The American Association of Petroleum Geologists Bulletin V 68, No. 8 (August 1984), P. 1040-1043, 4 Figs.Surface Gamma-Ray Logs: A Correlation Tool for Frontier Areas[footnoteRef:1] [footnoteRef:2] [1: Copyright 1984. The American Association of Petroleum Geologists. All rights reserved.1 Manuscript received, September 9, 1983; accepted, January 27, 1984. Presented at the AAPG Annual Convention, Dallas, Texas, April 1983, Poster Session I, AAPG New Technology.^edar Strat, Box 810, Ely, Nevada 89301.] [2: thank K. K. Mayden and S. R. Crook, who have assisted me in the field. My thanks go also to C. A. Sandberg, I. J. Witkind, D. A. Sprinkel, S. Stobaugh, J.]

ALAN K. CHAMBERLAIN[footnoteRef:3] [3: R. Bushman, and L. F. Hintze, who critically reviewed the manuscript; to T. Loyola and J. Bowcutt, who drafted the illustrations; and to N. Van Voorhis, who typed the manuscript. I am grateful to my wife who encouraged me as I developed these concepts. American Stratigraphic Co. kindly permitted use of their lithologic logs.]

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ABSTRACTSurface gamma-ray logs or scintillometer profiles of measured sections provide an excellent correlation tool in frontier areas where well control is sparse but outcrops are abundant. Subtle changes in lithology that might have been missed during customary section measuring are commonly detected by surface gamma-ray measurements. Furthermore, gamma-ray measurements across concealed strata may give clues to the lithology of the buried rock.A portable scintillation counter is used in conjunction with lithologic descriptions. Five-foot stratigraphic intervals provide optimum results.Surface gamma-ray logs have been successfully used to correlate and trace Paleozoic and Mesozoic units in Nevada, Utah, and Kentucky.INTRODUCTIONSurface gamma-ray logs, also known as scintillometer profiles, are made from gamma-ray radiation measurements using a hand-held scintillation counter. As one measures a stratigraphic section, the data are noted and plotted in graphic form. Such logs facilitate correlation of measured surface sections with well logs because gamma- ray log signatures for the same lithologic units are very similar.During a routine measurement of a stratigraphic section, surface gamma-ray logs can help in detecting subtle lithologic changes or in determining what units underlie covered intervals that are mainly talus. Such logs are especially useful in areas where well control is sparse and outcrops are abundant (e.g., the Overthrust belt and the Basin and Range province, Figure 1). In such areas, surface gamma-ray logs provide surface-generated well-type data where few well data are available. This makes it possible to objectively correlate surface with subsurface data.THEORYNatural gamma radiation in sedimentary rocks is generally a reflection of potassium-40, uranium, and thorium natural radioactive elements abundant in clays (Wood et al, 1974). Sedimentary rock with high clay content, such as organic black shale, will commonly emit higher gamma radiation than those with less clay content, such as quartz sandstone and dense carbonate rocks. The lithology of concealed units can be inferred from surface gamma-ray logs, because soils developed on these units emit a level of radiation similar to that of the rocks from which the soils are derived.TECHNIQUESurface gamma-ray logs are derived by recording natural radiation at each measurement station and plotting the data in graphic form. A hand-held scintillation counter is used. Two basic types of scintillation counters are availabledigital readout and dial-meter. The digital model seems to be more efficient for measured sections because it averages the readings. Absolute measurements of gamma radiation are not as important as the relative changes; consequently, tedious calibration of the instrument is not necessary as long as the same instrument is used for the entire measured section. In fact, the log signatures will be the same even though the absolute amount of radiation detected may vary because of the different calibrations. The gamma radiation can be recorded at any interval, depending on the detail desired. My experience suggests that a 5-ft (1.5-m) interval is the most efficient, chiefly because I use a 5-ft Jacobs staff to measure sections. The interval also produces a good average of surface radiations. Best results are achieved when the scintillation counter is carried waist high for the 5-ft intervals, but it should be placed nearer the outcrop for shorter intervals.Gamma-ray measurements are conveniently recorded by use of a portable tape recorder, freeing the hands to carry the Jacobs staff and other equipment. Such a technique allows continuous study and measurement of the outcrop. It would be slow and tedious to record in a field notebook every measurement over long measured sections. It is helpful to devise a numbering system that correlates the measured section with the radiation record. My technique is to record three things at each station: (1) the interval number, (2) the counts per second, and (3) a description of the lithology of the 5-ft (1.5-m) interval. Because the data are recorded in digital form, they may easily be transcribed directly onto computer. Basically, the result is a digitized gamma-ray log and lithologic description of the measured section. The data then can be manipulated easily to produce isopach maps, facies maps, and other computergenerated maps.After the data are transcribed onto paper, they can be plotted onto graph paper at any desirable scale. Instead of plotting American Petroleum Institute (API) units, counts

Figure 1Index map of wells and measured sections in Nevada and Utah used in surface-to-subsurface correlations where surface gamma-ray logs are especially useful. See Figures 2-4 for identification of wells and measured sections.per second are used. For compatibility with available American Stratigraphic Co. well logs, a scale of 1 in. = 100 ft (2.5 cm = 30 m) is used for the vertical dimension and 1 in. = 50 counts (2.5 cm = 50 counts) is used for the horizontal. Plotting the gamma radiation first, aids the preparation of a lithologic log in areas with partially covered slopes.EXAMPLESNevada

2Suntide Petroleum Inc.Measured Section3Northwest ExplorationNo. A-1 Suntide-Sinclair Nevada Federal SE NE Sec. 23, T18N, R57EHamilton, White Pine Range, Nevada Sec. 30, T17N, R56ENo. 5-A Jake's Wash Spot NW SW Sec. 1, T14N, R60EFigure 2Surface-to-subsurface correlation of Chainman Shale, northern White Pine Range, White Pine County, Nevada. Mississippian Chainman Shale sandstone in measured section was found by correlating surface gamma-ray log with gamma-ray logs of nearby wells.Gamma-ray logs prepared from surface outcrops have been extremely useful in correlating Paleozoic strata within the Basin and Range province where well control is sparse and outcrops are abundant. Gamma-ray signatures derived from strata ranging in age from Cambrian to Tri- assic are distinctive and allow for excellent correlation between mountain ranges. For example, in the northern White Pine Range, western White Pine County, Nevada, (Figure 1, point 2), distinctive radiation changes separate the Joana Limestone (Mississippian) from the overlying Chainman Shale (Mississippian) (Figure 2, column 2). Comparable radiation changes distinguish discrete litho-

2Measured SectionHamilton, White Pine Range, Nevada Sec. 30, T17N, R56E

Figure 3Surf ace gamma-ray profile of upper part of Mississip- pian Chainman Shale and lower part of Pennsylvanian Ely Limestone in northern White Pine Range, Nevada.

logic units within the Chainman Shale, such as the 150-ft- thick (46-m) sandstone unit 2,000 ft (610 m) above the base of the section. This sandstone correlates well with a sandstone in the same interval of a nearby well.Surface gamma-ray logs can help identify subtle lithologic changes. For example, in the upper part of the same measured section (2 in Figure 2), the nature of the contact between the Chainman Shale and the overlying Ely Limestone (Figure 3) is difficult to see because of detritus. On the surface gamma-ray log, one can detect a gradual change in the level of radiation, from a high (nearly 150 counts per second) in the shale, approximately 4,100 ft (1,250 m) above the base of the section, to a low (about 50 counts per second) at 4,500 ft (1,372 m) above the base of the section in the limestone. The detritus in the covered interval gradually becomes more calcareous upward. This suggests a gradational contact, implying a gradual transgression of the sea.In other measured sections in the Basin and Range province, surface gamma-ray logs aid in the recognition of gradual fining and gradual coarsening upward sequences, allowing better identification and interpretation of the depositional environments.UtahOn the east edge of the Basin and Range province, the Jurassic Eviii Creek Limestone overlies the Triassic- Jurassic Nugget Sandstone. Not only is there a major change in the gamma-ray signature between the Nugget Sandstone and the overlying Twin Creek Limestone, but members within the TWin Creek Limestone show distinct gamma-ray log signatures, which can be recognized and correlated between measured sections and logs of nearby wells (Figure 4).Appalachian BasinOthers have successfully used surface gamma-ray logs in correlating units in the Devonian and Mississippian shales of the Appalachian basin. Provo et al (1978) found that the Three Lick Bed of the Ohio Shale has a distinctive gamma-ray signature that is easily recognized in both outcrop and subsurface. Ettensohn et al (1979) discuss how surface gamma-ray logs can help correlate homogeneous black shales. More recently, Kepferle et al (1981) demonstrate how a surface gamma-ray log of a measured section of the Chattanooga Shale closely resembles the gamma- ray profile of the same interval in a nearby well.SUMMARYSurface gamma-ray logs are made by measuring gamma- ray radiation emitted by rocks of a measured section. The level of radiation reflects the clay contents of a rock unit. Such logs are easy to produce by using a hand-held scintillation counter and recording the gamma radiation over lithologic units at regular increments. The data are then plotted in log form with lithologic descriptions. Surface gamma-ray logs are easily correlated with well logs because gamma-ray signatures are essentially the same in the subsurface. It may be helpful in the future to identify or define rock units not only on lithologic characteristics and fossil content, but also on the basis of gamma-ray signatures. Surface gamma-ray logs may be useful for solving correlation problems.REFERENCES CITEDEttensohn, F. R., L. P. Fulton, and R. C. Kepferle, 1979, Use of scintillometer and gamma-ray logs for correlation and stratigraphy in homogeneous black shales: GSA Bulletin, v. 90, part I, p. 421-423; part II, p. 828-849.Kepferle, R. C., P. E. Potter, and W. A. Pryor, 1981, Stratigraphy of the Chattanooga Shale (Upper Devonian and Lower Mississippian) in Vicinity of Big Stone Gap, Wise County, Virginia: U. S. Geological Survey Bulletin 1499,20 p.

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1042Surface Gamma-Ray LogsProvo, L. J., R. C. Kepferle, and P. E. Potter, 1978, Division of black Ohio Shale in eastern Kentucky: AAPG Bulletin v. 62, p. 1703-1713. Wood, R. D., P. A. Wichmann, and H. B. Watt, 1974, Gamma-ray- neutron log, in Review 1: Dresser Industries, Inc., Houston, Texas, p. 1-19.

OLfds CreekMambarWatton CanyonBoundary Bldg*MambarRichMambarSlldar oc*MambarGypsum SpringMambarMeasured SectionEmigration Canyon. UtahColorado Energetics, Fuelco, Impel Corp.No. 13-3 Weber Coal Co.Measured SectionDevil's Slide, Weber Canyon, UtahAmoco Production No. 1 Mills Wl UnitNW NE Sec. 10, T14N, R120WSec. 21, T1N, R2ESOOOft(1524m)4500ftNUGGET(1372m)SANDSTONE0) conn)Spot NW SW Sec. 3, T2N, R5ENW SE SW Sec. 23, T4N, R3EGamma-Bay Log50100150200(API Units)Gamma-Bay Log (counts par sacond)Figure 4Surface-to-subsurface correlation of Triassic Nugget Sandstone and members of overlying Jurassic Twin Creek Limestone, northern Utah and southwestern Wyoming.Gamma-Bay Log(API units)samGamma-Bay Log (e>Alan K. Chamberlain1043

RESUMOLogs de raios gama de superfcie ou perfis scintillometer de sees medidos fornecer uma ferramenta de correlao excelente em reas de fronteira onde o controle bem escasso mas afloramentos so abundantes. Mudanas sutis na litologia que poderiam ter sido perdidas durante a medio seo habitual so comumente detectados por medies de raios gama de superfcie. Alm disso, as medies de raios gama em todo estratos escondido pode dar pistas sobre a litologia da rocha enterrado.Um contador de cintilao porttil usado em conjunto com as descries litolgicas. Intervalos estratigrficos Cinco ps proporcionar os melhores resultados.Logs de raios gama de superfcie tm sido utilizados com sucesso para correlacionar e traar unidades Paleozico e Mesozico em Nevada, Utah e Kentucky.

INTRODUOLogs de raios gama de superfcie, tambm conhecidas como perfis scintillometer, so feitos a partir de medies de radiao de raios gama usando um contador de cintilao de mo. Como se mede uma seo estratigrfica, os dados so anotados e plotados em forma grfica. Esses logs de facilitar a correlao de seces de superfcie medidas com perfis de poos, porque assinaturas de registo de raios gamma para as mesmas unidades litolgicas so muito semelhantes.Durante uma medio de rotina de uma seo estratigrfica, superfcie registos de raios gama pode ajudar na deteco de mudanas litolgicas sutis ou na determinao de que as unidades esto subjacentes intervalos cobertos que so principalmente tlus. Tais registros so especialmente teis em reas onde o controle bem escasso e afloramentos so abundantes (por exemplo, o cinto Overthrust ea provncia bacia e da escala, Figura 1). Nessas reas, os registos de raios gama de superfcie fornecer dados bem do tipo de superfcie onde gerou alguns dados de poos esto disponveis. Isto faz com que seja possvel correlacionar objectivamente superfcie com dados de subsolo.

TeoriaRadiao gama natural em rochas sedimentares geralmente um reflexo de potssio-40, urnio e trio - elementos radioativos naturais abundantes em argilas (Wood et al, 1974). Rocha sedimentar com alto teor de argila, tais como xisto preto orgnico, vai comumente emitem radiao gama maior do que aqueles com menor teor de argila, como o arenito de quartzo e densas rochas carbonticas. O lithology de unidades escondidas pode ser inferida a partir de registos de raios gama de superfcie, porque os solos desenvolvidos nestas unidades emitem um nvel de radiao semelhante das rochas a partir do qual so derivados os solos.

TecnicaLogs de raios gama de superfcie so derivados atravs da gravao de radiao natural em cada estao de medio e desenho dos dados em forma grfica. Um contador de cintilao de mo usado. Dois tipos bsicos de leitura de contadores de cintilao esto disponveis digital e disque metros. O modelo digital parece ser mais eficiente para sees medidos porque ele calcula a mdia das leituras. Medidas absolutas de radiao gama no so to importantes quanto as mudanas relativas; por conseguinte, a calibrao do instrumento tedioso no necessrio desde que o mesmo instrumento usado para toda a seco de medida. Na verdade, as assinaturas de registo ser o mesmo, embora a quantidade absoluta de radiao detectada pode variar devido s diferentes calibraes. A radiao gama pode ser gravado em qualquer intervalo, dependendo do detalhe desejado. Minha experincia sugere que um intervalo de 5 ps (1,5 m) o mais eficiente, principalmente porque eu uso pessoal a 5 ps de Jacob para medir sees. O intervalo tambm produz uma boa mdia de radiaes de superfcie. Os melhores resultados so obtidos quando o contador de cintilao realizada cintura alta para os intervalos de 5 ps, mas ele deve ser colocado mais perto do afloramento para intervalos mais curtos.Medies de raios gama so convenientemente registrados pelo uso de um gravador porttil, libertando as mos para levar a equipe do Jacob e outros equipamentos. Tal tcnica permite o estudo contnuo e medio do afloramento. Seria lento e tedioso para gravar em um caderno campo a cada medida ao longo sees longo medidos. til para conceber um sistema de numerao que correlaciona a seo medido com o registro de radiao. A minha tcnica para gravar trs coisas em cada estao de: (1) o nmero de intervalos, (2) as contagens por segundo, e (3) uma descrio da lithology do intervalo de 5 ps (1,5 m). Como os dados so gravados em formato digital, eles podem facilmente ser transcritas diretamente para o computador. Basicamente, o resultado um registo de raios gama digitalizados e descrio litolgica da seco de medio. Os dados, em seguida, pode ser manipulado facilmente para produzir mapas ispacas, facies mapas e outros mapas gerados por computador.Depois de os dados so transcritos para o papel, que podem ser representados graficamente em papel de grfico em qualquer escala desejvel. Em vez de planejar American Petroleum Institute (API) unidades, conta toras americana estratigrfica Co. assim, uma escala de 1 pol. = 100 ps (2,5 cm = 30 m) usado para a dimenso vertical e 1 pol. = 50 contagens (2,5 cm = 50 contagens) utilizado para a horizontal. Determinao da radiao gama em primeiro lugar, ajuda a preparao de um registo litolgica em reas com declives parcialmente cobertos.

ExemplosNevadaLogs de Gamma-ray preparados a partir de afloramentos superficiais tm sido extremamente til em correlacionar estratos do Paleozico no interior da provncia bacia e da escala em que o controle bem escasso e afloramentos so abundantes. Assinaturas de raios gama provenientes de estratos com idade variando de Cambrian para Trissico so distintas e permitem excelente correlao entre cadeias de montanhas. Por exemplo, no norte da Faixa de White Pine, Branco ocidental Pine County, Nevada, (Figura 1, ponto 2), mudanas de radiao distintivas separar o Limestone Joana (Mississippian) a partir da sobreposio Chainman Xisto (Mississippian) (Figura 2, coluna 2) . Mudanas de radiao comparveis distinguir unidades litolgicas discretas dentro do Chainman Shale, tais como as unidades de espessura (46 m) arenito 150-FT-2000 ps (610 m) acima da base da seo. Isto correlaciona-se bem com arenito um arenito no mesmo intervalo de um poo prximo.Logs de raios gama de superfcie pode ajudar a identificar mudanas sutis lito-lgicos. Por exemplo, na parte superior da mesma seco de medio (2 na Figura 2), a natureza do contacto entre o Chainman xisto e sobrejacente Ely Calcrio (Figura 3) difcil de ver devido a detritos. No registo de raios gama a superfcie, pode-se detectar uma alterao gradual do nvel de radiao, de uma alta (cerca de 150 contagens por segundo) em xisto, aproximadamente, 4100 ps (1,250 m) acima da base da seco, para uma baixa (cerca de 50 contagens por segundo) a 4500 ps (1,372 m) acima da base da seco em calcrio. O detritos no intervalo coberta gradualmente torna-se mais para cima calcria. Isto sugere um contato gradativo, o que implica uma transgresso gradual do mar.Em outras sees medidos no province Basin and Range, gamma-ray superfcie registra ajuda no reconhecimento de multar gradual e engrossamento sequncias ascendente gradual, permitindo uma melhor identificao e interpretao dos ambientes deposicionais.