Log Calibration Research Articles

Acoustic measurements on cores as a tool for calibration and quantitative interpretation of sonic logs : D. P. Marion & F. M. Pellerin, SPE Formation Evaluation, 9(2), 1994, pp 100–104

Acoustic measurements on cores as a tool for calibration and quantitative interpretation of sonic logs : D. P. Marion & F. M. Pellerin, SPE Formation Evaluation, 9(2), 1994, pp 100–104

Acoustic Measurements on Cores as a Tool for Calibration and Quantitative Interpretation of Sonic Logs

Summary An upscaling technique is developed for acoustic measurements on cores to present core data as a log for improved comparison between laboratory and sonic measurements. The method combines a laboratory study performed over a wide range of frequency, pressure, and saturation conditions on a series of core samples and construction of a synthetic log that includes upscaling of core measurements at the sonic wavelength and simulation of the log response for various spacing of the tool receivers. A comparison of core and sonic measurements in a carbonate reservoir illustrates the benefit of comparing core and log measurements at the same scale. Agreement between punctual core and sonic measurements is within ± 10%, whereas core data upscaled at the level of sonic measurements are within ± 2% of the sonic. In addition, results of quantitative interpretation of the sonic data in terms of porosity also confirm that sonic measurements can provide accurate estimates of petrophysical properties when properly calibrated with core data.

Integration of Sedimentological, Paleontological, Petrophysical, Paleomagnetic, and 3-D Seismic Data to Construct a 3-D Geological Model of the C Sands Reservoir, Maui Field, Offshore New Zealand

Integration of core descriptions with paleontological data and a core-derived paleomagnetic reversal stratigraphy has been used to interpret a conceptual depositional model for the Eocene C sands reservoir of the Maui gas-condensate field. The C1 sands were deposited as highstand, regressive shoreline sands that are partitioned by thin, tight transgressive deposits. Lowstand valleys were incised into these sediments in the eastern part of the field. On the basis of the paleomagnetic reversal stratigraphy, the fill of these valleys is demonstrably of a different age to laterally equivalent shoreline sediments. Dipmeter data indicate that the early fill of these valleys predominantly comprises flood-tidal sediments, deposited as relative sea level began to rise. This model has been extended from the cored wells by careful calibration and correlation of wireline logs from all wells. Both the landward and seaward pinch-out of the shoreline sands have been traced from amplitude displays of the three-dimensional (3-D) seismic data. The lateral extent of the lowstand valley fills is also defined by the 3-D seismic data. The well and seismic data have been combined within a sequence stratigraphic framework to construct 3-D geological models of the field that are consistent with the dynamic performance of the reservoir more » and will be used to construct a 3-D reservoir simulation model. « less

Spatial 87Sr/ 86Sr variations in formation water and calcite from the Ekofisk chalk oil field: implications for reservoir connectivity and fluid composition

The 87Sr/ 86Sr ratios of formation waters from the chalk of the Ekofisk oilfield were measured using a new technique: extracting salt residues from the interstices of core samples. In the Danian Ekofisk Formation, the chalk matrix itself has 87Sr/ 86Sr and δ 13C values characteristic of Danian seawater, even though δ 18O data indicate the presence of varying quantities of diagenetic carbonate cement. In the Ekofisk Formation, porewater 87Sr/ 86Sr values are very constant (range: 0.70786-0.70810). The formation water probably contains locally derived Sr from carbonate, plus a small contribution from noncarbonate minerals with higher 87Sr/ 86Sr. In the underlying Tor Formation, chalk samples containing a mixture of primary chalk and later cement, have 87Sr/ 86Sr values that rise with depth to a maximum of 0.70865, much higher than contemporaneous seawater. Porewater 87Sr/ 86Sr values increase even more dramatically with depth to a maximum of 0.70988. Both cement and waters have been influenced by Sr with a high 87Sr/ 86Sr probably derived from underlying sedimentary rocks. Comparison with waters brought to the surface together with produced oil indicates that the depthwise increase in 87Sr/ 86Sr reflects an increase in salinity from ∼50,000to∼140,000 mg/l total solute content. This suggests that Zechstein evaporites may be the source of the radiogenic Sr in the Tor Formation. The difference in isotope composition between carbonates and waters in the Ekofisk and Tor Formations indicates that their hydrochemical systems have evolved differently during diagenesis. This is due to the barrier effect of the Ekofisk Tight Zone, a hardground at the base of the Ekofisk Formation which effectively splits the reservoir vertically into two. This study thus provides a new way of determining the positions of permeability barriers in oil reservoirs. Also demonstrated for the first time is the existence of vertical variations in salinity of water in oil. Such variations could have important implications for the calibration of resistivity logs and calculation of the amount of oil in place.

Porosity Calibration of Modern Porosity Logs and Old Neutron Logs

Porosity Calibration of Modern Porosity Logs and Old Neutron Logs

Secondary Recovery from a Stromatoporoid Buildup: Devonian Duperow Formation, Ridgelawn Field, Montana: ABSTRACT

Ridgelawn field is located in Richland County, Montana, in the western part of the Williston basin. It is a multiple-pay field, with production from ordovician, Devonian, and Mississippian carbonates. Discovered in 1980, the field was recently unitized in the Devonian Duperow Formation for purposes of secondary recovery by waterflood. In this part of the Williston, the Duperow consists of a repetitive succession of shoaling-upward carbonate cycles, each deposited under increasingly restricted conditions on a shallow marine shelf. Production at Ridgelaw occurs from dolomites within one of these cycles, cycle IIIa. Three separate, laterally continuous porosity zones (here termed a, b, and c, from lowest to highest) are recognized and mapped individually in the field. The reservoir has a lensoidal geometry; porous dolomite thins and grades laterally into tight carbonate. The Duperow pool at Ridgelawn is a solution gas drive reservoir. Computer log analysis of the Duperow pay interval indicates an average true porosity of 11.8% and an average initial water saturation of 17.7%. Net pay, defined as greater than 5% crossplot porosity, averages 16.6 ft across the field. Petrographic analysis and log calibration suggests that different facies in each of the three porosity zones were preferentially dolomitized to create reservoir-qualitymore » rock; each is now a sucrosic dolomite with intercrystalline porosity. Porosity can be occluded (most often in the upper two zones b and c) by both calcite and anhydrite cements. The lowermost zone, a, is related to a stromatoporoid/coralline bank, and has excellent but highly variable porosity and permeability. The two upper zones, b and c, are more finely crystalline dolomite and represent shallower water depositional facies. Maps for each zone, including porosity, porosity-feet, net pay, and water saturation were constructed and used for equity determination in the unit.« less

Structural and stratigraphic configuration of the late Miocene Stage IVC reservoirs in the St. Joseph field, offshore Sabah, NW Borneo

The St. Joseph field is a large, structurally and stratigraphically complex oil field situated offshore West Sabah. The paper describes the main geological characteristics of the field, highlighting the benefit of 3D seismic and core/well log data in developing a comprehensive three-dimensional geological model, which is being used to guide field development. Structurally the field is situated along a major Lower Pliocene wrench fault zone (Bunbury-St. Joseph ridge) and comprises three distinctive areas : (1) NW Flank is a structurally simple area, dipping uniformly (at ca. 20°) to the NW, which contains the majority of recoverable oil reserves (ca. 95%). (2) Crestal Area is a structurally complex zone characterised by intense faulting, steeply dipping beds and incomplete stratigraphic sequences. Minor oil reserves are present which were discovered by the first exploration well (SJ-1) in 1975. (3) SE Flank is an area of moderate structural complexity with negligible oil reserves. 3D seismic data has significantly increased the quality of the structural definition of the field as a result of: (1) ability to map individual intra-Stage IVC reservoir intervals, (2) better fault definition, particularly in the structuraly complex areas, and (3) identi­ fication of stratigraphic features, such as slump scars. Practical benefits include im­ proved definition of the boundary between the NW Flank and Crestal Area, better delineation of the NE extent of the field (by faulting/slump scars) and improved location of development wells and drilling jackets. Stratigraphically the main NW Flank reservoir (Upper Sand Unit) comprises a complex sequence of shallow marine sandstones and shales (late Miocene, Stage IVC), which display marked lateral variations in sand development, reservoir quality and shale layer thickness/continuity. Sedimentological studies of ca. 1600 ft of core (including 800 ft of continuous core from well SJ-7) and palaeontological data indicate deposition in a storm/wave-influenced shallow marine (neritic) environment. The main reservoir units comprise several stacked coarsening/fining upward sequences reflecting repeated progra­ dation and transgression of coastal/delta front sand bodies. This subdivides the Upper Sand Unit into thirteen distinctive sub-units, which can be correlated field-wide. Log calibration of the four main facies types and correlation of the genetic sequences have enabled construction of a three-dimensional reservoir geological model of the Upper Sand Unit. This thick (700 - 900 ft I 27 - 43 m ), heterogeneous sequence is effectively a single, connected reservoir but with predictable lateral variations in sand and shale distribution:

Photochemical fluorimetric analysis of phenylbutazone and its degradation products

A room temperature photochemical spectrofluorimetric (RTPF) method has been developed for the assay of phenylbutazone (PB), and its major degradation products. Fluorescence spectral properties of PB, its degradation products, and their photoproducts are reported, as well as the optimal irradiation times (ranging from 4 to 45 min), which correspond to maximum fluorescence signals of photoproducts. Linear log—log calibration plots were obtained over a 50- to 1000-fold range of concentration, and limits of detection ranged between 1 ng/ml and 1.2 μg/ml. This has been shown to be a convenient technique, in terms of simplicity, short measurement times, sensitivity, and precision.

Depositional Systematics and Exploitation of Morrow Valley-Fill Complexes in Cheyenne County, Colorado: ABSTRACT

The Lower Pennsylvanian Morrow Formation in Cheyenne County, Colorado, contains sandstone reservoirs interpreted to be valley-fill deposits. These reservoirs are exceptionally permeable (up to 20 darcys), thick (20-70 ft, 6-21 m), and highly productive (up to 650 bbl/day, 103 m/sup 3//day). Previous depositional interpretations of the Morrow in southeast Colorado have spanned the spectrum of fluvial and deltaic environments, with little predictive success. Detailed sedimentologic analysis of 65 cores reveals that the Morrow is composed of a series of multistory, northwest to southeast-trending valley-fill complexes incised into marine limestone and shale. Valleys are associated with at least six distinct regionally extensive unconformity horizons indicated by weathering profiles, roots, coal, and calcrete deposits. The valleys are incised up to 80 ft (25 m) and are filled by coarse, trough cross-stratified sandstone grading upward and laterally into rippled fine sandstone or rooted flood-plain mudstone. The sequences are capped by transgressive burrowed sandstone which grades into marine mudstone. These lithofacies have unique well-log characteristics, allowing calibration of logs for correlation. Each sandstone body within a valley complex is the amalgamated deposit of laterally migrating channels. Channel stacking is documented by multiple scour lags, repeated fining-upward grain-size profiles, vertical changes in sedimentary structures, andmore » changes in transport vectors. These stacked channel deposits are well-interconnected reservoirs and are continuous along valley axes. Use of the valley-fill concept in conjunction with core-derived log calibration and transport vectors can improve predictability and substantially reduce drilling risk.« less

Porosity Calibration of Neutron Logs, SACROC Unit (includes associated papers 15799 and 15846 )

Summary A major field study has been initiated at the Scurry AreaCanyon Reef Operators Committee (SACROC) unit to provide a detailedgeologic description of the reservoir. To supply porosity information to the digital database, a new technique has been developed thattransforms neutron deflection logs into accurate neutron porosity logs. The technique uses computer programs to analyze statistically the neutronlog to be rescaled as well as nearby modern porosity logs. An equationis derived that transforms neutron log deflection into porosity. Oncethe porosity response of a particular tool is determined in areas ofgood porosity control, that response can be applied successfully inareas of little or no control. Introduction The SACROC unit reservoir description project (RDP)is a major field study and has as its overall goal thecreation of a digital geologic database of wide scope andrelevance to the operation's current and foreseen needs. The purpose of the RDP is to provide support to theengineering staff at this major CO2 and waterflood siteand to assist the geologists in infill wellsite selection. To accomplish these goals, an immense amount of data mustbe incorporated into the system. This paper describes a major part of the system: thegeophysical well log processing. All gamma ray andneutron logs have been digitized, but only the neutronlogs are undergoing transformation. They are being rescaledin terms of calibrated limestone porosity to provide adense three-dimensional control on the porositydistribution. The SACROC unit covers about 50,000 acres [200 x 10(6)m2], the major part of the giant Kelly-Snyder field(Fig. 1). The reservoir is in the Canyon limestone, a late Pennsylvanian stratigraphic reef or carbonatebuildup(Fig. 2). Although composed for the most partof clean limestone, it also contains shale, shaleylimestone, dolomite, and chert. Having produced over 10(9) bbl [160 x 10(6) m3] of oil, the unit has been converted to tertiary recovery methods(CO2 injection). If reliable porosity logs could beobtained for each of the 1,600 + wells in the unit, fluidmovement would be much more predictable, and floodefficiency could be improved. Throughout the unit is a total of 400+ modern neutronporosity logs(mostly sidewall neutron porosity [SNP]). The remaining 1,200 + wells have been logged with theold-style neutron deflection logs. Core control is limitedto 124 wells, 95 of which have a cored interval greaterthan 30 ft [10 m]. Preliminary tests showed that the old neutron logs couldbe transformed to accurate limestone porosity if sufficientmodern porosity-log control existed in nearby offset wells. Because virtually all of the porosity control exists in thenorthern one-third of the unit, accurate porosity transforms could be developed in that area, but the remainingtwo-thirds of the wells would have to be treateddifferently. We decided to develop carefully the most accuratetransforms possible for well logs in the area of good controland then to apply the appropriate transforms, accordingto the particular neutron tool used, throughout the restof the field. A survey of the log files indicated that 11 differentlogging companies have used at least 70 different neutronlogging tools at SACROC since the field's discovery in1948. Each tool was given an alphanumeric tool code sothat as transform equations were developed and storedalong with other well data, they then could easily berelated to the particular service company and toolconfiguration used. This would eventually permit anygroup of transforms to be called up and compared to others. With nearly 1,000 logs transformed to date, this automaticcross-referencing system has been an obvious advantage. Also, as logs are processed in those areas that lack control, the tool code permits the proper, previouslygenerated transform to be selected and used automatically. Digitization As each well is digitized, additional information to be usedin various phases of the project is recorded with a keypunch. Fig. 3 is a printout of these data for a typical well. After a log is digitized, a playback plot of the log asdigitized is overlaid on the original log for quality-controlpurposes. After the playback plot passes quality control, a second plot is requested with all of the backups shiftedto their correct position and any neutron-zero baseline shifts removed. The result is a continuous trace of neutrondeflection in inches from neutron zero. At this point, the log is ready for transform development. Porosity Transform Development The relationship between neutron log deflection andporosity was presented by Brown and Bowers. JPT P. 468^

Regional Variations of Porosity and Cement: St. Peter and Mount Simon Sandstones in Illinois Basin

Petrographic study of over 230 thin sections of cuttings, cores, and outcrop samples from the Ordovician St. Peter and Cambrian Mount Simon Sandstones of the craton-center Illinois basin yielded a series of maps that show a fairly regular distribution of primary and secondary porosity and cements. Both primary and secondary porosity are present in the St. Peter Sandstone. Primary porosity is dominant from outcrop to 4,000 ft (1,219 m), whereas secondary porosity is dominant at depths greater than 4,000 ft. The overall porosity decline with burial is best described by an exponential equation: ^phgr = 30.8 exp (-0.00032 d), where ^phgr equals porosity and d is depth in feet. However, two linear equations provide the best fit of primary (above 4,000 ft, 1,219 m) and secondary porosity (below 4,000 ft). The chronology of porosity development appears to be closely related to the models proposed by P. J. C. Nagtegaal and includes early cementation, leaching of cement and framework, stabilized framework, and framework collapse. Cements in the St. Peter are depth-dependent. They define distinct regions, are mappable, and include calcite, dolomite, silica overgrowths, chert, and chalcedony. Virtually everywhere in the Mount Simon Sandstone, porosity is secondary and results from dissolution of authigenic and replacement cements, framework grains, and fractures. In the subsurface, porosity reaches a maximum of 18% at 5,000 ft (1,524 m) and drops rather sharply to 8% below 5,000 ft. As with the St. Peter Sandstone, the overall decline is best described by an exponential equation of the form: ^phgr = 31.08 exp (-0.00026 d). Cements in the Mount Simon include quartz and feldspar overgrowths, hematite and kaolinite, carbonate, chlorite, and microquartz (chert). Basinwide maps show that they have a fairly regular distribution pattern in the basin. We suggest that in the future, basinwide maps of cement and porosity types will be abundant. Such maps should enhance understanding of diagenesis, especially where map pattern can be related to fluid and burial history. Maps of cements and porosity types can also aid the calibration of wireline logs, help find diagenetic traps, and help the design of drilling fluid systems and well-completion procedures. The routine use of cuttings rather than cores will hasten the advent of such maps.

Determination of cobalt by pyrogallol chemiluminescence

The chemiluminescence reaction involving pyrogallol in alkaline hydrogen peroxide solution is described. With reaction conditions selected for the determination of Co(II), the detection limit for Co(II) was at least two orders of magnitude below the detection limit of all other species tested. The results suggest that Ca, Mg, Mn, and Fe are the most likely interferents for environmental and biological samples. The log—log calibration graph for Co(II) is linear from a detection limit of 0.5 ng/ml up to 10 μg/ml.

Determination of hydrogen peroxide in a flow system with microperoxidase as catalyst for the luminol chemiluminescence reaction

Hydrogen peroxide is determined by a chemiluminescence method with a reagent containing 100 μM luminol and 3 μM microperoxidase at pH 10 (carbonate buffer). Microperoxidase is superior to hematin as a catalyst. The method uses an automated flow injection system with a throughput of 2 samples per minute. The log—log calibration plot is linear (slope 1.3) from the detection limit, 3 × 10 -9 M up to 10 -5 M H 2O 2. The background emission is low. Impurities in the carrier stream and from some plastics may cause elevated background unless precautions are taken.

Openhole Crossplot Concepts A Powerful Technique in Well Log Analysis

Summary A multitude of crossplot concepts for well logging parameters is available to the formation evaluation specialist. Raw and/or calculated logging parameters can be crossplotted on linear, semilogarithmic, and other exponential scales. Input data include resistivity, acoustic, and nuclear logging measurements and, if available, core, test, and production data. Applications of specific crossplot concepts allow recognition of log calibration problems, normalization of basic log measurements, and determination of lithologic reservoir characteristics in clastic and complex mineralogy. Additional information includes primary and total porosity, formation-water salinity, distinction between oil and gas, estimation of reservoir grain sizes, prediction of irreducible water saturation and, hence, anticipated water cut, heavy mineral analysis, and other data. This discussion focuses on advantages and limitations experienced with numerous crossplot techniques. Frequent reference is made to formation evaluation problems encountered in the North Sea area. Introduction Crossplotting of porosity logging data has been used since the early 1960's.1–4 Today, an extremely large variety of two- and three-dimensional crossplots (z plots) is available. While computer processing allows quick and easy data handling, hand plotting still provides an effective check for the experienced log analyst. Monomineral Porosity Model Assuming one studies a reservoir rock of known lithology which is clean and/or shale corrected, then each porosity can be expressed. as follows5,6:b, fN= function of [rock matrix, porosity, amount and type of fluid in pore space (mud filtrate, oil, gas)] .t=function of (rock matrix, porosity, amount and type of fluid in pore space, compaction, secondary porosity). Mental recollection of these and other basic log response functions greatly assists in understanding, defining, applying, and evaluating the potential of the numerous crossplotting techniques. Binary (Two-Mineral) Porosity Model Complex Reservoir Rocks Two basic mineral constituents are present. The mathematical solution requires, in addition to the unity equation, only two of the three basic porosity response functions listed as follows. Ambiguity exists in certain crossplots due to nonlinear dolomite response of neutron-type logs.b, fN=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale).t=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale, secondary porosity).1.0=porosity+S(bulk volume matrix components). Complex Reservoir Rocks Two basic mineral constituents are present. The mathematical solution requires, in addition to the unity equation, only two of the three basic porosity response functions listed as follows. Ambiguity exists in certain crossplots due to nonlinear dolomite response of neutron-type logs.b, fN=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale).t=function of (type and bulk volume percentage of rock matrix components, porosity, fluid in pore space, shale, secondary porosity).1.0=porosity+S(bulk volume matrix components).

Geology-Petrophysics of Levelland 12-Acre Tertiary Pilot, West Texas: ABSTRACT

The Levelland 12-acre (4.8 ha.) pilot was drilled during the latter part of 1972 on a double five-spot pattern, and underwent waterflooding from March 1973 until August 1979. On August 3, 1979, carbon dioxide flooding was initiated. The purpose of this study was to define a stratigraphic zonation for the San Andres Formation (Permian) in and around the pilot, to obtain an accurate, quantitative, reservoir description to aid engineers achieve a historical match of the waterflood, and to evaluate the tertiary recovery efficiency of carbon dioxide flooding. Excellent core control demonstrated that sedimentary deposition had occurred in an arid coastal carbonate-evaporite province similar to that which exists today on the Trucial Coast, Persian Gulf. Stratigraphic zonation was made using core data in conjunction with log data. A quantitative reservoir description was achieved by the following steps: (1) porosity calibration of the gamma ray-neutron logs; (2) permeability calculation from the calibrated porosity using available core data; (3) determination of the permeability cutoff for net pay; (4) determination of connate-water saturation from applicable native state cores; (5) determination of the reservoir zonation, and (6) computer generation of ^phgrH, KH, and H maps for individual reservoir zones and combinations of reservoir zones. /P> End_of_Article - Last_Page 938------------

Determination of traces of chloride in solution by microwave-induced plasma emission spectrometry using chlorine generation

The method described for the determination of traces of chloride in solution by microwave-induced argon plasma emission spectrometry is based on the evolution of chlorine from a potassium permanganate solution in sulphuric acid and measurement of the chlorine molecular emission at 257 nm. The detection limit is 10 ng of chloride and the log—log calibration plots are linear over ranges of 0.025–100 μg of chloride, depending on the orientation of the plasma. Potential interfering elements are shown not to present any major problems.

Application of the electrochemiluminescence of luminol to the determination of copper

The electrochemiluminescence of aqueous alkaline luminol solutions has been studied by using rapid alternate positive and negative electrical pulses. Traces of copper are shown to change the proportion of the light emitted during the positive and negative pulses, measurement of which gives a linear log—log calibration plot for 1 × 10 -7 to 6 × 10 -6 M copper in glycine buffer at pH 10.15. Out of ten common cations tested only Hg(II), Pb(II) and Mn(II) interfered.

Principles of Log Calibration and Their Application to Log Accuracy

Abstract Accurate measurement and recording of log data is necessary for effective well evaluation. Calibration data auxiliary to the log, when reviewed with a knowledge of the calibration procedure, will demonstrate the degree of log accuracy. The general method for calibrating logging systems is described. Variations of the general method are described and examples cited. Use of calibration data to confirm log accuracy is discussed and a field example of profitable accuracy control is given. The Appendix presents the necessary detail information that is needed to use calibration data for accuracy control of Induction-Electric/ gamma ray logs. Introduction Improved logging techniques have made use of log data for formation evaluation possible on a broad basis. Because of these advances, the accuracy of log measurements has become of prime importance.inaccurate well logs lead to a confused evaluation picture and the advantages of the new techniques are lost. Fig. I represents a case history which illustrates this point. R., analysis with the first run Induction log and Sonic log indicated high water saturation (Table 1). The Induction log had not been calibrated recently and no calibration records were available. On test the well flowed clean oil at 500 B/D. The remainder of the zone was drilled and logs recorded with complete and accurate sets of calibration records. Differences in shale conductivities tended to confirm first run Induction log miscalibration rather than invasion perturbations such as an annulus effect. R, analysis with the second run Induction log gave the expected saturation values. In this case a productive zone was not passed by because of a faulty log, but an expensive drill-stem test was necessary to confirm the presence of oil. If log accuracy can be demonstrated by the calibration records which usually accompany the log then calibration data may be a valuable tool to the oil company in maintaining accuracy control. This paper discusses the theory and methods of log calibration and their application to log accuracy. Hopefully, this information will help oil company personnel to understand and make profitable use of calibration data. Calibration Theory and Methods Fig. 2 describes a basic logging system. To calibrate such a system, it is necessary to relate the magnitude of a voltage (recorder signal) to the magnitude of the measured parameter; i.e., resistivity, conductivity, acoustic travel time, etc. The general method used by logging service companies consists of standardizing the recorder's voltage response when the sensing component (sonde) is in one or more environments for which the magnitude of the desired log parameter is accurately known. Such environments are termed "standard media". As an illustration, suppose a hypothetical resistivity system is to be calibrated over a moderate range of resistivities (Fig. 3). One might proceed by first placing the sonde in a standard medium of low resistivity such as salt water, the magnitude of which is accurately calculated from the salinity and temperature. The recording device is adjusted to read this resistivity. The sonde is next placed in a standard medium of high resistivity such as fresh water and the recorder is again adjusted to read the known high value. By alternating between the standard media and making appropriate adjustments, the sensitivity and scale of the recorder is set. JPT P. 817ˆ

PROCEDURES FOR THE DIRECT EMPLOYMENT OF NEUTRON LOG DATA IN ELECTRIC LOG INTERPRETATION

It is well‐known that under suitable borehole conditions there is a relationship between porosity and the deflection of a neutron log curve. This relationship finds practical use, calibration of the neutron deflections being made by reference to porosities measured on cores. It is shown that theoretically core analysis is not mandatory for neutron log calibration and that the calibration may be achieved by direct plotting of suitable neutron and electric log data. For this purpose the concept of the neutron deflection corresponding to a formation factor of unity is introduced. It is shown that this concept makes it possible to use a combination of electrical and radioactivity log data to locate zones of oil saturation in carbonate rock formations and even to estimate the oil saturation. Application of the method to sandstones is also considered. In oil‐base mud logging, neutron‐log data may be conveniently combined with resistivities read off induction logs to give information bearing on the location of saturated zones and the estimation of connate water salinities. Finally a combination of electrical and radioactivity log data is theoretically capable of contributing to the quantitative elucidation of the “dirty sand” problem.