Cambridge Arch Research Articles

Reservoir characterization and static earth model for potential carbon dioxide storage in Upper Pennsylvanian cyclothems, Nebraska, United States

This study estimates the carbon storage potential of interbedded shales and carbonate rocks (cyclothems) in the Pennsylvanian Lansing and Kansas City groups (LKC) on the Cambridge arch in southwestern Nebraska. This effort is essential to the development of a CO2 storage strategy for the Integrated Midcontinent Stacked Carbon Storage Hub project as part of the Department of Energy–National Energy Technology Laboratory’s Carbon Storage Assurance Facility Enterprise initiative. We present a static earth (SE) model representing the 250-ft (76-m)-thick LKCs. This model is based on vintage (mostly pre-1970) well logs from the Sleepy Hollow field (Red Willow County, Nebraska) as well as a new (June 2019) stratigraphic test well drilled expressly for the purpose of the present study. Interpretations of advanced petrophysical logs and cores from this new well were crucial ingredients in the development of the geologic framework for SE model development. Gamma-ray (GR) logs readily differentiate carbonate and mudstone units within the LKC, allowing the differentiation of three GR facies for use in a facies model. Carbonate rock units, which are composed of multiple textures, were correlated across the field. We capture the heterogeneity of these carbonates during petrophysical modeling using effective porosity logs along with Gaussian random function simulation conditioned by the three-dimensional facies model. The SE model was used in computing carbon storage estimates for each LKC carbonate zone over an area of 1 mi2. In total, supercritical CO2 storage is estimated at 602,157 t/mi2 (232,494 t/km2) when using a deterministic saline storage efficiency factor of 0.1.

Regional Seismic Evaluation of Basement Control on Paleozoic Sedimentation in Ohio and West Virginia: ABSTRACT

Paleozoic sediments of the central Appalachian lie upon a complexly deformed basement that continued to deform during the Paleozoic. In West Virginia basement is cut by an aulocogen known as the Rome trough. Seismic data reveal that the Rome Trough is an asymmetrical graben with a faulted eastern margin, and generally rotational western margin. Differential subsidence across the western margin decreases exponentially following the early Cambrian inception of the Trough. Displacement of the East Margin fault appears coupled to west-flank rotation. Minor inversion of Trough structure is also observed following the initial phase of rapid subsidence. Inversion influences local sedimentation patterns and results in minor structures that may influence hydrocarbon distribution. Regional seismic evaluation has been extended further west into Ohio to define regional scale subsidence history and local structural reactivation. Three seismic lines have been analyzed in Ohio that extend northwest from West Virginia to the Ohio COCORP line. Seismic expressions of the basement is marked by a relatively coherent reflection event. The basement faults in the distal part have similar offsets and therefore less influence on the sedimentary features. This analysis evaluates the structural affects of Coshocton zone, Cambridge Arch and Burning Spring anticline on the depositional historymore » of Paleozoic sedimentation in Ohio.« less

Current and Emerging Fossil-Energy Resources in Nebraska

Nebraska recently celebrated its 50th year of petroleum production and additional energy reserves are emerging from uranium, coal-bed methane, and geothermal resources. Oil was discovered originally in the Forest City basin of southeastern Nebraska during 1939 with cumulative production of almost 11 million bbl. The Cambridge arch area and flanking basins of southwestern Nebraska were proven productive in 1959, although not until 1959 was significant production discovered. This area has a cumulative production of almost 103 million bbl of oil. The Denver basin Cretaceous sands, discovered in 1949 in the Nebraska panhandle, have a cumulative production of over 330 million bbl of oil and nearly 280 billion ft[sup 3] of gas. Development and enhanced recovery continue, and significant reserves have been identified in Permian and Pennsylvanian rocks deeper within the Denver basin. A Niobrara Chalk play across western Nebraska has several gas wells waiting market access with potential for horizontal completions. Permian and Pennsylvanian rocks also have additional reserves in both southwestern and northwestern Nebraska. Two in-situ mining units in a world-class uranium deposit in basal Tertiary rocks of northwestern Nebraska have produced over 0.5 million lb of yellow cake. Coals present at depth in southeastern Nebraska are being evaluatedmore » for coal-bed methane. A state-wide inventory of geothermal resources identified areas with above-normal temperatures. Fossil energy exploration in Nebraska continues to be encouraged by new production and shallow depths.« less

Petroleum Exploration and Production in Nebraska: Historical and Geological Patterns

Nebraska recently celebrated petroleum anniversaries: 100 years of exploration and 50 years of production. The Forest City basin recorded the state's first production in 1939. Shale-to-carbonate facies change occurs for both Devonian- and Ordovician-age reservoirs in a belt transverse to anticlinal structures which represent basement reactivation. Cumulative production from the Nebraska portion of the Forest City basin is almost 11 million bbl. Oil was discovered along the Cambridge arch in southwestern Nebraska in 1948, although it wasn't until 1959 that significant exploration/production occurred. Most fields produce from Pennsylvanian carbonates. However, Nebraska's largest field (Sleepy Hollow) also produces from a basal sand which was reworked and isolated during Pennsylvanian time. Other Pennsylvanian and Cambrian sands are productive in the area but at much lower rates. Cumulative production for southwestern Nebraska is almost 103 million bbl of oil. Only minimal gas is produced. Production was established in 1949 in the Denver basin from the bar and channel sands in the upper Dakota (Cretaceous). A cumulative of over 330 million bbl of oil and nearly 280 billion ft{sup 3} of gas has been produced. Exploration and enhanced recovery continue in these reservoirs. Wildcat drilling in Nebraska has provided sufficient data to outline stratigraphicmore » relationships and delineate major structures. Reservoir conditions are excellent, both stratigraphic and structural traps are present, and the requirement for an oil window may be overcome by the current popularity of long-distance migration.« less

Petroleum Potential of Lower and Middle Paleozoic Rocks in Nebraska Portion of Mid-Continent: ABSTRACT

Central North America during the Paleozoic was characterized by northern (Williston) and southern (Anadarko) depositional regimes separated by a stable Transcontinental arch. Nebraska lies on the southern flank of this arch and contains the northern zero edges of the lower and middle Paleozoic rocks of the southern regime. Most of these rocks are secondary dolomites with zones of excellent intercrystalline porosity. The Reagan-LaMotte Sandstones and the overlying Arbuckle dolomites are overlapped by Middle Ordovician rocks toward the Transcontinental arch. Rocks equivalent to the Simpson consist of a basal sand (St. Peter) and overlying interbedded gray-green shales and dolomitic limestones. An uppermost shale facies is present in the Upper Ordovician (Viola-Maquoketa) eastward and southward across Nebraska. The dolomite facies extends northward into the Williston basin. The Silurian dolomites, originally more widely deposited, are overlapped by Devonian dolomites in southeastern Nebraska. Upper Devonian rocks exhibit a regional facies change from carbonate to green-gray shale to black shale southeastward across the Mid-Continent. Mississippian carbonates overlap the Devonian westward and northward across the Transcontinental arch. Pennsylvanian uplift and erosion were widespread, producing numerous stratigraphic traps. Sands related to the basal Pennsylvanian unconformity produce along the Cambridge arch. Arbuckle, Simpson, Viola, and Hunton production ismore » present in the Forest City basin and along the Central Kansas uplift. Although source rocks are scarce and the maturation is marginal, current theories of long-distance oil migration encourage exploration in the extensive lower and middle Paleozoic reservoirs in this portion of the Mid-Continent.« less

Microearthquakes near the Sleepy Hollow oil field, southwestern Nebraska

Abstract Between April 1982 and June 1984, 311 earthquakes were detected, and 176 earthquakes were located in the vicinity of the Sleepy Hollow oil field, southwestern Nebraska. The oil field is being injected with water to enhance oil production, and the earthquakes may be related to production. There is geologic evidence, however, to suggest that the earthquakes are tectonic in origin and are related to uplift along the axis of the Cambridge Arch, on which the field is located. First motion data from these events do not yield a well-constrained fault plane solution. Furthermore, first motion detections are inconsistent for earthquakes with nearly coincident hypocenters, indicating that more than one faulting mechanism is active in the oil field. Injection pressures do not correlate well with earthquake occurrence.

A Classification of Thrust Fronts: REPLY

Unlike Root (1987), I do not have first-hand experience of the Appalachians. Hence, my use of the Appalachians as an example of a type 1 buried thrust front was entirely based upon interpretations from publications. I hope that the lit­ erature is not misrepresented in this discussion. The main point of Root's (1987) criticism centers on the Burning Springs anticline and Cambridge arch. These structures are, however, anomalous (Rodgers, 1%3). The trends of the Burning Springs anticlme and Cambridge arch (approximately north-south to north-northwe st-southsoutheast) are divergent from the general Appalachian structural trend (northeast-southwest) (Rodgers, 1963, his Figure 1), and they do not follow or represent the regional trend of the Appalachian thrust front. The oblique trend closely follows the lateral termination of salt in the upper Salina Group (Rodgers, 1963). The same cannot be said for the main area of plateau deformation where folding, albeit weaker, continues beyond the salt termination in the Appa­ lachian Plateau area northeast of the Burning Springs anti­ cline (Davis and Engelder, 1985, their Figure 6). As Root suggests, the Burning Springs anticline-Cambridge arch trend may be an example of a type 2 buried thrust front; it is also more akin to the termination of a blind oblique ramp than a frontal thrust.

Thickness Trends and Structure of Berea Sandstone (Mississippian) in Washington County, Ohio: ABSTRACT

The Berea Sandstone has been a drilling target for hydrocarbon production in Ohio for over 100 yr. Although extensively produced, the Berea still rewards the driller with new commercial production. Due to its shallow depth and low cost of completion, the Berea has undergone a renaissance in interest in recent years. This paper presents the results of a study of more than 3,500 geophysical and driller's logs in Washington County in southeastern Ohio. Structure contours show major trends, such as the Burning Springs anticline and the Cambridge arch, trending almost north-south. In the western third of the county, structural trends change, however, to a more complex, less continuous pattern with a predominant northeast-southwest trend. Isopach trends delineate a system of ch nnel sands trending roughly east-west with sandstone thicknesses ranging from 0 to greater than 25 ft (8 m). Both structures and thalwegs are relatively narrow and thickness changes rapidly, both parallel to and normal to the thalweg. Although no new interpretations of Berea structure End_Page 244------------------------------ or depositional environment is intended, this study graphically shows that even in a mature province, low-cost, closely spaced data can indicate new minor structures and delineate new areas for profitable and productive testing with the drill. End_of_Article - Last_Page 245------------

Structures of Southeastern Ohio: ABSTRACT

A review of structure data in southeastern Ohio indicates that the Burning Springs anticline and the Cambridge arch are the only valid structures of regional extent in the area. Recent mapping by the authors has clarified the relation between these elements. The Burning Springs anticline previously has been shown to be the result of thin-skinned thrusting on a Silurian salt glide plane. The salt, now identified as the Salina F4, pinches out beneath the structure. The structure follows the western limit of the salt into southern Monroe County and there dies out. The Cambrian arch follows the pinchout of the Salina E salt; east of the pinchout, elevations of the Pittsburgh coal (Pennsylvanian) are about 300 ft higher than in the west. There is only a gentle southeastward dip below the salt. The structure is interpreted to be the result of movement of a southeastward-thickening block of supra-Salina rocks northwestward along a salt glide plane. A postulated near vertical tear fault (or series of faults) marks the western limit of this movement. The Parkersburg-Lorain syncline, often mentioned as lying west of the Cambridge arch, also is not present below the salt in the study area. Production of hydrocarbons from Salina (Silurian), Oriskany (Devonian), and Berea (Mississippian) zones and from several Pennsylvanian sands appears to be associated with the Cambridge feature for at least 75 mi of its extent. The northward extension of the Burning Springs anticline into Ohio apparently localized production from Pennsylvanian and Mississippian sandstones in Washington and Monroe Counties. End_of_Article - Last_Page 1891------------

Effect of Cambridge-Chadron Structural Trend on Paleozoic and Mesozoic Thickness, Western Nebraska

The Cambridge arch and the Chadron arch are parts of a structural system which crosses Nebraska in a northwesterly direction. Computer-plotted data on the Paleozoic-Mesozoic sequence indicate that variations in the stratigraphic thickness of individual units have been created by erosion, nondeposition, and thickening related to the trend of the structural system, but collectively the sedimentary sequence is of similar thickness throughout. This uniform thickness is caused by areas of erosion and nondeposition subsequently becoming sites of thicker deposition. The weak Paleozoic and Mesozoic erosional and depositional trends coincide in position with strong pre-Tertiary arching, and probably indicate that the system parallels a zone of weakness within the Precambrian basem nt. The pre-Tertiary arching differs from earlier events in that there is no compensating deposition on sites of earlier stratigraphic thinning.

Paleotopography: Key to Locating Concealed Structure and Related Petroleum Traps: ABSTRACT

Recognition that bedrock structure is reflected by modern topography in many areas has helped in successful exploration for petroleum reservoirs. However, application of this relation in other areas has proved unsuccessful. In such areas, older geologic patterns are concealed by divergent younger ones. Mapping of buried topography related to the older structural patterns may be an exploration technique deserving consideration. Two established oil regions in Nebraska--one in the southern part of the panhandle (D-J basin) and the other in the southwestern part of the state (Cambridge arch area)--have no recognizable relation between modern topography and underlying petroleum reservoirs. However, mapping of buried unconformities shows a definite relation between the occurrence of oil fields and paleosurfaces. In western Nebraska, oil and gas fields producing from Cretaceous sandstones coincide with paleotopographic ridges on the pre-Tertiary surface with striking regularity. Similarly in southwestern Nebraska, oil fields producing from Pennsylvanian strata coincide fairly well with paleotopographic highs on the pre-Cretaceous surface. This relation between established oil reservoirs and overlying paleotopography in 2 entirely different geologic regimes indicates a predictability pattern that should be utilized in future development in these and other petroleum-producing regions. End_of_Article - Last_Page 613------------

Epeirogenic and Climatic Controls of Early Pleistocene Fluvial Sediment Dispersal in Nebraska

The change from Pliocene to Pleistocene fluvial sedimentation in Nebraska is denoted by gravel with relative enrichment of mechanically weak rock species and a two-fold increase in largest clast size. These changes in fluvial sediments suggest modification in degradational energy affecting detritus apparently related to deterioration of climate in the early Pleistocene. Cooler Pleistocene climates with increased moisture resulted in greater discharge and carrying capacity for streams headed in the Rocky Mountains and flowing across Nebraska. These streams carried granitic detritus eastward toward the continental glacier margin in easternmost Nebraska. There, streams flowing off ice sheets carrying sedimentary and metamorphic detritus derived from the ice joined the east-flowing streams from the mountains. Detritus derived from continental glaciers in easternmost Nebraska, therefore, was not transported westward, but instead, was mixed with Rocky Mountain–derived detritus and transported southward along the ice-front margin. Even though the drainage basin of the Platte River system came into existence in the Tertiary, the present course of the Platte River dates only from mid-Pleistocene time. Widespread occurrence of lower Pleistocene braided channel deposits east of the Chadron–Cambridge Arch that contain Laramie Range–derived anorthosite indicates repeated channel switching and meandering during times of aggradation over surfaces of minimal relief. Relations of these gravels to the Chadron–Cambridge and Siouxana Arches suggest that uplift on these structures was sufficient to deflect and control the course of streams headed in the Laramie Range and flowing across the plains. Activity on the Chadron–Cambridge Arch also is suggested by the distribution of earthquake epicenters, modern drainage patterns, and the relation of the pre-Pleistocene bedrock surface to the arch and the profile of the Platte River. The presence of knickpoints on rivers crossing the arch suggests that these rivers are maintaining a course antecedent to a spasmodically rising arch. Rivers are entrenched now, but during the early Pleistocene when streams carried a heavy load of sand and gravel, similar activity along the Chadron–Cambridge Arch would have been adequate to spill the aggrading stream over its fan and divert it southward where it could follow a new course. Eastward-flowing streams heading in the Rocky Mountains were controlled by changes in discharge of streams and movement on epeirogenic structures during the early Pleistocene.

Developments in Eastern and Northwestern Colorado and Western Nebraska in 1966

The total drilling in eastern and northwestern Colorado and western Nebraska declined from 979 wells in 1965 to 779 wells in 1966. This decrease of 20.5% was caused by a sharp decline in exploratory and development drilling in central and western Nebraska, and indicates lack of success in extending production northward in the Nebraska panhandle and along the Cambridge arch. Exploratory drilling declined by 76 wells in Nebraska and by 52 wells in Colorado; development drilling increased by 32 wells in Colorado and decreased by 104 wells in Nebraska. Discoveries in northwestern Colorado from reservoirs of Cretaceous age and southeastern Colorado from reservoirs of Mississippian and Pennsylvanian ages should lead to increased exploratory work. The Denver basin activity shoul continue at about the same level, whereas central Nebraska drilling activity may increase with the discovery of oil in Reagan Sandstone northwest of Sleepy Hollow. Geophysical activity will probably increase in northwestern Colorado in search of structural trends in the Dakota and in southeastern Colorado in search of structural trends in the Mississippian.

High-Lights of Exploration and Development in Colorado and Western Nebraska, 1962: ABSTRACT

During 1962, 645 wells were drilled in western Nebraska, a 35 per cent decrease from the record year of 1961. The 381 exploratory wells resulted in 33 discoveries (success ratio of 8.7 per cent). Important developments in 1962 include the following: the expansion of the Cambridge arch productive area; the northward and northeastward spread of the search for Paleozoic oil into sparsely drilled north-central Nebraska; the rapid expansion End_Page 119------------------------------ of Lansing production at Silver Creek field; and a renewal of the search for Minnelusa oil in northwestern Nebraska. Oil production in Nebraska was 24,691,779 barrels, a 4 per cent increase from 1961. During 1962, 810 wells were drilled in Colorado, representing a 19 per cent increase from 1961. The 449 exploratory wells produced 45 successes (exploratory success ratio 10.1 per cent). Important developments in 1962 include the following: a surge of exploratory and development drilling in Washington County; the increasing search for pre-Cretaceous objectives around the flanks of the Denver basin; and the continuing search for various stratigraphic and structural objectives in western Colorado. Oil production in Colorado totaled 42,345,559 barrels, a 4.9 per cent decrease from 1961. Geophysical activity increased markedly around the fringes of the Denver basin but decreased enough on the Cambridge arch and in the Paradox basin to show a 12 per cent over-all decrease. In 1963, exploratory activity is expected to taper off somewhat from its 1962 pace. End_of_Article - Last_Page 120------------

Sleepy Hollow Field of Red Willow County, Nebraska: ABSTRACT

The Sleepy Hollow field, on the southwest flank of the Cambridge arch, is a recently discovered oil field of major importance which produces from multiple pay zones at a relatively shallow depth. The oil accumulation is partly controlled by a structural nose, trending south-southwest across the field. A large area of limited reversal may be mapped near the center of the field. Good porosities across the southern half of the field are responsible for production from the Lansing-Kansas City limestones. The principal producing limestone of this group is the C zone. The build-up of porosity in the C zone is a part of a porosity trend which can be traced for a considerable distance east and west of the field. Production from the basal sand is controlled by an oil-water contact on the flanks of the structure and by a pinch-out or truncation of the sand at the northeast end of the field. Cumulative production from the Sleepy Hollow field exceeds 10 million barrels. The Lansing-Kansas City has contributed approximately 3 million barrels and the basal sand has produced approximately 7 million barrels. The wells in the field average 3,500 feet in depth and can be completed for less than $40,000. End_of_Article - Last_Page 116------------

Developments in Colorado and Western Nebraska in 1963

There were 100 fewer tests drilled in Colorado and western Nebraska in 1963 than in 1962. This was a decline of 8.8%. The over-all success ratio, however, remained nearly constant, increasing by 0.3% to 29.6%. Exploratory drilling in the Cambridge Arch area of Nebraska suffered a sharp decline. This was not reflected by the success ratio in the area which increased from 4.2% in 1962 to 11.5% in 1963. In Nebraska, oil and gas production declined from 30,703,000 bbls. and 24,200 MMCFC in 1962, to 26,912,000 bbls. and 15,470 MMCFG in 1963. This production decline was mirrored in Colorado by a decline from 42,477,000 bbls. and 173,230 MMCFG in 1962, to 37,859,700 bbls. and 159,650 MMCFG during 1963 (production figures approx.). Oil and gas reserves also declined during this period with considerably fewer reserves discovered than produced. Several of the most active 1962 exploration plays either waned such as the Washington D-J and the Cambridge arch Pennsylvanian, or did not continue into 1963. Exploration activity increased in eastern and southeastern Colorado. In northwestern Colorado, a good deal of interest has developed in the bituminous sands and oil shale deposits. Some rank wildcats were drilled in this part of the state but most success came from deeper drilling in established fields. There was a 51% decrease in geophysical activity in Colorado and western Nebraska during 1963. This decrease was sharpest in Nebraska with a 77% decline and in southwestern Colorado with an 80% decline. Exploratory activity in 1964 should continue at about the 1963 level with activity concentrated in northwest and eastern Colorado. Activity in the Cambridge Arch area of Nebraska should increase.

Developments in Colorado and Western Nebraska in 1962

During 1962, 645 wells were drilled in western Nebraska, a 35% decrease from the record year of 1961. The 381 exploratory wells resulted in 33 discoveries (success ratio of 8.7%). Important developments in 1962 include the following: the expansion of the Cambridge arch productive area; the northward and northeastward spread of the search for Paleozoic oil into sparsely drilled north-central Nebraska; the rapid expansion of Lansing production at Silver Creek field; and a renewal of the search for Minnelusa oil in northwestern Nebraska. Oil production in Nebraska was 24,691,779 bbls., a 4% increase from 1961. During 1962, 810 wells were drilled in Colorado, representing a 19% increase from 1961. The 449 exploratory wells produced 50 discoveries (exploratory success ratio 11 per cent). Important developments in 1962 include the following: a surge of exploratory and development drilling in Washington County; the increasing search for pre-Cretaceous objectives around the flanks of the Denver basin; and the continuing search for various stratigraphic and structural objectives in western Colorado. Oil production in Colorado totaled 42,345,559 bbls., a 4.9% decrease from 1961. Geophysical activity increased markedly around the fringes of the Denver basin but decreased enough on the Cambridge arch and in the Paradox basin to show a 12% over-all decrease. In 1963, exploratory activity is expected to taper off somewhat from its 1962 pace.

Pennsylvanian and Lower Permian of Northern Denver Basin, Colorado, Wyoming, and Nebraska

Permo-Pennsylvanian sedimentation in the northern Denver basin was influenced by the positive areas of Frontrangia (ancestral Front Range) and the Cambridge arch. Near these sources, coarse clastics interfinger with shales and limestones of the basin. The Cambridge arch was first covered by marine Pennsylvanian sediments during Desmoinesian time. Frontrangia contributed coarse arkosic sediments from Morrowan into Leonardian time. A northwest trend of Frontrangia and the Cambridge arch is indicated by isopach and lithofacies data. The Transcontinental arch was a positive pre-Morrowan feature which also was very slightly positive during Desmoinesian and Missourian time and at the end of the Pennsylvanian Period. The Hartville, Wyoming, area was basinal until uplifted near the end of Wolfcampian time as indicated by truncated Wolfcampian sediments. lsopach maps indicate that the Las Animas arch was mildly positive during Morrowan, Atokan, and Desmoinesian time. On the east flank of the Laramie Mountains the Casper Formation includes sediments of Atokan through Wolfcampian Age. Sediments of questionable Morrowan Age may be present in the basal Casper or in underlying tongues of the Fountain. At the type locality strata as young as Leonardian are believed to be present in the upper Casper. The Ingleside Formation is the Colorado equivalent of the Casper and near the Wyoming-Colorado boundary line also includes sediments of Atokan through Wolfcampian Age. South of the Wyoming-Colorado boundary line basal Ingleside intertongues with the Fountain Formation and at the type locality the Ingleside is entirely Wolfcampian. The Hartville Formation is divided into seven members ranging in age from Atokan to Leonardian, inclusive. Wolfcampian fusulinids occur 70-80 feet below the subsurface shale marker in Cheyenne County, Nebraska. The Satanka Formation consists of the predominantly red sediments that lie between the Casper or Ingleside Formation below and the Opeche Shale or the Lyons Sandstone above. Lateral equivalents of the Satanka are the uppermost Casper of the type locality, the Cassa Member of the Hartville Formation, the middle part of the Lyons Sandstone at the type locality and rocks of Leonardian Age in northwestern Kansas. Lithofacies maps of Pennsylvanian and early Permian sediments indicate decreasing clastic ratios in progressively younger strata. Pre-Desmoinesian clastics decrease in particle-size toward the center of the Denver basin, whereas in sediments deposited following burial of the Cambridge arch, during Desmoinesian time, the clastic particles become smaller eastward across the basin. The present structure of the northern Denver basin is abruptly asymmetrical, much steeper on the west than on the east side, and slightly elongate in a northerly direction. The deepest part of the basin lies only 18-20 miles east of the Permo-Pennsylvanian outcrop. Isopach maps of the Pennsylvanian and Lower Permian Series show relatively greater subsidence in the center of the basin than on the flanks, and also indicate a general southward tilting. About one well per 400 square miles has penetrated the Lower Permian-Pennsylvanian section. These objectives in the northern Denver basin must be considered largely unexplored.

Ackman Field and Environs, Southwestern Nebraska

Discovery of the Ackman field, Red Willow County, Nebraska, in late 1959 encouraged exploration in that area to the extent that numerous other fields were found. Most of the production from these fields is derived from the Lansing-Kansas City Group, but basal Pennsylvanian sands have also contributed important reserves, particularly in the Sleepy Hollow field approximately 4 miles northeast of Ackman. The Ackman field was discovered as a result of seismic and subsurface work. Structurally, the field is related to the Cambridge arch, the axis of which is about 10 miles northeast. The Cambridge arch has undergone several important periods of diastrophic movement during its geological history, the most significant of which occurred at the end of Mississippian time. Ackman field structure is low-relief with some evidence of basement adjustment on the east side. Sediments within the field area range from Tertiary to Pennsylvanian; Triassic rocks are absent either by truncation or non-deposition. Production at Ackman is confined to beds of Pennsylvanian age. Six limestone zones within the Lansing-Kansas City Group and one limestone zone in the Shawnee Group are productive. There is no basal Pennsylvanian sand developed at Ackman. The thinly interbedded limestones and shales of the Lansing-Kansas City Group in southwestern Nebraska were apparently laid down by rapidly fluctuating shallow seas. Organic micro-build-ups over localized micro-structural-topographic features along with post-depositional modifications have resulted in the formation of competent reservoirs. The age of the productive basal sand in the Sleepy Hollow field is controversia because of its stratigraphic position. Indirect evidence favors Pennsylvanian age. A bar-complex origin is indicated for the sand body. Cumulative production of the Ackman field on November 30, 1961, was 961,230 barrels of oil with a variable water-cut and a negligible gas content. The field apparently does not have a water drive. An area of 2,040 acres has been proved productive on a 40-acre spacing program. Fifty-two wells have been drilled in the field; fifty are currently producing.

Developments in Colorado and Western Nebraska in 1961

During 1961, 678 wells were drilled in Colorado, representing a 3.6% decrease from 1960. The 260 new-field wildcats produced 30 discoveries (wildcat success ratio: 11.5%). Important developments in 1961 included the following: the surge of leasing and drilling activity in the Washington County part of the Denver basin during the last quarter; 3 wildcat discoveries and encouraging shows in southeastern Colorado; the first successful well in Pitkin County in the southern part of the Piceance basin; the decrease in exploratory activity in the Colorado part of the Paradox basin. Oil production in Colorado totalled 44,512,070 barrels, a 6.2% decrease from 1960. During 1961, 998 wells were drilled in western Nebraska, a 10.2% increase over 1960. The 481 new-field wildcats resulted in 34 discoveries (wildcat success ratio: 7.7%). Important developments in 1961 included the following: the expansion of the Cambridge arch productive area outward from its concentration in Red Willow County; the spread of the search for Paleozoic oil into sparsely drilled north-central Nebraska; the first production from Permian rocks; continued success in the Nebraska part of the Denver basin. Oil production in western Nebraska was 23,789,843 barrels, a 0.6% decrease from 1960. Geophysical activity increased in the Cambridge arch area of south-central Nebraska and decreased in the Paradox basin of southwestern Colorado. In 1962, exploratory activity is expected to maintain its 1961 pace, with decreases in some areas compensated by increases in other areas.