Rio Blanco County Research Articles

A new fossil katydid of the genus Arethaea Stål (Orthoptera: Tettigoniidae) with exceptionally preserved internal organs from the Eocene Green River Formation of Colorado

Orthoptera (grasshoppers, crickets, and katydids) are abundant and diverse in the lower–middle Eocene Green River Formation of the Piceance Basin in northwestern Colorado, though remain almost entirely unstudied. Here, we describe a new species of the katydid genus Arethaea Stål (Tettigoniidae: Phaneropterinae: Insarini) from the Parachute Creek Member at a locality near Meeker in Rio Blanco County. Arethaea solterae Heads, Thomas & Hedlund sp. nov. is the first fossil member of the genus and the first tettigoniid described from the Green River Formation. The holotype and only known specimen is remarkable for the exceptional preservation of internal organs including the anterior midgut (ventriculus) and what appear to be a testis, accessory glands, and possible fat body.

Geochemical studies of the Green River Formation in the Piceance Basin, Colorado

The Eocene Green River Formation contains the largest oil shale deposits in the world and is a welldocumented example of a lacustrine depositional system. In addition, mineral resources associated with oil shale in the Piceance Basin nahcolite [NaHCO3] and dawsonite [NaAl(CO3)(OH)2)] are of current and potential economic value, respectively. Detailed geochemical analysis across the basin can aid in the understanding of the depositional environment, sedimentary processes, and water-chemistry evolution in this system. Quantitative geochemical data for Green River oil shale from the Piceance Basin of Colorado were collected by inductively coupled plasma optical emission spectroscopy and mass spectrometry as part of this study. The basin margin is represented by samples from exposures at Douglas Pass (Garfield County) and the basin center area is characterized by core samples from two drilled wells: the Shell 23X-2 and John Savage 24-1 (Rio Blanco County). Major elements and groups of elements are used as proxies for clastic influx (Si, Al, K, Ti), carbonate deposition (Ca, Mg), salinity (Na), paleo-productivity (P), and redox state (Fe, S), respectively. Minor and trace elements reinforce observations based on major elements, including Rb, Zr, Nb for clastic influx and Mn, Sr for carbonate. Trace elements are used to characterize redox conditions (As, Mo, U, V, Co, Ni, Cu, Zn) and salinity (Rb/K, B/Ga). Chemical distinctions between the basin margin and the basin center, in terms of these components and total organic carbon concentrations, support the model of a permanently stratified lake through most of the depositional interval. A primary purpose of the study was to conduct more extensive sampling to confirm conclusions of a previous reconnaissance study. Geochemical data from this study indicates elevated Na around the basin margin occurring earlier than in the deeper basin. Early in the history of Lake Uinta, the salinity may have been elevated first in the shallower marginal waters, due to increased evaporation, which then led to elevated salinity in the basin center through transport of saline density currents. Other indicators of salinity (Rb/K, B/Ga) do not track Na content in intervals where clay minerals are absent due to diagenetic alteration under hypersaline conditions but may be used to indicate the salinities at which authigenic Na-bearing minerals begin to form. Most Na-rich samples show high proportions of clastic constituents (Si, Al, K, Ti) compared to conventional carbonate constituents (Ca, Mg). Redox-sensitive period IV transition metal elements (V, Co, Ni, Cu, Zn) show only local occurrence of significant enrichment relative to average shale abundances. Analysis of Fe/Al ratios for this dataset suggests that the depletion of these elements may be related to source rocks depleted in mafic constituents, with apparent redox-related enrichments subdued by this effect. The basin margin samples reflect generally oxic bottom waters, with some intervals deposited under more reducing, possibly dysoxic to anoxic conditions. The basin center results indicate more reducing conditions, with Mo and U enrichment factors suggesting operation of a particulate shuttle mechanism that scavenged Mo on Fe/Mn-oxyhydroxides that redissolved at depth, with Mo precipitating along with sulfides and/or organic matter at or near the sediment/water interface.

Unpaved Road Dust Control in the Piceance Creek Basin in Rio Blanco County, Colorado

AbstractRoad dust from unpaved roads (dirt roads) is a major source of airborne particulates; the loss of those fines accelerates the deterioration of roads. As a result, road dust emissions are a major concern of the users and managers of dirt roads. Exxon Mobil has been concerned with the dust emissions and rapid road deterioration of the access roads in its natural gas production facility in the Piceance Creek Basin north of Rifle, Colorado. As part of the access road management plan for Exxon Mobil Piceance Creek, a research project was initiated to investigate the effectiveness of several chemical dust suppressants in reducing dust emissions, thus prolonging the intervals between periodic maintenance. Rio Blanco County and the Bureau of Land Management participated in this project. Using the Colorado State University Dustometer and its associated dust measurement protocol, it was found that all chemical suppressants decrease dust emissions and that magnesium chloride (MgCl2) was the most effective du...

New Plants from Moffat and Rio Blanco Counties, Colorado

Described as new from Moffat and adjacent Rio Blanco counties, Colorado, are 4 taxa: Chrysothamnus nauseosus (Pallas) Britton var. linearifolius S.L. Welsh & S. Goodrich, var. nov.; Erigeron nematophylloides S.L. Welsh & A. Huber, sp. nov.; Lepidium montanum Nuttall var. diffusum S.L. Welsh & S. Goodrich, var. nov.; and Iva axillaris Pursh var. parvifolia S.L. Welsh & A. Huber, var. nov.

Notice of release of White River Germplasm Indian ricegrass

White River Germplasm Indian ricegrass ( Achnatherum hymenoides (Roem. & Schult.) Barkworth [Poaceae]) has been released for use in rangeland seedings. This plant material traces from a seed morph collected in northwestern Colorado in Rio Blanco County, near the White River. White River Germplasm displays good germinability in a species characterized by high levels of seed dormancy. We anticipate this material will be used in the Upper Colorado Plateau of eastern Utah and western Colorado and the Basin Province of southern Wyoming.

Carbon Dioxide Injection and Resultant Alteration of the Weber Sandstone, Rangely Field, Colorado (1)

Geologic interpretations made during the current enhanced oil recovery project at Rangely field (Rio Blanco County, Colorado) have shown the relationships between alteration of reservoir mineralogy and texture, changes in produced water composition, and increased problems with oil production. The bottom-hole pH of Weber brine has decreased from approximately 7.5 to 4.5 since the initiation of CO[2] injection into the Pennsylvania-Permian Weber Sandstone in the central portion of the field. Much of the Weber is partially cemented with carbonate minerals, including ferroan calcite and ferroan dolomite, with authigenic clay minerals, predominately illite and mixed-layer smectite/illite, also present. These minerals are affected by the lowered pH of the CO[2] flood. Changes in the chemistry of produced water are associated with alteration of reservoir mineralogy. The CO[2] flood has caused substantial increases in the concentrations of iron, calcium, and magnesium in the produced brine from the dissolution of carbonate cements and, perhaps, authigenic clays and detrital feldspars. Produced water from some wells has experienced large changes in barium and sulfate concentrations. An increase in bromide concentration indicates that previously segregated portions of the reservoir, which contained very saline original formation brine, have been accessed by the carbonated water. Hypotheses based on the water-chemistry changes were confirmed in core-flood experiments. These experiments also indicated that there was no net change in permeability following CO[2] injection.

Tight-Sand Development Potential in the Southern Rulison Area, Garfield County, Colorado

Summary Reservoir characterization and simulation of natural gas production were used to assess the production potential for an extension area within the Piceance Creek basin (Garfield County, CO). Many of the basin's reservoirs are lenticular sands characterized as having low porosities, low permeabilities, and relatively high water saturations. One such horizon is the Mesaverde group, which consists of discontinuous sandstone lenses distributed within a shale matrix and is present throughout the Piceance Creek basin. Prior studies of the commercial Rulison field in the east-central portion of the basin indicated that compressional forces created an anticlinal nose aligned northwest/southeast through part of the field. These forces created a conductive natural fracture system that enhances the permeability of the Mesaverde. This paper shows a relationship between Rulison field and an area south of Rulison that encompasses about 36 sq miles [93.2 km]. The area has been sparsely drilled (averaging one well per two sections and nonuniformly spaced); thus, the possibility exists for further commercial development through infill drilling. The southern Rulison area is characterized in structure, percent sand, and critical reservoir parameters including permeability. Production rates, cumulative gas production, and drilling and completion data are used to infer permeabilities by history matching with a dual-porosity reservoir simulator. Results include a reservoir description of the area and economic evaluations of several investment scenarios that will help to assess the potential resource. Introduction The Piceance Creek basin of northwest Colorado contains an estimated 33 Tcf [934 × 10–9 m3] of maximum recoverable gas and is the setting for the U.S. DOE's Multiwell Experiment (MWX). The Natl. Petroleum Council estimated the volume of recoverable gas at 12.9 Tcf [365 × 10–9 m3), assuming a gas price of $2.50/Mcf [$8.83/100 M], a 15% discounted cash flow rate of return, and the current level of technology. The basin comprises Garfield, Mesa, and Rio Blanco counties with minor extension into Delta, Gunnison, and Moffat counties. The Tertiary Age Wasatch formation and Cretaceous are Mesaverde group are typically the gas-productive horizons throughout the basin. Situated near the center of the basin in Garfield County is the Rulison field. Discovery of the field and its early development occurred in the late 1950's and early 1960's, with sparse drilling in the southern sections of Township 6 South, Range 94 West (T6S, R94W) and scattered drilling throughout T7S, R94W and T7S, R95W. Wasatch completions have generally been restricted to T6S, R94W with development on 160 acres/well [65 ha/well]. The Mesaverde has been drilled on 320 acres/well [130 ha/well] and is the focus of this study. Most of these wells penetrated the entire lenticular section of the Mesaverde (Williams Fork), with a few deeper wells reaching the marginal marine deposits in the lowermost Mesaverde. Northwest Exploration (now part of the Williams Co.) further developed the area during the early 1980's. Thirteen Mesaverde wells were completed in T6S, R94W. For this investigation, T6S, R94W will be referred to as the Rulison field and T7S, R94W and T7S, R95W as the southern area. The geology and natural gas production of the study area were evaluated to determine why the southern area had not been developed to the extent of the Rulison field. The Rulison field has been well characterized; therefore, the present effort concentrates on the southern area. In appraising the southern area as a developmental prospect, similarities to the Rulison field are identified and quan-tified. Fig. 1 is a generalized map of the study area, the two townships of interest are southwest of Rifle, just south of the Colorado River.

Carbon Dioxide Injection and Resultant Alteration of Weber Sandstone (Pennsylvanian-Permian), Rangely Field, Colorado: ABSTRACT

Geologic interpretations made during the current EOR (enhanced oil recovery) project at Rangely field (Rio Blanco County, Colorado), have made interesting connections between alteration of reservoir mineralogy and texture, changes in produced water composition, and increased production problems. Carbon dioxide is being injected into the Weber Sandstone in portions of Rangely field. The carbon dioxide injection is part of a very successful tertiary recovery project initiated in late 1986. The bottomhole pH of Weber brine has decreased from approximately 7.5 to 4.5 with the addition of CO/sub 2/. Changes in the chemistry of produced water are associated with alteration of reservoir mineralogy. The CO/sub 2/ flood has caused a substantial increase in the concentrations of iron, calcium, magnesium, and strontium in the produced brine. The amount of increase is directly related to the volume of CO/sub 2/ produced in each well. This increase resulted from the dissolution of carbonate cements, authigenic clays, and detrital feldspars. An increase in the calculated scaling potential of the produced water is a result of this change in chemistry. Hypotheses based on the water-chemistry changes were confirmed in pressure-cell and core-flood experiments. Core-flood experiments also indicate no net change in permeability following carbon dioxide injection:more » the increase in permeability due to the dissolution of carbonate cements is being offset by a decrease caused by migratory clays plugging pore throats. The clays, which coat the authigenic carbonates, are liberated when the carbonates are dissolved.« less

Paleotectonic, Stratigraphic, and Diagenetic History of Weber Sandstone, Rangely Area, Colorado: ABSTRACT

Rangely field is in Rio Blanco County, Colorado, on a doubly plunging anticline of Laramide age. The Rangely structure is asymmetrical with the steepest flank to the southwest. The Permo-Pennsylvanian Weber Sandstone is the primary producing formation with cumulative production exceeding 670 million bbl of oil. The Weber is a subarkosic arenite deposited in an eolian regime. It interfingers with the alluvial Maroon Formation in the southern and southeastern portions of Rangely field. Isopach maps of the Pennsylvanian formations suggest a paleotectonic platform in the Rangely area and a Permo-Pennsylvanian north-south-trending arch west of the Laramide-age Douglas Creek arch. Hydrocarbons migrated into the Rangely area prior to the Laramide orogeny and were stratigraphical y trapped at the Weber-Maroon transition zone. Subsequent Laramide structure localized and hydrocarbon accumulation. Diagenetic history of the Weber Sandstone differs between the Uinta and Piceance basins. Weber diagenesis in the Uinta basin is dominated by silica precipitation and porosity appears to be residual primary. Weber diagenesis in the Piceance basin includes dissolution of detrital material and precipitation of a complex sequence of carbonate cements. Weber porosity in the Piceance basin appears to be both residual primary and secondary. The boundary between these two diagenetic regimes appears to coincide with the Pennsylvanian paleoarch. End_of_Article - Last_Page 853------------

Storm-Dominated Shoreface Deposits, Sego Sandstone (Campanian), Northwestern Colorado: ABSTRACT

Characteristics of progradational shoreface units have been described from several sandstones in the Cretaceous Western Interior seaway. Little work has been done to document the characteristics of shoreface deposits in a transgressive setting. Outcrop studies of the Sego Sandstone (Campanian), in Rio Blanco County, Colorado, indicate that an initial episode of progradation was followed by a stagnation of the system in which minor transgressions occurred. Transgression of the foundering coast is evidenced by landward movement and storm breaching of a thinned shoreface unit and by the aggradation of a thick associated tidal flat and lagoonal complex. Three lithofacies are recognized in the shoreface deposits from 80 measured sections; each is composed of very fine-grained, laminated sandstone. (1) Stacked sequenced, 1 to 1.5 m (3.3 to 5 ft) thick, of interbedded sandstone and rippled sandy mudstone. The sandstone beds have 0.5 to 1 m (1.6 to 3.3. ft) thick, imbricated wedge-planar laminations that thin rapidly to the northwest into broad low-angle trough and antiform bedding before pinching out into the mudstone as rippled and burrowed beds. (2) Sandstone with low-angle trough cross-stratification and imbricated wedge-planar sets, but no associated mudstone. This facies ranges up to 5 m (16 ft) thick, but is most commonly 2 to 3 m (6 to 10 ft) thick. (3) Sandstone in superposed accretionary sets ranging from 0.5 to 3 m (1.6 to 10 t) thick and 8 to 30 m (26 to 100 ft) wide. Facies 1, which always forms the base of a sequence, is a stacked series of washover fans that thinned rapidly beyond the confined shoreface breach and spread landward into adjacent lagoonal lows. Facies 2 is a stabilized shoreface and is interrupted along strike by facies 3, the longshore-derived fill of storm channels. The stacking and lateral occurrence of filled channels indicates that active zones of weakness existed, and that net landward movement of the shoreface occurred as result of storm washovers. A thick (80 m, 260 ft) sequence of tidal and paludal sediments is found above the shoreface. Facies include: (1) ripple-stratified, bioturbated humates, and very fine-grained sandstone, interbedded with thin (1 to 30 cm, 0.4 to 12 in.) continuous coals. (2) Thin-bedded fine-grained sandstone and mudstone with flaser bedding and starved current ripples. (3) Very fine-grained, laminated sandstone with low-angle through cross-stratification and accretionary sets; nearly identical to facies 2 and 3 of the shoreface but thinner (1 m, 3.3 ft). (4) Small to large scale, stacked accretionary sets of fine-grained sandstone ranging to 10 m (33 ft) thick (individual sets 0.1 to 3 m, 4 in. to 10 ft thick), containing rippled foresets and abundant Ophiomorpha. Mud clast conglomerates are found in ome of the smaller channel forms. Study of 30 measured sections indicates that lagoonal deposits (facies 1) become sandier upward and are overlain by sand tidal flats and mixed tidal flats (facies 2). A system of meandering tidal creeks eroded into the contemporaneous tidal flat and lagoon facies. These creeks coalesced to form a central tidal inlet (facies 4). A subsequent transgression reworked shoreface sandstones (facies 3). Influx of sediment by longshore currents reinitiated progradation of the system. Meander-belt and floodplain facies were deposited over the tidal flat-lagoon complex during this regressive phase. End_of_Article - Last_Page 526------------

Paleotectonic, Stratigraphic, and Diagenetic History of Rangely Area, Colorado: ABSTRACT

Rangely field is located in Rio Blanco County, Colorado, on a Laramide doubly plunging anticline asymmetrical to the southwest. The Pennsylvanian Weber Sandstone is the primary producing horizon, with cumulative production exceeding 650,000,000 bbl. The Weber is a sub-arkosic arenite deposited in an eolian regime. It interfingers with the alluvial Maroon Formation in the southern and southeastern portions of Rangely field. Lower Pennsylvanian stratigraphy suggests a paleotectonic high in the Rangely area. Hydrocarbon migration into the Rangely area probably occurred prior to the Laramide with stratigraphic entrapment at the Weber-Maroon transition. The subsequent Laramide structure localized the hydrocarbon accumulation. Diagenetic history of the Weber Formation differs between the Uinta and Piceance basins. Weber diagenesis in the Uinta basin is dominated by silica precipitation; porosity appears to be residual primary. Weber diagenesis in the Piceance basin includes dissolution of matrix material and precipitation of a complex sequence of carbonate cements. Weber porosity in the Piceance basin appears to be secondary. The boundary between these two diagenetic regimes seemingly coincides with the present Douglas Creek arch. A diagenetic model is proposed for the Rangely area. The model assumes a paleotectonic high in the Rangely area or early expression of the Douglass Creek arch. Silica precipitation commenced after Weber deposition throughout the Rangely area. Pre-Laramide expression of the Grand Hogback may have caused sufficient faulting to permit fluid communication between the Eagle Valley Evaporites and the Weber Formation. Saline solutions from the Eagle Valley Evaporites had sufficient head or diffusion drive to replace the Weber Formation fluid in the Piceance basin. The saline solutions halted silica precipitation in the Piceance basin and initiated precipitation of the carbonate cements. Precipitation of silica continued in the Uinta basin. Development of secondary porosity in the Piceance ba in occurred prior to or simultaneously with oil migration. End_of_Article - Last_Page 1346------------

Study of a Possible CO2 Flood in Rangely Field

Summary A study program for CO2 injection in the Rangely field, Rio Blanco County, CO, is reported. The work used laboratory measurements as well as compositional-model calculations to investigate the phase behavior of mixtures of Rangely reservoir oil and CO2 with two different quantities of light gas contamination. The results showed that a CO2 flood in the field is technically feasible under current reservoir pressure and that a 10 mol 070 nitrogen content in the CO2 was detrimental to the process. Introduction The Rangely field was discovered in 1933; waterflooding was begun in 1958. The original oil-in-place (OOIP) in the Rangely Weber Sand Unit was approximately 1.6 billion STB (250×106 stock-tank m3). The work reported in this paper was aimed at evaluating the potential of tertiary CO2 flooding in the reservoir. Because some of the possible CO2 sources contain nitrogen, the effect of flooding on process efficiency was investigated. Another objective was to determine the effect of CO2 slug size on oil recovery. Three types of laboratory studies were conducted:measurement of the phase behavior of the CO2 gas/Rangely reservoir oil system,core displacement experiments at 160°F (71°) and 2,600 psi (17.9 MPa) using Rangely Weber cores that were conditioned with regard to wettability by an aging process, andslim tube experiments to determine minimum miscibility pressure between CO2 gas and reservoir oil. Each study was performed with two different CO2 gases. The first gas was about 95 mol % CO2 and 5 mol % CH4. The second gas contained 85 mol % CO2, 5 mol % CH4, and 10 mol % N2. Rangely Reservoir Oil/CO2 Physical Property Measurements The physical property data were obtained from constant composition expansion (CCE) and vapor/liquid equilibrium experiments (VLE). CCE experiments were conducted to determine the phase envelope (bubble point/dew point envelope). VLE experiments yielded vapor /liquid equilibrium constants (K-values). Both CCE and VLE experiments were performed using Rangely reservoir oil and two different CO2 gases, designated Gases 1 and 2. The compositions of the gases and Rangely-reservoir oil are shown in Table 1. CCE Experiments Figs. 1 and 2 show the pressure/composition diagrams for Rangely oil/Gas 1 and Rangely oil/Gas 2 systems, respectively, at 160°F (71°C). Both systems exhibited a larger single-phase region below 3,000 psi (21 MPa) than a previous system studied.1 Saturation pressure (bubble- and dew-point pressures) for the Gas 2/reservoir oil system were substantially higher than those for the Gas 2/reservoir oil system. This increase is due to 10 mol % N2 in Gas 2. The maximum percent increase in saturation pressure due to N2 contamination occurred at 70 mol % gas concentration. At that concentration, bubblepoint pressure for Gas 2 was 4,720 psi (32.5 MPa) compared with 2,690 psia (18.5 MPa) for Gas 1. These data and those given in Ref. 1 show that the light gases CH4 and N2 decrease the solubility of CO2 in reservoir oils at a given pressure. Rangely Reservoir Oil/CO2 Physical Property Measurements The physical property data were obtained from constant composition expansion (CCE) and vapor/liquid equilibrium experiments (VLE). CCE experiments were conducted to determine the phase envelope (bubble point/dew point envelope). VLE experiments yielded vapor /liquid equilibrium constants (K-values). Both CCE and VLE experiments were performed using Rangely reservoir oil and two different CO2 gases, designated Gases 1 and 2. The compositions of the gases and Rangely-reservoir oil are shown in Table 1. CCE Experiments Figs. 1 and 2 show the pressure/composition diagrams for Rangely oil/Gas 1 and Rangely oil/Gas 2 systems, respectively, at 160°F (71°C). Both systems exhibited a larger single-phase region below 3,000 psi (21 MPa) than a previous system studied.1 Saturation pressure (bubble- and dew-point pressures) for the Gas 2/reservoir oil system were substantially higher than those for the Gas 2/reservoir oil system. This increase is due to 10 mol % N2 in Gas 2. The maximum percent increase in saturation pressure due to N2 contamination occurred at 70 mol % gas concentration. At that concentration, bubblepoint pressure for Gas 2 was 4,720 psi (32.5 MPa) compared with 2,690 psia (18.5 MPa) for Gas 1. These data and those given in Ref. 1 show that the light gases CH4 and N2 decrease the solubility of CO2 in reservoir oils at a given pressure.

Biogenic hydrocarbon contribution to the ambient air of selected areas

In response to suggestions that biogenic emissions are responsible for high hydrocarbon concentrations described in several reports, a short-term sampling program was initiated in the reported areas to test this hypothesis. Limited numbers of whole-air samples were collected in Tedlar bags and analyzed by gas chromatography (GC) with flame ionization detection. Tulsa air was found to contain an average of 0.2% isoprene of the total nonmethane hydrocarbon (TNMHC) load. Rio Blanco County, Colorado, and Smoky Mountain air, respectively, averaged about 2 % and 4 % biogenic hydrocarbon of the total nonmethane hydrocarbon loads. Isoprene appears to be a dominant olefin in rural and remote areas. Although the tests were of short duration, results suggest monoterpenes and isoprene constitute only minor components in these areas relative to anthropogenic hydrocarbons.

Mechanical properties of cores obtained from the unleached saline zone, piceance creek basin, Rio Blanco county, Colo : Horino, F G; Hooker, V E USBM RI 8297, 1978, 21P

Mechanical properties of cores obtained from the unleached saline zone, piceance creek basin, Rio Blanco county, Colo : Horino, F G; Hooker, V E USBM RI 8297, 1978, 21P

USBM Pilot hole ‘X’, horse draw, Rio Blanco county, Colorado, Vol 1. Section 2: Permeability testing : Brown, A; Heley, W USBM OFR 103(2)-77, March 1977, 26P

USBM Pilot hole ‘X’, horse draw, Rio Blanco county, Colorado, Vol 1. Section 2: Permeability testing : Brown, A; Heley, W USBM OFR 103(2)-77, March 1977, 26P

USBM pilot hole ‘x’, horse draw, rio blanco county, colorado. Vol 1. Section 1: Geology and geophysics : Readdy, L A; Pents, D L USBM OFR 103(1)-77, March 1977, 44P

USBM pilot hole ‘x’, horse draw, rio blanco county, colorado. Vol 1. Section 1: Geology and geophysics : Readdy, L A; Pents, D L USBM OFR 103(1)-77, March 1977, 44P

Developments in Eastern and Northwestern Colorado

Developments in Eastern and Northwestern Colorado

Evaluation of Low-Permeability Gas-Bearing Formations in Rio Blanco County, Colorado

This evaluation was conducted in connection with an experiment in stimulating formations with multiple nuclear explosions. The values for reservoir properties were obtained by core and computerized log analyses for the nuclear stimulated well and by analysis of pressure buildup tests on another well nearby. Gas production was predicted on the basis of the values obtained. Introduction The existence of major volumes of gas in thick, low-permeability sandstone reservoirs in Colorado, Wyoming, and Utah has tantalized producers for decades. Conventional efforts to obtain commercial production from these resources have been generally production from these resources have been generally unsuccessful, not only because the permeabilities are extremely low (on the order of several microdarcies to several tens of microdarcies) but also because the gas-bearing sandstone beds are generally scattered throughout sand/shale intervals greater than 1,000 ft thick. Typical tight gas reservoirs are found in the Tertiary Fort Union and the Cretaceous Mesaverde formations of the Piceance Basin in northwestern Colorado. These formations have been drilled and conventionally fractured in Rio Blanco County without successfully obtaining commercial production. New gas production operations in this area have been sparked, however, by the potential for successfully stimulating gas production with multiple nuclear explosions. The 94,000-acre Rio Blanco Unit area (Fig. 1) has been established and a nuclear stimulation treatment has been carried out in Well RB-E-01 with three 30-kt explosives. Emplaced at depths of 5,839, 6,230, and 6,690 ft, these explosives should have effectively fractured a 1,300-ft gross section of the Fort Union and Mesaverde formations from 5,530 to 6,830 ft, subsurface. The evaluation of a stimulation treatment's effectiveness is, of course, dependent upon a knowledge of reservoir characteristics, particularly permeability and net pay thickness. The development of such knowledge for conventionally commercial reservoirs is facilitated by a rather extensive base of proved empirical and theoretical relationships. The extent to which this base is applicable to low-porosity and low-permeability reservoirs, such as those in the Fort Union and Mesaverde formations, is uncertain. Because of this uncertainty, greater than normal effort was made to evaluate cores, logs, and production tests. This report presents the reservoir property values obtained and the predictions of gas production based upon these values. production based upon these values. Area Geology The Piceance basin is a northwest trending structural downwarp (Fig. 1). It contains a sedimentary section dating from lower Paleozoic to Tertiary with a maximum thickness of about 30,000 ft. The east-west cross-section (Fig. 2) shows the asymmetric nature of the basin, with gentle dips on the west and steep dips on the east. The surface rock throughout the unit area is the Tertiary Green River formation, which is composed of oil shales, marlstones, and sandstones. The Wasatch formation, consisting of brightly colored clays and shale with minor sandstone, underlies the Green River formation and overlies the Fort Union formation.

Successful Stimulation of a Thick, Low-Pressure, Water-Sensitive Gas Reservoir By Pseudolimited Entry

Thick, water-sensitive gas reservoirs with very low pressures and permeabilities are especially difficult to exploit. Nonetheless, a permeabilities are especially difficult to exploit. Nonetheless, a pseudolimited-entry technique been successful in stimulating such a pseudolimited-entry technique been successful in stimulating such a reservoir in northwestern Colorado. The method incorporates liquid carbon dioxide in a fracture fluid consisting of two percent potassium chloride in fresh water. Introduction With an increased demand for natural gas, operators in the Rocky Mountain region are devoting more attention to certain gas-bearing Tertiary and Cretaceous formations in the area - formations that, until the past 15 years, had been largely considered non-commercial. Typical of these are reservoirs occurring in the Green River, Wasatch, Mesa Verde, Mancos and Dakota sequences and their geologic equivalents throughout the region. Portions of these formations are often thick and porous enough to contain substantial gas reserves, but are also so impermeable that payout of well expenses is long or impossible without effective stimulation. To make matters worse, formation pressures are frequently less than 1,000 psi and the reservoir rock may be susceptible to damage from extraneous water. Fortunately, the advent of air drilling and improved hydraulic fracturing techniques has corresponded to the expanding gas markets, and these innovations are largely responsible for the success to date in providing commercial gas completions in these formations. providing commercial gas completions in these formations. They are not cure-alls, however, and billions of cubic feet of potential reserves will be untapped until new or improved stimulation techniques are developed. To this end, stimulation by chemical and nuclear explosives has gained attention. Continental Oil Co. owns or operates leases on considerable acreage located on the Douglas Creek Archin Rio Blanco County, northwestern Colorado (Fig. 1). in the early 1960's, the company began working to perfect a hydraulic fracturing technique applicable to Mancos "B" formation gas wells in the area. The Mancos "B" reservoir rock not only has low permeability but also is thick, water-sensitive, and of permeability but also is thick, water-sensitive, and of subnormal pressure. By applying principles only recently developed, these problems were overcome with a pseudolimited-entry fracturing technique, which is pseudolimited-entry fracturing technique, which is discussed here. The Mancos "B" Reservoir For the most part, the Cretaceous Mancos formation consists of a gray to black bentonitic marine shale, and on the Douglas Creek Arch it is about 4,000 ft thick. Somewhat below midway through the formation, the shale becomes thinly laminated with a very fine-grained, argillaceous, quartz sandstone containing 10 to 20 percent carbonates. This sand-shale sequence, which is about 400 ft thick, has been termed the Mancos "B" zone at Douglas Creek and is gas-productive on the Central Arch. Fig. 2 shows a section of typical Mancos "B" core. The top of the Mancos "B" is encountered 2,000 to 3,000 ft below the surface, depending on structural position and surface elevation. The average effective permeability over the 400-ft gross interval is about 0.7 md, with porosities of 10 to 11 percent and 50 percent water-saturation as determined from core data. Significantly, the reservoir pressure is an abnormally low 437 psia (0.17 psi/ft). In addition, the formation rock is extremely susceptible to damage from extraneous fluids. JPT P. 185

Age Distributions of Spruce and Fir in Beetle-killed Forests on the White River Plateau, Colorado

In the 1940's an epidemic of the Engelmann spruce beetle (Dendroctonus engelmannii Hopk.) killed almost all the spruce (Picea engelmannii (Parry) Engelm.) and most of the fir (Abies lasio- carpa (Hook.) Nutt.) larger than 10 cm in diameter breast height, on the White River Plateau, in northwest Colorado. Reconstruction of the age distributions of spruce and fir at the time of the beetle attack were made by counting all the dead trees, stumps and live trees in two stands and estimating the ages of these trees at the time of the beetle attack. The resulting age distributions indicate a reduction in spruce reproduction in the late 1700's, while fir reproduction was relatively unchanged. An epidemic of Engelmann spruce beetles in the early 1700's is suggested to explain these age distributions. Fire, climatic change, or grazing and trampling by cattle are less likely hypotheses. INTRODUCTION AND BACKGROUND The White River Plateau is a subalpine plateau located in north- western Colorado, north of Glenwood Springs, in Garfield and Rio Blanco counties. The plateau is bordered on the south and southeast by the Colorado River, on the northeast by the Yampa River, and on the north by the north fork of the White River. The central part of the Plateau is drained by the south fork of the White River. Most of the present surface of the Plateau lies between 3050 and 3350 m elevation and may be described as rolling, broken here and there by rocky out- crops and deep canyons cut by the major streams draining the area. The vegetation is composed of about 50% forests of Engelmann spruce (Picea engelmannii (Parry) Engelm.) and subalpine fir (Abies lasio- carpa (Hook.) Nutt.) and about 50% meadow. Forests of spruce and fir with patches of meadow predominate on north and northeast ex- posures while meadows containing isolated spruce trees and patches of several spruce and fir predominate on south and southwest exposures. In the 1940's, following a severe wind and blowdown, an epidemic of Engelmann spruce beetles (Dendroctonus engelmanni Hopk.) killed almost all spruce larger than approximately 10 cm diameter breast height. The beetles killed the larger trees first and then pro- gressed through smaller trees. Between 1940 and 1944 the average diameter of trees killed was 53 cm; in 1945, the average diameter of trees killed was 43 cm, and in 1946, 38 cm. The average diameter of live spruce at the end of 1946 was 34.3 cm. By 1948 most of the spruce larger than 10 cm diameter breast height - were dead (Massey and Wygant, 1954). Some of this beetle-killed spruce was lumbered in the early 1950's. Preliminary reconnaissance in 1962 and 1963 indicated that at the time of the 1940 spruce beetle epidemic the forests were not composed of an uneven-aged stand of trees, as one might expect if the forest had