Devonian Formations Research Articles

The Devonian System in China

SummaryThe Devonian System of China occurs in the Tianshan–Hingan* Geosyncline of North China, the Yangtze Paraplatform of South China and in the geosynclinal areas of West China. Devonian formations are entirely absent from the Sino-Korean Paraplatform and the Tarim Platform, but along the border of the Tarim Basin and the slopes of Qilianshan (Chilienshan) are continental sequences rich in vertebrate and plant fossils. On the basis of differences in biostratigraphic characteristics the Devonian is divisible into 8 regions: 1. Junggar–Hingan (Dzungar–Khingan), 2. South Tianshan (South Tienshan), 3. Qilianshan (Chilienshan), 4. Longmenshan (Lungmenshan), 5. South China, 6. South-eastern China, 7. West Sichuan–North Xizang (West Szechuan-North Tibet), 8. Himalaya–West Yunnan.Using both lithology and fossil content, five principal facies (types) may be further differentiated: 1. Hingan type (eugeosynclinal facies), 2. Baoxin type (miogeosynclinal facies), 3. Xiangzhou type (platform near-shore facies), 4. Nandan type (platformal off-shore facies), 5. Cuifengshan (Chuifeng shan) type (continental facies).The Devonian of South China occurs in various facies, but has a well-defined principal stratotype. It has been divided by Chinese geologists into eight stages, two in the Upper Devonian, two in the Middle Devonian and four in the Lower Devonian.The Devonian sequences of the geosynclinal regions of West China are characterized by facies which contain graptolites and tentaculites, and have largely been affected by regional metamorphism. The Devonian biota of the Junggar–Hingan Region is essentially an endemic one, but it is associated with a few European and North American elements. Marine pyroclastics are the principal rocks. General stratigraphical columns of those regions are given with a correlation with other parts of China. The continental Devonian of China is unique especially in the light of its vertebrate content. Red sandstones are widespread in the Lower Devonian, and intercalated with argillaceous limestone and mudstone. This sequence is rich in endemic East Asiatic faunal elements including agnatha (Galeaspida, Polybranchiaspida), Antiarchi, etc. Middle and Upper Devonian lithostratigraphic units consist principally of light coloured quartzose sandstone formations in Southeastern China, with some red beds in South and North China, while their fauna and flora are cosmopolitan in character. Their exact geological ages vary, however, as clearly shown in the South China region, especially towards the top of the Upper Devonian (with Asterolepis) and in the early Middle Devonian (with Bothriolepis).

Silurian-Devonian Embayment into Appalachia: ABSTRACT

Several Silurian and Devonian formations, traced along central Appalachian outcrop belts, undergo anomalous changes in lithology, thickness, and faunal content in the area of Pendleton County, West Virginia. Hematitic sandstones are replaced by nonred sandstones, other sandstones change to shales, shale formations thin and pinch out, and Catskill red beds are replaced by nonred clastics. These changes represent a move from nearshore to more offshore deposition within an embayment along the western shoreline of Appalachia. The principal Silurian sandstones, the Tuscarora, Keefer, and Williamsport, all undergo facies changes in this area, becoming finer grained, argillaceous, and/or calcareous. Hematitic nearshore sandstones of the Rose Hill are replaced by nonred sandstones in the embayment. The Rochester and Rose Hill shales are thickest in, and largely confined to, the embayment; younger shales, the Big Mountain and Mandata, are replaced by sandstones south of the embayment. McKenzie shales contain more interbedded limestones in the embayment, and the fossiliferous middle Tonoloway Limestone contains a different fauna there than in adjacent areas. During Middle Devonian time, thicker, deeper water Needmore Shale beds in the area were flanked by shallower water facies of the Onesquethaw. The Mahantango Formation also is thickest in this area and thins abruptly to the south. Throughout this time two provenances supplied detritus, one east of West Virginia's eastern panhandle and the second in central Virginia. The Pendleton embayment was situated between these two regions that occasionally prograded westward, accentuating the embayment. For the final Devonian regression these areas are End_Page 846------------------------------ referred to as the Fulton and Augusta lobes of the Catskill delta, separated by Grant bay. End_of_Article - Last_Page 847------------

Use of scintillometer and gamma-ray logs for correlation and stratigraphy in homogeneous black shales: Summary

INTRODUCTION Widespread, thick, uniform black-shale units may defy all but the coarsest stratigraphic subdivision. The devonian and Mississippian black-shale formation such as the Ohio, New Albany, and Chattanooga Shales of the eastern and central United States are good examples. Exposures of these black organic-rich shale formations appear lithologically uniform except for a few interbeds of greenish-gray shale. These black shales make up as much as one-fourth of the Devonian sedimentary sequence in the Appalachian basin and are important sources of natural gas Avila, 1976). Yet, because most means of stratigraphic control are absent and paleontologic correlation is laboriously difficult in these shales, internal subdivision and correlation among Devonian-Mississippian black shales of some areas are wholly inadequate. In parts of Ohio, Indiana, and Tennessee, widespread bentonites or thick green-shale beds are present in the Devonian-Mississippian black-shale sequence, providing a basis for its subdivision (Campbell, 1946; Hass, 1956; Hoover, 1960; Conant and Swanson, 1961; Lineback, 1968, 1970). In eastern Kentucky (Fig. 1), however, similar marker beds are either very thin or absent. Thus, the Devonian-Mississippian black-shale sequence of eastern Kentucky has not been divided previously, because apparent homogenity impeded attempts to compare and correlate the sequence with subdivision in the better known black-shale sequences of adjacent states.

Mid‐Devonian movements in the Northeastern Lachlan Fold Belt

Abstract Calculations of the angular discordance between the Upper Devonian Lambie/Catombal Groups and underlying Lower Devonian formations at 130 separate locations in six areas in the northeastern Lachlan Fold Belt show that the unconformity is of a low angle except for one locality, near Limekilnes (100°). Fewer than 3% of the calculated angular discordances exceed 30°, and 73%” are less than 20°. Attempts to discover a mid‐Devonian fold direction from the restored orientation of the Lower Devonian beds, after the Upper Devonian beds have been rotated to horizontal, have been unsuccessful. Scatter of the restored bedding poles, either primary, or introduced by deformation or imprecision inherent in the measurement technique, camouflages any consistent mid‐Devonian fold axis. Although there was demonstrable uplift, tilting, and erosion in the mid‐Devonian, limb dips on any mid‐Devonian folds do not exceed 30°. From consideration of our data, and the interpretation of angular unconformities, we conclude ...

Geochemical soil surveys over cambrian and lower devonian formations in the belgian ardennes as a tool for geological mapping

A geochemical soil survey was made over outcropping Cambrian and Lower Devonian formations in the Belgian Ardennes. About 300 soil samples were collected in an area of 2.5 × 1.1 km2.Analysis of rock samples from scarce, but stratigraphically well-known outcrops show that the Cambrian rocks are richer in copper than those of the Lower and Upper Gedinnian (Lower Devonian). The Upper Gedinnian rocks are more nickeliferous than the other horizons.In the well-drained soils, copper and lead distributions permit precise location of the boundary between Cambrian and Lower Gedinnian formations. The nickel distribution seems to be a good stratigraphic indicator for distinguishing Upper from Lower Gedinnian. Zinc distribution seems unrelated to underlying bedrock geology.Regression analysis confirms that the content of Cu, Pb and Ni is essentially a function of lithostratigraphy. However, the distribution of these elements is more influenced by pedological factors in the poorly drained soils.

Lower Devonian biostratigraphic correlation in Eastern Australia and Western North America

Savage, N. M.: Lower Devonian biostratigraphic correlation in Eastern Australia and Western North America. Correlation of Lower Devonian marine strata of Eastern Australia and Western North America can be attempted using several fossil groups. In general the faunas are similar, with many species common to both areas. Graptolites occur infrequently, but are of great value because of their precise zonal significance. Several of these graptolites occur in the classic type sequences of Bohemia but are absent from the Rhineland stages. For this reason, and because of the Bohemian affinities of the shelly faunas, the Bohemian stage names are preferable to the Rhenish names for use in Eastern Australia and Western North America. Cono-donts are less precise index fossils than graptolites, but are considerably more abundant and therefore have greater application. Brachiopods are valuable if viewed as assemblages. In terms of the Rhenish stages, most Eastern Australian and Western North American early Lower Devonian formations appear to have been placed too high. Recent evidence from northern Canada throws new light on the problem of correlating Rhenish and Bohemian classic early Lower Devonian stages.

Geology and Petroleum Possibilities in and Around Gulf of St. Lawrence: ABSTRACT

The Gulf of St. Lawrence is the largest embayment on the Atlantic Coast of North America between Florida and the Labrador Sea. In this region, surface geology provides the main control for defining four structural units which can be extended into or across the Gulf using published geophysical data and available well control. Prospective beds are found in early, middle, and late Paleozoic formations which in turn overlie Precambrian, Taconic, and Acadian basement. Acadian basement rocks underlying the Maritimes basin, Sydney basin, Fundy basin, etc., are exposed in parts of Nova Scotia, and continue south beneath the great Mesozoic and Tertiary sedimentary wedge of the Scotian basin. Production has not been obtained in the Anticosti basin, but most Ordovician formations have yielded gas and oil shows. The thick, gently dipping, early Paleozoic section is fairly prospective. Many seeps and shows, but insignificant oil recoveries, come from Devonian formations in the Gaspe fold belt; the Silurian also may be prospective in this structurally complex area. The Cambro-Ordovician section in both the klippe and autochthonous sequences in the western Newfoundland fault belt is prospective; seeps, shows, and very minor oil production have come from Ordovician porous zones. In the Maritimes basin, the presence of one small oil and gas field, many shows and seeps, oil shale and albertite, pronounced structures, and a thick sedimentary section have maintained interest and act vity in this large intermontane basin of Late Devonian to Permian sedimentary rocks. Carboniferous End_Page 2149------------------------------ sandstones and carbonate rocks provide the best prospects. Although estimated values for oil and gas in place in these four structural units may be quite large, the Paleozoic age of the sedimentary section, the low grade metamorphism in some parts of the area, the generally poor porosity at depth, and the large area under water where operations are very expensive all reduce considerably the commercial potential. End_of_Article - Last_Page 2150------------

Environmental Significance of Certain Yellow-Brown Dolomites, Great Basin, California and Nevada: ABSTRACT

Within the dominantly gray limestone and dolomite sequences of the Great Basin, sporadic yellow-brown dolomite is present as discrete beds, lentils, or wisps--the latter imparting a mottled or pseudobrecciated appearance. These distinct deposits, studied in Ordovician, Devonian, Pennsylvanian, and Triassic formations, are fine grained, are usually unfossiliferous, and posses a less than 5 percent terrigenous fraction of well-rounded quartz and sparse feldspar. Laminations, desiccation polygons, and possible relict bird's-eye structures also are apparent. Stratigraphically, these deposits commonly overlie dolomite sequences several hundreds of feet thick and are in turn overlain by limestone sequences of varied thicknesses. The persistent occurrence of these yellow-brown dolomites at the top of dolomitized sequences possibly indicates that the yellow-brown lithology represents supratidal or penecontemporaneous dolomitization from which seepage-refluxion brines ultimately descend. The underlying rocks commonly are coarser grained, contain normal marine fossils, and reveal gravity-controlled dolomitization structures. In some cases where yellow-brown deposits are incorporated in a completely dolomitized interval, the beds below the lowest yellow-brown horizon reveal multiple dolomitization. End_of_Article - Last_Page 770------------

Phytoplankton Abundance and Diversity During the Late Devonian and Early Mississippian of Ohio: ABSTRACT

The Upper Devonian Chagrin and Cleveland Shales contain a diverse and abundant organic-walled microplankton assemblage of acritarchs and leiospheres with associated spores, whereas the Lower Mississippian Bedford Shale phytoplankton assemblage is greatly reduced. Ten-gram samples were examined at intervals of 10 ft or less for the entire 573 cores through the 3 formations, to determine microplankton abundance and diversity. In these samples, spores are the most abundant element, followed by leiospheres and then acritarchs. More than 50 species of acritarchs and leiospheres, mostly new, have been identified in 62 samples of the Upper Devonian section, whereas one Gorgonosphaeridium species occurs abundantly in all Upper Devonian samples and is present in most Lower Mississippian samples. General acritarch diversity decreases slightly up-section in the Chagrin Shale, and increases slightly in the basal Cleveland Shale; the decrease is more marked in the upper Cleveland Shale, and is most notable in the Bedford Shale. Acritarch abundance also lessens up-section through the Upper Devonian formations, with minima in the upper fourth of the Chagrin Shale and the upper half of the Cleveland Shale. Acritarchs are very scarce in the Bedford Shale. As total phytoplankton abundance shows a marked decrease up-section, from the middle of the Chagrin Shale to the top of the Bedford Shale, a drop in net primary productivity is indicated for the Late Devonian and Early Mississippian of Ohio. End_of_Article - Last_Page 812------------

Palaeomagnetic Results from Palaeozoic Sediments of Northern Australia

Summary A palaeomagnetic study was made of 52 samples from an Upper Devonian formation and 97 samples from three Cambrian formations. In most cases, the application of demagnetization procedures failed to reveal other than a recent magnetization, presumably acquired by chemical means during weathering processes. Seventeen samples, four of which had been collected a decade earlier, appeared to yield the direction of original magnetization of the Middle Cambrian Hudson Formation. Assigning unit weight to sample mean results, the palaeomagnetic pole position for the period of deposition is calculated to be 19E, 18 N with A,, = 13. The data for the Jinduckin Formation have been re-assessed using sample results, and these give a slightly revised pole position for the Lower Ordovician at 25 E, 13 S with Ags = 11. The first results from a recent palaeomagnetic survey of the Palaeozoic of northern Australia indicated the widespread occurrence of a sometimes dominant secondary magnetization (Luck 1970; McElhinny & Luck 1970). The importance of this effect was revealed during measurements on 149 samples from four other sedimentary formations collected in the same general region (Table 1). The natural remanent magnetization (NRM) of most of these samples was diiected along the Earth's present field. A similarly directed secondary component, presumably representing a viscous remanent magnetization (VRM), had been readily removed from most samples of the Jinduckin Formation by the application of partial demagnetization techniques (Luck 1970). In contrast, the present paper reports that recently acquired magnetization could not be cleaned from the Blatchford Formation, the Hart Spring Sandstone, the Cockatoo Formation or from most samples of the Hudson Formation. This hard, secondary component was taken to represent a chemical remanent magnetization (CRM) imposed by weathering processes. In favourable circumstances, the effect might be neutralized by a chemical method such as devised by Collinson (1965) for separating the red pigment from the specularite of red beds. This technique may be regarded as a means of partial demagnetization (Collinson 1967) if the secondary magnetic component resides in the pigment. Disc-shaped (3.5 cm x 1.0 cm) and cylindrical (2.8 cm x 2-8 cm) specimens were cut from most samples. The discs were investigated using an astatic magnetometer

Correlation of the middle devonian formations of Australia

Correlation of the middle devonian formations of Australia

Defining lithostratigraphic boundaries in homogeneous strata—A case study

Confusion exists in the literature as to: ( 1) which strata are included in the Esopus Formation in New York, U.S.A.; ( 2) what is their aggregate thickness; ( 3) where the unit's lithostratigraphic boundaries are; and even (4) what these strata should be called. The apparent homogeneity of Esopus beds and paucity of fossils therein account for much of this confusion. Data from detailed field investigations and laboratory analyses were assembled and examined with the aid of a digital computer. Results of these studies show that the Onesquethaw Esopus Formation, ranging from about 200 ft. in thickness near Catskill to about 250–275 ft. near Kingston, New York, may be subdivided into two or three members on the basis of field and petrographic characteristics. X-ray diffraction data and emission-spectrographic analyses support this subdivision. The essentially non-calcareous Esopus beds, thus, are clearly separable from the underlying Port Ewen Limestone and Glenerie Limestone, and from the overlying Schoharie Limestone and Onondaga Limestone, all upper Lower Devonian formations. Of 36 variables tested, the following Pearson correlations with extreme levels of significance (0.0005, or 99.95%) were found to characterize Esopus beds among the sequences studied: the mineral ratios, (kaolinite:illite) and (quartz:feldspar), (kaolinite:illite) and (dolomite:feldspar), (dolomite: feldspar) and (quartz:illite); and, the trace element pairs, B-V, Ba-Ga, Ba-Rb, Ba-Sc, Be-V, Cr-Sr, Cu-Rb, Cu-Sr, and Ni-V. Members of the Esopus Formation are also characterized by geochemical and mineralogical relationships and occurrence trends. Methods used in this study should be applicable throughout the world.

Agnostoid Trilobites in a Devonian Formation in West Malaysia

Agnostoid Trilobites in a Devonian Formation in West Malaysia

Stable Carbon Isotope Investigation of Natural Gases from Sacramento and Delaware-Val Verde Basins--Possible Igneous Origin: ABSTRACT

Investigations of chemical and stable carbon isotopic character of gases from major fields in the Sacramento basin, California, and the Delaware-Val Verde basins of West Texas and New Mexico indicate a strong possibility of a high-temperature, igneous-type origin for some of these gases. Geomagnetic surveys of the Sacramento basin indicate the occurrence of 2 types of volcanics. Thin, near surface flows and deep-seated basalt plugs are observed which appear to be related to the occurrence of gas in the basin. Analysis of coproduced methane and carbon dioxide from a given well yields information concerning the thermal history of the gas. Isotopic equilibrium or quenching temperatures were determined and interpreted on a regional basis. Gases from the Sacramento Valley are produced from sandstone lenses in alluvial fan-type deposits from Late Cretaceous to Eocene in age. In the south-central part of the basin, the Marysville Buttes constitute a topographic break which has a volcanic plug jutting about 2,000 ft above the surrounding country. Considerable amounts of gas have been produced from sandstone bordering this feature. Secondary folds and faults attending the intrusion exist at a radius of several miles from the core. Gases produced in wells surrounding this cylindrical shaft of igneous rock exhibit a temperature dependence as a function of the distance of production from the central core. Delaware and Val Verde basin gases from the deep Ellenburger and Devonian formations are dry and composed almost entirely of methane and carbon dioxide. The basin gas data suggest a possible igneous origin, centered about the Marathon thrust zone of the Ouachita structural belt to the southwest. End_of_Article - Last_Page 369------------

A palaeomagnetic survey of South American rock formations - Studies on rock formations from Northeast Brazil

Devonian, Carboniferous and Xriassic formations from Piaui and Maranhao States in the northeast of Brazil have been studied and palaeomagnetic pole positions deduced. During the Devonian and Carboniferous the south pole appears to have moved away from S. America in a southeasterly direction from the vicinity of Rio de Janeiro: in the Triassic it was situated near the present position of the south pole relative to S. America. These results are in good agreement with those from other formations of the same age from other parts of the continent. Thermal cleaning has been carried out. The Devonian formations were almost completely remagnetized by the Mesozoic or Tertiary geomagnetic fields and the primary magnetization is very weak. Polar wander of 40 to 50° appears to have occurred during the time interval under investigation, i.e. M. Devonian to Triassic.

Devonian Palaeomagnetic Pole for Europe and N. America

THE position of the Devonian pole relative to Britain, was originally deduced from palaeomagnetic measurements on the Old Red Sandstone formation from S. Wales1. It was apparently confirmed later by measurements on many other Devonian formations from Europe and western Russia2. We label these poles “DI” in Table 1. Palaeomagnetic work on Devonian lavas from the Midland Valley of Scotland yields, however, a completely different pole3 which is labelled “DII” in Table 1 (ref. 4).

AEROMAGNETIC STUDY OF COYANOSA

The Coyanosa field is situated seventeen miles northwest of Fort Stockton within the eastern part of the Delaware basin adjacent to the southwestern corner of the Central Basin Platform. Deep production, discovered by Mobil Oil Corporation in May, 1963, now covers an area of 30 sq mi of Devonian and Ellenburger formations. Devonian products have a market value currently estimated at almost $500 million and Ellenburger at $400 million.

Successfully Cycling a Low-Permeability, High-Yield Gas Condensate Reservoir

Abstract The Headlee Devonian field in West Texas produces a rich (325 bbl/MMcf) gas condensate from a depth of 11,900 ft. Over 50,000,000 bbl of hydrocarbon liquids have been recovered during 8 years of cycling operations. The average core properties are 5.8 percent porosity and 0.11 to 0.30 md permeability. The dewpoint pressure of 4,375 psia is only 881 psi below, the average reservoir pressure. This high dewpoint pressure and the low formation permeability result in flowing BHP's below the dewpoint pressure. A method for calculating the volume of retrograde liquid around the producing wellbore and its effect on producting rates is presented. Also presented is a prediction technique combining areal sweep behavior and a stratification analysis that matches past performance, indicating a recovery of 3.5 times that expected under primary pressure depletion. Introduction The Headlee Devonian field was unitized in Dec., 1957, for the purpose of conducting cycling operations. Gas injection began on the effective date of the unit. Flowing BHP's below dewpoint pressure are experienced because of low formation permeability and a high dewpoint pressure. A retrograde liquid saturation of 45 percent occurs within 50 psi of the dewpoint pressure. It was, therefore, desirable to derive a method of calculating the volume of this retrograde liquid around the producing wellbore and its influence on producing rates. It was also desirable to compare actual injected gas production with a prediction of sweep efficiency and injected gas production. History The Headlee Devonian field was discovered in Aug., 1953. It is encountered at a depth of approximately 11,900 ft. The field is located in Ector and Midland counties immediately east of the city of Odessa, Tex. Fig. 1 is a structure map contoured on the top of the Devonian formation. Although the field is limited on the east by decreasing permeability and water and on the west by water, no evidence of water influx has been noted. it contains 14,000 productive acres. Formation stimulation with large volume fracture treatments was required to establish commercial production rates. Generally, 300 gal of brine per foot of net pay with 2 lb/gal of sand as propping agent were used in the treatments. The operators in the field were aware that a great loss in liquid recovery would occur if cycling operations were not put into effect before the reservoir pressure declined below the single-phase pressure. At the operators' request, production from the reservoir was restricted to six producing days on Nov. 1, 1956, by the Texas Railroad Commission. At that time there were 15 statewide producing days. The field was unitized in Dec., 1957, for the purpose of conducting cycling operations. Gas injection, using a nine-spot pattern with wells on 80-acre spacing, began on the effective date of the unit. Less than 2 percent of the ultimate recovery was produced prior to unitization. A production-injection balance was attained at the end of 1959 and the field was restored to statewide producing days in Jan., 1960. Data obtained prior to unitization indicated that the reservoir fluid existed in a single phase; however, it was inconclusive as to whether a bubble point or a dew point was encountered on isothermal pressure reduction at reservoir temperature. The field was classified as an oil reservoir. Subsequent to unitization, sufficient additional reservoir fluid data were obtained to establish conclusively that the reservoir fluid encountered a dew point. The field was reclassified as a non-associated gas field effective July 1, 1962. On Jan. 1, 1966, there were 163 wells (111 producers, 39 injectors, 13 shut-in) in the unit. Cumulative unit liquid production at that time was 52,800,000 bbl (Fig. 2). The reservoir fluid was initially produced through conventional separation facilities to atmospheric storage tanks. Since Dec., 1961, absorption and stabilization facilities have been used. The well effluent is now separated in the field at either 1,200 or 750 psi for ease of measurement and movement to additional facilities. The separator liquid yield is in the order of 300 to 350 bbl/MMcf of full well stream. JPT P. 41ˆ

Application of Well Control Technology to Drilling Problems in the Delaware Basin

Abstract Wells drilled to test deep Devonian or Ellenburger formations in the Delaware Basin will encounter 5 to 7,000 ft of abnormally pressured Wolfcamp and Pennsylvanian formations. These formations contain many gas bearing sections which require mud weights from 12 to 16.7 lb/gal to balance. Frequently, these gas bearing formations are noncommercial, but of sufficient volume to be troublesome if not controlled. Since drilling with mud weights which overbalance formation pressures is inherently expensive, a great incentive exists to utilize techniques for controlling these formations with as light mud weights as possible. Successful use of light mud weights to drill Wolfcamp and Pennsylvanian formations requires a knowledge of the detection, interpretation and control of gas in the annulus. This paper discusses these aspects of well control procedures and proposes many cost-saving features that may be incorporated into drilling programs which utilize reliable well control procedures. Introduction A knowledge of well control procedures is essential to conducting a safe drilling operation. Protection of life and property is dependent on the ability to control a threatened blowout. However, if the ability to control a threatened blowout reaches a certain stage of accomplishment, then it is possible to effect major changes in a drilling program - all designed to reduce drilling costs. The need for reliable well control procedures has been receiving increased attention in the past few years.1-3 Recent advancements in drilling technology1-6 have pinpointed mud weight as the major variable in an attempt to reduce drilling costs. The major conclusion drawn is to keep mud weight as light as possible to achieve minimum drilling costs. To effect this recommendation requires an accomplished well control program. Many adverse things can occur while killing a well. The most costly item is loss of circulation and/or stuck drill pipe. A secondary concern is suspension of drilling operations while controlling the well. Minimum drilling costs cannot be achieved if the cost of adverse items outweigh benefits derived from drilling with light mud weights. The purpose of this paper is to reiterate those factors which are necessary to achieve reliable well control and to propose certain drilling procedures that can be achieved with a sophisticated state of well control. Prior to the publication of O'Brien and Goins1 in 1960, virtually all articles on well control technology were restricted to discussions about blowout prevents and how to avoid blowout conditions through crew training. O'Brien and Goins were the first to go beyond the statement "put the well on a choke and raise the mud weight". The next significant contribution to well control technology was made by Records in 1962.2 Using the concept of maintaining a constant bottom-hole pressure, Records presented a calculation procedure for determining the rate at which to expand the gas as it was circulated up the annulus. Records' contribution was doubly significant in that he also designed a specialized piece of equipment7 for precision control of surface pressure which regulates pressure directly rather than indirectly through volume control. A more recent publication by Schurman and Bell3 proposes an approximate method for a more rapid determination of the back-pressure schedule to be used to control gas expansion in the annulus. After describing their procedure, they made the statement, "The results (well control) depend strongly on the experience and understanding of the (choke) operator." This statement is equally applicable to any subject; but in the field of well control, inexperience and misunderstanding often result in a catastrophe.

MIDDLE DEVONIAN SALT FORMATIONS AND THEIR BROMIDE CONTENT, ELK POINT AREA, ALBERTA

The Prairie Evaporite formation, or First Salt, of the Elk Point area contains three types of halite: chevron, clear, and brown. The chevron halite is thought to be primary and to have grown upwards from the Devonian sea floor. The origin of the clear and brown varieties is not known. Where these types occur together, the clear halite has a tendency to contain more bromide than either chevron halite or brown halite. The total range for bromide in halite from the Prairie Evaporite is from 0.004 to 0.02 weight percent (wt.%).The Cold Lake and Lotsberg formations, the lower two salts, consist of clear halite which is so low in bromide (0.0004 wt.%) that it could not have crystallized from sea water, neither could this halite have crystallized from sea water previously saturated by dissolving halite. It is probable that the lower two salts resulted from solution of halite in fresh water, with subsequent crystallization. Solution may have occurred during discharge of meteoric groundwater, while the basin was structurally low, subsequent surface evaporation having promoted crystallization. In this way, salt could have been recycled without removal from the area. Alternatively, surface runoff, and downward movement of surface runoff, may have been important in the recycling of halite.Groundwater in Devonian formations of central Alberta, and spring waters from Devonian formations in Manitoba, are saline and relatively high in bromide (0.001–0.10 wt.% Br). The bromide content cannot be explained simply by solution of halite in meteoric waters. Either these brines are modified remnants of seawater, or else there is some undefined source of bromine causing enrichment.