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Gas Production from the Smoky Hill Member
At the present time, the Niobrara Chalk is the basis for a major gas play in northwestern Kansas, northeastern Colorado, and southwestern Nebraska. The occurrence of natural gas in Niobrara rocks has been known since 1912, with the first significant production in 1918 from the Beecher Island field, Yuma County, Colorado (Lockridge, 1977, p. 271). Following several decades of relative inactivity, interest in the Niobrara was renewed in the 1970s and more than 30 fields are actively under production. Most commercial production is from reservoirs less than 854 m (2800 ft) below the surface. Lockridge and Scholle (1978) reported that the pay zone is a primary reservoir, with high porosity and low permeability, and that gas accumulation is nearly all controlled by structural closure. Those authors, together with Rountree (1979) and Rice and Shurr (1980), noted the regional potential of the Niobrara as a gas reservoir outside the area of present drilling activity.
Energy resources in the play area consist largely of biogenic gas (Rice and Claypool, 1981, p. 14) that resulted from bacterial decomposition of organic matter. The productive interval, approximately 6 to 15 m thick, is near the top of the Smoky Hill Member and apparently corresponds to the upper chalk unit (Scott and Cobban, 1964) of the Smoky Hill Member at Pueblo, Colorado, and to a 6-m-thick interval of granular chalk 8.5 m below the top of the Smoky Hill at Locality 21, Logan County, Kansas. According to Hann (1981, p. 148), hydraulic fracture stimulation is required for economic production in the target interval, which is known as the Beecher Island zone. Shallow drilling depths and favorable gas prices combine to make the production of Niobrara gas feasible economically.
Summary and Conclusions
1. The Smoky Hill Chalk Member crops out extensively in bluffs and badlands of the Smoky Hill River drainage basin of Trego, Gove, and Logan counties, Kansas. A composite section, based on detailed measurements of strata in 12 key exposures and several supplementary sections, is 182 m (596 ft) thick. The member lies conformably and gradationally on the Fort Hays Limestone Member, and is overlain conformably and gradationally by the Sharon Springs Member of the Pierre Shale.
2. The Smoky Hill consists mainly of obscurely to well laminated, fissile, fecal-pellet-speckled, shaly weathering chalk that is mostly olive gray to dark olive gray or olive black and weathers to various, mostly pale shades of yellow, orange, or brown. Laminations are owing mainly to vertical variations in fecal-pellet abundance. In the upper part of the section crinkly structure is common, and many non bioturbated chalk beds lack laminations. All beds of chalk have relatively high content of organic carbon, and a few organic-rich beds form prominent dark-colored bands on little-weathered exposures. Beds of light olive gray bioturbated chalk are scattered irregularly through the lower half of the member. These beds are also fecal-pellet speckled but lack the laminae seen in many nonbioturbated chalk beds. In the upper half of the member, beds of chalk having somewhat granular appearance are the apparent equivalent of bioturbated beds, but contain only sparse evidence of a burrowing infauna. Both bioturbated and granular chalk beds are more resistant to erosion than nonbioturbated beds, and form cap rocks in chalk badlands. Well-preserved macroinvertebrate skeletal remains are common through much of the composite section, and where most abundant form conspicuous shelly zones. Near the middle of the member concentrations of the pelagic crinoid Uintacrinus socialis occur in thin lenses of hard, brittle limestone that are limited apparently to the zone of Clioscaphites choteauensis. The Smoky Hill contains more than 100 bentonite seams, many of which are now represented only by concentrations of iron oxide and gypsum. In little-weathered exposures, the bentonites are mostly pale shades of gray or bluish gray, whereas in weathered exposures various shades of orange, gray, brown, olive, pink, or white prevail. Iron disulphide, commonly weathered to iron oxide and jarosite, occurs as oblate spheroidal bodies developed around large inoceramid valves or along bentonite seams. Several intervals of stratified chalk are characterized by nodules of crystalline, polycuboidal pyrite. Nodular chert is a common epigenetic feature of highly weathered chalk beds that lie directly beneath the Ogallala Formation (Miocene and Pliocene).
3. Compilation of the composite section was facilitated through use of 23 marker units, which are defined on the basis of bioturbated chalk beds, bentonite seams, and organic-rich chalk beds, either singly or in combination. These marker units are spaced sufficiently close together to permit precise determination of stratigraphic position of all but some of the most stratigraphically limited exposures. The marker units have wide geographic extent, and several are recognizable as far to the west as the Pueblo, Colorado, section.
4. Macroinvertebrate fossils serve to delineate a sequence of biostratigraphic zones, including (ascending): Inoceramus (Volviceramus) grandis Range Zone (Upper Coniacian), Inoceramus (Cladoceramus) undulatoplicatus Range Zone (Lower to Middle Santonian), Clioscaphites vermiformis Range Zone (Middle Santonian), Clioscaphites choteauensis Range Zone (Middle Santonian), and Inoceramus balticus s.l. Range Zone (Upper Santonian-Lower Campanian). Several species of ammonites used in the standard Western Interior zonal scheme have not been recorded in the Kansas section, so the Kansas zonation is less refined than in Colorado.
5. Stratified chalks of the Smoky Hill are mostly foraminiferal pelmicrite with packstone or wackestone texture. Fecal pellets, consisting almost entirely of little-altered coccoliths, have been flattened by compaction into oblate spheroids having average maximum diameter of 0.12 mm. Tests of planktonic foraminifers are preserved mostly with walls intact or only slightly altered, and the chambers are usually filled with one to a few crystals of sparry calcite. Accessory carbonate grains include prisms and fragments of inoceramid bivalves and oysters. The matrix is micritic, and comprises a heterogeneous mixture of coccoliths, crystals of secondary calcite, and noncarbonate (detrital) particles. The chalks also contain fish scales and bones; wisps or flakes and angular, siltsized grains of black carbonaceous matter; minute framboids of pyrite or their oxidized equivalent both in the matrix and inside foraminifer chambers; and rare calcispheres. Principal detrital grains are quartz and clay materials. The stratified chalks have strongly preferred, bed-parallel orientation of elongate and tabular grains.
In the upper part of the composite section many chalk beds lack laminae, are not bioturbated, and contain few skeletal remains of macroinvertebrates. These rocks are related genetically to laminated chalk, and are similar petrographically. Insoluble residue content of laminated and nonlaminated chalk ranges from 5.8 to 52.5 percent, averaging 31.4 percent. Most of the rocks are best termed impure chalk; a few beds are better termed chalky marl.
6. Bioturbated chalks are mainly foraminiferal and pelletal micritic wackestones or packstones. The most highly bioturbated beds lack laminations, but all gradations were recorded between bioturbated chalk and laminated chalk. Bioturbated chalk contrasts with nonbioturbated chalk in having less well compacted fecal pellets, generally smaller number of crushed foraminifers, greater abundance of foraminiferal fragments, more common alteration of foraminiferal test walls, smaller amount of black organic carbon, lack or near lack of stratified grain fabric, and fewer pyrite framboids in the matrix. In bioturbated chalk both fecal pellets and matrix contain more secondary calcite than the nonbioturbated chalks. Granular chalk beds have microscopic features intermediate between those of bioturbated and nonbioturbated chalk, but are apparently related genetically to the bioturbated chalk. Granular and bioturbated chalks contain minor amounts of detrital mineral grains and have lower average insoluble residue content than the nonbioturbated chalks.
7. The most obvious diagenetic feature in Smoky Hill chalks is compaction of allochems and horizontal, sediment-filled burrows. Neomorphic calcite and interstitial cement occur in all samples although the amounts vary according to the degree of bioturbation. Foraminifer chambers are filled mostly with sparry calcite, although crushed specimens are usually filled with microspar or (rarely) micrite. Diagenetic dissolution of aragonitic skeletal remains was a major source of carbonate for secondary calcite. Some secondary calcite was derived from pressure solution, which is manifest in incipient microstylolites in many samples and in well-developed microstylolites in the Uintacrinus limestone, but this was not a major source of carbonate for cementation and void-filling. Dissolution of coccoliths was an insignificant source of secondary calcite, but many foraminifer tests are partly corroded and probably furnished a substantial amount of carbonate for secondary mineralization. Cement and neomorphic overgrowths on coccoliths are best developed in bioturbated rocks, suggesting that bioturbation enhanced the lithification process. Overall, Smoky Hill chalk beds are not well cemented, and large volumes of carbonate were not required to produce the observed diagenetic effects. Although the bioturbated beds may reflect slower deposition than the nonbioturbated beds, the Smoky Hill composite section lacks hardgrounds.
8. Smoky Hill muds were deposited on the stable eastern shelf region of the Western Interior foreland basin during the regressional part of the Niobrara cycle. In Kansas the depositional cycle began after a prolonged episode of nondeposition, during which Carlile strata were truncated by submarine erosion, which lasted longest along the more easterly portions of the basin. In Kansas, initial deposits of the Niobrara cycle comprise the Fort Hays Limestone Member, which rests sharply on the Carlile, and incorporate at the base material reworked from the Carlile. The transgressive maximum occurred during deposition of the Fort Hays, which contains little detritus derived directly from terrigenous source areas. In the Smoky Hill, larger quantities of detritus reflect increased influence of terrigenous source areas during a protracted regressional event. The Niobrara chalks become generally less pure upward stratigraphically as well as westward toward the principal source area of landderived detritus.
9. Principal components of the Smoky Hill chalk beds are pelagic elements, especially including coccoliths, fecal pellets, and tests of planktonic foraminifers. Chief among benthic constituents are remains of inoceramids and oysters, with only minor amounts of skeletal remains derived from other macro invertebrate groups. Secondary carbonate material consists primarily of neomorphic calcite overgrowths on coccoliths and foraminifer tests, crystals of calcite cement occurring mostly in the chalk matrix, and sparry calcite fillings of foraminifer tests. Black organic matter, as angular silt-sized grains or as wisps and flakes, occurs in all little-weathered chalk samples, and is interpreted as plant debris. Much of the organic carbon is apparently of marine origin. Interstitial reducing conditions, especially in the nonbioturbated chalks, were responsible for relatively high organic carbon content and abundance of pyrite framboids in the chalk.
10. Smoky Hill muds accumulated in waters having a probable depth range of 150 to 300 m. In western Kansas, water depth apparently was greatest during the latter half of Smoky Hill deposition, although regional stratigraphic relationships suggest general regression at this time. In addition to eustatic sea-level rise and subsidence caused by isostatic adjustment for water and stratigraphic loads, tectonic subsidence may have been a factor in producing this greater depth. Alternatively, a minor transgressive pulse during the overall regression could have produced the same result.
11. Regional patterns of biotic distribution suggest a climatic gradient from south to north across the Western Interior region. For the most part Kansas lay in a mild-temperate belt, which was influenced by occasional incursions of subtropical waters from Tethys.
12. Biotic data suggest that Smoky Hill deposition took place in marine water of less-than-normal salinity, but probably not less than 30 to 31 ‰.
13. During Smoky Hill deposition the sea floor consisted mostly of watery oozes that excluded most infaunal organisms and supported a primary benthos (inoceramids, rudists) comprising forms adapted to life on soft substrates. Purer oozes accumulated at times of minimum terrigenous detrital influx and supported a limited, mostly soft-bodied infauna. Bottom currents were weak, and bottom waters were usually poorly oxygenated. At times of better circulation, increased oxygen levels fostered increase in diversity and numbers of benthic organisms, which are now preserved in shelly zones. In the upper part of the member, nonlaminated (and nonbioturbated) chalks, which contain few macroinvertebrate fossils, represent a nearly anoxic (perhaps intermittently anoxic) benthic environment. Smoky Hill laminae do not appear to be of seasonal origin. Winnowing of bottom muds by weak currents explains the unequal thickness and erratic distribution of laminae.
14. Smoky Hill fossils are preserved as calcareous skeletal remains (most abundant), molds, borings, combinations of calcareous and organic matter (rare), and chitin (rare). The most abundant fossils are inoceramids and oysters. Inoceramids, rudists, and conchs of dead cephalopods were hosts for a limited variety of attached and boring forms, including oysters, cirripeds, serpulids (rare), and sponges (rare). The inoceramids and rudists manifest growth forms signifying adaptation to life on soft substrates, and commonly grew to extraordinarily large size. Oysters commonly encrusted both exposed and buried sides of their hosts, and commonly have preferred orientation related to current direction or possibly to growth in the direction of newly created substrate. Hosts encrusted only by oysters in the spat stage of growth indicate burial before oysters could mature. Pectinids and bakevelliids are very sparse, and may be epiplanktonic forms. Pelagic crinoids occur as articulated specimens only on undersides of Uintacrinus limestone lenses, where disintegration was prevented because of reducing conditions. Restricted stratigraphic occurrence and enormous numbers of individuals suggest crinoid death by mass mortality. Cephalopods were all nektonic forms, some perhaps nektobenthonic, and present no evidence of death by predation. Concentration of scaphites in only a few, limited stratigraphic intervals suggests strong control by water chemistry, and is probably related to peak salinity. Trace fossils are less diverse than in the Fort Hays and are rare in the upper part of the Smoky Hill. Poor circulation of bottom waters and paucity of suitable substrates were the major factors that restricted development of the macro invertebrate infauna.
15. Smoky Hill benthic communities were characterized by low diversity assemblages, the basic elements of which were inoceramids and epizoic oysters. Inoceramids and sparse rudists were "islands" on the soft sea floor, and served as the principal substrates for all other epifaunal species. At times, improved circulation of bottom waters fostered increase in numbers and diversity of benthic macroinvertebrates. Decreased circulation resulted in reduction of diversity and numbers of individuals nearly to zero. High proportions of bivalves with articulated valves, consistency of the basic assemblages, minimum of shell fragmentation, and wide size range of individuals at a given horizon indicate that Smoky Hill assemblages represent fossil communities. Except for vagile infaunal burrowers (worms?), all benthic macroinvertebrates were suspension feeders. Smoky Hill muds contained a plentiful food supply, which was not utilized extensively by infaunal deposit feeders because of unsuitable substrates and marginally oxygenated bottom waters. Teeth of shell-crushing sharks are rare and bite marks on shells were not observed, suggesting that predation was not a major factor in benthic community life.
16. During Smoky Hill deposition the seawater was relatively deep, slightly below normal salinity, and had a temperature characteristic of mild-temperate to subtropical climates. The sea floor was almost perfectly flat, the surficial deposits were soft and watery, and the depositional rate approximated 0.036 mm/year. Bottom currents were weak and bottom waters were mostly oxygenated poorly. The benthic scene was also dark, monotonous, and hostile to many groups of marine organisms.
Acknowledgments
It is a great pleasure to acknowledge here the help that was received from friends and colleagues during the course of this study. Virgil B. Cole, who during the thirties spent many months mapping surface structure in the Niobrara outcrop, conducted me through the Smoky Hill section of Graham County and pointed out many of the marker bentonites that were used in those days as structural datums. He also shared with me his extensive knowledge of Smoky Hill localities in the study area. Assistance with the field work was given ably by Craig B. Hatfield, Gary F. Stewart, Phil Heckel, Mark Nickelson, and Calvin James. Erle G. Kauffman assisted in the identification of inoceramid bivalves and has discussed with me on several occasions problems of Niobrara correlation and depositional environment. Rick Zakrzewski has kindly permitted me to examine fossil collections in the Sternberg Museum at Hays. J. D. Stewart generously shared with me his extensive knowledge of Smoky Hill invertebrate fossil occurrences. Landowners in the study area permitted full access to their properties, without which the work could not have been completed. Drafting, typing, and photographic services were furnished by the Department of Geology, Indiana University. In this regard, special thanks are extended to James Tolen, Karen Walker, and George Ringer, respectively. The Kansas Geological Survey furnished financial and logistical support for the several summers during which the main portions of field and laboratory work were undertaken. Those especially helpful in this regard were Daniel F. Merriam, William W. Hambleton, W. James Ebanks, and Lila Watkins. For purposes of comparative analysis several days were devoted to study of the Niobrara section at Pueblo, Colorado, during the summer of 1976. That part of the research was supported by the Earth Sciences Section, National Science Foundation, Grant DES75-06542. Acknowledgment is made also to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research during the final phases of field work in the summer of 1978. Lastly, I wish to acknowledge my wife, Marjorie, for assisting me during the final trips to the field in November 1980 and May 1982, and for her encouragement throughout the course of this study.
To all of the above-named persons and institutions, I extend my profound thanks.
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Kansas Geological Survey, Geology
Placed on web Feb. 20, 2015; originally published Dec. 1982.
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