Seismic Work Research Articles

Lone Pine Field--Exercise in Structural Geology: ABSTRACT

Lone Pine field, in the northwest part of North Park basin, Colorado, lies between the Park Range on the west and the Sheep Mountain-Delaney Butte thrust on the east. The oil field underlies a surface anticline mapped by Hale in 1965. Seismic work by a major oil company condemned this prospect, and it was farmed out to Burton-Hawks Exploration Company, who drilled the discovery well in November 1971. In general, the structural configuration and drilling depths conformed closely with Hale's cross section interpretation. The discovery well had a gas blowout from the Dakota Sandstone while coming out of the hole for a drill-stem test. After the well was controlled the Lakota Formation was drilled and completed as an oil producer with an I.P.F. of 515 BOPD. Subsequent drillin has found nine more oil wells and one gas well, all south of the discovery. The northern productive limit appears to have been found, with two dry holes just north of the discovery; however, the southern extent of the field remains to be determined. Present oil production extends 1.25 mi along both flanks of the structure, with a gas cap separating the two flanks. Some minor oil production is obtained from the Dakota Sandstone oil ring in two wells. The Lone Pine structure is a tightly folded, complexly faulted anticline, with 25-45° dips on the east flank and 45-60° dips on the west flank in the oil column. An oil column of approximately 300 ft has been established. Stratigraphic thickness of the Lakota is 70-75 ft, with 60-65 ft of effective pay. Average porosity of the Lakota is 18 percent. Drilling depths range from 2,400 to over 3,000 ft depending on topography and hole deviation. Wells are spaced about 800 ft apart along the east flank; optimum spacing on the west flank is yet to be determined. Each east-flank well proves up an estimated 300,000 bbl of recoverable oil, and west flank wells approximately 150,000-200,000 bbl of oil. A strong water drive hopefully will assure flowing wells throughout the life of the ield. Detailed structural analysis has been essential in guiding development drilling. Large scale, precise structural cross sections are built through each well. Logs are examined closely to determine even the smallest faults. All possible surface data are utilized. Concentric fold form, constant bed thickness and bed length, and a detailed analysis of fault behavior are combined to map as closely as possible the structural attitude in development wells while drilling. The target area for each well is a bank of production (oil ring) only 300-500 ft wide, and attempts are made to penetrate the oil ring near its midpoint to avoid water fingering from below and gas fingering from above. In addition, because of strong tendency for hole deviation in the steeply dipping strata, and the desire to penetrate the Lakota pay broadside where possible, the wells must be located at the surface (away from the axis) about 400 ft from the desired location at total depth. Frequent directional surveys while drilling have been necessary, particularly in the early development where unpredictable structural complexities were found. Despite the problems and additional expense of this type of development drilling, Lone Pine field should prove very profitable. Some of the wells are capable of flowing over 1,000 BOPD; however, all wells are choked back to 100-150 BOPD to prevent premature water fingering, or possible gas fingering. End_of_Article - Last_Page 906------------

Structural Setting of Hydrocarbon Accumulations in Folded Southern Appalachians

Two small oil fields in Lee County, Virginia, and one small gas field in Scott County, Virginia, have been discovered in the Valley and Ridge province of the Southern Appalachians. Very little drilling has been done, however, except in proximity to these fields and, similarly, little seismic work has been done. Hence, the intermediate and deep subsurface structure in this structurally complex belt is little known. The two oil fields are within the Cumberland overthrust block, and the gas field is close to it. One deep well drilled in 1967, near the crest of the Powell Valley anticline and near the Rose Hill oil field, penetrated the Pine Mountain overthrust at shallow depth, as expected. However, it also penetrated a previously unknown fault, later named the Bales fault, at 6,400 ft, below which 1,450 ft of beds were duplicated. The probable position of basement there is -10,400 ft, which is about 1,500 ft deeper than its probable position at wells 12 mi farther northwest and normal to the regional strike. Thus, the basement surface seems to be southeastward sloping and relatively smooth, and basement movement does not seem to be involved in the arching of the Powell Valley anticline. Data from a recent seismic line across the Valley and Ridge province support this interpretation. The Bales fault is interpreted to be an imbricate fault, ramping upward from a sole fault in Lower Cambrian rocks, that probably extends northwest at least to the northwest edge of the Cumberland block and southeast beneath most of the Valley and Ridge province. It may extend even farther southeast beneath the Blue Ridge province. The large slice of the crust above the sole fault is believed to have slid northwestward as a unit down a very gentle slope before internal stresses in the moving slice caused rupturing by many imbricate faults. Structural traps for hydrocarbons should exist in the Valley and Ridge province. If the interpretation advanced here of the mechanism of faulting is correct, compressive stresses during mountain building should not have been strong enough to squeeze out the hydrocarbons.

Petroleum Developments in New Zealand During 1972

Seismic prospecting offshore increased 130%, to 34,141 line-mi. No seismic work was done on land. Drilling on land reached an all-time record with 11 wells completed and an aggregate footage of 56,470 ft. This is an increase of 39% over the previous year. All wells were abandoned, but 3 had gas shows of which 1 tested up to 1.0 MMCFGD with minor amounts of condensate. For 1973, however, a sharp decline in drilling activity is anticipated. Concession areas decreased both on land and offshore. By the end of the year, land licenses covered 36,088 sq mi, and offshore licenses 339,803 sq mi. A further decrease of land concessions is anticipated for 1973, as some licenses probably will expire without being renewed immediately. The Kapuni field continued its upward trend in gas and condensate production; 12,484.22 MMcf of dry gas and 1,118,133 bbl of condensate were produced--an average of 34.2 MMCFGD and 3,000 b/d of condensate. New reserve figures indicate a proved recoverable reserve of 655 Bcf of raw wet gas, equivalent to 460 million million BTU. An agreement for the development of the offshore Maui gas field, which probably will include a government participation, is anticipated for 1973. In New Plymouth, the small Moturoa oil field ceased production on July 31.

Developments in New York in 1972

In 1972, 147 wells were drilled in New York. This is an increase from 1971 drilling and is due largely to the sharply increased prices offered for crude and natural gas in this area. The current price for Bradford crude is $5.48/bbl, the highest paid since 1920. The current offer for new intrastate gas is $0.50/Mcf, nearly double the price of 2 years ago. The only exploratory successes consisted of 1 small oil-field extension, 1 small Oriskany new-pool discovery, and 1 small Medina field extension. One Oriskany confirmation had an initial open flow of approximately 50 MMcf, which equals the record open flow recorded in New York. Oil production in 1972 was 1,018,323 bbl, and gas production amounted to 3,678,561 Mcf. The first Onondaga reef production went on line in 1972, and 2 of the 3 reef fields are now producing gas. Geophysical activity totaled 73 weeks of reflection seismic work--the highest since 1962. Renewed drilling for Onondaga reefs is anticipated in 1973, as well as much increased drilling in the older oil and gas producing areas, because of the significant recent increases in wellhead prices for oil and gas in this area and the increasing demand for these resources.

Subandean Basin Oil Fields, Ecuador and Colombia: ABSTRACT

The Subandean basin of Colombia and Ecuador covers an area of approximately 50,000 sq mi, and is part of a foreland which extends from the Guyana shield to the Andes Mountains. The present structural basin is asymmetric and plunges toward the south. More than 30,000 ft of Paleozoic to Tertiary sedimentary rocks are present. During the Early Cretaceous, a marine transgression buried a basal sandstone member (Caballos in Colombia, Hollin in Ecuador). Later fluctuations in sea level resulted in deposition of marginal marine-fluvial sandstones of the Upper Cretaceous. Major petroleum reserves are trapped in large anticlines and faulted structures in these Cretaceous sandstones. Texaco began exploration for petroleum reserves in the trackless jungles of the Upper Amazon region of South America in 1941. During the early 1940s, the Orito anticline was discovered and mapped in detail by geologic field parties. In 1963, the decision to drill Orito was made, and Texaco acquired an aggressive partner, Gulf Oil, which earned one-half interest in the Texaco holdings by drilling the Orito No. 1 discovery well. Following the Orito field discovery, exploration was accelerated and geophysical exploration began in 1965 with air magnetics and analogic seismic work. The helicopter gave support to portable geophysical crews, and later the helirig opened the entire area for exploratory drilling. The helicopter also figured prominently in the subsequent construction of two pipelines across the Andes Mountains. Production began in Ecuador in August 1972, climaxing the long, arduous exploration and development of the Texaco-Gulf holdings in the Oriente. End_of_Article - Last_Page 769------------

Geology of Offshore Areas of The Canadian Arctic Islands Based On Geophysical Data

Abstract Surface geologic exposures in the Arctic Islands are excellent, as is the case in most of the world's deserts. The sparse subsurface information from bore holes is limited to only a few islands. The geology of the inter-island areas is much more speculative. Preliminary interpretations based on erosion and isostatic readjustments after the melting of the continental ice sheet may have to be revised after a review of the geophysical data which are now becoming available. Hypotheses concerning block faulting with horst and graben development represent a conservative approach, whereas the consideration of rift and drift hypotheses gains popularity. Recently obtained reconnaissance data indicate the possibility that a distinct geologic section exists offshore compared to the onshore boundaries. This conclusion is based on regional geophysical data obtained by government and industry. Magnetic and gravity surveys coyer much of the Arctic Archipelago Seismic profiles in marine areas of sourthern Arctic Islands indicate a wide range of large structures, and refraction probes are useful in defining the velocity, and thus the possible geologic age of these sediments. Introduction THE CANADIAN ARCTIC ISLANDS have attracted the venturous for more than 300 years and even in our own time we have seen men in quest of a northwest passage. Continental borderlands have become major oil search areas throughout the world since the development of a concept that the offshore is more than just an extension of onshore geology. Continental shelves often present a whole new and very thick sedimentary section. Integration of the large volume of data compiled by the Canadian Government with newly acquired data from industry stimulates renewed interest in the offshore and inter-island areas of the Arctic Archipelago. The Canadian Arctic Islands are on the north flank of the North American craton. Major extensions of the Precambrian basement are noted, such as the Minto Arch in the southwest and the Boothia Uplift on the south, as seen in Figure 1. Hachured areas indicate Paleozoic sediments comprising the stable shelf and geosynclinal trough of the Franklinian geosyncline. The thickest portion of this earlr depositional area along the north edge of the Paleozoic outcrop has been compressed and faulted by late Paleozoic deformation. Most of the exposed area of the Franklinian geosyncline (Kerr, 1967a) is of miogeosynclinal suite. Eugeosynclinal sediments are exposed in Northern Ellesmere Island (Trettin, 1967). The central portion of the Arctic Islands has been called the Sverdrup Basin. This depositional basin is about 700 miles long and 250 miles wide, and is composed of Mesozoic and Cenozoic sedimentary sequences. The Paleozoic Franklinian geosyncline accumulated about 40,000 feet of carbonates, shales and some sands. An additional 40,000 feet of shales and sands are present in the Mesozoic Sverdrup Basin. Excellent geologic exposures facilitated aerial photographic mapping of the islands, and aerial field mapping was carried on during the short summer seasons. Aeromagnetic gravity and seismic work followed on shore and, although logistics are difficult good results were generally obtained.

Geology of Oil Fields of West Siberian Lowland

Since 1959 more than 114 oil and gas fields estimated to contain more than 4 billion tons (28 billion bbl) have been discovered in the West Siberian Lowland, a topographic and structural basin east of the Ural Mountains. About 1,750,000 sq km (675,000 sq mi), an area nearly three times that of Texas, is considered prospective. The bottom of the basin consists of igneous and metamorphosed Paleozoic rocks at an average depth of 3,000 m (10,000 ft) in the south, and 4,000 m (13,000 ft) in the north. All productive strata are sandstones of Late Jurassic and Cretaceous ages. All fields discovered to 1970 are on anticlinal structures, found mainly by geophysical work, including magnetic, gravity, and seismic profiling. Detailed seismic work was begun only in 1964. Producing structures consist of irregularly shaped areas of closure on large anticlinal arches. Most fields are medium size, covering less than 100 sq km (25,000 acres), and contain less than 30 m (100 ft) of pay sandstone. Average depth of the wells is 2,500 m (8,000 ft). Several pools are large in area, but medium size in reserves. A few pools contain over 300 million tons (2.1 billion bbl), of which the largest is Samotlor with slightly more than 2.5 billion tons (15.1 billion bbl). Halbouty et al. included eight other West Siberian fields among the giant oil fields of the world. Several lar e gas fields have been discovered in the northern part of the basin, with proved reserves of 9 trillion cu m (315 trillion cu ft). The basin is still largely unexplored, and reserves of oil and gas will be at least doubled and probably quadrupled in the next few years. The oil fields are about 700 km (400 mi) north of a thickly populated farming and industrial area crossed by several railroads, and are accessible by large rivers which are navigable in summer. Although the fields are about 2,700 km from Moscow, transporting the oil to market seems no more difficult than in the Alberta basin. The gas fields are more remote, but construction of a 2.5-m (100 in.) line has already begun.

Developments in Southeastern States in 1970

Drilling operations in the southeastern states of Alabama, Florida, Georgia, and Mississippi declined from 786 wells in 1969 to 668 wells in 1970, a decrease of 15%. The decrease is due primarily to the subnormal number of wells drilled in the Wilcox area of Mississippi. Of the total completions, 311 were exploratory tests which resulted in 42 discoveries. Development drilling during 1970 included 357 completions netting 204 oil wells and 17 gas wells. Deep drilling in the Jurassic trend of Mississippi and Alabama continues to highlight the drilling activity and registered a 20.4% success in exploratory drilling and 68.8% success in development drilling. Probably the most significant discovery during 1970 was the Smackover oil discovery at Jay field in Santa Rosa County, Florida, thus establishing the first Jurassic oil production in Florida. The discovery of Cotton Valley and Hosston oil production in Leflore County, Mississippi, extended production from these formations 47 mi north. Drilling in the Eocene Wilcox trend of southwestern Mississippi declined from 468 wells in 1969 to 263 wells in 1970. An increase in geophysical activity was registered in 1970 resulting largely from an increase in seismic work in Mississippi and Florida. Exploration is expected to continue at an active pace in the southeastern states in 1971 with the greatest emphasis in the Jurassic trend.

Case History of the University Block 9 (Wolfcamp) FieldA Gas-Water Injection Secondary Recovery Project

On the whole, the Wolfcamp has responded well to this system of alternating gas injection and water injection - better than it would have responded to natural depletion or waterflooding. Operations are expected to improve substantially with injection rates scheduled on the basis of simulated reservoir material balancing rather than stock-tank material balancing. Introduction The University Block 9 (Wolfcamp) reservoir, located 8 miles south of Andrews, Tex., was discovered in 1953. The limestone, domal feature at a depth of 8,400 to 8,500 ft was discovered as a result of seismic work in the area. A pressure-maintenance, miscible-displacement, peripherally patterned project was begun shortly after unitization in 1960, and the alternate gas-water injection phase of operations has been continued to the present time. Thus far, the project has yielded more than 2 1/4 times the estimated primary reserves and has been a very successful and profitable operation. The results of the field performance should be beneficial to others considering similar secondary recovery projects. Geological and Reservoir Descriptions The Wolfcamp reservoir of the University Block 9 field (Fig. 1) was discovered by the Atlantic Refining Co.'s No. 1-A University "9AGH". Located on the eastern edge of the Central Basin Platform, the domal, limestone feature (Fig. 2) appears to have been deposited as a combination of draping and carbonate buildup over the underlying Pennsylvanian shelf carbonates. Of the three Wolfcamp zones, the lowest zone is the most productive. It consists of 4 to 45 ft of organic limestone with intercrystalline-to-vugular porosity and has an oil-water contact at approximately 5,370 ft subsea. The nonproductive middle zone is composed of 10 to 40 ft of shaly limestone with irregularly deposited shale stringers. The productive upper zone consists of approximately 10 ft of limestone with intercrystalline porosity. In 1957–58 an engineering committee made a study of the reservoir for secondary recovery purposes. They made material balance and volumetric calculations to determine the original reserves of oil and volume of the reservoir. Statistical analyses of cores, capillary pressure data, reservoir fluid, and well logs were used pressure data, reservoir fluid, and well logs were used to determine the basic reservoir data applied in the material balance and volumetric calculations. Respective material balance and volumetric calculations were 90.32 million and 90.70 million reservoir bbl for reservoir volume, and 50.8 million and 51.0 million STB for original oil in place. Primary Performance Primary Performance The reservoir performance from the initial pressure of 3,500 psig to the saturation pressure of 1,775 psig indicated a performance typical of fluid-expansion drive. Below the bubble-point pressure the reservoir performed by solution-gas drive with no evidence of performed by solution-gas drive with no evidence of water encroachment. The reservoir performance with respect to water production, GOR, bottom-hole pressure, and oil production is shown in Figs. 3 and 4. pressure, and oil production is shown in Figs. 3 and 4. JPT P. 1485

Developments in Southeastern States in 1969

Drilling operations in the southeastern states of Alabama, Florida, Georgia, and Mississippi increased from 726 wells in 1968 to 786 wells in 1969, an increase of 7.7%. Of the total completions, 418 were exploratory tests which resulted in 38 discoveries. Development drilling during 1969 included 368 completions netting 177 oil wells and 18 gas wells. Deep drilling in the Jurassic trend of Mississippi and Alabama continued to increase, registering a 21% success in exploratory drilling and a 77% success in development drilling. The most significant developments were a Smackover gas-condensate discovery in Holmes County, Mississippi, which extended Jurassic production 36 mi northwest, and a deep H2S-rich gas-condensate discovery in Rankin County, Mississippi. The Eocene Wilcox trend of southwestern Mississippi remained the most active drilling area in the southeastern states with 59.5% of all completions. An increase in geophysical activity was registered in 1969 resulting largely from an increase in seismic work in Alabama and Florida. Exploration is expected to continue at an active pace in the southeastern states in 1970 with the Jurassic and Wilcox trends being the most active. Also, an increase in activity is expected in the Black Warrior basin in 1970 following a major lease play in 1969.

PROGRESS OF STUDIES, BASIN STUDY GROUP, BUREAU OF MINERAL RESOURCES

The Basins Study Group is part of the Subsurface Section of the Bureau's Petroleum Exploration Branch and was formed in 1962 to collect and review available basic data on the sedimentary basins of Australia and Papua-New Guinea. The Core and Cuttings Laboratory forms the second part of the Subsurface Section, and the Laboratory's technical staff contribute to basin reviews by carrying out analyses of various kinds, and assist in the collection of data principally by providing thin sections of various sedimentary formations.Recent activities of the Basins Study Group include a review of the Sydney Basin, and an increased effort to assemble basic data on all sedimentary basins, with particular emphasis on the Canning and Carnarvon Basins.The review of the Sydney Basin is nearing completion. It was undertaken with the co-operation of the Geological Survey of New South Wales and received generous support from petroleum exploration companies active in the Basin. The review included detailed petrological examination of twelve wells and selected outcrop samples. The results confirmed the previously-held opinions that the reservoir characteristics of Sydney Basin sediments are generally unfavourable. At present there are no indications of untested onshore areas where an improvement in reservoir properties may occur. The Bureau petrologists detected the rare mineral dawsonite in eight wells; the mineral occurred mostly in Permian sediments, both in marine and non-marine rocks, but it was recorded also from Triassic rocks in the Kurrajong Heights No. 1 well. The review of geophysical data from the Sydney Basin was concentrated mainly on seismic work. The magnetic tapes of three surveys were replayed and considerable improvement in records was effected. Record sections of all seismic surveys were reduced photographically to a horizontal scale of 1:50,000 and the reductions were spliced to provide easily-managed cross-sections. The geophysical review is nearing completion and structure contour maps and isochrons are in preparation.The collection of basic data is done for each sedimentary basin as it becomes available, but present emphasis is on assembling data from Western Australian basins: all seismic traverses in the onshore parts of the Canning and Carnarvon Basins have been plotted at 1:250,000 scale, and with the co-operation of the Geological Survey of Western Australia, bibliographies of the Canning, Carnarvon and Perth Basins have been compiled for issue as Open-file Records. Bibliographies of the Papuan and Ipswich-Clarence Basins have also been compiled.

Drilling in the Norwegian Part of the North Sea

To perform effectively in the tough environment of the North Sea it is necessary to become aware of the physical conditions, the economic latitudes and restrictions, and the government regulations, and to take these factors into account in the early stages of planning. Introduction The North Sea is most often defined as that portion of the sea lying between the parallels of 51 degrees N and 61 degrees N and the meridians 2 1/2 degrees W and 8 1/2 degrees E. It is bounded on the west by the British Isles and on the east by Norway, Denmark, Germany, Holland, Belgium and a few miles of France. In the north it is open to the Atlantic Ocean, or rather the Norwegian Sea, and in southern end is open through the Straits of Dover and the English Channel to the Atlantic. The Skagerak in the east between Norway and Denmark connects the North Sea with Kattegat, and thus with the Baltic Sea. Of the total 275 million inhabitants of Western Europe, 130 million live in the countries bordering the North Sea. For these people the North Sea is a main fishing ground as well as a principal route for national and international shipping. The ports along the shores-London, Antwerp, Rotterdam, Amsterdam, Bremen and Hamburg-are among the busiest in the world and have become market places of great importance to world trade. Through the centuries die people in the area surrounding the North Sea have developed a highly advanced technology in a number of different fields, and enjoy a standard of living surpassed by only the U. S. and Canada. To maintain this status, an increase in gross national product and per-capita income is needed, and for this, cheap energy is of paramount importance. When, on Aug. 14, 1959, the now-famous well, Schlocteren No. 1, in the Groeningen province of the Netherlands reached strata enormously rich in natural gas, visions of a new era for Western Europe became clear to the European industrial entrepreneurs as well as to the politicians. Almost instantly the prospect of the North Sea area as a place to search prospect of the North Sea area as a place to search for new oil and gas deposits became apparent. And as the legal problems were solved internationally by the Continental Shelf Convention of 1958 (duly ratified and binding from June 10, 1964), and nationally by the different countries bordering the North Seaconcessions were granted for exploration and exploitation. On the Norwegian part of the continental shelf the seismic work started in 1963, and the first well was spudded in 1966. The Challenge of the North Sea Before a driller enters a new area, he tries to familiarize himself with the environment in which he will operate. One would think that for this area, where so much trading and fishing has been carried on by well trained and educated people, all relevant data would be readily available. But it is not. Nothing like a "Driller's Manual to the North Sea" exists, and those operating in this area have gained their experience the hard and expensive way. Many critical situations have been experienced, and even some serious disasters. The loss of the jack-up rig Sea Gem in British waters in December, 1965, with the loss of 13 lives, was the first shock for the public. JPT P. 1259

Reply to 'Old Red Sandstone of Kintyre'

SIRS,—Drs Friend and Macdonald in a recent paper of the Old Red Sandstone of Kintyre (1968) make a series of far—reaching generalisations about the tectonics of the region, which in the light of our work need severe modifications. Although the final results of our seismic and geological investigations await completion of research, certain salient features can be presented now. The ground survey in the large measure confirms the local Lower O.R.S. stratigraphy established by Friend and Macdonald (1968), although our impression is that the unit which they call the New Orleans Conglomerate is unconformable on the Glenramskill Formation and is in turn unconformably overlain by the Bastard Sandstone, hence the lack of conglomeratic beds in Glen Hervie to which they draw attention (p. 271). As regards the surface separating the Dalradian and the Lower O.R.S. of Kintyre, seismic results obtained so far by us indicate a far more complex situation than a simple downwarp proposed by Friend and Macdonald. The basis of the seismic work rests on the determination of local velocities of P. waves in the Lower O.R.S. and Dalradian shown in the following table

Developments in Southeastern States in 1968

Drilling operations in the southeastern states of Alabama, Florida, Georgia, and Mississippi declined slightly from 764 wells in 1967 to 726 wells in 1968, a decrease of 5%. Of the total completions 410 were exploratory tests which resulted in 28 discoveries. Development drilling maintained a moderate pace with 161 successful wells out of a total of 316 drilled. Deep drilling in the Jurassic trend of Mississippi and Alabama continued to increase sharply in 1968. The most significant developments were a rich gas-condensate discovery from the Norphlet Formation in Escambia County, Alabama, extending Jurassic production 90 mi southeast, and the discovery of several excellent Smackover fields in Clarke and Wayne Counties, Mississippi. The Eocene Wilcox trend of southwestern Mississippi continued to dominate the drilling statistics with 66% of all the wells drilled in the southeastern states. A notable increase in geophysical activity was registered in 1968 resulting largely from a sharp increase in seismic work in the Alabama portion of the Jurassic trend. Exploration is expected to continue at an active pace in the southeastern states in 1969 with the Jurassic and Wilcox trends being the most active.

THE COEFFICIENT OF COHERENCE: ITS ESTIMATION AND USE IN GEOPHYSICAL DATA PROCESSING

The coefficient of coherence between two stationary time series was introduced by Wiener in 1930. It is related to the signal‐to‐noise ratio, to the minimum prediction error, and has important invariance properties. As an estimate of this parameter, most geophysicists have used the so‐called “sample coherence.” An approximate distribution of the sample coherence for Gaussian data has been derived by N. R. Goodman. We have tested this distribution by means of Monte Carlo experiments for validity and robustness (insensitivity to the Gaussian assumption). It has passed the tests. The Goodman distribution provides a means of constructing estimates of the true coherence which are better than the widely used sample coherence. It can also be used to calculate confidence intervals. Finally, it forms a basis for choosing the lag window and data window necessary for best estimation of the true coherence. For good estimates of the true coherence, two precautions must be observed: 1. The cross‐spectrum and power spectra of the two time series must be smoothly varying over the width of the spectral window. 2. The ratio of the length of the data window to the lag window must be large. For most seismic work the second requirement severely limits the spectral resolution. Examples show that large errors can result if this resolution is not sufficient to satisfy the first requirement. In many geophysical studies the parameter of interest is the signal‐to‐noise ratio. Because of its relation to the coherence, the Goodman distribution provides a basis for its estimation as well.

Progress of Exploration in North Sea

After an account of the occurrences of oil and gas in the countries adjoining the North Sea the paper outlines the regional structural features of northwestern Europe. The outcrop geology of the North Sea floor is described and followed by an account of the stratigraphic succession in the Tertiary, Cretaceous, Jurassic, Triassic, and Permian formations. Geophysical surveys have included gravity and magnetic surveys which define the main basins, and seismic work which has shown that salt structures ranging from non-piercing salt pillows to steep-sided diapirs are developed through most of the southern North Sea. Drilling has led to discovery of three gas fields of commercial size in the basal Permian sandstone up to the time of compilation of this paper. One additional dis overy since has been made in the Bunter Sandstone.

THE BARROW ISLAND OILFIELD

Barrow Island lies 35 miles off the northwest coast of Australia, 800 miles north of Perth, and has an area of 90 square miles. Miocene Trealla Limestone crops out over most of the island. Eocene Giralia Calcarenite is found in some valleys of the crestal region and Quaternary sand covers the island fringes.The Barron No. 1 well was located at the highest point in the sub-surface, as indicated by seismic refraction work, and in the crestal area of a gently dipping surface anticline. It was mapped as having at least 80 feet of vertical closure with an area of 24,000 acres. This well was completed as a new field discovery in August, 1964, with an initial production rate of 985 BOPD from sand in the Jurassic. In February, 1965, Barrow No. 4 was completed as a shallower pool discovery with an initial production rate of 125 BOPD from a Lower Cretaceous sand. By December, 1966, 33 wells had been drilled for a total footage of 155,409 feet.Barrow No. 1 is the deepest well at 9875 feet although it is considered that the sedimentary section may exceed 20,000 feet. The known sequence begins with Upper Jurassic siltstones. These pass upwards into a predominantly sandstone sequence containing minor shale and siltstone, and are followed by the dark-coloured marine shales and siltstones and minor sandstone of Lower Cretaceous age. In the Upper Cretaceous and Tertiary, fossiliferous, micro-crystalline Limestones and calcareous siltstones were deposited.The field contains two petroliferous intervals. The deeper interval is within the Upper Jurassic and contains a number of small irregular pools with good reservoir characteristics, these, however, are not commercial in themselves. The shallower interval contains the widespread "Windalia Sand" which has an average pay thickness of 44 feet and generally poor reservoir characteristics. Drilling is now developing the "Windalia Sand" pool which covers 24,700 acres and contains reserves currently estimated at 114 million barrels recoverable by primary methods. Structure at this pay horizon is a broad north plunging nose truncated at the south by a down-to-the- south fault. One hundred and forty-four development wells will he drilled in 1967.

Developments in Upper Gulf Coast of Texas in 1966

Exploratory drilling in Railroad Commission District 3 increased, but significant discoveries decreased. The discovery percentage was 18. A total of 667 development wells was drilled. Of 409 producers, 1 of every 6 was a gas well. The success ratio was 1:15, and the success percentage 61. Wildcat drilling increased in the Miocene, Frio, and Yegua trends. The Frio trend continued to be the main objective, followed by the Yegua. The Wilcox and Yegua trends led in crew-weeks of seismic activity. Major seismic work in the offshore areas was completed in June. As a result of this, increased drilling is expected.

Determination of Mooring System Design Data - Offshore British Columbia

Abstract An outline is presented of basic data required in the design of a mooring system for a floating drilling vessel. For offshore British Columbia, field measurements to determine the range of soil conditions, anchor performance in the various types of soil and current velocities were required to supplement the available data. Planning techniques, equipment used and procedures followed in accumulating this information are described to serve as possible guide lines for others entering new offshore areas where similar information is needed. Within the area of interest, three general classifications of soils were present:coarse gravels with clay fill,fine sands, andvery hard silts and clays with shear strengths up to 47,000 lb/sq ft immediately below the mud line. A mooring problem was not expected in the gravels and fine sands; however, applicable data on the performance of anchors in soils with shear strengths greater than 200 lb/sq ft was not available. It was necessary to establish the performance of anchors in these soils; otherwise, anchor piles would have to be used. Anchor tests proved conclusively that lightweight-type anchors in the range of weights tested could develop satisfactory holding power in all types of soil encountered. A brief summary of the meterological and oceanographic conditions off the coast of British Columbia and examples of tabular and graphic methods of presenting maximum and average conditions are included in the Appendix. Introduction Prior to the actual design of a mooring system for a floating drilling vessel, considerable data pertaining to the local environmental conditions must be established. The most significant factors may be listed as follows.Meteorological: magnitude, direction, frequency of occurrence and duration of storm winds.Oceanographic:range of water depths,normal seasonal tidal fluctuations and magnitude and frequency of occurrence of maximum storm tides,magnitude, direction, period, frequency of occurrence and duration of storm waves, andnormal and maximum storm fluctuations of current velocity and direction (for deep draft semi-submersible vessels, information on the current velocity variation with water depth should also be obtained).Miscellaneous:variations in the ocean floor soil conditions and related anchor performance must be considered; andother factors such as the proximity of dangerous rocks or reefs, floating hazards such as icebergs and the expected advance warning of severe storms should be considered in the determination of the design safety factor and the speed in which the system can be set and retrieved. In addition to these environmental factors, information on vessel reactions to wind and current at various approach angles is necessary to determine the strength of the required system. To determine flexibility of the mooring lines, vessel heave, surge, pitch, roll and yaw responses to waves of different periods, heights and approach angles must be known. These vessel characteristics are normally obtained by model tests. Regarding the specific area of interest off the coast of British Columbia, a preliminary investigation into the availability of mooring system design data showed that sufficient information was available on winds, tides and waves (Appendix). Water depths had been recorded and some soil samples had been obtained during previous seismic work. Vessel characteristic data for the Sedco 135 vessel were available from model tests.1 A review of the available information showed that individual anchor holding capacities in the order of 300,000 lb would be required to maintain the vessel on location during the maximum anticipated storm. Although a 30,000- or 40,000-lb U.S. Navy Light Weight Type (LWT) anchor is capable of developing the required holding capacity, the shear strength of the soil must be low enough to permit penetration of the anchor flukes, yet high enough to provide sufficient resistance to anchor drag.

Developments in South Texas in 1965

Although drilling activity decreased slightly in South Texas during 1965, there was a substantial increase in production of gas and gas liquids. Total drilling decreased 7.7%, which included a 10% decline in development wells and a 2.2% decline in exploratory wells compared with 1964. Geophysical activity decreased 3.2% in 1965, and oil production declined slightly. Increases took place only in production of gas (up 14.7%) and gas liquids (up 30.9%). Exploration for the deep Jurassic, Edwards (Cretaceous), and Wilcox (lower Eocene) production continued with little success. The shallow lower Eocene (Poth Sand) play continued with better results; however, as in the past, most exploration and discoveries were in the more prolific Oligocene-Miocene trend. The most significant increase in activity during 1965 resulted from offshore leasing, seismic work, and drilling. This high level of activity probably will continue.