and 1975. Contour interval is 2750 kPa. (After Jardine et al.)<ref name=pt06r58>Jardine, D., Wilshart, J. W., 1987, Carbonate reservoir description, in Tillman, R. W., Weber, K. J., eds. Reservoir Sedimentology: SEPM Special Publication 40, p. 129–152.</ref>]]

===Unconformities and subcrops===

===Unconformities and subcrops===

==Mapping thicknesses==

==Mapping thicknesses==

[[file:subsurface-maps_fig4.png|thumb|{{figure number|4}}(a) Cross section and (b) net pay Isopach map of the Strachan gas field, western Canada. Contour Interval is 100 ft. (From <ref name=pt06r55>Hriskevich, M. E., Faber, J. M., Langton, J. R., 1980, [http://archives.datapages.com/data/specpubs/fieldst2/data/a012/a012/0001/0300/0315.htm Strachan and Ricinus West gas fields], Alberta, Canada, in Halbouty, M. T., ed., Giant Oil and Gas Fields of the Decade 1968–1978: AAPG Memoir 30, p. 315–327.</ref>.)]]

[[file:subsurface-maps_fig4.png|thumb|{{figure number|4}}(a) Cross section and (b) net pay Isopach map of the Strachan gas field, western Canada. Contour Interval is 100 ft. (From Hriskevich et al.)<ref name=pt06r55>Hriskevich, M. E., Faber, J. M., Langton, J. R., 1980, [http://archives.datapages.com/data/specpubs/fieldst2/data/a012/a012/0001/0300/0315.htm Strachan and Ricinus West gas fields], Alberta, Canada, in Halbouty, M. T., ed., Giant Oil and Gas Fields of the Decade 1968–1978: AAPG Memoir 30, p. 315–327.</ref>]]

Interpretations of [[Depositional environments|depositional trends]], [[Predepositional structure|pre-]] and [[Syndepositional structure|syndepositional]] structural development, and reservoir [[storage capacity]] are based in large part on thickness information. An accurate meaning of thickness is critical in these and other analyses (see [[Conversion of well log data to subsurface stratigraphic and structural information]]).

Interpretations of [[Depositional environments|depositional trends]], [[Predepositional structure|pre-]] and [[Syndepositional structure|syndepositional]] structural development, and reservoir [[storage capacity]] are based in large part on thickness information. An accurate meaning of thickness is critical in these and other analyses (see [[Conversion of well log data to subsurface stratigraphic and structural information]]).

===Isochore===

===Isochore===

A contour map of equal values of true vertical thickness is an ''isochore map.''<ref name=pt06r141>Tucker, P. M., 1988, Seismic contouring—a unique skill: Geophysics, v. 53, n. 6, p. 741–749, DOI: 10.1190/1.1442509.</ref> Note that in common practice, isochore maps are informally referred to as “isopach” maps, a term that properly should be restricted to true stratigraphic thickness.

A contour map of equal values of true vertical thickness is an ''isochore map.''<ref name=pt06r141>Tucker, P. M., 1988, Seismic contouring—a unique skill: Geophysics, v. 53, n. 6, p. 741–749, DOI: [http://library.seg.org/doi/abs/10.1190/1.1442509%20 10.1190/1.1442509].</ref> Note that in common practice, isochore maps are informally referred to as “isopach” maps, a term that properly should be restricted to true stratigraphic thickness.

===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: 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: 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, 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 [[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>

[[file:subsurface-maps_fig5.png|thumb|left|{{figure number|5}}Porosity-weighted average water saturation map for Layer 2 of a Middle Eastern carbonate reservoir.]]

[[file:subsurface-maps_fig5.png|thumb|left|{{figure number|5}}Porosity-weighted average water saturation map for Layer 2 of a Middle Eastern carbonate reservoir.]]

A variety of maps are used to predict or monitor reservoir performance.

A variety of maps are used to predict or monitor reservoir performance.

[[file:subsurface-maps_fig6.png|thumb|{{figure number|6}}Porosity thickness (ϕH) maps for the B and C zones from the San Andres Formation reservoir, Jordan field, Ector and Crane Counties, Texas. Contours in PV fraction-feet. (After <ref name=pt06r81>Major, R. P., Holtz, M. H., 1989, Effects of geologic heterogeneity on waterflood efficiency at Jordan field, University Lands, Ector and Crane counties, Texas: 64th Annual Technology Conference of the Society of Petroleum Engineers, San Antonio, TX, Oct., SPE 19874, p. 633–640.</ref>.)]]

[[file:subsurface-maps_fig6.png|thumb|{{figure number|6}}Porosity thickness (ϕH) maps for the B and C zones from the San Andres Formation reservoir, Jordan field, Ector and Crane Counties, Texas. Contours in PV fraction-feet. (After Major et al.)<ref name=pt06r81>Major, R. P., Holtz, M. H., 1989, [https://www.onepetro.org/journal-paper/SPE-19874-PA Depositionally and Diagenetically Controlled Reservoir Heterogeneity at Jordan Field]: 64th Annual Technology Conference of the Society of Petroleum Engineers, San Antonio, TX, Oct., SPE 19874, p. 633–640.</ref>]]

===Permeability===

===Permeability===

===Productivity index===

===Productivity index===

To avoid [[Production problems#Water-gas coning|coning]], sand production, pipe collapse, or other harmful effects, wells might not be produced at their [[Absolute open flow|maximum wide-open flow]] rates. Therefore, the ability of a well to produce is usually determined by a [[Production testing#Single-point tests|productivity index]] (PI).<ref name=pt06r66>Kimmel, J. D., Dalati, R. N., 1987, Potential tests of oil wells, in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 32-1–32-16.</ref> The PI is a measure of the stock tank barrels (STB) of oil produced per day per psi drawdown under [[Steady state conditions|steady state]] or [[Pseudosteady state conditions|pseudosteady state flow conditions]]. Changes will show on periodic maps of PI during reservoir life indicating trends in reservoir depletion or formation damage.

To avoid [[Production problems#Water-gas coning|coning]], sand production, pipe collapse, or other harmful effects, wells might not be produced at their [[Absolute open flow|maximum wide-open flow]] rates. Therefore, the ability of a well to produce is usually determined by a [[Production testing#Single-point tests|productivity index]] (PI).<ref name=pt06r66>Kimmel, J. D., Dalati, R. N., 1987, [https://www.onepetro.org/book/peh/spe-1987-32-peh Potential tests of oil wells], in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 32-1–32-16.</ref> The PI is a measure of the stock tank barrels (STB) of oil produced per day per psi drawdown under [[Steady state conditions|steady state]] or [[Pseudosteady state conditions|pseudosteady state flow conditions]]. Changes will show on periodic maps of PI during reservoir life indicating trends in reservoir depletion or formation damage.

===Solution gas to oil ratio===

===Solution gas to oil ratio===

Engineers forecast [[Reserves estimation|ultimate recoverable reserves]] by applying material balance equations or [[Production histories#Decline curve analysis|decline curve analysis]] to production history records. For example, in a [[Drive mechanisms and recovery#Solution gas drive|depletion]]-type reservoir, the solution gas to oil ratio is sometimes plotted versus cumulative oil production on semilog paper.<ref name=pt06r37>Garb, F. A., Smith, G. L., 1987, Estimation of oil and gas reserves, in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 4-1–40-38.</ref> If such a curve shows a good straight-line relationship, the curve can be used to predict the trend of a cumulative gas or cumulative oil plot to estimate ultimate recovery.

Engineers forecast [[Reserves estimation|ultimate recoverable reserves]] by applying material balance equations or [[Production histories#Decline curve analysis|decline curve analysis]] to production history records. For example, in a [[Drive mechanisms and recovery#Solution gas drive|depletion]]-type reservoir, the solution gas to oil ratio is sometimes plotted versus cumulative oil production on semilog paper.<ref name=pt06r37>Garb, F. A., Smith, G. L., 1987, [https://www.onepetro.org/book/peh/spe-1987-40-peh Estimation of oil and gas reserves], in Bradley, H. B., ed., Petroleum Engineering Handbook: Richardson, TX, Society of Petroleum Engineers, p. 4-1–40-38.</ref> If such a curve shows a good straight-line relationship, the curve can be used to predict the trend of a cumulative gas or cumulative oil plot to estimate ultimate recovery.

The solution gas to oil ratio (GOR) is the amount of dissolved gas that will evolve from the oil as the pressure is reduced to atmospheric from some higher pressure. GOR is usually expressed in units of SCF gas/STB oil. A barrel of oil and its solution gas at reservoir conditions of temperature and pressure will usually “shrink” as the fluid is produced and brought to stock tank conditions (normally reported at [[temperature::60&deg;F]] and 14.7 psia). As GOR changes during reservoir life, GORs for individual wells can be mapped periodically to identify areas of the reservoir receiving or not receiving pressure support and serving as indicators for reservoir management action.

The solution gas to oil ratio (GOR) is the amount of dissolved gas that will evolve from the oil as the pressure is reduced to atmospheric from some higher pressure. GOR is usually expressed in units of SCF gas/STB oil. A barrel of oil and its solution gas at reservoir conditions of temperature and pressure will usually “shrink” as the fluid is produced and brought to stock tank conditions (normally reported at [[temperature::60&deg;F]] and 14.7 psia). As GOR changes during reservoir life, GORs for individual wells can be mapped periodically to identify areas of the reservoir receiving or not receiving pressure support and serving as indicators for reservoir management action.

[[file:subsurface-maps_fig7.png|thumb|left|{{figure number|7}}Cumulative oil production map for the A, B, C, and D zones from the San Andres Formation reservoir, Jordan field, Ector and Crane counties, Texas. Contours in MSTB/year/acre. (After <ref name=pt06r81 />.)]]

[[file:subsurface-maps_fig7.png|thumb|left|{{figure number|7}}Cumulative oil production map for the A, B, C, and D zones from the San Andres Formation reservoir, Jordan field, Ector and Crane counties, Texas. Contours in MSTB/year/acre. (After Major et al.)<ref name=pt06r81 />]]

===Water cut===

===Water cut===