Reflection Seismograph Research Articles

Introduction to this special section: Seismic resolution

The science of modern seismology was born more than 100 years ago (1889) when the first teleseismic record was identified and the seismograph was developed ( Ben-Menahem, 1995 ). In 1921, earth exploration was revolutionized when a team led by Clarence Karcher conducted the first field tests of the reflection seismograph in Oklahoma City ( Dragoset, 2005 ). That experiment showed that the subsurface can be imaged using seismic data. Businesses boomed as the seismic method started establishing its track record in finding hydrocarbons. Over the last century, the seismic method has emerged as the cornerstone of exploration geophysics, providing us with increasingly accurate characterizations of the subsurface and enabling us to better discover and describe hydrocarbon prospects, geothermal anomalies, seafloor hazards, aquifers, and much more.

Direct detection of onshore hydrocarbon microseepages by remote sensing techniques

Remote sensing to detect hydrocarbon microseepage onshore, has the advantage of recognizing marginal and submarginal low relief structural prospects and all stratigraphic traps that are overlooked by reflection seismograph, besides its fast speed and low cost. The hydrocarbon‐induced surface alterations of soil and sediments and associated anomalous vegetation that can be identified from remote sensing imagery are reduction of ferric iron (red bed bleaching), conversion of mixed‐layer clays and feldspars to kaolinite, increase of carbonate content and anomalous spectral reflectance of vegetation. Conventional remote sensing data mostly cover 0.4–2.5 μm wavelength region with broad bandwidth that cannot characterize the absorption features caused by hydrocarbon microseepage. High spectral resolution imaging data are demanded.

Recovering Corporate Assets Through Environmental Lawsuits

Over the past decade many environmentally motivated restrictions and withdrawals have been carried out at the expense of private property owners, most of whom seemed to have no effective recourse. This paper (1) summarizes a recent case in which a Michigan oil operator and several mineral owners fought back, winning a judgment worth $120 million from the State of Michigan, and (2) analyzes the essential geotechnical procedures, particularly those involving stochastic exploration risk analysis, required to determine and to justify the plaintiff`s claims for monetary damages. The intent is to provide encouragement and guidance to other private property owners, especially those involved in mineral resources, who may be contemplating similar legal actions. The Fifth Amendment to the US Constitution prohibits the taking of private property for public use without just compensation, and the 14th Amendment forbids the state to deprive any person of property without due process of law. Nevertheless, most knowledgeable attorneys generally conceded that the law concerning takings of private property by state regulatory restriction has been murky and uncertain at best.

History of Petroleum Exploration in California and the West Coast

California's main oil and gas basins consist of the inland Sacramento and San Joaquin and the Los Angeles, Ventura, and Santa Maria basins adjacent to the coast and extending offshore. The state's total oil production to 1991 is approximately 22.8 billion bbls. Producing formations range in age from basement Jurassic to Pleistocene, but production is mainly from thick multiple sand zones of Miocene and Pliocene age. The first oil discovery was in the eastern Ventura basin in 1875. By the turn of the century, 22 fields, including several giants in the San Joaquin Valley, had been discovered by drilling near oil seepages. The most important event of the 1920s was the discovery of several giant oil fields in the Los Angeles basin, drilled on topographic highs suggestive of underlying anticlines. State production rapidly increased to 850,000 BOPD, or 40% of all US production. The 1930s saw the advent of the reflection seismograph, responsible for the state's largest oil field (Wilmington) in the Lost Angeles basin and the state's largest gas field (Rio Vista) in the Sacramento basin. A number of important fields were found under the San Joaquin Valley floor. Geological thinking in the late 1930s and 1940s resulted inmore » the discovery of large stratigraphic traps in the San Joaquin Valley (e.g., East Coalinga) and at Santa Maria from fractured shale, plus two new small producing basins, the Cuyama and the Salinas. Offshore exploration, consisting of seismic work, ocean-bottom sampling, and coreholing, revealed the presence of a number of anticlines in the Ventura basin, paralleling the Santa Barbara coast. The first offshore discovery was made in 1959 on state lands followed by several major fields on federal lands in the late 1960s. Elsewhere along the West Coast, exploration in Oregon and Washington has yielded only minor gas.« less

COMMENT ON J. C. KARCHER’STHE REFLECTION SEISMOGRAPH

In the late summer of 1973, the American Institute of Physics requested that John Clarence Karcher prepare a “first person account” of his life and career, detailing in particular his efforts and contributions toward the perfection of the reflection seismograph. Dr. Karcher’s memoir on this subject gives many fascinating descriptions of geophysical history as he remembers it. That this relatively short scientific history should leave out some of the prominent co‐workers in the field and fail to give due credit to others is completely understandable. My Comment may fill in a few gaps.

Reservoir Management in South Louisiana With the Pulsed Neutron Log

Summary This paper illustrates through case histories how the pulsed neutron log aids proper reservoir management in strong water drive reservoirs in south Louisiana. An economic analysis performed on the results of remedial operations using pulsed neutron logs clearly shows the benefits of risk reduction when compared with those remedial operations without pulsed neutron logs. Introduction The studied area, southwest of New Orleans (Fig. 1), provides an excellent illustration of reservoir management using the pulsed neutron log. The geologic structures in the area studied consist of deep-seated salt domes, which were delineated by reflection seismograph. The productive sands are of the Miocene and Pliocene ages. As wells go off production, remedial work must be performed either to initiate production in a new interval or to restore production to the same interval. The pulsed neutron log has become an integral part of the remedial work program in the studied area to ensure proper reservoir depletion. The first pulsed neutron log in the area was run in 1964, and since that time the yearly average number of pulsed neutron logs has risen to 35 to 40% of the total number of remedial operations. The log is being used to a large extent in the studied area due to the characteristics of the reservoir drive, which as a whole is a strong water drive. However, several sands exhibit a "bottom water" drive, which results in the coning of water to the perforations. This premature water break through limits the radius of depletion that can be achieved. Since the oil/water contact varies throughout the reservoir according to the extent of coning and amount of production, pulsed neutron logs are the most efficient means of monitoring the depletion of the reservoir and the movement of the oil/water contact. Other applications of the pulsed neutron log have been to ensure the proper depletion of membered sands to aid in the evaluation of low-resistivity sands, to identify the sealing nature of small faults, and to evaluate the effects of permeability barriers on recovery. Theory Neutron bursts with energy levels of 14 MeV are emitted every 1,000 microseconds. These high-energy neutrons are captured primarily by chlorine but also by the rock matrix, hydrogen, and materials in the wellbore. After capture of the neutrons, fixed-energy-level gamma rays are emitted from the capturing nucleus. It is these gamma rays that are counted during the 1,000-microsecond cycle. The number of capture events is proportional to the number of uncaptured neutrons at any time. Therefore, what is measured is the intensity and the rate of decay of the capture gamma ray flux. Many papers and articles have been written on the subject of evaluating pulsed neutron logs. The water saturation is calculated using this equation: (1) To calculate a quantitative SW, values for porosity, water salinity, shale content, and hydrocarbon type must be assigned or derived from the pulsed neutron log using nearby clean (nonshaly) water sands. After a water saturation has been calculated, a check for possible errors can be made by comparing the water saturation calculated from the pulsed neutron log with the original water saturation of the open hole electric log. A significant difference in these water saturations would indicate an error in calculation or the possibility of a depleted zone. JPT P. 419^

Erawan Field, Gulf of Thailand: A History of Applying Evolving Geophysical Technology to a Complex Geologic Structure: ABSTRACT

The Erawan gas field, with estimated recoverable reserves of 1.5 tcf of natural gas, was discovered in 1972. The drilling locations have all been selected on the basis of complicated reflection seismograph results. The productive section is a Tertiary sandshale sequence of fluvial to shallow-marine origin, and individual sand units rarely exceed 50 ft (15 m) in thickness. The Erawan structure is a complexly faulted graben, with fault block rotation producing an anticlinal attitude. High fault density (200 to 500 m separation) and thin productive beds result in many separate hydrocarbon traps. Commercially productive sands occur at depths between 5,000 and 9,000 ft (1,525 to 2,740 m) subsea. Union Oil Co. of California acquired the acreage in 1968 and End_Page 597------------------------------ shot the first seismic survey the same year. These data were adequate to confirm a thick sedimentary section, and they contained sufficient structural indications to select the initial drill site. By 1974, four different seismic surveys had been conducted. More and more faulting became apparent with improving data quality. To resolve the structural pattern, a high-resolution seismic survey was conducted in 1976. These data first demonstrated the very complicated fault patterns now known to exist. Additionally, processing of this survey proved that migration is essential for fault definition, although dips are relatively shallow. Paradoxically, it was observed that the best seismic correlations across the many faults were predominantly low frequency. Consequently, a low-frequency sei mic survey was initiated in 1979. Interpretation of this survey made it possible to correlate gross sedimentary units over large areas of the structure. These correlations were consistent with well control. In 1979, development plans were completed and a 3-D seismic survey was conducted prior to installation of permanent production facilities. This survey comprised 5,000 line-km of data with subsurface coverage of 25 m in the dip direction and 75 m along strike. Nonstandard processing techniques are being tested, both for Erawan field data and other structures in the Gulf of Thailand. Wavelet processing, utilizing a recorded far-field signature, has shown significant improvement in some cases. Migration before stack has significantly enhanced the data by better fault definition and impro ed correlations across the faults. Development plan for Erawan field consists of five platforms with 12 well slots per platform. Gas will be transported by pipeline to Bangkok and condensate will be separated and offloaded via a single point mooring system. Gas production of 250,000 mcf is planned with first production in the fall of 1981. The development program will continue to be heavily dependent upon detailed analysis of 3-D and other seismic reflection data for optimum positioning of boreholes in both horizontal and vertical dimensions. End_of_Article - Last_Page 598------------

Advanced Seismic Techniques For Delineation of Petroleum Reservoirs

Introduction Seismic inversion yields synthetic sonic logs from seismic reflection data recorded at ground surface. Synthetic sonic logs are similar to, and have most of the properties of, long source-receiver borehole sonic logs. They provide a new area/or application of seismic digital process technology. The synthetics have been found to be particularly useful for the delineation of petroleum reservoirs, including the extent, thickness and porosity of the reservoir rocks. In a typical survey, field data, obtained from seismic line profiles across reservoirs, yield a sequence of synthetic sonic logs. Continuous vertical cross sections which display colour-coded transit time units can be interpreted in terms of lithology and porosity within the limits of interpretation of the single sonic curve and the resolution of the method. Sonic velocity changes are related to porosity and, by establishing the velocity of the matrix rock, the porosity within it can be estimated from synthetic sonic logs. Applications include optimizing development drilling patterns to obtain maximum production, the design of secondary recovery programs, the positioning of injection wells and the location of offshore drilling platforms to ensure maximum recovery. Introduction Delineation and definition of a known new petroleum reservoir is generally the responsibility of the production engineer or geologist, who must rely on a few well logs to make his determination. The advantages of the superb vertical resolution of the logs are offset by poor horizontal sampling; well logs can be obtained only by first drilling expensive boreholes, which are therefore usually spaced at rather wide intervals. The field was probably discovered with the reflection seismograph, which provides excellent horizontal sampling, but poor vertical resolution. Also, the conventional seismograph merely maps stratigraphic boundaries and pro- vides little or no confirmation about the actual rock units. Hence, it is of little help in determining reservoir properties. As a result, the development, geologist prefers to rely on samples and well logs, as they yield the desired degree of resolution and type of information. At the borehole location, both well logs and seismic traces sample the same geological column. In fact, a seismic reflection trace is merely a transform of the sonic and density information on borehole logs. For years, geophysicists have computed synthetic seismograms from sonic and density logs. A well constructed synthetic seismogram will match a field-recorded seismogram very closely, and aids in correlating seismograph information to borehole geology at the tie point. The reverse process, of computing a synthetic sonic log from a seismic trace, is also theoretically possible, but it has not worked well in the past, because of a difference "in quality of source signals. A sonic log is a relatively pure signal, recorded directly from the rock at the level of investigation. A seismic reflection signal, generated at the ground surface, must travel down through the· earth to the level of investigation and back again. In transit, it is often contaminated by external noise, distorted by wave pro" pagation phenomena, and part of it may be lost or attenuated in the transmission.

Seismic method

The fifty years from 1930 to 1980 almost completely cover the entire history of the reflection seismograph exploration industry. During its growth from infancy to maturity there have been many major technological breakthroughs and innumerable evolutionary advances. At the conclusion of any time period, there is a tendency to believe that the peak has been reached, only to see significant breakthroughs which start a whole new round of amazing developments. So it is today with a veritable explosion of channel capacity in the recording systems and three‐dimensional surveys finally just becoming a practical reality. The prospects for major improvements in the resolving power of the seismic exploration method have probably never been better.

Principles of Sedimentology

This book has been written in response to the rapid expansion and diversification of sedimentology during the last 30 years. We believe that a critical synthesis of this field is a matter of importance and urgency. The study of sedimentology involves a dynamic interplay with many of the other branches of geology. Its tools range from those used to measure the submicroscopic to ones involving the use of radar, the reflection seismograph, and Earth satellites. In the introduction of Part I we define the wide scope of sedimentology, as we see it, a scope which may be broader than that of some of our colleagues. Actually sedimentology is such an unfamiliar term to some geologists that it has not yet become a heading in the index section of the Bibliography and Index of Geology. This book is intended to be used as a text in undergraduate and graduate courses of sedimentology. In writing this book we have aimed primarily at junior and senior students. Our style of writing in the early chapters is consistent with lower-level undergraduate courses, but we believe that as the student advances he or she progressively should become accustomed to a more professional style. Therefore, the later chapters are written at a professional level. To those who simply scan the book, this lack of uniformity may seem perplexing, but it is deliberate and has the student in mind. Beginning graduate students in sedimentology will also find this text useful. Chapters 8 to 14, especially Chapters 8, 10, 11, and 12, are central to the professional geologist employed in oil and gas exploration or exploitation, and in the study of groundwater or economic geology either in industry or government. Chapter 9 may be of interest to geologists concerned with environment.

Buried Structures in Marine Exploration: ABSTRACT

Examples are presented of what most geologists call structures. Improvements in reflection seismograph techniques and data processing have produced deeper penetration and better resolution of data which have led to the location of many in the Gulf of Mexico and other marine areas. Some of these structures are buried diapirs--some are related to growth fault development. In either situation, local movement ceased and the structures were overridden by sediments prograding over a subsiding basin. Recognition of these undrilled structures has sparked intense bidding competition in recent offshore lease sales in the Gulf of Mexico and the Atlantic offshore. Buried structures will be important targets in offshore exploration for years to come and will contribute substantially to the nation's reserves of oil and gas. End_of_Article - Last_Page 1282------------

Land seismic work in Australia

In the evermore urgent search for oil and gas reserves, one scientific aid stands out. The reflection seismograph is the industry's most important tool for resolving complex sub-surface geological problems. In Australia the search for oil has been in progress for the past one hundred years. Prior to the 1950's the exploration effort was, at best, spasmodic. It is significant that continuous exploration for oil reserves commenced with the introduction of the seismic tool in Westerg Australia during the 1950's. In the ensueing thirty years of application, in excess of 215 crew years or some 250,000 line kilometres of seismic coverage have been recorded onshore in Australia. As we move into the 1980's, it is instructive to review our past experiences and consider how we may apply the lessons of the past to the improved tools of the present and the challenge of the future. There have been several reviews of the seismic method since its introduction in the 1930's. During this period predictions were made regarding the future progress of the science. Historically, most of these predictions have come true, at an accelerated rate and with the development of a technology, not known, at that time.

Developments in New York in 1978.

In 1978, 482 wells were completed in New York. In known oil and gas fields, 161 oil and 195 gas wells were completed. Gas well completions were down 20% from 1977 because of a lack of pipeline capacity in the far southwestern part of the state. Exploratory drilling resulted in 13 new gas-field discoveries, 1 deeper pool gas discovery, and 15 extensions to known gas fields. All the discoveries are in the Silurian Medina Sandstone in the western part of the state. The deeper pool discovery is in the Devonian Oriskany Sandstone in western New York. One extension was to an Ordovician Queenston field in central New York and the rest were extensions to Medina fields in the western part of the state. In a dry Medina test in Allegany County, a 1,652-ft Devonian black shale core was cut. In eastern New York, Gulf drilled a deep wildcat which was dry. Oil production in 1978 was 852,462 bbl and gas production amounted to 13,000 MMcf. The price for New York stripper crude held steady at $14.77/bbl during 1978. Gas prices varied up to $2.25/Mcf. During 1978, 70 crew-weeks of reflection seismograph work and 18 crew-weeks of gravity work were accomplished in New York. Leasing continued in areas of possible Medina and Queenston gas production in western and central New York; 7 state reforestation tracts totaling 12,274 acres were leased at an average price of $7.15/acre during 1978. Exploration and development in Medina and Queenston gas-producing areas will continue. A few deep tests as well as Devonian black shale and Oriskany structural tests will be drilled in 1979. Leasing and drilling in the New York part of Lake Erie are delayed until completion of an environmental impact study.

MAXIMUM LIKELIHOOD FILTERING OF REFLECTION SEISMOGRAPH DATA

Two types of adaptive multichannel filters applied to exploration reflection seismograph data are the maximum likelihood filter and the Wiener filter. Derived equations for the two types of filters demonstrate that the maximum likelihood filter equations are identical in form to the Wiener equations and are solved by the same matrix inversion techniques. Application of the maximum likelihood filter to common‐depth‐point (CDP) data demonstrates that the output reflection quality is sensitive to reflection alignment on input traces. Presently, sufficient coherency of reflections on input traces can be achieved only on CDP traces with a time window in which the reflection signal is dominant or on vertical array data for which reflection alignment is straight‐forward.

Developments in New York in 1977

In 1977, 538 wells were completed in New York. In known oil and gas fields, 119 oil and 243 gas wells were completed. Gas well completions were down 20% from 1976 because of a lack of pipeline capacity in the far southwestern part of the state. Exploratory drilling resulted in 5 new gas field discoveries and 8 extensions. Four of the discoveries are in the Silurian Medina sandstone in the western part of the state. The other discovery is in the Devonian Oriskany Sandstone in west-central New York. Six of the extensions are to Medina gas fields in western New York; 1 extension is to an Ordovician Trenton gas field in north-central New York and the other is an extension to an old, shallow Upper Devonian sandstone oil and gas pool in western New York. Two deeper pool tests were drilled to test Middle Ordovician and Upper Cambrian objectives but found no deep production. The COST G-2 stratigraphic test was drilled in the Georges Bank area off Massachusetts to a TD of 21,874 ft. Oil production in 1977 was 813,313 bbl and gas production amounted to 10,363 MMcf. The price for New York stripper crude was raised from $13.82 to $14.77/bbl during 1977. The price for new, intrastate, gas in 1977 was $1.33/Mcf summer and $1.73/Mcf winter. During 1977, 64 crew-weeks of reflection seismograph work were accomplished in New York; 15 weeks of this was marine seismic work in the New York part of Lake Erie. Leasing continued in areas of possible Medina and Queenston gas production in western and central New York; 15 state reforestation tracts totaling 19,239 acres were leased at an average price of $14.73/acre during 1977. Exploration and development in Medina and Queenston gas-producing areas will continue. Studies of Devonian black shales indicate that they may have some potential for future gas production in selected areas. Drilling for gas in the New York part of Lake Erie was approved by the legislature and a bill signed into law by the governor in 1977. Plans for implementing the law are now under way.

Developments in New York in 1976

In 1976, 537 wells were completed in New York. This was an 8% increase from 1975 drilling. In known oil and gas fields, 126 oil wells and 304 gas wells were completed; 270 of the gas well completions were in Chautauqua County in the southwestern corner of New York. Exploratory drilling (44 tests) resulted in 8 new gas field discoveries, one new-pool discovery, and 11 gas field extensions. Seven of the discoveries were in the Silurian Medina sandstone in the western part of the state. The other discovery is in the Ordovician Queenston formation in central New York. The new-pool discovery is in the Devonian Oriskany Sandstone in west-central New York. Ten of the extensions are to Medina gas fields in western New York. One extension is to Queenston gas production in central New York. Two deeper pool tests were drilled to Cambrian sandstones in western New York but found no deep production. The COST G-1 stratigraphic test was drilled in the Georges Bank area off Massachusetts to a TD of 16,044 ft. Oil production in 1976 was 857,293 bbl and gas production amounted to 9,250 MMcf. The price for New York stripper crude fluctuated from $13.07/bbl to $11.05/bbl and then to $13.82/bbl during 1976. The price for new gas in 1976 was $1.07 and $1.47, summer and winter rates, respectively. During 1976, 42 crew-weeks of reflection seismograph work were accomplished in New York, down from 46 crew-weeks in 1975. Leasing was confined mainly to areas in western New York where the Medina sandstone is productive. Gulf leased an estimated 50,000 acres in the southern Catskill area in eastern New York. An estimated 4 million acres was under lease at the end of 1976. Severe gas curtailment during the winter of 1976-77 has heightened the demand for additional gas supplies. Drilling for more Medina and Queenston gas will continue and the New York part of Lake Erie may be opened for leasing in 1977.

History of the Rio Bravo Field

History and reservoir performance of the Rio Bravo field, Kern County, Calif., are summarized. The Rio Bravo and Vedder zones have produced more than 100 million bbl of oil since 1937 with 125 wells completed in these zones. Efforts made to solve some rather unusual operating problems and to promote recovery are described. promote recovery are described. Introduction The Rio Bravo field is located about 15 miles north-west of Bakersfield, Kern County, Calif. (See Fig. 1.) The zones of principal importance are the Rio Bravo and the Vedder; and, although a deeper Helbling and a shallower sequence of Pliocene gas sands are of economic importance, this report is limited to the Rio Bravo-Vedder reservoirs. These other zones, as well as field stratigraphy, have been discussed in previously published papers. The Rio Bravo field is classed as published papers. The Rio Bravo field is classed as a major oil field, having producing 108 million bbl of oil. As of Aug. 1, 1974, total gas sales have amounted to 56.8 Bcf. Discovery was based on reflection seismograph surveys. The discovery well was Union Oil Co. Kernco 1-34 (now Rio Bravo Unit Kernco 85-34), completed Nov. 4, 1937, at a total depth of 11,302 ft in the Rio Bravo sand. Initial production was 2,400 B/D of oil; 39.6 deg. API; 0.7 percent of water; and 1,260 Mcf/D of gas from 11,220 to 11,302 ft. After 30 days, production was 2,588 B/D of oil and 4,306 Mcf/D of gas. This was the first commercial producer in California below 11,000 ft, and for a short period it was the deepest producing well in the world. Although the first few wells were limited to the Rio Bravo sand, the underlying Vedder sand was opened to production in April 1938 in The Superior Oil Co. Wagner 2 well. Development ensued rapidly with the use of large steam-powered rigs, and by the end of 1941, when the U.S. entered World War 11, 97 wells had been completed, Also by that time, except for some edge-well drilling (mainly in 1954, 1955, and 1956), development was substantially complete. Structure and Geology Structurally, the Rio Bravo-Vedder pool consists of an elongated, asymmetrical dome with axis trending in a generally northwest-southeast direction. The accumulation of hydrocarbons, as in the somewhat similar Greeley field adjacent to the southeast, is controlled by anticlinal folding that occurs along the same directional trend. A major fault, also trending in a northwest-southeast direction, bounds a portion of the easterly limits of production in both fields. The Rio Bravo and Vedder are of Miocene age, the former overlying the latter, and separated by a thin, apparently competent, shale break. The Rio Bravo is a blanket sand covering the entire area of the field, which is about 2,300 acres, while the area of the Vedder is somewhat less. As seen from Fig. 2, which shows contours on top of the Rio Bravo, closure in this sand is about 400 ft. The 11,200-ft contour was the approximate average depth of the original water-oil contact. The two north-south normal faults through the easterly limits of Section 28 have not had a major effect on production. Reservoir Rock and Fluid Characteristics The Rio Bravo sand varies between 40 and 70 ft in thickness, and, in general, 75 percent or more of the zone is productive. P. 145

THE STATUS OF GEOPHYSICAL KNOWLEDGE IN THE EAST COAST AND EASTERN BASINS OF INDIA

The major problem in geophysical exploration for hydrocarbons in the east coast basins of India (Mahandi, Krishna-Godavari, Palar and Cauvery Basins) is to obtain persistent and correlatable reflection seismic data from prospective zones at depth. From the geological and well data a considerable thickness of pre-Tertiary and Tertiary sediments is known to be present in these basins. As these areas are covered with a blanket of alluvium, the basin configuration and sub-surface structural features are to be inferred from geophysical data. Although gravity, magnetic and refraction surveys have broadly outlined the basin configuration, suitable seismic reflection techniques are yet to be evolved for detailed mapping of the structures. Experiments using offset spreads and geophone grouping parameters derived from noise studies have yielded encouraging results in a limited area of the Cauvery basin.The problems of the eastern basins (Assam, Tripura and Bengal) are varied in nature. The structures in the shelf area of Assam, have been successfully mapped using reflection seismograph, but the tectonic complexity of sub-thrust regions of the Naga Hills and the steeply folded belts of Tripura present formidable logistics and technical problems for delineating the sub-surface features. Some theoretical studies have been made for developing field techniques to improve the signal-to-noise ratio in these areas.The problem in the Bengal basin is the absence of continuous and correlatable reflection events from the prospective Mio-Pliocene section. The use of digital data acquisition and processing techniques has not substantially improved data quality from this section to date.

The application to hard rock mineral investigations of a portable seismograph with detectors adjustable for coherent reflections

An investigation to determine the suitability of a portable reflection seismograph in mapping rock type interfaces in the Canadian Shield involved the recording of approximately 5,000 ft. of linear profile over the contact between Proterozoic basal conglomerates and Archean basic complex. In the interpretation, the multiple reflections which occur are removed from the field records, and reflecting horizons to depths of 1,100 ft. have been identified and these correlate well with the geological interpretation based on surface outcrop and drilling. The results of applying the seismic technique to this area, point the way to the wider use of the method to structural problems in mining exploration.

Evolution of Smackover Play, Texas to Florida: ABSTRACT

The Smackover limestone is the primary exploration target in the updip part of the entire Gulf Coast area, extending from East Texas to Florida. Accordingly, the Smackover provides most of the new reserves being found in this area and most major companies and many independents have virtually stopped Cretaceous exploration to concentrate on the Jurassic reservoirs (Smackover and Cotton Valley). Smackover exploration started on a major scale during the 1940s in southern Arkansas where relatively shallow (above 9,000 ft) oil was found. Production was predominantly from oolitic limestones on relatively simple anticlinal structures that were located by reflection seismograph. Nearly 20 years later, in the 1960s, major Smackover exploration began in East Texas. There, a different set of problems confronted explorationists--more radical facies changes, predominantly dolomite reservoirs, gas condensate-sulfur rather than oil pays, abundant inert gases, complex intrusions, greater depths (over 12,000 ft), and erratic porosity developments! Nearly 5 years later, in the late 1960s, Smackover exploration expanded to Mississippi, Alabama, and most recently, to Florida. There, producing rates are higher and multipays of both oil and gas are common in the Cotton Valley and Norphlet sandstones, in addition to the Smackover. Broad limits of the favorable producing facies in the Smackover are fairly well recognized in Mississippi and presently are being extended eastward into Alabama and Florida where recent discoveries by Humble have started hot lease plays. Although local structure and stratigraphic variations are extremely complex, the regional picture of Smackover deposition is viewed as a simple wedge of predominantly carbonate rocks rimming the Gulf Coast, and underlain by along the downdip area and an old Paleozoic landmass on the north in the updip area. Most Smackover fields have been round in areas of post-Smackover (Buckner anhydrite) thins, seismic salt swells, and very low relief Lower Cretaceous structures. Most of the recently discovered fields cover less than 1,200 acres; however, pay sections of more than 100 ft are not uncommon, yielding large reserves over small geographic areas. Consequently, several tests commonly are required to pinpoint the accumulation. End_of_Article - Last_Page 152------------