Northwestern Colorado Research Articles

Absolute stress measurements at the rangely anticline, Northwestern Colorado: Reply to Discussion by Nichols and Farrow

Absolute stress measurements at the rangely anticline, Northwestern Colorado: Reply to Discussion by Nichols and Farrow

Absolute stress measurements at the rangely anticline, Northwestern Colorado: Discussion of R. V. De La Cruz and C. B. Raleigh's

Absolute stress measurements at the rangely anticline, Northwestern Colorado: Discussion of R. V. De La Cruz and C. B. Raleigh's

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.

Absolute stress measurements at the rangely anticline, Northwestern Colorado

Absolute stress measurements at the rangely anticline, Northwestern Colorado

Absolute stress measurements at the rangely anticline, Northwestern Colorado

Absolute stress measurements at the rangely anticline, Northwestern Colorado

Developments in Eastern and Northwestern Colorado in 1972

Exploratory drilling in 1972 decreased 30.8% in Colorado's most active area, the Denver basin, but there were substantial increases in exploratory drilling elsewhere in the report area, as well as significant increases in development drilling throughout the state. Consequently, total drilling activity increased 7.7% in 1972, and for the second consecutive year Colorado was the most active state in the Rocky Mountain region. Colorado was one of only two Rocky Mountain states to show an increase in oil production in 1972. Significant exploratory wells were completed in the Denver, North Park, and Piceance basins and on the Las Animas arch. Development drilling was highlighted by strong infield programs in Spindle, Third Creek, Lowry, Irondale, and Dragoon fields. Developmen of the giant Wattenburg gas field is awaiting Federal Power Commission approval of an application from Panhandle Eastern Pipeline Company to market the gas.

Determining Subsurface Stratigraphic Position by Fluid-Entry Analysis During Air Drilling

The most common methods of determining subsurface stratigraphic position while drilling with mud are difficult or impossible to use in air-drilling operations because drill cuttings may be powdery, and the extremely fast drilling rate remains unchanged as different formations are penetrated. Moreover, air-drilled holes are commonly completed in 4 to 10 days and logs are run only at total depth. These factors demand a wellsite specialist and a rigorous attempt to monitor tops, if formation depths cannot be accurately predicted. A different type of subsurface stratigraphic surveillance, applicable only to air-drilling, has proved successful. Rather than attempting to monitor proved successful. Rather than attempting to monitor the rock characteristics of marker zones, the method monitors their fluid content. Since air compression is stopped and the well is opened to the atmosphere while drillpipe connections are made, drilling with air is like conducting a series of 30-ft formation tests. This fact, together with the fact that bottom-hole air pressures rarely reach 500 psi and bottoms-up times pressures rarely reach 500 psi and bottoms-up times last only a few minutes, makes it possible to observe the type and depth of fluids feeding in from penetrated formations. By comparing fluid-entry depths with logs, the fluid content of specific formations in an area can be determined. It follows that this information may provide a means of fixing the geologic position of a well by monitoring fluids that enter the wellbore during drilling. Thus, by comparing, the nature and distances between fluids entering the wellbore with known fluid-entry characteristics for the area, the subsurface stratigraphic position can be deduced. This technique is termed fluid-entry analysis. The method has been applied by Continental Oil Co. on the thickly faulted Douglas Creek Arch in northwestern Colorado and has provided sufficiently accurate subsurface control to allow substantial savings in spite of otherwise unpredictable formation depths. In air drilling to the Mancos "B" gas reservoir, it became evident that specific up-hole formations contributed fluids to the wellbore in a fairly predictable manner. From this, a system was devised to predictable manner. From this, a system was devised to monitor subsurface position during drilling. In planning a well, tops are predicted from the structure contour map and surface elevation. The well is drilled with air or mist, and depths at which water and gas occur, as well as increases in water and gas, are noted. Fluid-entry data are compared with known fluid-entry characteristics and the original predictions are revised as information is received. predictions are revised as information is received. After the adjusted total depth is reached and a flow test is taken, the well is logged. The only data immediately required are formation productivity and tops, so specially trained personnel are not required on location. The company drilling foreman reports this information and a completion decision is made. Following are the results of using this procedure during the drilling of 40 wells: By not drilling 2593 ft of useless hole in 9 wells that were structurally higher than predicted, $11,000 was saved. Ten wells were drilled deeper than originally planned and were stopped at the correct depth planned and were stopped at the correct depth without interrupting drilling, saving approximately $21,000. Only one log was required and it was not necessary to re-enter and deepen any well after logging. Specially trained personnel were required on location for only 17 man-days as compared with an estimated 100 man-days if conventional surveillance methods had been attempted. Using the method, casing, was not set in any well that was later found noncommercial. Gas feed-in from the Mancos "B" alone might be sufficient to bottom the well properly however, should it be necessary to mud up before reaching the Mancos "B" because of water flow, or should the well not produce gas or flow less than expected at the predicted Mancos "B" depth, the lack of up-hole predicted Mancos "B" depth, the lack of up-hole fluid-entry data would dictate that drilling be halted and that a log be run to determine where the well is geologically. This could require a geologist or engineer at the wellsite and additional expense if it were necessary to re-enter the well and deepen after logging, or if the well had already been drilled too deep. Fluid-entry analysis can be applied wherever two or more marker formations contribute fluids consistently over a large enough area to allow application of the method to a number of wells. The drilling fluid must be air or air mist and fluid feed-in should be of low enough pressure and volume to prevent having to mud up, at least until enough interval has been drilled to determine up-hole stratigraphy. P. 1222

Absolute stress measurements at the rangely anticline, Northwestern Colorado

Five different methods of measuring absolute state of stress in rocks in situ were used at sites near Rangely, Colorado, and the results compared. For near-surface measurements, overcoring of the borehole-deformation gage is the most convenient and rapid means of obtaining reliable values for the magnitude and direction of the state of stress in rocks in situ. The magnitudes and directions of the principal stresses are compared to the geologic features of the different areas of measurement. The in situ stresses are consistent in orientation with the stress direction inferred from the earthquake focal-plane solutions and existing joint patterns but inconsistent with stress directions likely to have produced the Rangely anticline.

Developments in Eastern and Northwestern Colorado

Exploratory and development drilling in eastern and northwestern Colorado in 1971 was up from 1970 and Colorado was the most active state in the Rocky Mountain area. Colorado oil production was up more than 4%. Significant exploratory wells were completed in the Denver basin, North Park basin, Sand Wash basin, and on the Las Animas arch. Development drilling, up substantially from 1970, was highlighted by extensive development programs in Wattenberg, Peoria, Hombre, and Rangely fields.

Upper Cretaceous Foraminifera from the Upper Mancos Formation, the Mesaverde Group, and the Basal Lewis Formation, Northwestern Colorado

Systematic descriptions, five new species, Ammobaculites masteri, A. pagodensis, Cyclammina haydeni, Scutuloris batteni, Eoeponidella yampensis, three faunal assemblages (lagoonal, estuarine, near-shore marine)

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

Developments in Eastern and Northwestern Colorado and Western Nebraska in 1970

Eastern and northwestern Colorado and western Nebraska experienced a drop in total drilling activity in 1970. The Denver basin received most attention, with several significant discoveries and an improved success rate. Increased activity in that area is indicated for 1971. Exploratory drilling and success declined in both southeastern and northwestern Colorado; development drilling remained about the same in the former area and decreased drastically in the latter, though with an increased success rate.

Earthquakes and Fluid Injection: ABSTRACT

Earthquakes have been clearly linked to subsurface fluid injection in two places--near Denver at the Rocky Mountain Arsenal's waste-disposal well, and at the Rangely oil field in northwestern Colorado. The theory linking fluid pressure to earthquakes is based on the effective stress concept, i.e., that increases of pore pressure reduce the effective normal stress across existing or potential fracture surfaces. In both cases, evidence exists for substantial tectonic shearing stresses in the reservoir rock prior to injection. Although the initial shear stress was below the critical value necessary to cause failure, fluid injection relieved a fraction of the frictional resistance to shear fracture, and earthquakes resulted. There is presently no way to determine before drilling whether injection at a given site will produce earthquakes. At Denver and Rangely the earthquakes appear to be located entirely along pre-existing faults. At Rangely, the earthquakes have been drastically reduced in frequency by reducing pore pressures in the hypocentral region. In placing injection wells, existing faults should be avoided. Seismic surveillance during injection can provide early warning of inadvertently triggered earthquakes and palliative measures can be taken. The Rangely experience suggests that seismic activity due to waterflooding in oil fields may be controlled without seriously disrupting production of oil. End_of_Article - Last_Page 2090------------

Lower Tertiary Deltas and Petroleum, Rocky Mountain Region: ABSTRACT

Paleocene and Eocene lacustrine deltas are present in several Rocky Mountain intermontane basins. Deltaic deposits of the Eocene Green River Formation in the Red Wash-Raven Ridge area, northeastern Utah, have been studied intensively. Recognition of the Red Wash delta is based on distribution of (1) fluviatile red and green shale facies (Wasatch Formation), distributary and nearshore sandstone facies, and lacustrine oolitic, ostracodal limestone facies (Green River Formation); (2) geographic orientation and spatial dimensions of thick sandstone bodies projecting into the lake beds, indicating positions of entry into Lake Uinta of south-flowing streams; and (3) sedimentary structures that are in accord with a deltaic environmental interpretation (although they are present lso in other environments). An estimated 100 million bbl of oil reserves (ultimate production) is trapped in a series of discrete sandstone bodies within the Red Wash delta. Variations in petroleum chemistry from pool to pool indicate local source beds and short-distance migration. Several billion bbl of oil in Wasatch-Green River outcrops (not including oil shale) attest to the almost incredible petroleum-generating power of Eocene lake deposits. In the Piceance Creek basin, northwestern Colorado, Douglas Creek sandstones (basal member of the Green River Formation) were deposited at the mouth of a southwestward-flowing river, and facies associations similar to those of the Red Wash area are present. The lobate shape of sandstone deposition is not as distinctively developed as in Red Wash. An estimated 250 billion cu ft of gas is trapped at the up-dip edge of porous intervals on Piceance Creek anticline. Oil saturation is common in outcrops and is present in the subsurface although high wax content and low reservoir temperature prevent commercial production. A similar facies association in the Wasatch-Green River section is present in the Washakie basin, southern Wyoming. A river flowing northwestward deposited thick sandstone beds that interfinger with lake beds in the vicinity of the basin axis and along the west flank of the basin. Oil and gas shows have been reported in these beds, but no commercial production has been developed. Paleocene lakes in several basins have shoreline deposits, in part deltaic, that contain oil and gas fields and are targets for future exploration. Examples are the Fort Union Formation in the Big Piney-La Barge area, Waltman Shale lake beds in the Wind River basin, and Fort Union lake deposits in the Big Horn basin. The lobate configuration of Fort Union coarse clastics on the west flank of the Big Horn basin is very suggestive of deltaic deposits. End_of_Article - Last_Page 851------------

Developments in Eastern and Northwestern Colorado and Western Nebraska in 1969

Total drilling for the year nearly doubled from 1968. The increase occurred in the Denver basin of Colorado and Nebraska and southeastern Colorado. New discoveries declined despite the increased drilling. Denver basin production has been extended southwest 30 mi by the Ironhorse discovery. Five new Mississippian fields on the Las Animas arch will give added interest to this area.

Silver Analysis of Seeded Snow by Atomic Absorption Spectrophotometry

An experimental weather modification program using silver iodide as the cloud seeding agent is being conducted over the Park Range in northwestern Colorado. The measure of silver content in target area precipitation can be used as an aid in tracking the silver iodide plume and evaluating seeding results. The ultramicroquantities of silver in snow samples are concentrated by a solvent-extraction technique with an organic complexing reagent, and silver content is then determined by atomic absorption spectrophotometry. This analytical method is capable of detecting silver iodide activities in seeded snow. The results obtained using this technique on Park Range target area snow samples are discussed.

Developments in Eastern and Northwestern Colorado and Western Nebraska in 1968

Total drilling in eastern and northwestern Colorado and western Nebraska is the same as 1967. The Denver basin again received the most drilling, but wildcat drilling has declined in the Colorado part of the basin and increased significantly in the Nebraska part. Development drilling in the Denver basin was more successful than in 1967. Southeastern Colorado experienced an increase in wildcat drilling activity and maintained a success of 12.5%. The increase in geophysical activity in southeastern Colorado indicates that more wells will be drilled in that area in 1969. Drilling in northwestern Colorado was similar to last year, but a decline in the success percentage of wildcat drilling indicates a probable decline in drilling for that area in the coming year.

Environmental Interpretation of Upper Part of Mesaverde Formation, Northwestern Colorado, from Outcrop, Core, and Subsurface Study: ABSTRACT

Various lines of evidence, such as the vertical succession of gross lithologic character, textures, and sedimentary structures, the fauna, and the geometry of rock stratigraphic units, indicate that the upper part of the Hayden Gulch outcrop section of the Mesaverde Formation in northwestern Colorado is a sequence of former barrier islands and lagoons intertonguing with overlying offbeach marine shale of the Lewis Formation. The collective criteria used to recognize the different depositional environments at the outcrops were readily apparent in a core taken along depositional End_Page 730------------------------------ strike from the outcrop section; the nearly perfect vertical sequence of lithology in a core compensates for the loss of lateral exposure in outcrop. Correlation of the outcrop section with electric logs permitted mapping of the distribution of the rock-stratigraphic units, and the resultant geometric interpretation precluded a delta interpretation for this particular stage of Mesaverde deposition in this area. End_of_Article - Last_Page 731------------

The Oil Shale Industry's Water Problems

An oil shale industry producing 2 million bpd of oil can be expected to require each year the diversion of about 750,000 acre-ft for direct use, plus use by the population and affiliated industries which oil shale development will add to the economy of the upper Colorado River basin. Of this, about 500,000 acre-ft would be consumed, and about 250,000 acre-ft returned to the river. The total lawfully usable water supply in the upper basin is a good deal less than the total visible supply. This is due to interstate commitments to down-stream users. The effect, particularly in Colorado, is that there is not enough uncommitted water available for use in the state of origin to supply the anticipated water requirements of the oil shale industry. These requirements must be met in large part by purchasing existing water rights and perhaps reducing the state's agricultural economy in some degree as a partial offset to benefits to be derived from the new industry. The ultimate remedy, which may well occur, is importation of several million acre-feet annually, into the Colorado River basin from other watersheds.

Biostratigraphy of Niobrara-Equivalent Part of Mancos Shale (Cretaceous) in Northwestern Colorado

Study of microfossils, particularly Foraminifera, from the part of the Cretaceous Mancos Shale in northwestern Colorado that is equivalent to the Niobrara Formation of the High Plains region has determined that these fossils can be used to establish biostratigraphic subdivisions and interpret paleogeographic conditions. The microfossils used in the investigation came from two surface sections and one well in northwestern Colorado and one surface section in northeastern Colorado. The microfauna contains several species in common with beds of the same age in the Gulf Coast and Great Plains areas, and also contains forms which have been reported from equivalent beds in Alberta and the Arctic Slope of Alaska. The microfossils are predominantly calcareous and represent a limited number of planktonic and benthonic forms. Samples taken throughout each of the sections were studied and plotted systematically to determine the stratigraphic distribution of the microfossils. Concentrations of larger numbers of fossils were found in the lower part of the Niobrara-equivalent strata. This lower part is characterized in both northwestern and northeastern Colorado by a variety of benthonic calcareous foraminifers, notably species of Globorotalites and Bifarina, and Pleurostomella austiniana. It is termed informally the Globorotalites interval. Further study may make possible the subdivision of this interval on the basis of the stratigraphic distribution of Globorotalites and Bifarina. The microfauna of the upper part of the section is more limited and is characterized particularly by some species of Gavelinella, notably G. talaria. This upper part is termed the Gavelinella interval. Throughout the Niobrara-equivalent section the planktonic species Heterohelix lobulosa and Hedbergella loetterlei are common, and in most samples the planktonic forms predominate. Interpretation of the environment of deposition of the northwestern Colorado beds has been based on (1) the ratio of planktonic to benthonic foraminifers, (2) modern analogues of foraminiferal genera found as fossils, (3) size and abundance of microfossils, and (4) variety of genera and species. The evidence indicates that large volumes of clay-size clastic sediment were supplied to northwestern Colorado during the deposition of the Niobrara-equivalent part of the Mancos Shale. This fine-grained clastic material diluted the population of foraminifers found as fossils in the sediment (in comparison with the living population) and may have restricted the variety of genera and species. The absence of keeled foraminifers in Niobrara-equivalent beds in northwestern Colorado indicates colder water temperatures (probably less than 20° C) in this region than in areas on the south and east at the same time. The colder water temperatures, as well as the distribution of fine-grained clastic sediment, probably resulted from a combination of a southward-directed, nearshore circulation system along the western boundary of the western interior seaway and the possible upwelling of deeper, colder water. Ratios of planktonic to benthonic Foraminifera and modern analogues of some genera indicate that the seaway was probably on the order of 200 m deep in northwestern Colorado during deposition of the Niobrara-equivalent beds.