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Wattenberg field (view source)

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==Reservoirs==

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Most of the production from Wattenberg comes from Cretaceous reservoirs (Dakota, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, Hygiene Sandstone, and Terry Sandstone). [[:file:M125-WattenbergField-Figure7|Figure 7]] is a north-south stratigraphic cross section through the central Wattenberg area. Location of the section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]]. The Wattenberg High paleostructure trends west to east across GWA and curves to the northeast in the eastern part of the mapped area. This feature was discussed by Weimer and Sonnenberg<ref name=WeimSon1982 />. This Wattenberg High is well illustrated on [[:file:M125-WattenbergField-Figure7|Figures 7]] and [[:file:M125-WattenbergField-Figure8.jpg|Figures 8]]. The thin area shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]] is interpreted to be because of erosional truncation and onlap of units in the upper Niobrara.

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Most of the production from Wattenberg comes from Cretaceous reservoirs (Dakota, Muddy (J) Sandstone, Codell Sandstone, Niobrara Formation, Hygiene Sandstone, and Terry Sandstone). [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north-south stratigraphic cross section through the central Wattenberg area. Location of the section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]]. The Wattenberg High paleostructure trends west to east across GWA and curves to the northeast in the eastern part of the mapped area. This feature was discussed by Weimer and Sonnenberg<ref name=WeimSon1982 />. This Wattenberg High is well illustrated on [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]] and [[:file:M125-WattenbergField-Figure8.jpg|Figures 8]]. The thin area shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]] is interpreted to be because of erosional truncation and onlap of units in the upper Niobrara.

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[[file:M125-WattenbergField-Figure7|center|framed|300 px|{{Figure number|7}}South to north stratigraphic cross section through Wattenberg Field. Section is datumed on top of the Niobrara Formation. Dramatic thinning of the Niobrara occurs as a result of erosional truncation and onlap onto a paleostructural feature known as the Wattenberg High. Location of the cross section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]].]]

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[[file:M125-WattenbergField-Figure7.jpg|center|framed|300 px|{{Figure number|7}}South to north stratigraphic cross section through Wattenberg Field. Section is datumed on top of the Niobrara Formation. Dramatic thinning of the Niobrara occurs as a result of erosional truncation and onlap onto a paleostructural feature known as the Wattenberg High. Location of the cross section is shown in [[:file:M125-WattenbergField-Figure8.jpg|Figure 8]].]]

[[file:M125-WattenbergField-Figure8.jpg|center|framed|300 px|{{Figure number|8}}Isopach map of J Sandstone to top of Niobrara interval. The Wattenberg High paleostructure trends west to east across Wattenberg and curves to the northeast in the eastern portion of the mapped area. CMB is an abbreviation for the Colorado Mineral Belt. GWA = Greater Wattenberg Area. Contour interval 50 ft.]]

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===Muddy (J) Sandstone===

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The Muddy (J) Sandstone ranges in thickness from 40 to 150 ft across Wattenberg Field and consists of two members: an older Fort Collins Member and a younger Horsetooth Member<ref name=WeimSon1982 /><ref name=WeimSon1989 /><ref> MacKenzie, D. B., 1965, [https://archives.datapages.com/data/bulletns/1965-67/data/pg/0049/0002/0150/0186.htm Depositional environments of Muddy Sandstone, western Denver basin, Colorado]: AAPG Bulletin, v. 49, p. 186–206.</ref><ref> Weimer, R. J., S. A. Sonnenberg, G. B. C. Young, 1986, [https://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0100/0143.htm Wattenberg Field, Denver Basin, Colorado], ‘’in’’ C. W. Spencer, and R. F. Mast, eds., Geology of tight gas reservoirs: [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg24.htm AAPG Studies in Geology 24], p. 143–164.</ref> ([[:file:M125-WattenbergField-Figure7|Figures 7]], ([[:file:M125-WattenbergField-Figure10.jpg|10]], [[file:M125-WattenbergField-Figure11.jpg|11]], ([[:file:M125-WattenbergField-Figure12.jpg|12]]). The Fort Collins consists of very fine- to fine-grained sandstone containing marine trace fossils and is interpreted to be a delta front deposit. The Horsetooth consists of fine- to medium-grained, well-sorted, cross-stratified sandstone. This sandstone is interpreted to have been deposited as part of a valley-fill deposit of fresh to brackish water origin. The sandstones contain carbonized wood fragments and are intercalated with siltstone and mudstone. The valley-fill model is shown in [[:file:M125-WattenbergField-Figure13.jpg|Figure 13]]. The sequence stratigraphy of the Muddy (J) Sandstone reservoir in Wattenberg field was discussed in detail by Weimer and Sonnenberg<ref name=WeimSon1989 />. Regional distribution of the Muddy (J) and valley-fill systems have been published by Weimer and Sonnenberg<ref name=WeimSon1989 />, Dolson et al.<ref> Dolson, J. C., D. S. Muller, M. J. Evetts, and J. Stein, 1991, [https://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0003/0000/0409.htm Regional paleotopographic trends and production, Muddy Sandstone (Lower Cretaceous), Central and Northern Rocky Mountains]: AAPG Bulletin, v. 75, no. 3, p. 409–435.</ref>, and others.

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The Muddy (J) Sandstone ranges in thickness from 40 to 150 ft across Wattenberg Field and consists of two members: an older Fort Collins Member and a younger Horsetooth Member<ref name=WeimSon1982 /><ref name=WeimSon1989 /><ref> MacKenzie, D. B., 1965, [https://archives.datapages.com/data/bulletns/1965-67/data/pg/0049/0002/0150/0186.htm Depositional environments of Muddy Sandstone, western Denver basin, Colorado]: AAPG Bulletin, v. 49, p. 186–206.</ref><ref> Weimer, R. J., S. A. Sonnenberg, G. B. C. Young, 1986, [https://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0100/0143.htm Wattenberg Field, Denver Basin, Colorado], ‘’in’’ C. W. Spencer, and R. F. Mast, eds., Geology of tight gas reservoirs: [https://archives.datapages.com/data/alt-browse/aapg-special-volumes/sg24.htm AAPG Studies in Geology 24], p. 143–164.</ref> ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], ([[:file:M125-WattenbergField-Figure10.jpg|10]], [[file:M125-WattenbergField-Figure11.jpg|11]], ([[:file:M125-WattenbergField-Figure12.jpg|12]]). The Fort Collins consists of very fine- to fine-grained sandstone containing marine trace fossils and is interpreted to be a delta front deposit. The Horsetooth consists of fine- to medium-grained, well-sorted, cross-stratified sandstone. This sandstone is interpreted to have been deposited as part of a valley-fill deposit of fresh to brackish water origin. The sandstones contain carbonized wood fragments and are intercalated with siltstone and mudstone. The valley-fill model is shown in [[:file:M125-WattenbergField-Figure13.jpg|Figure 13]]. The sequence stratigraphy of the Muddy (J) Sandstone reservoir in Wattenberg field was discussed in detail by Weimer and Sonnenberg<ref name=WeimSon1989 />. Regional distribution of the Muddy (J) and valley-fill systems have been published by Weimer and Sonnenberg<ref name=WeimSon1989 />, Dolson et al.<ref> Dolson, J. C., D. S. Muller, M. J. Evetts, and J. Stein, 1991, [https://archives.datapages.com/data/bulletns/1990-91/data/pg/0075/0003/0000/0409.htm Regional paleotopographic trends and production, Muddy Sandstone (Lower Cretaceous), Central and Northern Rocky Mountains]: AAPG Bulletin, v. 75, no. 3, p. 409–435.</ref>, and others.

[[file:M125-WattenbergField-Figure10.jpg|center|framed|300 px|{{Figure number|10}}Type log for Wattenberg Field. Curves shown are gamma ray, spontaneous potential, resistivity, and neutron/density. Black bar in depth track indicates cored interval. Fort Collins Member designated F.C.M., and Horsetooth Member designated H.M. Note perforated interval 8160–8173 ft and associated neutron-density crossover gas effect.]]

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[[file:M125-WattenbergField-Figure11.jpg|center|framed|300 px|{{Figure number|11}}Cored photographs for the Muddy (J) Sandstone from the Amoco #1 Rocky Mountain Fuel Well (Sec. 8, T1N-R67W). Log of cored interval shown in [[:file:M125-WattenbergField-Figure9.jpg|Figure 9]]. Muddy (J) interval is subdivided into Fort Collins and Horsetooth members. LDF = lower delta front; UDF = upper delta front; LSE = lowstand surface of erosion; TSE = transgressive surface of erosion. The well was perforated in the upper delta front, the main Muddy (J) pay in Wattenberg Field.]]

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[[file:M125-WattenbergField-Figure12.jpg|center|framed|300 px|{{Figure number|12}}Isopach map of Muddy (J) Sandstone interval. The Muddy (J) Sandstone ranges in thickness from 160 ft to less than 60 ft across the mapped area. The thickest area is to the northeast, and thinnest area is to the west. See [[:file:M125-WattenbergField-Figure7|Figure 7]] for cross-section A-A’. Contour interval 20 ft.]]

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[[file:M125-WattenbergField-Figure12.jpg|center|framed|300 px|{{Figure number|12}}Isopach map of Muddy (J) Sandstone interval. The Muddy (J) Sandstone ranges in thickness from 160 ft to less than 60 ft across the mapped area. The thickest area is to the northeast, and thinnest area is to the west. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. Contour interval 20 ft.]]

[[file:M125-WattenbergField-Figure13.jpg|center|framed|300 px|{{Figure number|Figure 13}}Valley-fill model for the Muddy (J) Sandstone (from Weimer and Sonnenberg<ref name=WeimSon1982 />). (A) Deposition of Fort Collins Member, wave-dominated deltaic progradation overlying Skull Creek Shale, (B) truncation of older marine and deltaic deposits with sea level lowering, and (C) sea level rise and back filling of valley networks with fluvial and marine strata (Horsetooth Member).]]

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Most of the production comes from the Fort Collins Member (fine-grained delta front sandstone), which ranges in thickness from less than 25 ft to more than 100 ft ([[:file:M125-WattenbergField-Figure11.jpg|Figures 11]], ([[:file:M125-WattenbergField-Figure12.jpg|12]], [[:file:M125-WattenbergField-Figure13.jpg|13]], [[:file:M125-WattenbergField-Figure14.jpg|14]], [[:file:M125-WattenbergField-Figure15.jpg|15]]). Thin areas in the Fort Collins are caused mainly from erosional incisement by Horsetooth valleys. The main pay section in the Fort Collins varies from 10 to 20 ft in thickness. The Fort Collins sandstones consist of approximately 80% quartz, 10% argillaceous matrix, 5% rock fragments, and 5% feldspar. Clay is the main matrix material, which is both detrital and diagenetic. The sandstones are bioturbated, and the amount of bioturbation decreases vertically in abundance. Higher energy conditions in the upper Fort Collins winnow out the clay fraction. Trace fossils change vertically from largely deposit feeders (lower Fort Collins) to suspension-feeding organisms (upper Fort Collins). Porosity is mainly intergranular with minor microporosity found in the matrix. The most important diagenetic event in the Fort Collins is formation of quartz overgrowths. The percentage of quartz overgrowths decreases as the clay content increases. Pores are triangular in shape because of the quartz overgrowths. [[:file:M125-WattenbergField-Figure15.jpg|Figure 15]] illustrates location of wells that have cumulative production in excess of 500 MMCFG. The production sweet spot coincides with an isopach thick in the Fort Collins Member. Also important to production is the Wattenberg temperature anomaly, which is represented by the vitrinite reflectance contours.

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[[file:M125-WattenbergField-Figure14.jpg|center|framed|300 px|{{Figure number|14}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. The Fort Collins ranges in thickness from 25 to 100 ft across the mapped area. The thickest area is located south of Greeley. See [[:file:M125-WattenbergField-Figure7|Figure 7]] for cross-section A-A’. The best production occurs between Broomfield and Greeley where the total Fort Collins interval is 75–100 ft thick. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

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[[file:M125-WattenbergField-Figure14.jpg|center|framed|300 px|{{Figure number|14}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. The Fort Collins ranges in thickness from 25 to 100 ft across the mapped area. The thickest area is located south of Greeley. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. The best production occurs between Broomfield and Greeley where the total Fort Collins interval is 75–100 ft thick. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

[[file:M125-WattenbergField-Figure15.jpg|center|framed|300 px|{{Figure number|15}}Isopach map of the Fort Collins Member of the Muddy (J) Sandstone. Contour interval is 25 ft. Also shown are Muddy (J) wells, which have cumulative production greater than 500 MMCFG. The thickness in Fort Collins is the sweet spot area for production. Also shown are Ro values for shales above the Muddy (J). Vitrinite reflectance contours modified from Higley and Cox<ref name=HigCox>Higley, D. K., and D. O. Cox, 2007, Oil and gas exploration and development along the front range in the Denver Basin of Colorado, Nebraska, and Wyoming, ‘’in’’ D. K. Higley, compiler, Petroleum systems and assessment of undiscovered oil and gas in the Denver Basin Province, Colorado, Kansas, Nebraska, South Dakota, and Wyoming—USGS Province 39: USGS Digital Data Series DDS–69–P, 41 p.</ref> and Smagala et al.<ref name=Smag>Smagala, T. M., C. A. Brown, and G. L. Nydegger, 1984, Log-derived indicator of thermal maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming, in J. Woodward, F. F. Meissner, and J. C. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: RMAG, Denver, Colorado, p. 355–363.</ref>. GWA = Greater Wattenberg Area. The best Muddy (J) production generally coincides with the highest maturity areas and thickest Fort Collins.]]

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The Horsetooth Member ranges in thickness from less than 25 ft to more than 125 ft across Wattenberg Field ([[:file:M125-WattenbergField-Figure16.jpg|Figure 16]]). The sandstones are fine to medium grained, well sorted, and rounded and consist of 57%–80% quartz, 5%–10% rock fragments, and 5% feldspar. Porosity is intergranular with minor microporosity and intraparticle porosity. The Horsetooth Member is thickest in the northeast, southeast, and southwest parts of the mapped area. Diagenesis is similar to the Fort Collins Member with abundant silica cement along with smectite, chlorite, and kaolinite clay pore fillings. The clays can contribute to low-resistivity pay in the Horsetooth Member.

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[[file:M125-WattenbergField-Figure16.jpg|center|framed|300 px|{{Figure number|16}}Isopach map of the Horsetooth Member of the Muddy (J) Sandstone. The Horsetooth ranges in thickness from 25 to 125 ft across the mapped area. The thickest area is located north of Greeley. See [[:file:M125-WattenbergField-Figure7|Figure 7]] for cross-section A-A’. Probable flow directions in valley-fill systems are shown by red dashed lines. The thick areas on this map coincide with thin areas on [[:file:M125-WattenbergField-Figure10.jpg|Figure 10]]. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

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[[file:M125-WattenbergField-Figure16.jpg|center|framed|300 px|{{Figure number|16}}Isopach map of the Horsetooth Member of the Muddy (J) Sandstone. The Horsetooth ranges in thickness from 25 to 125 ft across the mapped area. The thickest area is located north of Greeley. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. Probable flow directions in valley-fill systems are shown by red dashed lines. The thick areas on this map coincide with thin areas on [[:file:M125-WattenbergField-Figure10.jpg|Figure 10]]. GWA = Greater Wattenberg Area. Contour interval 25 ft.]]

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The best production in the Muddy (J) in Wattenberg comes from the Fort Collins Member ([[:file:M125-WattenbergField-Figure15.jpg|Figures 15]], [[:file:M125-WattenbergField-Figure17|17]]). The Horsetooth valley fills are also productive in Wattenberg but in isolated areas (largely because of intense diagenesis). In areas outside of Wattenberg, the Horsetooth is the more important Muddy (J) reservoir. [[:file:M125-WattenbergField-Figure14.jpg|Figures 14]]] and [[:file:M125-WattenbergField-Figure16.jpg|16]] show wells that have cumulative production of greater than 500 MMCFG to date. These wells are located along the basin synclinal axis and mainly where the Fort Collins Member is the thickest.

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The best production in the Muddy (J) in Wattenberg comes from the Fort Collins Member ([[:file:M125-WattenbergField-Figure15.jpg|Figures 15]], [[:file:M125-WattenbergField-Figure17.jpg|17]]). The Horsetooth valley fills are also productive in Wattenberg but in isolated areas (largely because of intense diagenesis). In areas outside of Wattenberg, the Horsetooth is the more important Muddy (J) reservoir. [[:file:M125-WattenbergField-Figure14.jpg|Figures 14]]] and [[:file:M125-WattenbergField-Figure16.jpg|16]] show wells that have cumulative production of greater than 500 MMCFG to date. These wells are located along the basin synclinal axis and mainly where the Fort Collins Member is the thickest.

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[[file:M125-WattenbergField-Figure17|center|framed|300 px|{{Figure number|17}}Isopach map of the Horsetooth Member of the Muddy (J) Sandstone. See [[:file:M125-WattenbergField-Figure7|Figure 7]] for cross-section A-A’. Wells shown on map are wells that have cumulative production values greater than 500 MMCFG. Vitrinite reflectance contours modified from Smagala et al. .<ref name=Smag /> and Higley and Cox<ref name=HigCox />. GWA = Greater Wattenberg Area. The valley-fill deposits are generally tight via silica cementation, but areas northeast of Denver have good production.]]

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[[file:M125-WattenbergField-Figure17.jpg|center|framed|300 px|{{Figure number|17}}Isopach map of the Horsetooth Member of the Muddy (J) Sandstone. See [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] for cross-section A-A’. Wells shown on map are wells that have cumulative production values greater than 500 MMCFG. Vitrinite reflectance contours modified from Smagala et al. .<ref name=Smag /> and Higley and Cox<ref name=HigCox />. GWA = Greater Wattenberg Area. The valley-fill deposits are generally tight via silica cementation, but areas northeast of Denver have good production.]]

The thickest area of Fort Collins appears to be associated with the Wattenberg High. Horsetooth valley-fill systems occur surrounding the area of thick Fort Collins. The valley-fill systems appear to be associated with topographic low areas ([[:file:M125-WattenbergField-Figure13.jpg|Figure 13]]).

===Codell Sandstone===

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The Upper Cretaceous Codell is a sandstone sequence of rocks that occurs between the main carbonate-producing episodes of the Cretaceous, namely the Niobrara and Greenhorn formations ([[:file:M125-WattenbergField-Figure7|Figure 7]]). The Niobrara and Greenhorn formations consist of chalks and marls and are thus deep-water carbonate deposits. The Codell probably represents a sea level lowering in between two of the highest sea levels during the Cretaceous. The interpreted depositional environment for the Codell Sandstone is as a shelf sand or shoreface<ref name=WeimSonnen1983>Weimer, R. J., and S. A. Sonnenberg, 1983, Codell Sandstone, new exploration play, Denver Basin: Oil and Gas Journal, v. 81, no. 22, p. 119–125.</ref>.

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The Upper Cretaceous Codell is a sandstone sequence of rocks that occurs between the main carbonate-producing episodes of the Cretaceous, namely the Niobrara and Greenhorn formations ([[:file:M125-WattenbergField-Figure7.jpg|Figure 7]]). The Niobrara and Greenhorn formations consist of chalks and marls and are thus deep-water carbonate deposits. The Codell probably represents a sea level lowering in between two of the highest sea levels during the Cretaceous. The interpreted depositional environment for the Codell Sandstone is as a shelf sand or shoreface<ref name=WeimSonnen1983>Weimer, R. J., and S. A. Sonnenberg, 1983, Codell Sandstone, new exploration play, Denver Basin: Oil and Gas Journal, v. 81, no. 22, p. 119–125.</ref>.

The Codell Sandstone is a major pay in the giant Wattenberg Field of the Denver Basin. The mid-Turonian Codell Sandstone Member of the Carlile Formation has long been a focus as an exploration and development target in the Denver Basin. The first oil production from the Codell dates back to 1901 and the Boulder Oil Field development<refname=HigCox />. Vertical well completions within the Wattenberg Field date back to 1981 and were hydraulic fracture stimulated. The drilling depths for the Codell in Wattenberg range from 4000 to 8200 ft. Although extensively drilled with vertical wells, the Codell is now being developed via horizontal wells in Wattenberg Field. The vertical wells also have a history of successful hydraulic refracturing<ref name=Birm2001>Birmingham, T. J., D. M. Lytle, and R. N. Sencenbaugh, 2001, Enhanced recovery from a tight gas sand through hydraulic refracturing: Codell Formation, Wattenberg Field, Colorado, ‘’in’’ D. S. Anderson, J. W. Robinson, J. E. Estes-Jackson, and E. B. Coalson, eds., Gas in the Rockies: Rocky Mountain Association of Geologists Special Publication, p. 101–116.</ref>. New horizontal wells (2011 to present) with initial production of 100–700 BOPD (GOR ~10,000 cf/bbl) indicate substantial remaining reserves in the formation.

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[[file:M125-WattenbergField-Figure19.jpg|center|framed|300 px|{{Figure number|19}}Isopach map of the Codell Sandstone, porosity greater than 6% using 2.68 matrix density; contour interval 5 ft. Porous Codell ranges in thickness from 5 to 25 ft across the mapped area. Thickest area is to the west. Codell pressure greater than 0.5 psi within orange dashed area. Synclinal axis at Codell level shown by dashed gray line. Outline of where Codell produces shown by dashed blue line. GWA = Greater Wattenberg Area.]]

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The Codell Sandstone in Wattenberg is an impermeable, bioturbated, fine-grained marine shelf sandstone<ref name=WeimSonnen1983 />. Occasional thin hummocky cross-stratified beds (1–2 ft thick) are present in cores and the outcrop sections of the Codell ([[:file:M125-WattenbergField-Figure20.jpg|Figures 20]], [[:file:M125-WattenbergField-Figure21.jpg|21]]). The Codell Sandstone in Wattenberg is poorly to moderately sorted, very fine- to fine-grained sublitharenites and subarkose. Slight to moderate compaction and ductile grain deformation is common. Partial dissolution of chert and mica has created minor amounts of secondary porosity. Cements include mixed layer illite/smectite, illite, chlorite, quartz overgrowths, and calcite. Detrital clay further limits reservoir quality. The Codell in Wattenberg is a classic low-resistivity, low-contrast pay (LRLC; [[:file:M125-WattenbergField-Figure7|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]).

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The Codell Sandstone in Wattenberg is an impermeable, bioturbated, fine-grained marine shelf sandstone<ref name=WeimSonnen1983 />. Occasional thin hummocky cross-stratified beds (1–2 ft thick) are present in cores and the outcrop sections of the Codell ([[:file:M125-WattenbergField-Figure20.jpg|Figures 20]], [[:file:M125-WattenbergField-Figure21.jpg|21]]). The Codell Sandstone in Wattenberg is poorly to moderately sorted, very fine- to fine-grained sublitharenites and subarkose. Slight to moderate compaction and ductile grain deformation is common. Partial dissolution of chert and mica has created minor amounts of secondary porosity. Cements include mixed layer illite/smectite, illite, chlorite, quartz overgrowths, and calcite. Detrital clay further limits reservoir quality. The Codell in Wattenberg is a classic low-resistivity, low-contrast pay (LRLC; [[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]).

[[file:M125-WattenbergField-Figure20.jpg|center|framed|300 px|{{Figure number|20}}(A) Type log Codell Sandstone, Wattenberg Field. Gamma ray, spontaneous potential, resistivity, neutron, and density curves shown. Codell is a low resistivity, low contrast pay. (B) X-ray diffraction data for the Dome Frank core<ref name=USGS>[http://geology.cr.usgs.gov/crc/ USGS Core Research Center], 2015, accessed February 4, 2021.</ref>.

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[[file:M125-WattenbergField-Figure21.jpg|center|framed|300 px|{{Figure number|21}}Core photographs of Dome Frank Codell core. Two distinct facies are present in core: (1) bioturbated sandstone and (2) low-angle cross stratified (hummocky beds). Unconformities are present at the top and base of Codell. From USGS Core Research Center<ref name=USGS />.]]

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The Codell has very little gamma-ray response and very little spontaneous potential (SP) response. Abnormally high porosity readings occur on density logs as a result of gas saturation ([[:file:M125-WattenbergField-Figure7|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]). Current development activity in Wattenberg Field includes horizontal wells with multistage hydraulic fracturing. The low resistivity is because of clay content of the sandstones. Values of porosity and permeability are 14% and 0.05 md for Niobrara/Codell wells in Wattenberg Field<ref name=HigCox /><ref> Higley, D. K., D. O. Cox, and R. J. Weimer, 2003, [https://archives.datapages.com/data/bulletns/2003/01jan/0015/0015.htm Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver Basin, Colorado]: AAPG Bulletin, v. 87, no. 1, p. 15–37.</ref>. Based on core testing, the Codell Sandstone in Wattenberg Field has an average porosity and permeability of 14% and 0.1 md, respectively<refname=HigCox />.

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The Codell has very little gamma-ray response and very little spontaneous potential (SP) response. Abnormally high porosity readings occur on density logs as a result of gas saturation ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure20.jpg|20]]). Current development activity in Wattenberg Field includes horizontal wells with multistage hydraulic fracturing. The low resistivity is because of clay content of the sandstones. Values of porosity and permeability are 14% and 0.05 md for Niobrara/Codell wells in Wattenberg Field<ref name=HigCox /><ref> Higley, D. K., D. O. Cox, and R. J. Weimer, 2003, [https://archives.datapages.com/data/bulletns/2003/01jan/0015/0015.htm Petroleum system and production characteristics of the Muddy (J) Sandstone (Lower Cretaceous) Wattenberg continuous gas field, Denver Basin, Colorado]: AAPG Bulletin, v. 87, no. 1, p. 15–37.</ref>. Based on core testing, the Codell Sandstone in Wattenberg Field has an average porosity and permeability of 14% and 0.1 md, respectively<refname=HigCox />.

The best Codell production coincides with an area of high heat flow in the Wattenberg Field that is centered south of Greeley and north of Denver ([[:file:M125-WattenbergField-Figure18.jpg|Figures 18]], [[:file:M125-WattenbergField-Figure19.jpg|19]]). The high heat area is reflected on temperature maps and by vitrinite reflectance data. The production “sweet spot” in the Codell coincides largely with high pressures and high heat flow ([[:file:M125-WattenbergField-Figure19.jpg|Figure 19]]).

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<ref name=Longmetal1998 /><ref name=Landetal2001 /><ref name=Kauff1977 />/><ref> Weimer, R. J., 1960, [https://archives.datapages.com/data/bulletns/1957-60/data/pg/0044/0001/0000/0001.htm Upper Cretaceous Stratigraphy, Rocky Mountain Area]: AAPG Bulletin, v. 44, p. 1–20.</ref><ref> Hattin, D. E., and C. T. Siemers, 1978, Upper Cretaceous stratigraphy and depositional environments of western Kansas: Kansas Geological Survey Guidebook 3, 102 p. (reprinted 1987, with modifications).</ref><ref> Hann, M. L., 1981, Petroleum potential of the Niobrara Formation in the Denver Basin—Colorado and Kansas, Master’s thesis, Colorado State University, Fort Collins, Colorado, 260 p.</ref><ref> Sonnenberg, S. A., and R. J. Weimer, 1981, Tectonics, sedimentation, and petroleum potential northern Denver Basin Colorado, Wyoming, and Nebraska: Colorado School of Mines Quarterly, v. 7, no. 2, 45 p.</ref><ref>Sonnenberg, S. A., and R. J. Weimer, 1993, Oil production from Niobrara Formation, Silo Field, Wyoming: Fracturing associated with a possible wrench fault system: The Mountain Geologist, v. 30, no. 2, p. 39–53.</ref><ref name=Barl1985>Barlow, L. K., 1985, Event stratigraphy, paleoenvironments, and petroleum source rock potential of the Niobrara Formation (Cretaceous), northern Front Range, Colorado, Master’s thesis, University of Colorado, Boulder, Colorado, 288 p.</ref><ref name=Barl1986>Barlow, L. K., 1986, An integrated geochemical and paleoecological approach to petroleum source rock evaluation, lower Niobrara Formation (Cretaceous), Lyons, Colorado: The Mountain Geologist, v. 23, p. 107–112.</ref><ref> Rodriguez, T. E., 1985, High-resolution event stratigraphy and interpretation of the depositional environments of the upper Smoky Hill Member, Niobrara Formation of the northwest Denver Basin, M.S. thesis, University of Colorado, Boulder, Colorado, 197 p.</ref>. Source rock geochemistry of the Niobrara was described by Clayton and Swetland<ref name=ClaySwet>Clayton, J. L., and P. J. Swetland, 1980, [https://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0010/1600/1613.htm Petroleum generation and migration in the Denver Basin]: AAPG Bulletin, v. 64, p. 1613–1624.</ref>, Meissner et al. <ref name=Meissetal1984 />, and Weimer<ref name=Weimer1996 />.

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The Niobrara represents one of the two most widespread marine invasions and the last great carbonate-producing episode of the WIC Basin<ref name=Kauff1977 /><ref name=Barl1985 /><ref name=Barl1986 />. The Greenhorn interval is the other time of widespread marine invasion and carbonate production. The dominant lithologies of the Niobrara Formation are limestones (chalks) and interbedded marls ([[:file:M125-WattenbergField-Figure7|Figure 7]]). The chalk-marl cycles are interpreted to represent changes from normal to brackish water salinities possibly related to regional paleo-climatic factors or sea level fluctuations. The chalk lithologies are thought to represent deposition in normal to near-normal marine salinities having a well-mixed water column and well-oxygenated bottom waters. The chalks reflect influx of warm Gulfian currents into the WIC seaway during relatively high sea levels. The chalk/limestone facies is composed of coccolith-rich fecal pellets probably derived from pelagic copepods, inoceramid and oyster shell fragments, planktonic foraminifer tests, micrite, clay, and quartz silt<ref> Lockridge, J. P., and P. A. Scholle, 1978, Niobrara gas in eastern Colorado and northwestern Kansas, ‘’in’’ J. D. Pruit, and P. E. Coffin, eds., Energy Resources of the Denver Basin: Rocky Mountain Association of Geologists Guidebook, p. 35–49.</ref>. The interbedded shale/marl cycles are interpreted to be caused by an increase in fresh water runoff caused by increased rainfall, which may be related to climatic warming. The fresh water runoff creates a brackish water cap and salinity stratification. Vertical mixing of the water column is inhibited causing anoxic conditions in the bottom waters. This enhances preservation of organic material and results in organic-rich source rocks. The decrease in water salinities is also suggested by oxygen isotopic values<ref name=Barl1985 />. The shalier intervals may reflect lower sea levels and greater influx of clastic material from the west. The chalks have previously been interpreted to represent higher sea levels during Niobrara time<ref name=Kauff1977 />.

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The Niobrara represents one of the two most widespread marine invasions and the last great carbonate-producing episode of the WIC Basin<ref name=Kauff1977 /><ref name=Barl1985 /><ref name=Barl1986 />. The Greenhorn interval is the other time of widespread marine invasion and carbonate production. The dominant lithologies of the Niobrara Formation are limestones (chalks) and interbedded marls ([[:file:M125-WattenbergField-Figure7.jpg|Figure 7]]). The chalk-marl cycles are interpreted to represent changes from normal to brackish water salinities possibly related to regional paleo-climatic factors or sea level fluctuations. The chalk lithologies are thought to represent deposition in normal to near-normal marine salinities having a well-mixed water column and well-oxygenated bottom waters. The chalks reflect influx of warm Gulfian currents into the WIC seaway during relatively high sea levels. The chalk/limestone facies is composed of coccolith-rich fecal pellets probably derived from pelagic copepods, inoceramid and oyster shell fragments, planktonic foraminifer tests, micrite, clay, and quartz silt<ref> Lockridge, J. P., and P. A. Scholle, 1978, Niobrara gas in eastern Colorado and northwestern Kansas, ‘’in’’ J. D. Pruit, and P. E. Coffin, eds., Energy Resources of the Denver Basin: Rocky Mountain Association of Geologists Guidebook, p. 35–49.</ref>. The interbedded shale/marl cycles are interpreted to be caused by an increase in fresh water runoff caused by increased rainfall, which may be related to climatic warming. The fresh water runoff creates a brackish water cap and salinity stratification. Vertical mixing of the water column is inhibited causing anoxic conditions in the bottom waters. This enhances preservation of organic material and results in organic-rich source rocks. The decrease in water salinities is also suggested by oxygen isotopic values<ref name=Barl1985 />. The shalier intervals may reflect lower sea levels and greater influx of clastic material from the west. The chalks have previously been interpreted to represent higher sea levels during Niobrara time<ref name=Kauff1977 />.

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The Niobrara consists of four limestone (chalk) units and three intervening marl intervals ([[:file:M125-WattenbergField-Figure7|Figures 7]], [[:file:M125-WattenbergField-Figure22.jpg|22]], [[:file:M125-WattenbergField-Figure23.jpg|23]], [[:file:M125-WattenbergField-Figure24.jpg|24]]). The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill Member. The chalk units are referred to in descending order as the A, B, C, and Fort Hays. Erosional unconformities exist at the top and base of the Niobrara. The upper unconformity removes the upper chalk bed in some areas of the Wattenberg Field. The B and C chalks are the main focus of horizontal drilling by operators in the field.

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The Niobrara consists of four limestone (chalk) units and three intervening marl intervals ([[:file:M125-WattenbergField-Figure7.jpg|Figures 7]], [[:file:M125-WattenbergField-Figure22.jpg|22]], [[:file:M125-WattenbergField-Figure23.jpg|23]], [[:file:M125-WattenbergField-Figure24.jpg|24]]). The lower limestone is named the Fort Hays, and the overlying units are grouped together as the Smoky Hill Member. The chalk units are referred to in descending order as the A, B, C, and Fort Hays. Erosional unconformities exist at the top and base of the Niobrara. The upper unconformity removes the upper chalk bed in some areas of the Wattenberg Field. The B and C chalks are the main focus of horizontal drilling by operators in the field.

[[file:M125-WattenbergField-Figure22.jpg|center|framed|300 px|{{Figure number|22}}Isopach Niobrara Formation; contour interval 50 ft. Axis of Wattenberg High shown in red. Niobrara ranges in thickness from 250 ft to more than 400 ft in mapped area. Red dashed lines show area of Niobrara producers. GWA = Greater Wattenberg Area.]]

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[[file:M125-WattenbergField-Figure24.jpg|center|framed|300 px|{{Figure number|24}}Type log for Niobrara Formation. Well was cored in the B and C chalk beds. Chalks generally have low gamma ray readings, high resistivity, and porosity development. This well was completed in the Codell Sandstone. Reservoirs in the Niobrara are the A, B, C, and Fort Hays chalk intervals. Source beds for the Niobrara include the Sharon Springs, Niobrara chalks, and also A marl, B marl, and C marl. The D marl is organic lean in Wattenberg area. The chalks have been targeted with vertical wells from 1981 through 2010. The play has now become a horizontal play targeting chalks and marls from 2010 to the present.

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The Niobrara ranges in thickness from 300 to 400 ft across the field area ([[:file:M125-WattenbergField-Figure22.jpg|Figures 22]], [[:file:M125-WattenbergField-Figure23.jpg|23]]). The thickness changes in the Niobrara were described by Weimer and Sonnenberg<ref name=WeimSon1982 />. The west to east thin area shown on [[:file:M125-WattenbergField-Figure22.jpg|Figure 22]] was thought to be caused by a paleostructural feature of similar orientation (i.e., basement fault block). The thinning is due in part to erosional truncation of the A chalk unit over the broad east–west trending area. [[:file:M125-WattenbergField-Figure7|Figure 7]] is a north–south stratigraphic cross section datumed on top of the Niobrara. Thickness changes in individual Niobrara chalks and marls units in the past have been noted by several authors and attributed to chalk bar formation (associated with bottom water currents), compensational stacking, paleotopography effects (low areas being isopach thicks and high areas being isopach thins), and submarine current erosion and scour. Numerous normal faults present within the Niobrara Formation create small circular isopach thins on the various isopach maps (bull’s-eyes). The normal faults create tremendous reservoir heterogeneity and potential problems for drillers trying to “stay in zone” in horizontal laterals. [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]] illustrates the location of the Wattenberg temperature anomaly superimposed on the Niobrara isopach map. The outline area of where the Niobrara produces is also shown on [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]].

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The Niobrara ranges in thickness from 300 to 400 ft across the field area ([[:file:M125-WattenbergField-Figure22.jpg|Figures 22]], [[:file:M125-WattenbergField-Figure23.jpg|23]]). The thickness changes in the Niobrara were described by Weimer and Sonnenberg<ref name=WeimSon1982 />. The west to east thin area shown on [[:file:M125-WattenbergField-Figure22.jpg|Figure 22]] was thought to be caused by a paleostructural feature of similar orientation (i.e., basement fault block). The thinning is due in part to erosional truncation of the A chalk unit over the broad east–west trending area. [[:file:M125-WattenbergField-Figure7.jpg|Figure 7]] is a north–south stratigraphic cross section datumed on top of the Niobrara. Thickness changes in individual Niobrara chalks and marls units in the past have been noted by several authors and attributed to chalk bar formation (associated with bottom water currents), compensational stacking, paleotopography effects (low areas being isopach thicks and high areas being isopach thins), and submarine current erosion and scour. Numerous normal faults present within the Niobrara Formation create small circular isopach thins on the various isopach maps (bull’s-eyes). The normal faults create tremendous reservoir heterogeneity and potential problems for drillers trying to “stay in zone” in horizontal laterals. [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]] illustrates the location of the Wattenberg temperature anomaly superimposed on the Niobrara isopach map. The outline area of where the Niobrara produces is also shown on [[:file:M125-WattenbergField-Figure23.jpg|Figure 23]].

A typical well log response for the Niobrara is shown in [[:file:M125-WattenbergField-Figure24.jpg|Figure 24]]. The chalk and marl intervals can be distinguished based on log response. In general, the chalks have a low gamma ray value than the marls. This is attributed to higher clay and total organic clay (TOC) in the marls compared to the chalks. The chalks tend to have higher resistivities than the marls because of the purer chalk lithology and presence of hydrocarbons. In addition, on neutron and density porosity logs, the curves track each other in the chalks and diverge because of increase in clay content in the marls.

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===Terry and Hygiene sandstones===

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Production occurs in the Upper Cretaceous (Campanian) Hygiene and Terry sandstones in the Wattenberg area ([[:file:M125-WattenbergField-Figure26.jpg|Figures 26]], [[:file:M125-WattenbergField-Figure27|27]], [[:file:M125-WattenbergField-Figure28.jpg|28]]). Most of the production is from the Terry Sandstone. The sandstones are interpreted to be marine sandstones enclosed in finer-grained mudstones<ref name=Porter1989 /><ref>Moredock, D. E., and S. J. Williams, 1976, Upper Cretaceous Terry and Hygiene Sandstones, Singletree, Spindle, and Surrey Fields, Weld County, Colorado, ‘’in’’ R. C. Epis, and R. J. Weimer, eds., Studies in Colorado field geology: CSM Professional Contribution 8, p. 264–274.</ref><ref name=PortWeim1982>Porter, K., and R. J. Weimer, 1982, [https://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0012/2500/2543.htm Diagenetic sequence related to structural history and petroleum accumulation: Spindle field, Colorado]: AAPG Bulletin, v. 66, no. 12, p. 2543–2560.</ref>. Sandstones are glauconitic, feldspathic litharenites. The depositional environment was interpreted by Porter<ref name=Porter1989 /> to be offshore marine bars ([[:file:M125-WattenbergField-Figure27|Figure 27]]). Coarsening upward sequences consist of three facies: bioturbated sandy mudstones; burrowed to nonburrowed, thin-bedded, rippled to cross-stratified sandstone; and fine- to medium-grained, cross-stratified sandstone. Sandstone facies are designated as sandy shelf, bar margin, and central bar. The marine bars in the Terry trend northwest-southeast. Bars are 2–4 miles wide and 8–16 miles long. The nature of the trap is stratigraphic. Source rocks are the Sharon Springs Member of the Pierre Shale and the Niobrara Formation.

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Production occurs in the Upper Cretaceous (Campanian) Hygiene and Terry sandstones in the Wattenberg area ([[:file:M125-WattenbergField-Figure26.jpg|Figures 26]], [[:file:M125-WattenbergField-Figure27.jpg|27]], [[:file:M125-WattenbergField-Figure28.jpg|28]]). Most of the production is from the Terry Sandstone. The sandstones are interpreted to be marine sandstones enclosed in finer-grained mudstones<ref name=Porter1989 /><ref>Moredock, D. E., and S. J. Williams, 1976, Upper Cretaceous Terry and Hygiene Sandstones, Singletree, Spindle, and Surrey Fields, Weld County, Colorado, ‘’in’’ R. C. Epis, and R. J. Weimer, eds., Studies in Colorado field geology: CSM Professional Contribution 8, p. 264–274.</ref><ref name=PortWeim1982>Porter, K., and R. J. Weimer, 1982, [https://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0012/2500/2543.htm Diagenetic sequence related to structural history and petroleum accumulation: Spindle field, Colorado]: AAPG Bulletin, v. 66, no. 12, p. 2543–2560.</ref>. Sandstones are glauconitic, feldspathic litharenites. The depositional environment was interpreted by Porter<ref name=Porter1989 /> to be offshore marine bars ([[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]). Coarsening upward sequences consist of three facies: bioturbated sandy mudstones; burrowed to nonburrowed, thin-bedded, rippled to cross-stratified sandstone; and fine- to medium-grained, cross-stratified sandstone. Sandstone facies are designated as sandy shelf, bar margin, and central bar. The marine bars in the Terry trend northwest-southeast. Bars are 2–4 miles wide and 8–16 miles long. The nature of the trap is stratigraphic. Source rocks are the Sharon Springs Member of the Pierre Shale and the Niobrara Formation.

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[[file:M125-WattenbergField-Figure26.jpg|center|framed|300 px|{{Figure number|26}}Structure contour of Terry Sandstone marker bentonite bed. Contour interval 100 ft. Map shows Terry (green dots), Hygiene (red dots), and Terry/Hygiene commingled (pink dots) production. Vast majority of production comes from Terry Sandstone (green dots). Type log shown in this figure. Production curve shown in [[:file:M125-WattenbergField-Figure27|Figure 27]]. Field names from scout ticket reports. The Surrey, Spindle, and Singletree fields are now combined together into the Spindle Field. GWA = Greater Wattenberg Area. These fields are regarded as unconventional accumulations of stratigraphically trapped oil and gas. Fields are shallow pool accumulations in the Wattenberg Field.]]

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[[file:M125-WattenbergField-Figure26.jpg|center|framed|300 px|{{Figure number|26}}Structure contour of Terry Sandstone marker bentonite bed. Contour interval 100 ft. Map shows Terry (green dots), Hygiene (red dots), and Terry/Hygiene commingled (pink dots) production. Vast majority of production comes from Terry Sandstone (green dots). Type log shown in this figure. Production curve shown in [[:file:M125-WattenbergField-Figure27.jpg|Figure 27]]. Field names from scout ticket reports. The Surrey, Spindle, and Singletree fields are now combined together into the Spindle Field. GWA = Greater Wattenberg Area. These fields are regarded as unconventional accumulations of stratigraphically trapped oil and gas. Fields are shallow pool accumulations in the Wattenberg Field.]]

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[[file:M125-WattenbergField-Figure27|center|framed|300 px|{{Figure number|27}}(A) Type log for Terry and Hygiene sandstones, Wattenberg Field area. (B) Paleography for Upper Campanian. (C) Offshore shallow shelf model for sand bar development. Modified from Porter and Weimer<ref name=PortWeim1982 />.]]

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[[file:M125-WattenbergField-Figure27.jpg|center|framed|300 px|{{Figure number|27}}(A) Type log for Terry and Hygiene sandstones, Wattenberg Field area. (B) Paleography for Upper Campanian. (C) Offshore shallow shelf model for sand bar development. Modified from Porter and Weimer<ref name=PortWeim1982 />.]]

[[file:M125-WattenbergField-Figure28.jpg|center|framed|300 px|{{Figure number|28}}Production curve for Terry and Hygiene sandstones from Wattenberg—Terry, Spindle, Singletree, Surrey, Hambert, New Windsor, and LaPoudre fields.]]

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