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A ''reservoir rock'' is any porous and permeable rock capable of ''potentially'' containing hydrocarbons in its pore system. This statement implies that not all reservoir rocks qualify as pay. In some reservoirs, there may be intermediate pay types or a continuum between pay and nonpay intervals. This situation may include reservoir units that have differing fluid saturations or pore geometries, or that are present at different elevations above the Basic open hole tools [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water%20contact hydrocarbon-water contact].

A ''reservoir rock'' is any porous and permeable rock capable of ''potentially'' containing hydrocarbons in its pore system. This statement implies that not all reservoir rocks qualify as pay. In some reservoirs, there may be intermediate pay types or a continuum between pay and nonpay intervals. This situation may include reservoir units that have differing fluid saturations or pore geometries, or that are present at different elevations above the Basic open hole tools [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water%20contact hydrocarbon-water contact].

The production methodologies—primary, secondary, and enhanced recovery—affect the definition of pay. For example, beds with limited lateral continuity may qualify as pay under primary production, but may not be waterfloodable at contemplated injector-producer well spacings, thus disqualifying them as pay under secondary production. Thus, there are two separate but related questions regarding pay determination: first, the delineation of [[reservoir quality]] rock, and second, the classification of that part of a reservoir quality interval as pay.

The production methodologies—primary, secondary, and enhanced recovery—affect the definition of pay. For example, beds with limited [[lateral]] continuity may qualify as pay under primary production, but may not be waterfloodable at contemplated injector-producer well spacings, thus disqualifying them as pay under secondary production. Thus, there are two separate but related questions regarding pay determination: first, the delineation of [[reservoir quality]] rock, and second, the classification of that part of a reservoir quality interval as pay.

==Pay determination techniques==

==Pay determination techniques==

! Procedure

! Procedure

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| 1. Geologically characterize reservoir || [[Core description]], wireline log calibration, lithofacies determination, depositional environment analysis

| 1. Geologically characterize reservoir || [[Core description]], wireline log calibration, [[lithofacies]] determination, depositional environment analysis

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| 2. Determine reservoir properties || Core analysis (porosity, [[permeability]], fluid saturation), wireline log analysis (porosity, fluid saturation)

| 2. Determine reservoir properties || Core analysis (porosity, [[permeability]], fluid saturation), wireline log analysis (porosity, fluid saturation)

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|}

The simplest, yet most useful, method for combining this information is a composite log, which displays the different classes of data in a format in which each data set is readily correlated by depth. From a detailed reservoir profile log, pay zones can be identified and correlated to uncored wells using well log curves that are calibrated to core data. Examples of this type of procedure can be found in Connolly and Reed,<ref name=pt06r19>Connolly, E. T., Reed, P. A., 1983, Full spectrum formation evaluation: Canadian Well Logging Society Journal, v. 12, p. 23–69.</ref> Harris,<ref name=pt06r48>Harris, D. G., 1975, [https://www.onepetro.org/journal-paper/SPE-5022-PA The roles of geology in reservoir simulation studies]: Journal Petroleum of Technology, May, p. 625–632.</ref> Hearn et al.,<ref name=pt06r51>Hearn, C. L., Ebanks, W. J. Jr., Tye, R. S., Ranganathan, V. 1984, Geological factors influencing reservoir performance of the Hartzog Draw field: Journal of Petroleum Technology, v. 36, Aug., p. 1335–1344, 10, 2118/12016-PA.</ref> and Hietala and Connolly.<ref name=pt06r53>Hietala, R. W., Connolly, E. T., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0200/0215.htm Integrated rock-log calibration in the Elmworth field, Alberta, Canada, Part II—well log analysis methods and techniques], in Masters, J. A., ed., Elmworth—Case Study of a Deep Basin Gas Field: [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], p. 215–242.</ref>

The simplest, yet most useful, method for combining this information is a composite log, which displays the different classes of data in a format in which each data set is readily correlated by depth. From a detailed reservoir profile log, pay zones can be identified and correlated to uncored wells using well log curves that are calibrated to core data. Examples of this type of procedure can be found in Connolly and Reed,<ref name=pt06r19>Connolly, E. T., and P. A. Reed, 1983, Full spectrum formation evaluation: Canadian Well Logging Society Journal, v. 12, p. 23–69.</ref> Harris,<ref name=pt06r48>Harris, D. G., 1975, [https://www.onepetro.org/journal-paper/SPE-5022-PA The roles of geology in reservoir simulation studies]: Journal Petroleum of Technology, May, p. 625–632.</ref> Hearn et al.,<ref name=pt06r51>Hearn, C. L., W. J. Ebanks, Jr., R. S. Tye, and V. Ranganathan, 1984, Geological factors influencing reservoir performance of the Hartzog Draw field: Journal of Petroleum Technology, v. 36, Aug., p. 1335–1344, 10, 2118/12016-PA.</ref> and Hietala and Connolly.<ref name=pt06r53>Hietala, R. W., and E. T. Connolly, 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0200/0215.htm Integrated rock-log calibration in the Elmworth field, Alberta, Canada, Part II—well log analysis methods and techniques], in J. A. Masters, ed., Elmworth—Case Study of a Deep Basin Gas Field: [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], p. 215–242.</ref>

An important component of effective pay determination is a systematic, sedimentologically based reservoir zonation. This procedure provides a direct method of evaluating the validity and representativeness of core measurements in relation to the actual distribution of porosity, permeability, and fluid saturations within the reservoir. Core description should be integrated with well logs for calibration and correlation to uncored wells. Discussion of calibration techniques can be found in Connolly and Reed,<ref name=pt06r19 /> Hietala and Connolly,<ref name=pt06r53 /> and Sneider and King.<ref name=pt06r128>Sneider, R. M., King, H. R., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0200/0205.htm Integrated rock-log calibration in the Elmworth field, Alberta, Canada—Part I, Reservoir rock detection and characterization], in Masters, J. A., ed., Elmworth—Case Study of a Deep Basin Gas Field, [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], p. 205–214.</ref>

An important component of effective pay determination is a systematic, sedimentologically based reservoir zonation. This procedure provides a direct method of evaluating the validity and representativeness of core measurements in relation to the actual distribution of porosity, permeability, and fluid saturations within the reservoir. Core description should be integrated with well logs for calibration and correlation to uncored wells. Discussion of calibration techniques can be found in Connolly and Reed,<ref name=pt06r19 /> Hietala and Connolly,<ref name=pt06r53 /> and Sneider and King.<ref name=pt06r128>Sneider, R. M., and H. R. King, 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0200/0205.htm Integrated rock-log calibration in the Elmworth field, Alberta, Canada—Part I, Reservoir rock detection and characterization], in J. A. Masters, ed., Elmworth—Case Study of a Deep Basin Gas Field, [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], p. 205–214.</ref>

Well and production tests are often taken over too large an interval in the wellbore to be precise in distinguishing pay and nonpay, especially in heterogeneous reservoirs. Spinner and temperature surveys can be good indicators of the loci of production where the borehole penetrates the reservoir if production rates are high enough. Electric logs can delineate hydrocarbon saturated intervals, but are not an effective tool for pay determination until they are calibrated with production tests, core analyses, or results from analogous reservoirs. The effective determination of pay relies on analyses from the physical sampling of reservoir and nonreservoir rocks. The different classes of information regarding reservoir behavior and pay determination may be irreconcilable or open to misinterpretation in the absence of a thoroughly understood geological framework.

Well and production tests are often taken over too large an interval in the wellbore to be precise in distinguishing pay and nonpay, especially in heterogeneous reservoirs. Spinner and temperature surveys can be good indicators of the loci of production where the borehole penetrates the reservoir if production rates are high enough. Electric logs can delineate hydrocarbon saturated intervals, but are not an effective tool for pay determination until they are calibrated with production tests, core analyses, or results from analogous reservoirs. The effective determination of pay relies on analyses from the physical sampling of reservoir and nonreservoir rocks. The different classes of information regarding reservoir behavior and pay determination may be irreconcilable or open to misinterpretation in the absence of a thoroughly understood geological framework.

! Criterion || Reservoir || Nonreservoir

! Criterion || Reservoir || Nonreservoir

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| Initial displacement pressure (psi) || <100 || >100

| Initial [[displacement pressure]] (psi) || <100 || >100

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| [[Capillary pressure]] (psi) (1% bulk volume Hg fluid saturation) || <300 || >500

| [[Capillary pressure]] (psi) (1% bulk volume Hg fluid saturation) || <300 || >500

[[Category:Geological methods]]

[[Category:Geological methods]]