27. gamma-ray well-log characteristics, leg 96

27. GAMMA-RAY WELL-LOG CHARACTERISTICS, LEG 961

J. M. Coleman, Louisiana State UniversityR. Constans, Chevron U.S.A. Inc

andA. H. Bouma, Gulf Research and Development Co.2

ABSTRACT

Gamma-ray well-log traces from six sites on the Mississippi Fan visited during Deep Sea Drilling Project (DSDP)Leg 96 were processed by Chevron's NFLECT program3 to provide data on grain-size trends and bedding thickness, inaddition to the common evaluation of lithologies of missed cored sections.

The midfan channel sites (621 and 622) and the lower fan channel site (623) contain the highest percentage of gam-ma-ray NFLECT-calculated "sand." Site 623 contains the highest percentage of gamma-ray NFLECT "silt" of all sites,and the lower fan overbank site (624), the highest percentage of "clay." The lower fan distal site (615) contains less than10% gamma-ray NFLECT sand but is very high in silt.

INTRODUCTION

During DSDP Leg 96, open-hole logging runs weremade using Schlumberger wireline sondes at six sites (615,620, 621, 622, 623, and 624) on the Mississippi Fan. Inaddition, Site 616 was logged through the drill pipe witha combined formation density/compensated neutron/gamma-ray tool. The site locations and types of logs runare shown in Figure 1. The data recorded from theseruns fill in missing information between the recoveredcore data and aid in the interpretation of the sedimenta-ry sequences deposited in the youngest fan lobe of theMississippi Fan.

The theoretical principles and applications of the var-ious logging tools used on this leg are summarized inSchlumberger log interpretation manuals (Schlumberg-er, 1972, 1974). Factors affecting log responses and thefundamentals of log evaluation are also presented by As-quith and Gibson (1982) and Helander (1983).

The configuration of the drilling operations on theGlomar Challenger and the unconsolidated nature of theshallow sediments penetrated presented a rather hostileenvironment for wireline electric logging. Only small-di-ameter logging tools could be employed because the ab-sence of a drilling riser system mandated that all toolsbe lowered through drill pipe into open hole. Use of sea-water as a drilling fluid rather than a mud system alsocontributed to the less than optimum logging conditions.Large washout zones and bridging of the borehole bysoft sediments hampered logging operations and raisedquestions as to the validity of some of the density andporosity measurements. Details of the logging operations,

Bouma, A. H., Coleman, J. M., Meyer, A. W., et al., Init. Repts. DSDP, 96: Wash-ington (U.S. Govt. Printing Office).

2 Addresses: (Coleman) Coastal Studies Institute, Louisiana State University, Baton Rouge,LA 70803; (Constans) Chevron USA Inc., 935 Gravier St., New Orleans, LA 70112; (Bouma,present address) Chevron Oil Field Research Co., P.O. Box 36506, Houston, TX 77236.

3 Chevron's NFLECT program was specifically designed for another well-log trace, buthas been adapted to gamma-ray traces for this study. The technical justification for such ad-aptation to gamma-ray traces has not been worked out completely and may pose questions inthin-bed corrections.

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Figure 1. Location of Mississippi Fan sites drilled on DSDP Leg 96with indications of the types of well logs run. C = caliper log, D= formation density log, G = gamma-ray log, I = dual-inductionlateral log, N = compensated neutron log, S = long-spaced soniclog.

hole conditions encountered, and tool failures at each ofthe logged sites can be found in the operations sectionof the site summaries (this volume).

The porosity values calculated from the density toolsgenerally range from 50 to 60%. Although these valuesappear to be in agreement with the porosity values cal-culated from physical-property studies (see Bryant, Wet-zel, Taylor, and Sweet, this volume) and the measuredporosities from sedimentologic studies of core slabs (seeColeman, Bouma, et al., this volume), the reliability ofthe log porosity data is questionable because of the large

547

J. M. COLEMAN, R. CONSTANS, A. H. BOUMA

variations in the log curves. Therefore, density and po-rosity characteristics from the logs will not be discussedin this chapter.

The gamma-ray log was used as the primary lithologylog because of the lack of a diagnostic signature recordedon the spontaneous potential log (SP). The gamma-raytool is a device that consists of a scintillation-type detec-tor that measures natural background gamma-ray emis-sions. These emissions are usually caused by decay ofpotassium, thorium, and uranium salts present in sandsand shales. Shales, particularly marine shales, have agreater gamma-ray emission level than sandstones, lime-stones, and dolomites (Helander, 1983). This differencemakes the gamma-ray tool useful for distinguishing shalesfrom nonshales. Slight variations in the diameter of theborehole do not appreciably affect the gamma-ray re-sponse, but large washout zones may cause a slight de-crease in the gamma-ray count. Caution must be em-ployed, therefore, when using the gamma-ray log as alithologic indicator in extremely washed out boreholes.The caved-in zones identified from the caliper log ap-pear to correlate with sandy cored intervals in the Mis-sissippi Fan sites logged with the gamma-ray tool. Thelowering of gamma-ray counts in these zones by bore-hole conditions serves principally as positive reinforce-ment of the interpretation of a sandy lithology.

Gamma-ray logging through drill pipe can reduce thegamma-ray radioactivity by as much as 30%. Althougha log with good diagnostic character was recorded throughdrill pipe at Site 616, the reduced values are not readilycomparable with the sites logged in open hole. There-fore, Site 616 logging results are not discussed here.

METHODSThe gamma-ray data were processed utilizing one of Chevron Geo-

sciences' computer well-log applications. The purpose of this particu-lar program (NFLECT) is to develop a trace that sharply defines bedboundaries and reflects a truer value of the static spontaneous poten-tial (SP) independent of layering effects. This is done by measuringthe amount and rate of change on the trace and by specifying thresh-olds that cause the trace to be reset to some specified value if thosethresholds are exceeded.

The computer processing of the gamma-ray data was executed inthe following manner: Data from the field tapes were transferred todisks accessible by an IBM mainframe computer. The data were re-corded on tape in feet and had to be converted to meters. The integrityof these data was checked by playing back the digits and matchingthem to the logs. After removing baseline drift, the gamma-ray tracewas "smoothed" using a 3-ft. running average window to filter outhigh-frequency information that is noise and not related to beddingsequence. The values of this trace were then inverted to simulate theresponse of an SP trace. The NFLECT program was run over thistrace to produce the "squared" gamma-ray NFLECT trace. Beds thatare not significant in a statistical sense are suppressed by the choice ofsuitable parameters. The same parameters were used at all sites to in-sure consistent interpretations.

A typical section of traces generated with these procedures is dis-played in Figure 2. The inflection trace was used with liberal limits topredict relative lithology. Gamma-ray values greater than 70 API indi-cate coarse sediments; values less than 20 API indicate fine sediments.The broad range of values that falls between these limits is consideredeither intermediate or mixed.

The NFLECT program was used to classify the log response intothe three categories described above on a 0.2-m interval, and thus logthickness could be obtained for each category in the hole. This capa-bility permits evaluation of the bedding-thickness variation in a verti-cal sense. In addition, the program sums up the categories over 10-m

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range, allowing an evaluation of the vertical change in grain-size dis-tribution. The gamma logs also allow an evaluation of the sharpnessof the bedding planes and shape of the intervals. These characteristicsof each logged hole are discussed later.

NFLECT ZONES AND LITHOLOGICINTERPRETATIONS

The gamma-ray NFLECT values were compared tocore photographs and descriptions and to various sedi-mentological analyses that were conducted by shore-basedscientists to obtain a better understanding of the rela-tionship between the NFLECT values and the sedimen-tology of the deposits at each site. It must be remem-bered, however, that the logged intervals do not repre-sent the entire cored interval because of surface pipe andoccasional inability of the logging tool to penetrate tothe bottom of the open hole. It is, therefore, often diffi-cult to correlate core recovery depth precisely with log-ging depth. For this reason, only core samples that fellinto large intervals of continuous NFLECT data wereused to determine the characteristics of the gamma-rayresponse.

NFLECT Values > 70 API (coarse sediments)

The average of 18 samples recovered from intervals inthis NFLECT category contain 65% sand, 30% silt, and5% clay. One sample contained 95% sand, 5% silt, and0% clay, the average grain size being in the medium sandrange. The finest grained samples generally consist of35% sand, 55% silt, and 10% clay. These grain sizeand mineralogical results indicate that sediments in thisNFLECT category consist of relatively clean sands con-taining scattered redeposited organics and mica.

On the core photographs and X-ray radiographs thesands appear massive and display little or no internalstructure. It has proven impossible, however, to deter-mine whether this lack of internal structure is a result ofthe coring disturbance or natural. A crude size grading

548

GAMMA-RAY WELL-LOG CHARACTERISTICS

is apparent in some cores; in a few instances, radiographsindicate small-scale cross-laminations in finer grained lay-ers within the overall sandy unit. The sands also containa few very thin clay laminations (generally less than afew centimeters thick, although one sand had a clay lay-er 8 cm thick). Another sample, displaying thin silty claylaminations alternating with graded sand layers, was tilt-ed at an angle of 20°.

Porosity measurements within these NFLECT valueswere rare, but values of 29 and 35.4% were recorded intwo samples from shipboard measurements. Roberts andThayer (1985) indicate that, in measurements on 50 thinsections of which only 20 fell within this NFLECT inter-val, porosity ranges from 20 to 35%. Estimates basedon the size and sorting indicate that an additional 26samples have porosities that range from 30 to 49% (Thayeret al., this volume). These samples were moderately wellto well sorted. The massive nature of the sands, the rela-tively high porosities, moderately well to well sorting,and lack of substantial clay content are probably char-acteristic of this NFLECT interval.

NFLECT Values 70 to 20 API (mixed sediments)The 70 to 20 NFLECT interval represents those sedi-

ments that fall between the finer grained clays and thecoarser sands. As expected, a wide variation in proper-ties was found. Textural averages of samples from thisinterval are 23% sand, 51% silt, and 26% clay. Sandcontent ranges as high as 69% and as low as 3%; clayvalues range from 1 to 67%. This unit is composed pri-marily of alternating thin sands and clays with abun-dant finely laminated silts. Thin, graded laminae, only afew millimeters to a few centimeters thick, are the mostcommon structures. The sand laminations rarely exceed15 cm in thickness, whereas the silt layers commonlyrange up to 20-25 cm thick. Mica and redeposited woodyorganic material are common in the silt and fine-sandunits (see site chapters, this volume). Thin-section anal-yses by Roberts and Thayer (1985) show that the sandsrange from fine to very fine in grain size, are moderatelywell sorted, and have porosities that range from 25 to41% and permeabilities that range from 0.5 to 8.0 dar-cies. Thus, this NFLECT interval represents the siltierthin-bedded sediments.

NFLECT Values <20 API (fine sediments)The < 20 NFLECT unit represents the finest grained

sediments present in the recovered cores. Average textur-al values, based on 22 samples, are 2% sand, 45% silt,and 53% clay. Sand content ranges as high as 10%, butmany samples contained no sand-sized particles. Claycontent ranges from a high of 80% to a low of 40%.The samples display one of two major types of stratifi-cation: either extremely thin-laminated silts and clays orrelatively thick-bedded (40 to 80 cm) clays. Disturbedstructures such as microfracturing and thin convolutelaminations are very common within this unit.

Although the NFLECT values do not necessarily cor-relate specifically with grain-size intervals, NFLECT val-ues greater than 70 represent the well-sorted coarser se-quences and, for convenience, these deposits are referred

to as "sands." NFLECT values less than 20 representthe finest-grained sediments, and they are referred to as"clays." Sediments with NFLECT values ranging from70 to 20 are referred to as "silts."

NFLECT CHARACTERISTICS

Sites 621 and 622Holes 621 and 622 were drilled in the midfan chan-

nel, Hole 621 in the thalweg of the channel and Hole622 on the inside concave or "point bar" side of thechannel (Site 621 and 622 chapters, this volume). Figure 3illustrates typical gamma-ray log responses of the sedi-ments in these two holes. The uppermost sediments werenot logged because of pipe in the hole, and thus the fin-er-grained "passive fill" of the channel is not well repre-sented. The overall log response at both sites shows afining-upward sequence, with gamma counts ranging froma high of 90 API units (NFLECT value of 10) near thetop of the holes to a low of 30 API units (NFLECT val-ue of 70) near the bottom. Most of the coarser-grainedintervals exhibit a blocky shape, with sharp bases andsharp tops. Most of the sands are both underlain andoverlain by clays.

Figure 4 illustrates the bedding-thickness variation foreach NFLECT interval at Site 621. With one exceptionat 90 m sub-bottom, the "sand" beds thin upward, es-pecially when combined with the uppermost unloggedintervals (Fig. 4A). Sand beds average 1.44 m in thick-ness (standard deviation 0.4). The silt beds do not showany particular trend and average 2.26 m in thicknesswith a standard deviation (SD) of 1.54 (Fig. 4B). Nearthe top of the hole there is some indication that pack-ages of thickening-upward sequences exist, each pack-age 12 to 15 m thick. The clay intervals thicken upwardfrom the base of the hole to a sub-bottom depth of 140m and then thin upward to the top of the hole (Fig. 4C).Clay beds average 2.05 m in thickness, with a standarddeviation of 1.29.

Grain-size trends at Site 621 were averaged over 10-mincrements (Fig. 5). Within the logged interval, no ap-parent trend is present within the sand- or silt-sized units.However, it must be remembered that the uppermost fine-grained unit at the top of the borehole was not logged.Clay percentages show a persistent increase uphole to124 m sub-bottom and then a rather dramatic decreaseto the top of the logged interval.

Figures 6 and 7 illustrate generally the same bed thick-ness and textural characteristics for Site 622 as were ob-served at Site 621, except that the vertical variations ingrain size are not quite as apparent. Sand beds average1.13 m in thickness (SD 0.29), silt and clay beds 1.90 m(SD 1.29) and 1.44 m (SD 0.64), respectively.

Site 620Hole 620 was drilled approximately 18 km east of the

midfan channel and generally contains sediments thatwere deposited in an overbank environment (see Site 620chapter, this volume; Fig. 1). The gamma-ray log responseis shown in Figure 8, and analysis of the log indicatesthat it is generally a silt-rich sequence, containing only

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thin sand and clay intervals. Bedding thicknesses in allthree NFLECT intervals show no trends (Fig. 9). Thesand beds average 0.98 m in thickness (SD 0.30), claybeds average 1.46 m (SD 0.62); the thickest beds are silt,averaging 2.83 m (SD 1.54). In a general sense, the siltbeds appear thickest near the top of the borehole. Grain-size trends illustrate that the silt percentage increases up-ward and that most of the sand is located near the bot-tom of the hole (Fig. 10).

Site 623

Hole 623 was drilled in the lower fan in a channel-overbank complex (Fig. 1; see Site 623 chapter, this vol-ume). In this region of the lower fan, only a single smallchannel appears to be active at any one time, and adja-cent to it numerous abandoned channels are present (Low-

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er Fan Introduction and Summary, this volume). Thinpackages of sand tend to be overlain by silt and thenclay, each package being 15 to 20 m thick (Fig. 11). Thesands are all underlain by clays and are capped by ratherthick-bedded silts. The sand beds average 1.48 m in thick-ness (SD 0.47); the silt beds show a high variability inthickness, averaging 2.71 m, but have a standard devia-tion of 3.52 (Fig. 12). Clay beds are thinner, averagingonly 1.25 m in thickness, and show a low standard devi-ation (0.49). Grain-size percentages averaged at 10-m in-tervals show the fining-upward packages that are char-acteristic of these sediments (Fig. 13).

Site 624

Site 624 is located west of Site 623 in an area removedfrom the channel-overbank complex (Fig. 1; Site 624chapter, this volume). This site displays the finest grainedsediments of all the wells logged, containing only 1.4 mof net sand, 31.6 m of net silt, and 92.0 m of net clayover the logged interval of 125 m. Figure 11 illustratesan example of the gamma-ray response at Site 624; notethat, with the exception of the single thin sand, the re-sponse is extremely spiked , indicative of alternating siltand clay layers. The thick-bedded nature of the layeringis illustrated by Figure 14; the average silt layer is 3.49 mthick (SD 3.08) and the average clay layer is 9.51 m thick(SD 9.94). The grain-size trend appears to fine upwardto a sub-bottom depth of 102 m; above that level, a slightcoarsening-upward trend is suggested (Fig. 15).

Site 615

Hole 615 was the deepest hole drilled on Leg 96 andwas located on the lower fan near the termination of thechannel at its distal end (Fig. 1). Two fan lobes were en-countered at the site (see Site 615 chapter, this volume).At a sub-bottom depth of 242 m, a prominent seismichorizon occurs that separates these two fan lobes; it isalso at this point that an increase in planktonic and ben-thic fauna occurs, probably related to a period of lowersedimentation rates. Figure 16 illustrates the gamma-raylog response for parts of this borehole. The lower fanlobe shows a thinning-upward trend in the sand layers

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••••• •••B •••

103 123 143 163Sub-bottom depth (m)

183


(Fig. 17A) and alternating packages of thickening-up-ward silt beds, each package being from 20 to 50 m thick(Fig. 17B). The clay layers tend to show a crude thicken-ing-upward trend, especially when evaluated on the thick-er beds (Figure 17C). The sand beds average 1.27 m thick(SD 0.43), the silt beds average 2.77 m thick (SD 1.90),and the clay beds average 1.57 m thick (SD 0.82). Figure18A shows the grain-size trend, and the fining-upwardtrend is apparent.

The upper lobe, from the seafloor to a sub-bottomdepth of 242 m, contains less sand and overall is thick-er bedded than the underlying fan lobe. The sand layersshow little or no trend (Fig. 19A); the silt layers are quitethick and do not appear to display any trend (Fig. 19B),and the same is true of the clay layers (Fig. 19C). The

1 0 0

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177 197

Figure 14. Bed thickness trends of (A) silt (NFLECT 70-20 API), and(B) clay (NFLECT <20 API) at Site 624.

sand beds average 1.03 m in thickness (SD 0.09), the siltbeds average 3.40 m (SD 2.88), and the clay layers aver-age 1.72 m (SD 2.21). A general fining-upward trend issuggested by the grain-size distribution (Fig. 19B), espe-cially if the upper unlogged part of the borehole is ex-amined.

The gamma-log response in this hole indicates thatthe sands are both underlain and overlain primarily byclays. The clay layers overlying the sands then grade up-ward into laminated silts, then into thinly laminated clays.This pattern is present in both fan lobes.

556


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90

80

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δ> 6 0 -CO

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082 102 122 142 162 182 202


GUZlSand EZDSilt ESJClay


SUMMARY

The well logs provide essential data in evaluation ofthe missing cored sections. Processed by the NFLECTprogram, they provide additional data on grain-size trendsand bedding thickness in each logged hole. Figure 20summarizes the sand-silt-clay percentages of each site.The channel sites in the midfan (Sites 621 and 622) andthe lower fan channel site (Site 623) contain the highestpercentages of sand. Of these three channel sites, Site623 contains the highest percentage of sand. Site 623contains the highest silt content and the least clay. Themidfan overbank site (Site 620) contains some sand, but

can be characterized primarily as a silt- and clay-rich se-quence. The lower fan overbank site (Site 624) containsthe highest percentage of clay and virtually no sand. Thelower fan site (Site 615), at the distal ends of the channelsystem, contains less than 10% sand, but a very highpercentage of silt.

ACKNOWLEDGMENTS

Appreciation is extended to Chevron USA, Inc., and Chevron Geo-sciences for providing the resources, both "hard" and "soft," for prep-aration of this study and for permission to publish the gamma-rayNFLECT displays. We acknowledge Phillip Sand Hansell IPs assist-ance in providing interpretive computer-processed log displays and hishelp with the many details of the operation of the Chevron Rapid WellLog System. A draft version of this manuscript was reviewed by E. G.Wermund and R. Moiola.

REFERENCES

Asquith, G., and Gibson, C , 1982. Basic Well Log Analysis for Geol-ogists: Tulsa (American Association of Petroleum Geologists).

Helander, D. B., 1983. Fundamentals of Formation Evaluation: Tulsa(Oil and Gas Consultants International Publications).

Roberts, H. H., and Thayer, P. L., 1985. Sedimentology of major de-positional environments encountered in DSDP Log 96 borings. InBouma, A. H., Normark, W. R., and Barnes, N. E. (Eds.), Sub-marine Fans and Related Turbidite Systems: New York (Springer-Verlag), pp. 331-339.

Schlumberger, 1972. Log Interpretation Manual/Principles (Vol. I):New York (Schlumberger Limited).

, 1974. Log Interpretation Manual/Applications (Vol. II):New York (Schlumberger Limited).

Date of Initial Receipt: 2 May 1985Date of Acceptance: 5 July 1985

557


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558

GAMMA-RAY WELI^LOG CHARACTERISTICS

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Figure 17. Bed thickness trends of (A) sand (NFLECT >70 API), (B)silt (NFLECT 70-20 API), and (C) clay (NFLECT <20 API) atthe Site 615 lower fan lobe.

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Figure 18. Sand-silt-clay percentages averaged over 10-m incrementsfor Site 615. (A) lower fan lobe, (B) upper fan lobe.

560


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100

621 622 620 623 624 615

FZZI Sand E 3 Silt EZZ3 Clay

Figure 20. Total sand-silt-clay percentages for Leg 96 sites.

561

27. gamma-ray well-log characteristics, leg 96

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