Log-Derived Indicator of Thermal Maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming

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1 C H A P T E R 2 1 Log-Derived ndicator of Thermal Maturity, Niobrara Formation, Denver Basin, Colorado, Nebraska, Wyoming THOMAS M. SMAGALA Consultant CHARLES A. BROWN CBW Services, nc. GARY L. NYDEGGER Consultant Revisiting and Revitalizing the Niobrara in the Central Rockies, J.E. Estes-Jackson, D.S. Anderson, eds. Denver, Colo.: Rocky Mountain Association of Geologists, 211.

2 355 LOG-DERVED NDCATOR OF THERMAL MATURTY, NOBRARA FORMATON, DENVER BASN, COLORADO, NEBRASKA, WYOMNG THOMAS M. SMAGALA', CHARLES A. BROWN2, AND GARY L. NYDEGGER3 ABSTRACT The Cretaceous Niobrara Formation in the Denver basin is a highly organic, hydrocarbon-source rock. Accurate values of vitrinite reflectance (RJ present in the Niobrara penetrated by 23 control wells were measured directly or derived by plotting the log of Ro from two or more vertically-distributed measurements versus depth. These vitrinite reflectance values when plotted against borehole resistivities for the "K" zone of the Niobrara in the wells, substantiate a linear loglog correlation between the two parameters that allows the mapping of organic maturation levels overa large part of the Denver basin through the use of a large number of available electric logs NTRODUCTON Vitrinite reflectance (Ro), as well as other rock derived indicators of the thermal maturity or organic material, has become a powerful tool in the exploration for oil and gas. Basin-wide maturation studies can provide clues by which to resolve parameters related to 1) the types of hydrocarbon present, 2) migration trends, 3) trapping mechanisms, 4) burial history reconstruction, 5) formation diagenesis, and 6) geothermal history. n most cases the lack of published data, cost of the analysis, and insufficient well cuttings or cores generally restrict the interpretation of a basin's maturity to the contouring of a relatively small number of vitrinitereflectance values over a large area. This paper presents an empirical technique which uses the logderived resistivity of a known organic source bed, the Niobrara Formation in the Denver basin, to predict an equivalent vitrinite reflectance in any well in which a resistivity log was run through the Niobrara. This technique can provide a moreextended analysis of an area using several thousand data points derived from resistivity logs rather than a few based on direct vitrinite measurements. The study area in the Denver basin is located in eastern Colorado, southeastern Wyoming and southwestern Nebraska,(Figure 1). The stratigraphic section to be discussed in this paper is shown on Figure 2. SQURCE*ROCK RESSTVTY MATURATON-LEVEL CONCEPT The conceptual basis for relating the resistivity of a source bed, such as the Niobrara, to the level of organic maturation is presented by Meissner (1978) in his discussion of the Bakken Formation of the Williston basin. Meissner demonstrated that the Bakken is a known source bed with organic- 'Consultant. *CBW Services, nc. 'Consultant. KANSAS - 4 Km Figure 1. Location map of Denver basin. W K Y MOUNTAN ASSOCATON OF GEOLOGSTS

3 356 THOMAS M. SMAGALA, CHARLES A. BROWN, AND GARY L. NYDEGGER TERTARY FORMATONS DENVER-ARAPAHOE FORMATON THCKNESS LL MEMBER t LARAME FORMATON TERRY SANDSTONE oz " - LATE CRETACEOUS < z c d 5- $ 2: MTTEN MEMBER SHARON SPRNGS MEMBER SMOKY HLL MEMBER FORT HAYS LMESTONE (TMPAS Ls.) COMLL SANDSTONE MEMBER GREENHORN LMESTONE -$E *s? - - e $g *c - -"D' ZONE MARKER EARLY CRETACEOUS -"KB ZONE MARKER < ) LYTLE EQUVALENT L JURASSC MMRSW FORMATON RALSTON CREEK FORMATON.in? CY) - PERMAN PENNSY LVANlAN FOUNTAlN FORMATON Figure 2. Generalized geologic column, Denver basin. Dotshdicate oil productive units (after Welmer, 1973). carbon content ranging from.65 to 1.33 weight percent and averaging 3.84 weight percent. Meissner was able to relate hydrocarbon shows, sonic-transit time, overpressuring, sub surface temperature and resistivity values to the area where the Bakken is a mature oil-source rock. Similarly, the Niobrara Formation of the Denver basin is documented to be an oilsource bed with organic carbon contents ranging from -43 to 5.8 weight percent and averaging of 3.2 weight percent (Rice, in publication). A visual microscopic kerogen examination of Niobrara core samples from five wells in northeastern Colorado by Rice indicated the following percentages of kerogen types, 1) amorphous, 44 to 5%, 2) herbaceous, 22 to 34%, 3) woody, 11 to 22%, and coaly, 11 to 12%. The Niobrara is composed of chalks and calcareous shales deposited in a deepwater, far-offshore environment. Figure 3 is a type log showing clean-matrix, chalk zones which are highly correlative and consistent over virtually the entire Figure 3. Type log of Nlobrara showing the "K" zone which was correlated throughout the Denver basin. Amoe+Chemplin 344 A-1, SW SW See. 25 TlON; R64W, Weld Co., Colorado. Denver basin, Lockridge and Scholle (1978) show that there is a reduction in porosity with increased depth of burial for the chalks of the Niobrara in the Denver basin as compared to European chalks (Figure 4). One consequence of this porosity reduction with increased overburden is that the resistivity of the chalk can be expected to increase according to the em pirical Archie equation (Archie, 1942). R, Rt = $93, n Where: Rt. = resistivity of the formation (ohm-meters) = resistivity of the formation water (ohmmeters) =pore space m and n = empirical exponents S, = water saturation in pore space ROCKY MOUNTAlNASSOClATlON OF GEOLOGSTS

4 LOG-DERVED NDCATOR OF THERMAL MATURTY <3 -J >. k 3- z W >!x V, 2- LT a ap or GAS WELL i DEPTH TO TOP NOBRARA - FEET Figure 4. Nlobrara porosity versus depth relationship, eastern flank of Denver basin compared wlth European chalks (after Lockridge and Scholle, 1978). However, it was observed in this study that the dominant factor causing increases in resistivity of the Niobrara is not the further reduction of porosity, but increasingly higher indigenous oil saturations within the clean-matrix chalks caused by the level of maturation and the production of hydrocarbons. n fact, from the analyses of formationdensity logs, it appeared that once significant maturation commenced, further porosity reduction is minimized. Conceptually, it is perceived by the authors that two criteria must be present to provide a quantification of maturity level with resistivities, 1) an extremely, low permeability matrix rock with a water-capillary system, (in this case, chalk) and 2) an interspersed andlor local source of organic material. THERMAL MATURATON MEASUREMENT.VTRNTE REFLECTANCE (Ro) The primary standard used for the determination of source rock maturity in this study is vitrinite reflectance. Vitrinite macerals are the primary constituents of humic coals and are common components -of many sedimentary kerogens. Vitrinite is formed by the diagenesis of lignin and cellulose from plant cells. Reflectance measurements are obtained from concentrated organic material which has been segregated from rock samples by laboratory techniques. The concentrated material is immersed in oil, and the percent reflectance from incident light is measured on identified vitrinite particles through the use of a microscope containing a calibrated photometer (Dow and OConnor, 1982). Measurements are normally made on a number of particles present in a single concentration sample, and ranges of values obtained from the sample are presented for interpretation in the form of a statistical plot. The stage of thermal maturity characterized by a given amount of vitrinite reflectance may be related to equivalent coal rank, or more importantly to zones of maturity in which hydrocarbons from specific types of organic matter are generated or destroyed (Figure 5). Figure 6 is a vitrinite-reflectance distribution plot from a single sample interval from a well used in this study (lainter,' 1982). When interpreting a single Ro distribution, such as Figure 6, it is necessary to recognize the primary vitrinite population. Rock samples from both cores and well cuttings may have secondary vitrinite populations that have been recycled from geologically older sediments. Samples from well cuttings have the additional problem of contamination by uphole cavings. The mean reflectivity of the primary uncontaminated vitrinite population is the value generally used to quantify the maturation of a rock sample. A primary rule in interpreting reflectance data is that Ro values will increase exponentially with a linear increase in depth, or in temperature when the geothermal gradient is linear. Unless there are complications such as faults, igneous intrusions, significant unconformities or changing geothermal gradients, the Ro values will plot as a straight line on a semilog graph (Figure 7), and the slope of the line will depend upon the geothermal gradient and the geologic age of the section. ROCKY MOUNTAN ASSOCATON OF GEOLOGSTS

5 358 THOMAS M. SMAGALA, CHARLES A. BROWN, AND GARY L. NYDEGGER PEAT 1 ORGANC MATTER TYPE LGNTE SUB- pn 4 6 5% g T B z - hg A 3 L 2,= at 2 MED- LOW- SEM w - 2- U - -META f AMORPHOUS (OL) LPTNTC CBALY (GAS) Figure 5. Zones of petroleum generation and destruction as related to the vitrinlte reflectance (Ro) and coal rank (after Dow, 1982). Vitrinite-reflectance data were obtained from 23 wells in the Denver basin (Clayton and Swetland, 198; Tainter, 1982; Rice, 1983; proprietary data) as shown in Table 1. Figure 7 is a semi-log plot of log Ro vs. linear depth for the Amoco-Champlin 344 A-1 (Sec. 25 TlON, R64W Weld County, Colorado). The Codell in this well has a Ro value of.66% as interpreted from the average line representing all data on semi-log plot even though the actual measured mean reflectivity of the primary vitrinite population is.75%. The interpreted Ro value from a Ro vs. depth profile is a more reliable indicator of the maturity level than a single Ro at a given depth..coal-rank data from outcrops and shallow mines in the Denver and Laramie formations was also used to calibrate vertical maturity profiles (Kirkham, 1978). All of the coals are of lignite to subbituminous rank and represent equivalent vitrinite reflectances of.3 to.45 percent (see Figure 5). n the Wattenberg field area, vitrinite-reflectance values were studied from seven wells in the Hygiene interval, and seven wells in the Skull Creek. When the data in this localized area are examined in conjunction with the surface coals of the Denver and Laramie formations, it is necessary to interpret a dogleg (i.e.: change in slope) of the Ro versus depth profiles. Figure 8 shows the data points in the Hygiene and Skull Creek and an interpreted profile to a surface vitrinite reflectance of.45%. This phenomena of dogleg maturation profiles may be caused by a change in the geothermal gradient (Hunt, 1979). RELATON OF NOBRARA RESSTVTY TO VlTRlNlTE REFLECTANCE n order to map organic-maturation levels on a regional basis.throughout the Denver basin with a limited amount of actual Ro measurements, a relation between 1) the vitrinite reflectance of the Niobrara Formation and 2) borehole log FT. AVG.%RO MEDAN S.DLV, PONTS -_ ) X ) K ) y( ,O VlTRlNlTE REFLECTANCE (Ro $6) Figure 6. Vitrinlte-reflectance distribution plot. Amoco-Champlin 344 A-, SW SW Sec. 25 TlON; R64W, Weld Co., Colorado (after Tainter, 1982). ROCKYMOUNTAN ASSOCATON OFGEOLOGSTS

6 LOG-DERVED NDCATOR OF THERMAL MATURTY 359 Table 1. Actual or derived Niobrara vitrinite reflectance and Niobrara K zone resistivity data. Vitrinlte data from Clayton and Swetland (1 98O), Rice (in publication), Tainter (1 982) and proprietary sources. VlTRlWlTf REFLECTAlCE ( R %) k L % LQS mb o N w. - m -TERTARY -FOX HtLLS -PERRE -HYGENE Location 6s-63W 1 2s-43W 2 1 N-54W 3 1 N-66W 4 2N-54W 5 2N-66W 6 2N-67W 7 2 N -67W 8 2N-68W 9 3N-51 W 1 3N -67 W 11 4N-46W 12 4N-65W 13 4N-67W 14 5 N -64W 15 5N-68W 16 8N-62W 17 9N-56W 18 9N-66W 19 1N-64W 2 14N-58W 21 15N-62W 22 16N-66W 23 K Zone Plot # Resistivity O 32. EST Actual or Derived Niobrara Vitrinite Reflectance, RQ oo NQBRARA -K ZQNE - -CODELL - -SKULL CREEK -LYONS Figure 7. Vltrlnlte reflectance versus depth plot. Amoco-Champlln 344 A-1, SW SW Sec. 25 T1 ON; R64W, Weld Co., Colorado (modified after Tainter, 1982). measured resistivity values representative of a single regionally-correlative Niobrara stratigraphic zone was developed. n this study only deep-reading resistivity devices, such as the laterolog, induction and long-spaced electric logs were used for mapping. The variability of bed resolution inherent in the tool design of the three types of resistivity tools, as well as improper tool calibration, may make local contouring of resistivities difficult, but have little effect on regional trends. For this study, the K zone, as shown on Figure 3, was selected for relating resistivity to Ro. Vitrinitereflectance values interpreted for the Niobrara from the 23 control wells and the corresponding resistivity values of the K zone were plotted (Figure 9). The statistical data spread indicates the existence of a valid empirical correlation. A listing of the data points used in Figure 9 is provided in Table 1. MATURTY MAPPNG BASED ON LOG-DERVED EQUVALENT VlTRlNlTE REFLECTANCE The significant maturity levels of vitrinite reflectance related to critical stages of hydrocarbon generation, as shown on Figure 5, are.5 to.6% (start oil generation),.8% (start wet gas generation), 1.% (start dry gas generation) and 1.2% (peak dry gas generation). These would correlate to Niobrara K zone resistivities of 16 ohmm, 35 ohm-m, 61 ohmm, and 1 ohm-m, respectively, as interpreted from Figure 9. Figure 1 is a contour map of equivalent vitrinite reflectance values based on their established relation to Niobrara K zone resistivities throughout the Denver basin. An average of four wells per township were selected as the resistivitydata base underlying the preparation of this map. ROCKY MOUNTAN ASSOCATON OF GEOLOGSTS

7 36 THOMAS M. SMAGALA, CHARLES A. BROWN, AND GARY L. NYDEGGER 9 c. VlTRlNlTE REFLECTANCE (Ro %)..2 a ' p.a 3P 4D lyl l l ~r SURFACE COALS ' ' 2 4 r - &SOW w TERRY-HYGENE NTERVAL SKULL CREEK Figure 8. Vitrinite reflectance versus depth plot of several wells and surface coal data, (Wattenberg field area located on Figure 1) showing dogleg maturation profile. n general, logderived equivalent vitrinite values representing the Niobrara K zone (Figure 1) increase toward the west in coincidence with increasing burial depth. The most apparent anomalous feature is a southwest-northeast trending wedge of high logderived equivalent vitrinite reflectance values that extends along the Front Range from Fort Collins to Denver and then northeastward to about 9N-58W in Weld County, Colorado. This feature crosses structural contours and is on trend with the Colorado Mineral Belt, a feature of Late Cretaceous to Early Tertiary ages (Warner, 198). These higher logderived equivalent vitrinite-reflectance values, are believed to have been produced by a localized increase in the temperature gradient along the Mineral Belt. The area showing high equivalent vitrinitereflectance values on the "K' zone Niobrara map in the vicinity of Denver is situated in the deepest part of the basin and is interpreted to be more mature due to both a greater depth of burial and a somewhat higher than normal geothermal gradient. An area with similar burial depth occurs in the Wyoming portion of the basin near Cheyenne, however, the geothermal gradients are lower here than the Denver area. These lower gradients are associated with lower logderived equivalent vitrinite reflectance values indicative of lower stages of thermal maturity. SUMMARY AND CONCLUSONS An accurate value of vitrinite reflectance in any subsurface stratigraphic zone may be made by plotting vertically-distributed values of log Ro versus depth and deriving a straight line relation passing through the depth of the zone to be calibrated. This value of vitrinite reflectance may be empirically correlated to electrical log resistivity of a known source bed with a matrix reservoir system. The correlation may be used to estimate logderived equivalent vitrinite reflectance in large numbers of wells for which logs are available, but for which actual vitrinite data is not. The logderived data is useful in mapping maturity patterns related to stages of hydrocarbon generation or destruction. Although the work presented defined such patterns of maturity in the Niobrara Formation of the Denver basin, the general technique may have application in other formation and areas. REFERENCES Archie, G.E., 1942, The electrical resistivity log as an aid in determining some reservoir characteristics: Journal of Petroleum Technology, V. 5, p Clayton, J.L., and P.J. Swetland, 198, Petroleum generation and migration in Denver basin: AAPG Bulletin, v. 64, p Dow, W.G., and O'Connor, D.., 1982, Kerogen maturity and type by reflected light microscopy applied to petroleum exploration; in, How to assess maturation and paleotemperatures: SEPM, Short Course No. 7, p Hunt, J.M., 1979, Petroleum geochemistry and geology: San Francisco, W.H. Freeman, p Kirkham, R.M., 1978, Coal mines and coal analysis of the Denver and Cheyenne basins, Colorado: Colorado Geological Survey Open-file report 78-9, 14 p. Lockridge, J.P. and Scholle, P.A., 1978, Niobrara gas in eastern Colorado and northwestern Kansas; in J.D. Pruit and P.E. Coffin, eds., Energy Resources of the Denver basin: Denver, Rocky Mountain Association of Geologists, p Malloj, William W., 1977, Oil and gas from fractured shalereservoirs in Colorado and northwest New Mexico: Rocky Mountain Association of Geologists, Special Publication No. 1, 38 p. Meissner, Fred F., 1978, Petroleum geology of the Bakken Formation Williston basin, North Dakota and Montana, it? Williston basin Symposium: Billings, Montana Geological Society, p Rice, Dudley D., (in publication), Occurence of indigenous biogenic gas in organic-rich, immature chalks of Late Cretaceous age, eastern Denver basin, in J. Palacas, ed., Carbonate Source Rocks: Tulsa, AAPG, Tainter, P.A., 1982, nvestigation of stratigraphic and paleostructural controls on hydrocarbon migration and entrapment in Cretaceous D and J sandstone of the Denver Basin: M.S. thesis, University of Colorado, Boulder, 235 p. Warner, L.A., 198, The Colorado lineament; in H.C. Kent and K.W. Porter, eds., Colorado Geology; Denver, Rocky Mountain Association of Geologists, p Weimer, R.J., 1973, A guide to uppermost Cretaceous stratigraphy, growth faulting and early Lararnide crustal movement: The Mountain Geologist, v. 19, p ROCKY MOUNTAN ASSOCATON OF GEOLOGSTS

8 ~ ~ LOG-DERVED NDCATOR OF THERMAL MATURTY 361 Figure 9. Niobrara vltrinite reflectance versus K zone resistivity, Denver basin. Figure 1 on following pages. ROCKY MOUNJAlNASSOClAJlON OFGEOLOGSTS

9 362

10 Figure 1. Contour map of log-deriwed, equivalent wltrinite-reflectance levels based on Niobrara K zone resistivities. Niobrara production after Mallory (1 977).