The density and neutron logs are two important physical measurements in formation evaluation. It was found through theoretical derivation that the density log and the neutron response physically relate to each other in sedimentary formations because of the existence of the element hydrogen. It is the physical dependence of the two logs that constitutes the usefulness of these two logs in determining the porosity, identifying lithology of the formation and detecting natural gas formation as widely used today. Based on the dependence, a correlation coefficient between density porosity and neutron porosity has been defined. The polarity (i.e, the sign) of the coefficient is different for gas-bearing zones and oil-bearing zones. The correlation coefficient can be extended to an approach for identifying natural gas formations since it is more sensitive to gas. The example of one well from Tarim basin in northwest China demonstrates that an excellent correlation between density and neutron logs does exist in log measurements. Thus, the physical dependence of those two logs is shown not only by theoretical derivation but also by field logs. The correlation polarity approach indicates a gadoil contact more accurately than the conventional overlay technique. This is verified with the RFT pressure data from the well.

This tutorial illustrates the benefits of modeling compensated neutron logging instrument responses. Recent progress has been made with speeding up model computations using a forward model based on diffusion theory. The commercially available modeling code, PC2, is applied to understanding and explaining the responses of neutron tools in horizontal wells. For examples, the influence and detectability of beds below and adjacent to the horizontal wellbore and the effects of non-axisymmetric invasion profiles are examined.

The response of Schlumberger's compensated neutron log (CNL)2 was investigated using Tittle's regression model to resolve variations reported in the literature and observed in the comparison of neutron porosity log data with core and other porosity log data. One objective of this investigation was to examine these differences in terms of the neutron macroparameters, slowing-down length and thermal-diffusion length, which include absorption cross section of the formation rock/fluid system. These neutron macroparameters were input to the Tittle regression model to predict the ratio-porosity response of the CNL. The utility of the Tittle regression is demonstrated using common reservoir minerals and mineral mixtures. Three field examples are presented; two have petrophysical data to support the validity of the regression model. The petrophysical data included routine core analysis and elemental analysis, including trace elements that are strong thermal neutron absorbers. In addition, thermal neutron absorption cross sections were measured by a nuclear reactor technique or a method using a chemical neutron source. Analysis was also made of the formation water, oil, and drilling fluids. The neutron porosity modeling code yields results in good agreement with field logs and makes possible an in-depth evaluation of expected CNL porosity-lithology response based on the slowing-down length and the diffusion length of the formation. The modeling and field examples reported here confirm that the macroscopic absorption cross section is an important parameter affecting CNL response and show that large variations in absorption cross section are not uncommon. Variations in absorption cross section can occur in many lithologies, and they can have large effects on log-derived porosity. The data presented also demonstrate the need for a neutron porosity tool that has a response dominated by porosity (hydrogen content) and not affected by absorption cross section. Any lithologic effect should be due to major rock constituents.

Monitoring reservoir fluid movements in the Prudhoe Bay Field on the North Slope of Alaska has been accomplished by the use of cased-hole compensated neutron logs, pulsed neutron logs, and gamma ray logs. Cased-hole nuclear logging data have been collected over time (timelapse) in a majority of the wells in the field and are utilized to make reservoir management decisions. Cased-hole compensated neutron logs are used to monitor gas contact movement, sand production across perforations, and filtrate dissipation when compared to the open-hole compensated neutron logs. Log examples of gas contact monitoring (gas underrunning shales, gas coning) as well as examples of sand production are included. Monitoring filtrate dissipation with neutron logs gives an indication of reservoir rock quality. Examples of how this was utilized to develop perforation strategies in the Sag River Sandstone are also included. The monitoring of water movement has been attempted by the use of repeat cased-hole pulsed neutron logs. The capture cross section of Prudhoe Bay formation oil and water provides limited range in measured sigma from oil and water bearing sands. This limited environment for pulsed neutron logs requires a base-monitor approach with particular attention to log repeatability and borehole changes. Log examples of attempts to monitor water movement in Prudhoe Bay are included as well as a discussion of the methods used to improve pulsed neutron log interpretation in this area of marginal applicability. Cased-hole gamma ray logs, generally run with compensated neutron logs or pulsed neutron logs, monitor radioactive scale precipitation across perforations, producing significant amounts of formation water. Log examples of gamma ray increases across perforations and their use in identifying water producing intervals are included.

Our research shows that boron in the rock man Trix occrs in quantities large enough to affect seriously log analysis in several formations along the Texas Gulf Coast. Examples of log-derived quantities affected by boron content include porosity (compensated neutron log), gas indicator (crossover of neutron and density porosity curves), shale indicator (neutron-density crossplots used in shaley sand interpretations), water salinity and saturation (pulsed neutron logs), and elemental analysis (derived from energy spectrum of gamma rays of capture). Our study of cores from the Frio Formation indicated no obvious geographic correlation. There was a clear trend toward higher boron content in Frio shales and shaley sands than in relatively clean sands. Boron also occurs in significant amounts in mud constituents such as bentonite, barite, and lignosulfonate. Boron also occurs in Gulf Coast Frio formation waters, but this does not directly affect thermal neutron logs or pulsed neutron logs run soon after drilling, because these logs respond mainly to conditions close to the well, within the invaded zone, where little or no formation water is present. It may be possible to correlate boron in the formation water with boron in the rock, but a lot more data would be needed to establish such a correlation. A reliable epithermal neutron log would go a long way toward solving the boron problem, but epithermal logs to date have been too sensitive to borehole environment. An approach suggested recently by D. V. Ellis et al. offers some hope that the problem could be solved by simultaneous use of data from the compensated neutron and pulsed neutron logs, which are affected differently by the boron content of the formation. Again, more data are needed to validate this approach.

This paper presents a method for calculating hydrocarbon density from logs. Empirical relations between bulk volume hydrocarbon and the volume of hydrocarbon seen by the density and neutron logs are derived. An approximate relation is developed for excavation effect on the neutron log. Approximating polynomials for the relationship between hydrocarbon density and the log response of the hydrocarbon seen by the density and neutron logs are derived. A calculator program is presented which computes hydrocarbon density, porosity, and water saturation using these relations. If the hydrocarbon is a mixture of oil and gas, a means of estimating oil and gas saturations is provided by the program. Finally, an example is given to illustrate the use of the program.

Sixty-seven density and neutron logs were run on shallow wells in a 360 acre area in the San Joaquin Valley of California. The heavy crude reservoir is a dirty sandstone and the shales within the unit are extremely silty. It was extremely important that the "porosity" logs were properly calibrated in order to accurately determine the magnitude of the reserves. A frequency histogram of the density and of the neutron porosity on each well was compared with the field total histograms. This comparison along with densityneutron cross-plots and maps of average porosity were used to calibrate the log response for each well. It was determined that in 39 wells (58.2%) a calibration correction was required in one or both of the "porosity" logs.

Oil and gas reservoirs in the Norwegian sector of the North Sea occur in rocks of Paleocene, Cretaceous, and Jurassic age. The reservoir rocks include shaly and mineralogically complex sandstones, chalks, and mixed carbonates. The formation fluids include gas, condensate, oil, and water of various salinities. For these diverse conditions modern logging programs and conventional log analysis techniques usually provide recognition of the hydrocarbon-bearing intervals. In many reservoirs additional data from cores and wireline or other production testing techniques are needed to develop acceptable logging parameters for accurate quantitative evaluations. Also, some fundamental problems of log interpretation have occurred, requiring special techniques to resolve them. Logging programs and log analysis techniques are different for exploratory wells and for development wells of the Norwegian North Sea. Exploratory wells require primarily a rapid and reliable detection of the hydrocarbon intervals and fluid contacts. The log analyses may be done at the well site by simplified techniques such as the comparison of compatibly-scaled logging curves by overlay methods and the use of overlay grids and crossplots for estimating porosity and water saturation, and by conventional calculations of porosity and water saturation using log interpretation charts and hand calculators. Complex log calculations are sometimes made for exploratory wells from log data digitized at the well site and transmitted to computing centers onshore. For development wells, the log analyses must provide accurate and detailed results for net thickness, porosity, and water saturation and accurate location not only of fluid contacts but also the saturation and extent of transition zones for the hydrocarbon reservoirs. Speed is not always essential, so great care can be taken to obtain the best possible results. The log calculations are usually obtained by computer processing of digitized log data. Preliminary log calculations and cross: plots of log readings from different devices and of log versus core data can be used to develop the log interpretation parameters. Often, the data from two or more wells can be combined for developing the logging parameters. Conventional log analysis techniques, with modifications as needed for special problems, are often used by personnel of the operating companies. Both manual and computer calculation procedures are used, and the calculated results are usually interpreted by considering other formation evaluation data from mud logging, drill cuttings, cores, and production testing. More complex log analysis techniques, usually designed to generate all logging parameters from log data alone, are available through well logging service companies and consulting firms. The log-calculated results are often interpreted without the benefit of formation evaluation data from sources other than wireline logging. In the following discussion, conventional log analysis techniques are reviewed briefly. Then, logging programs and special logging techniques for unusual log analysis problems are discussed separately for the Paleocene sandstones, Danian limestone and Jurassic sandstones. Finally, a short discussion is presented on some complex log analysis techniques available to the industry on a service-charge basis.

Logging of large diameter boreholes for formation evaluation and as an aid to construction has become a routine procedure at the United States Energy Research and Development Administration (ERDA), formerly Atomic Energy Commission (AEC), Nevada Test Site (NTS). Some of the same services are being used for commercial mine shaft construction. Log interpretation has been altered to account for conditions of 48 inch to 120 inch diameter boreholes, generally above the water table, and penetrating volcanic rocks. The methods are described and log examples and photographs of the borehole instruments are shown. This work was performed for the Nevada Operations Office under Contract AT(26-1)-618.

Lithology and porosity can be determined quantitatively in mixed lithology formations which support acoustic "shear" wave arrivals. Recent borehole measurements agree with early laboratory results1 and show the relationship between compressional and "shear" wave transit times to be primarily a function of lithology. The compressional wave to shear wave transit time ratio is essentially a constant for each rock type. Given the ability to predict percentage lithology from the transit time ratio, the compressional wave transit time may be lithology corrected for accurate porosity estimates.

A unique two-detector Neutron tool has been developed and introduced commercially. More than 600 open-hole logs and 150 cased-hole logs have since been recorded in the United States. This new tool records a Compensated Neutron Log (CNL*) for which the effects of hole conditions (mud cake and rough borehole walls) have been minimized and are generally small. Since the CNL may be run in combination with other logging tools, rig time is saved and wellsite interpretation becomes easier. Recorded with the Density log, the CNL delineates gasbearing intervals in sand-shale sequences. A simple crossplot method delineates shaly gas zones on which gas effect is less apparent. The new Neutron log investigates more deeply into the formation than the Density. This complicates the computation of porosity in gas-bearing zones, but makes gas zones easier to find. For instance, in most low-porosity gas zones, the porosity is given directly by the Density whereas the CNL shows an appreciable gas effect. The CNL has a greater shale effect than the epithermal neutron tools such as the SNP.* Studies, made in numerous wells where both types of Neutron logs were recorded, show that the CNL can be used in determination of both bulk volume fraction of shale and porosity. A study of CNL responses in different lithologies has been made. The results show greater matrix effect on the CNL than on other Neutron tools. Porosity derived from the NLDensity is potentially more accurate than that derived

After extensive field testing in its experimental stage, a Sidewall Acoustic Neutron logging instrument has been placed in limited field operation. The instrument in its present configuration yields a simultaneous recording of an acoustic interval transit time (At) curve, a neutron porosity curve, a caliper curve, and an optional gamma ray curve. The neutron curve is obtained from a conventional epithermal sidewall neutron pad mounted on a mechanical carrier equipped with a hydraulic retractor. Mounted on the same carrier but contacting the opposite side of the borehole is the sidewall acoustic section. The entire acoustic section, consisting of a transmitting transducer and two receiving transducers, is contained in a pad approximately two feet in length. The receiver spacing is 6.0 inches, producing an acoustic interval transit time curve with much sharper interface resolution than that obtained from a conventional acoustic logging device. The quality of information obtained from the sidewall acoustic has been found to be as good as or better than that obtained from conventional acoustic logging instruments. The simultaneous acoustic derived porosity and neutron derived porosity can be used for "quick look" detection of gas bearing strata. The resolution of the sidewall acoustic curve may be used for thin bed reservoir analysis. Due to the unique construction of the acoustic section, wave forms are obtained which have less borehole effect than those obtained with conventional acoustic sections and thus lend themselves to studies of the shear and compressional waves. Examples are shown of logs made under various conditions, and results are discussed. Information which may be derived from wave-form studies is also discussed.

Log interpretation of gas sands requires special consideration of gas effects on the responses of porosity logs in general use today-sonic, density, and neutron. Gas saturation near the wellbore in all types of sand invariably causes an increase in density log porosity and and a decrease in neutron log porosity. For poorly compacted, shallow sands and for some abnormal-pressure sands, gas saturations near the wellbore cause an increase in sonic log porosity. Shaliness is an additional factor important to the log interpretation of gas sands because sonic and neutron porosities reflect the water content of shale. As a result, three factors may affect porosity logs of gas sands-actual or "effective" porosity, gas saturation, and shaliness. Because of the gas and shaliness effects, more than one porosity log is required to detect and evaluate gas sands. Tixier et a1 have applied all three porosity logs (sonic, density, and neutron) to detect and evaluate gas sands." Effective porosities are obtained by simultaneous solution to response equations for these logs using a highspeed computer. Included also is a resistivity log (usually induction-electrical) to provide for geological correlation and interpretation of fluid saturation. Special studies of Miocene gas sands of a U. S. Gulf Coast field showed that a combination of only two porosity logs-density and neutron-plus an induction-electrical log has all the advantages of the more complex approach using four logs. These studies were applied to sands within a 3000-ft. interval of one well. The following Schlumberger logs of this interval were studied: 6FF40 induction-electrical log BHC sonic log with SP and caliper curves FDC density log with gamma ray and caliper curves SNP neutron log with caliper curve The logged interval included fine-grained, silty to slightly shaly, relatively thick blanket sands with known gasproduction capabilities. Conventional core data (porosity, permeability, and irreducible water saturation) were avail-

Californium-252 is an isotopic neutron source that has only recently become available for experimental well logging. One curie of 2 5 2 ~ f emits 4.4 × 109 neutrons per second by spontaneous fission, 300 times the emission rate of any other one-curie radioisotopic source. Californium-252 has several other advantages as a high yield source for well logging: very small physical size, low gamma and heat emission, and expected low cost relative to other sources. A 50-millicurie 252Cf source fabricated at the Savannah River Laboratory was made available by the U. S. Atomic Energy Commission to the U. S. Geological Survey for a feasibility study on well logging. The nuclear and physical characteristics of this source and some of the health physics aspects of its use in the field are discussed. The source was used to make epithermal neutron logs, which are compared with logs made with plutoniumberyllium and americium-beryllium sources in the same well. The high neutron flux available from 252Cf permitted the use of longer than usual spacing while maintaining a high count rate and excellent sensitivity. In addition, continuous activation logs were made utilizing a spacing of 5.5 feet from the source to detector. Aluminum-28 was identified as the chief radioisotope contributing to the log response. This new technique may provide a log more closely related to clay content than the natural gamma log. Stationary irradiation experiments were also carried out in boreholes, and sodium-24 and manganese-56 were readily produced and identified. Suggestions for additional research on logging applications and problems resulted from this feasibility study. Potential well logging applications not investigated include the activation of temporary depth markers and the use of stable tracers that can be activated at the site or in the well. The high neutron yield of Californium252 will facilitate in situ activation analysis for many elements as an aid to exploration for oil, water, and other minerals.

Gulf Coast pay sands are not always apparent on resistivity logs. High porosities, low formation water resistivities, and shaliness are common. The finer-grain and silty sands are characterized by high irreducible water saturations. The resistivities of clean water sands range from approximately 0.2 to 1.0 ohm-meter; shaliness increases this R, value. Silty pay sands may exhibit resistivities within this same range. Thus, recognizing pay zones with only a resistivity log is often difficulti f not impossible. The problem can be solved by a judicious combination of resistivity and porosity logs. Three types of porosity logs are now available: The BHC (sonic), FDC (density), and SNP (neutron). In addition, S P and Gamma Ray curves, and sidewall samples, are beneficial in the study of shaly sands. When all three porosity logs are available, determinations of, effective porosity and shale content, and distinction between gas and oil, are possible. Resistivity values can then provide a reasonable estimation of water saturation. When no gas is present, two porosity logs, i.e. density with either sonic or neutron, permit an estimation of effective porosity and shaliness. To a great extent, the neutron and sonic give nearly the same values in the absence of gas, and any one of the two can be combined with the density. The neutron has the advantage over the sonic at shallow depths, but unaccounted gas will make the neutron-derived porosity pessimistic. When only one porosity log is available, a minimum of information will be obtained. An R, log incorporating sonic and resistivity data has proved to be very effective for finding pay zones, whether gas or oil bearing, but good values of porosity cannot be obtained in shaly sands. The neutron is an effective substitute for the sonic except in gas sands. The use of density permits good porosity values, but the combination resistivity density is pessimistic in shaly sands and may miss some shaly producing intervals. Because the equations used in quantitative evaluation of shaly sands are complex, the computations are best handled by computer. Special shaly sand programs are used. The computed results can be presented both in tabulated listings and in analog form. The analog presentation provides, at a glance, variations in formation and fluid characteristics important in formation evaluation.

Logging research in the U.S.S.R. is carried out in a number of institutes, including (1) in Moscow the All Union Research Institute for Nuclear Geophysics and Geochemistry, the I.M. Gubkin Institute for the Petrochemical and Gas Industry, and the All-Union Scientific Research Institute of Geophysics (VNII) and (2) in various Branches of these institutes outside of Moscow, as for Example the Volga-Ural Branch of VNII. International conferences on well logging were held in Poland in 1962 and in 1965. A U. S. exchange delegation visit to the Soviet Union in 1965 included many aspects of well logging. From these meetings and the published Russian language literature on well logging, the author has compiled a review of recent developments in well logging in the U.S.S.R. The principal topics covered in this review are acoustical and nuclear logging methods and log interpretation. Acoustic logs are used in open and cased holes in much the same way as in the U. S. acoustic velocity logs for porosity; acoustic amplitude logs for studying the elastic properties of rocks, for fracture location and for indicating cement bonding of casing. A variety of radioactivity logs are in widespread use. The gamma-gamma log has been developed primarily for evaluation of cement behind pipe and in minerals prospecting; it is used to a lesser extent for formation density determination. Pulsed neutron logs are in widespread use and are run in both open and cased holes. Log interpretation work includes studies of high salinity formation waters and high alinity muds, identification of fractured reservoirs, and the use of computers for log analysis. Computers are used in gamma-ray log interpretation; to provide a lithologic log from the combination of several logs; and by means of "self-learning" programs to make statistical analyses of suites of logs for providing various geological and geophysical parameters.

Dual Porosity Logging records reservoir information in a simplified form. It is the product of a combination tool that enables simultaneous recording of linear porosities from acoustic and epithermal-neutron devices, with gamma ray and caliper curves. It makes possible the comparison of both acoustic and epithermal-neutron response normalized to a common linear porosity scale. The anomalous departures between "Dual Porosity" curves become guides for identification of particular lithologies and types of porosities. The limitation of each porosity device, when used singularly, becomes an advantage when studied in conjunction with the companion porosity curve. This paper is based on field examples of dual porosity logging compared with core studies to show the validity and utility of this new technique. The "Dual Porosity Log" compares formation response to both acoustic and neutron devices for the determination of porosity and lithology.

In sections of the upper Gulf Coast region, there are extensive gasbearing sands ranging in depth from 1000 feet to 5000 feet. Routine methods readily identify these sands when the formation waters are salty. However, when the waters are fresh or brackish, as is often the case, routine methods are not enough. The purpose of this article is to present a good method of finding shallow gas using Neutron and Sonic Logs.