The Art of Logging Horizontal Niobrara Wells in the Denver Basin

Title: The Art of Logging Horizontal Niobrara Wells in the Denver Basin

Author: Janet Marks, Halcon Resources

Publication: The Outcrop, March 2013, p. 8, 10, 14-21, 26-29, 32-37

Ms. Marks has a BS from the University of Illinois and MS from the University of Wyoming. She formerly performed mudlogged/well site geology on 33 horizontal wells throughout the Denver Basin for Columbine Logging from 2010-2012.

She would like to thank Columbine Logging and the various operators for the time spent drilling Niobrara wells that allowed her to gain this knowledge.

This article was part of a presentation for the AAPG-RMS in Grand Junction, September 2012.

Relatively recently, horizontal drilling into basin centered, continuous source rocks has exploded. Without pre-planning of the well path, it is easy to deviate from the formation sweet spots during drilling. Seismic and wellbore control are examined prior to drilling t the borehole to anticipate any faults. Using all the available data, the well site geologist helps make timely critical decisions on the borehole path when drilling fast in real time.

Calling the top of formations while drilling vertically and landing the intermediate casing within the exact part of the desired formation is important. After drilling horizontally out of the intermediate casing, it is necessary to know what bed within the formation the bit is located in with respect to the target zone.  The target is generally a 5-20 foot thick layer that has the greatest porosity, least gamma, and/or highest resistivity.

The well site geologist uses various tools to log critical geological data during real-time drilling of wells to provide formation evaluation, monitor drilling performance and determine the location of the bit in the pay zone.

The following article shows examples from the stratigraphic section drilled to the Niobrara target as well as through the target interval.

During the time of Niobrara deposition (Upper Cretaceous; late Turonian-earliest Campanian), the continents experienced highstand eustatic sea levels worldwide resulting in shallow epicontintental seas including the Western Interior Sea. To the west i in the Cordilleran region, the Sevier orogeny was causing subduction, volcanism and mountain building. Siliciclastic sedimentation and subsidence occurred in the western portion of the seaway. To the east was a relatively shallow shelf that was terrigenous sediment starved. Widespread pelagic and hemipelagic carbonate sedimentation rich in coccolith and planktonic foraminifera were deposited.

Cretaceous Transgressive/ Regressive Cycles

Figure 1: From Longman, Luneau, and Landon, 1998 The Niobrara Formation represents 4 large-scale, chalk rich sedimentary cycles reflecting synchronous transgressive/ regressive pulses largely eustatic in origin (Scholle and Pollastro, 1985). These carbonate benches were thick enough to form regionally extensive, homogeneous chalk and marl sequences. G.K. Gilbert (1895) was the first to describe these rock. He also was first to recognize these cycles as likely related to periodic variations in the Earth’s orbit or what is now known as Milankovich cycles.

Stratigraphy and Type Log

Figure 2: After Sonnenberg, 2011 and Longman, Luneau and Landon, 1998. This is a type log of northern Denver Basin showing gamma ray, combination resistivity and compensated neutron-density logs. This data is used to subdivide the Niobrara into ten intervals that can be correlated across the Denver Basin and into adjacent areas. The chalks exhibit lower gamma and higher resistivity. Also note the neutron/density cross over in the B and C Chalks, usually the horizontal drilling reservoir targets. The chalks are generally high in CaCO3 (70-80%), low in clays and very brittle leading to natural fracturing and greatly enhanced reservoir porosity. The marls are higher in gamma, higher in organic carbon and are thought to be the source rocks of the Niobrara. The marls have less CaCO3, more clay and are therefore more ductile for fracturing. Also note the Sharon Springs Member of the Pierre Shale just above the top of the Niobrara with the high gamma marker bed.
Figure 3: LPlot mud log of the Sharon Springs (base of Pierre Shale) to Niobrara A Marl. I am going to break from conventional wisdom and discuss these layers in the order that you see them as a well is drilled versus in ascending order of time and deposition. The Sharon Springs Bentonite Marker Bed occurs 15-30 feet TVD above the top of the Niobrara. The characteristic double peak (red arrow) of the gamma ray goes up to 250 API at the marker bed. At the top of the Niobrara is a thin unnamed marl, 15-30 feet TVD thick with a gamma around 150 API. The A Chalk (Upper Chalk) is generally 10-30 feet thick TVD with low gamma around 100-115 API. Beneath the A Chalk, the A Marl (Upper Chalky Shale) is 35-60 feet TVD thick where the gamma goes back up to 130-150 API. The higher gamma often is related to bentonite beds (high radioactive potassium) or high Uranium content associated with high organic content.
Figure 4: Pierre Shale: 6450’ TVD The Sharon Springs right above the top of the Niobrara. Note: all TVDs are approximate as these photographs are taken from numerous wells across the Denver Basin and I am correlating them back to the example mudlog. Note the pyrite (yellow arrow) and oil sheen (black arrow) that can sometimes be visible and the bentonite that fluoresces (red arrow). The Sharon Springs is a dark gray to black shale that organic rich, slightly bituminous, and deposited in an anoxic marine environment. When drilling above the Sharon Springs Marker the gamma is still below 100 API. As you approach the top of Niobrara, the Pierre Shale gets very platy and generally does not effervesce with HCl. The sandstone stringers that were abundant below the Tepee Buttes and just above this sample disappear and the shale appears organic rich. The Sharon Springs readily collapses and can give the drillers lots of problems. Big coarse platy chunks can be visible on the shaker screens. It is important to get the mud weight up around 10.5 to prevent sloughing. If the flaking off is abundant, the mud engineer may use Soltec or Baritrol (an asphalt product). This powder dissolves/melts if hot enough (above 126° F) and becomes part of the borehole wall cake. It is important to have the Intermediate Casing cover all of the Sharon Springs.
Figure 5: Pierre Shale: 6470′ TVD. The Sharon Springs Marker Bed with abundant bentonite (red arrows) that brightly fluoresces. This bentonite marker bed is generally 15’- 25’ TVD above the top of the Niobrara and usually has disseminated pyrite (yellow arrow). It is not thick, but because the drilling is at an angle, it appears thicker. The gamma correlation shows a characteristic highly radioactive-high gamma double peak up to 280-300 API for 2 feet. This correlates with the Ardmore Bentonite, a 1-2” bed of yellowish-orange bentonite with biotite that is interbedded with dark gray shale. The type section is named after a quarry in Ardmore, SD and is used as a marker for Cretaceous rocks of the Great Plains. The bentonite is a volcanic ash derived from the Sevier Orogenic Belt in Utah, Nevada, Idaho and Western Wyoming.
Figure 6: Sharon Springs Marker Bed under ultra violet light. You can see the pieces of bentonite under the microscope as in the previous figure, but when in the fluorescent light box, they glow. The sample will have little or no effervescence with HCI.
Figure 7: 6495′ TVD, Top of Niobrara. Note the framboidal pyrite crystals (yellow arrow), bentonite (red arrow) and the mottled chalk (blue arrow). The calcium carbonate of the Niobrara causes violent effervescence with HCI.
Figure 8: 6520′ TVD Niobrara A Chalk. Note the mottled chalk (blue arrow) and platy bentonite (red arrow). It is described as: Lt olive-black chalk, lt brown, lt gray, buff, m lt-m gy – m brn, sft – firm, subplty-sbblky, motstri, earthy luster, arg, calc mtx, cln fiz, tr pyr, sme-occ bent and bri yel mnrl flor. The Niobrara A Chalk is dark compared to B Chalk, i.e. it is not as clean as the B Chalk. The gamma drops from high (250 API) to low (110-115 API), but is higher gamma than B Chalk.
Figure 9: 6550′ TVD A Marl. The A Marl is dark gray to dark brown, silty, with little or no mottling. The gamma increases to 130-150 API. The marls were deposited in more anoxic conditions than the chalks and have higher organic content. The log description is: m dk brn-gy, mod sft-firm, sbplty- sbblky, lam, slty ip, gty, arg tex, org/calc mtx, tr pyr, sme bent w/ lt yel flor, sme dism pyr. In outcrop, it is pale yellowish-brown, fissile, chalky shale that contains many beds of bentonite and large concretionary masses of shaly limestone.
Figure 10: 6560′ TVD A Marl with 10% bentonite. About 10-15’ TVD below the top of the A Marl there is a bentonite layer (red arrow) that is pale grey to white gray with brilliant yellow fluorescence. There are abundant disseminated pinpoint cubic pyrite and framboidal pyrite (John Witner, personal communication, 2010). In addition there are very few fossils in the A Marl. Also note the Lubrabeads (black arrow) in the center of the photograph, an additive to help with sliding. In weathered outcrop, there are hundreds of thin layers of rusty red bentonite clay from the fall of ash from repeated eruptions of volcanoes to the west – Nevada and Utah. These ash deposits can be traced for miles across the chalk-marl beds. They are used as marker units in describing the stratigraphy of the formation (Hattin, 1981).
Figure 11: 6570′ TVD. Niobrara A Marl. Note the large framboidal pyrite crystal (yellow arrow) and the lack of fossils or bentonite.
Figure 12: LPlot mudlog of bottom of the A Marl to the B Chalk. The B Chalk Bench (Middle Chalk) is 25-35’ TVD thick in Denver-Julesburg Basin. The B Chalk is cleaner than the A Chalk with low gamma (70-80 API). Usually the gas goes up. The log description for the B Chalk is: m-m lt gy – brn, sft- firm, sme brit, sbplty-sbblky, mottled-striated, tr wh frag, earthy luster, clean fiz, tr pyr. The upper part of the B Chalk has characteristically more marl content, higher gamma and is a darker chalk. The bottom half of the B Chalk is the main target zone (between the two red arrows).
Figure 13: Oil shows under fluorescent light. 6300-6900’ MD. The Sharon Springs Member of the Pierre Shale (6300-6550’ MD) does have some weaker oil shows, but there are strong oil shows when drilling in both the chalks and marls of the Niobrara.
Figure 14: 6600’ TVD Niobrara B Chalk with White Specks (blue arrow). The white specks are coccolithrich fecal pellets, probably formed by pelagic copepods that thrived during times of high sea level as water circulated through the seaway (Longman, Luneau and Landon, 1998).
Figure 15: 6615’ TVD B Chalk with some bentonite with disseminated pyrite.
Figure 16: 6625’ TVD B Chalk Target Zone. The target zone is the bottom half of the B Chalk. It is an approximately 16’ TVD thick porous zone that has high resistivity and a cross over area between the neutron and density curves. It is mottled white (blue arrow).
Figure 17 6640’ TVD Niobrara B Chalk base/Top of B Marl with Inoceramus and trace bentonite with disseminated pyrite (red arrow). At the bottom of B Chalk/top of B Marl interface there are abundant Inoceramus and fossils (white arrows). In outcrop it is a gray hard platy chalk separated by beds of gray hard fissile chalky shale.
Figure 18 6650’ TVD B Marl with some white specks. Black arrow points to a walnut shell (Lost Circulation Material). The white piece on the right is a fossil. The B Marl is the most oilprone source rock of the Niobrara Formation.
Figure 19 6690’ TVD B Marl with abundant bentonite (light p pieces) with trace disseminated pyrite (red arrows). There is also some calcite and fossils (white arrow). In the B Marl, the bentonite is generally devoid of pyrite, or if present it has very fine grained pinpoint cubes (John Witner, personal communication, 2010).
Figure 20 B Marl Inoceramus (white arrow) in outcrop Near Pueblo Science Center. Note the interbedded, more massive bioturbated section (chalk) versus the fissle, laminated, nonbioturbated section (marl) where the Inoceramus are well preserved.
Figure 21: Close up of an Inoceramus in same outcrop with long structural prisms (white arrow). The brown is probably oxidized pyrite.
Figure 22: 6700’ TVD Bottom of B Marl with abundant Inoceramus prisms (white arrows). There is also abundant bentonite with trace pyrite that fluoresces (red arrows). You generally do not see this many fossils in the A Marl. Note Lubra beads (black arrow).
Figure 23 6720’ TVD C Chalk with Inoceramus prisms (white arrows). The C chalk does not have a distinct top, but is very interbedded with marl. This description is: 80% CHK: m – m lt gy – brn, sft – firm, sbplty -sbblky, mot-stri, rthy lstr, arg, calc mtx, cln fiz, v calc; 20% MRL: m dk – m gy – brn, mod firm – sft, brit, sbplty -sbblky, mot-lam, rthy lstr, slty ip, tr Inoc, n bent, v calc.


  • Sharon Springs – Ardmore bentonite marker with double peak high gamma and brilliant mineral fluorescence, very little effervescence with HCI, hot shale, anoxic, platy, dark gray to black shales, bituminous, large pieces on the shale shaker, 15-30’ TVD above top of Niobrara, not thick but drilling at a high angle
  • Top of Niobrara – violent effervescence with HCI indicating calcareous, lighter in color than the Sharon Springs, smaller pieces, 15-30’ TVD thick unnamed marl
  • A Chalk – not as clean as B Chalk, few fossils if any, some bentonite, mottled, lighter color, 10-30’ TVD thick
  • A Marl – large framboidal pyrite crystals and bentonite bed, little or no fossils, truly a marl, darker and higher gamma than chalks, silty, organic rich, 35-60’ TVD thick
  • B Chalk – cleaner than A or C Chalks, white specks, some fossils, target zone in lower half, mottled, lighter color, trace to some bentonite, base has some fossils + Inoceramus prisms, 25-35’ TVD thick
  • B Marl – more interbedded with chalk, bentonite with pinpoint disseminated pyrite, abundant fossils especially near the bottom with Inoceramus prisms, darker and higher gamma than chalks, silty, organic rich, 35-50’ TVD thick
  • C Chalk – interbedded with marl-especially in the top, some fossils, Inoceramus prisms, target zone also in lower half, 30-40’ thick
  • Chalks – 70-110 API gamma; Marls – 120-150 API gamma
The top of a Niobrara outcrop being measured by Kelly Bruchez who is completing his Master’s in Geology with Dr. Steve Sonnenberg at the Colorado School of Mines. The outcrop is located along the Blue River on the Jones Ranch approximately 3.5 miles south of Kremmling, Colorado. Photo by Larry Rasmussen.

References Cited
Gilbert, G.K. 1895, Sedimentary Measurement of Cretaceous Time, Journal of Geology, V.3, p. 121-127.

Hattin, Donald E., 1981, Petrology of the Smoky Hill Member, Niobrara Chalk (Upper Cretaceous), in Type Area, Western Kansas, AAPG B65 5, p. 831-849.

Kauffman, Earl G., 1977, Geological and Biological Overview: Western Interior Cretaceous Basin, RMAG, MG14, 3-4, p. 75-99.

Kauffman, Earl G., 1977, Second Day, Upper Cretaceous Cyclothems, Biotas and Environments, Rock Canyon Anticline, Pueblo, Colorado, RMAG, MG14, 3-4, p129-152.

King, Phillip B., 1959, 1977, the Evolution of North America, Princeton University Press, 197 p.

Longman, Mark W., Barbara A Luneau and Susan M. Landon, 1998, Nature and Distribution of Niobrara Lithologies in the Cretaceous MG35 4, p. 137-170.

Scholle, Peter A. and Richard M. Pollastro, 1985, Sedimentology and Reservoir Characteristics of the Niobrara Formation (Upper Cretaceous), Kansas and Colorado: Rocky Mountain Carbonate Reservoirs, a Core Workshop: SEPM Core Workshop 7, p. 447-

Sonnenberg, Stephen A., 2011, The Niobrara Petroleum System: A New Resource Play in the Rocky Mountain Region, Chap. 1 of Estes-Jackson, Jane E. and Donna S. Anderson, RMAG Revisiting and Revitalizing the Niobrara in the Central Rockies, p. 13-32.

Witner, John, 2010, personal communication, Columbine Logging.