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The data The wireline log is a graphic representation of continuous measurements that are normally stored in column format. As the depth wheel on the winch moves by one log datum, usually 1 centimetre, a measurement is recorded. Such an intensive sampling regime does not mean that the measurement has a resolution of 1 centimetre. A measurement at the tool's resolution, usually dependent on energy source to detector spacing, is recorded at each depth datum. This is a bit like a moving average filter, where the filter length is the source‐detector spacing. Sampling density must not compromise log resolution, so, given that file size is no longer an issue for computer storage and processing speed, 1 centimetre (or an equivalent in feet) is a practical standard.
Column format data
The three major benefits from a wireline log accrue even if the measurement is not calibrated (it is qualitative), although long term precision relies on some form of normalisation where at least the wiggly line appears somewhere on the page within a similar unit range. The human eye and the depth reference do the rest.
Depth accuracy Clearly, depth accuracy is very important, but it is often taken for granted. Both logger and geologist should recognise the need for depth measurement quality assurance during any logging programme. The provision of a test well is a big advantage in this process but, otherwise, a long tape measure will do the job if employed vertically while a sonde is lowered into a borehole. Depth errors are not always systematic. They can result from the logger misaligning his sonde depth datum with the geologist's surface datum, usually ground level, or failure to recheck that alignment on return to surface.
The depth of a log should be referenced to the post‐log check against the surface datum because the up‐log is run with the logging cable tight on the depth wheel.
Accuracy and precision One oilfield logging commentator stated that all mineral logs are qualitative in nature. While we have to commend the Oilies for their ability to cope with very challenging borehole conditions, we can say that, because mineral borehole conditions are relatively benign, we do record accurate, quantitative measurements much of the time.
As well as the three basic benefits of wireline logging for minerals, we can add, at the very least, quantitative logs of coal density, iron ore density, uranium and potash grade based on natural gamma as well as excellent sonic logs for seismic calibration and very high resolution acoustic and optical images.
Porosity is the key measurement in an oilfield play. It must be close to perfect. That is not the case in mineral logging. In fact, we are not much bothered about absolute porosity. In fact, we are not over concerned about the absolute accuracy of many of the logs we produce...exceptions include orientation data and sonic velocity. That might be an overstatement (of course) but the key to extracting value from mineral logs is very often empirically based. Beyond depth, thickness and correlation, empiricism is the critical link between a wiggly line and valuable extra knowledge. The key is not absolute accuracy but precision. Most remote physical property measurements, which are estimations after all, cannot be perfectly accurate, but they must be precise, and can be made precise. Thereafter, conversion to site‐specific quantitative parameters such as coal ash percentage or uniaxial compressive strength or lab‐based bulk density should be straightforward.
3
The wireline log is, at least, vertically precise. It is one event. A drill‐core analysis comprises multiple events.
Continuous, in‐situ, precise measurement offers the geologist and the mining engineer an objective framework on which to add subjective or laboratory‐based geological and geotechnical analyses. Where laboratory data are included, the continuous objective logs may be converted to more relevant parameters via empirical cross‐plot and regression. The wireline log tests the precision of laboratory measurement.
The sonic log The compression wave sonic log is exceptional because it is both precise and very accurate. No calibration is required (assuming that one accepts the accuracy of its timer and the physical layout of the sonde's transducers).
A full waveform sonic log
The log is based on deduction of a short travel time (transmitter to receiver‐1) from a longer one (transmitter to receiver‐2, for instance). The deduction removes all the variables in the sonic ray path except for the measured interval.
This high level of accuracy and precision, as well as the log's resolution and continuity, and its proven relationship with rock strength and rock elasticity, make it an important geotechnical tool. It will only operate in fluid‐filled boreholes. It will log through PVC casing. It is insensitive to caving.
Sampling volume The geologist and geotechnician should bear in mind that, in sedimentary rocks, the measured volume of some wireline logs is in the order of 100 times the volume of his core stick. In igneous and metamorphosed (non‐porous/denser) rocks it will be half that but still a substantial volume compared to core, and it is in these rocks where most lateral variation in mineralogy occurs. Because the logger is measuring a different and greater
volume than the laboratory technician, there will occasionally be some disparity in measured ore grade and possibly rock strength between the two.
The density log Density is tough to get right. It is a log of the concentration of electrons around the sonde based on gamma ray counts at a fixed distance from a radioactive source. Denser rocks contain more electrons which slow down and ultimately cause the absorption of a gamma ray through multiple collision. Lower density results in more counts, high density, in fewer counts.
The problem is that the relationship is not linear, that formation chemistry (in terms of Z/A ratio) is not constant and that the measurement has a relatively small volume, making it sensitive to environmental factors such as caving. The caving issue becomes more problematic at higher densities. Iron ore is difficult to log accurately in non‐cored boreholes.
4
The difficulties should not be overstated because equipment characterisation and duality of measurement (long and short spaced detectors) afford some environmental compensation.
From left: Gamma ray, televiewer image, dual density and caliper
In coalfield exploration, the logger should expect to produce very accurate density logs, within +/‐2%, based on a good calibration system and a test well reference. Accuracy will be reduced in dry boreholes, large diameter boreholes and caving. Compensation for mud weight and mud cake on the borehole wall is available from some contractors. The logging operation should be set up to meet the challenges expected at a particular site.
Lithology logging Wireline logs offer lithological analyses based on the physical properties of the major rock types. In sedimentary deposits, coal and iron ore, representing the two ends of the density spectrum, are easy to identify. The interburden, usually comprising a clastic mixture of quartz and clays, biochemical sediments like limestone, or evaporite minerals, will require more complex analysis. The natural gamma log is sensitive to grain size...high gamma indicates a high clay fraction because potassium forms part of clay mineral structure and radioactive cations of uranium and thorium are trapped by negatively charged surfaces within clay lattices. Sought‐after uranium and potash deposits exhibit higher gamma signatures. Clean (clay‐free) sandstone will normally be described by low gamma counts. Minerals of organic origin emit low gamma counts.
Coal lithology log
5
The density of clean sandstone is a function of its porosity. The same applies to limestone, dolomite, halite and anhydrite among others. This porosity may be measured by the neutron log, which is sensitive to H and so water fraction, and by conversion of density and sonic logs. Although absolute porosity is of little interest, the relationships between the three porosity logs (they will agree only in clean formations) is a useful descriptor of lithology.
Lithology from three porosity logs
As with oilfield logging, any estimation of porosity requires accurate logs. If there is a separation in clean sandstone, the logger is advised to believe the sonic‐based porosity. This is a good test of equipment calibration and borehole compensation. The logs should overlay in clean formations.
Resolution While mineral physical property logs can certainly be precise and most often accurate, within an acceptable tolerance, they have one important limitation...measurement resolution. The highest resolution of any physical property log (gamma, density, sonic, resistivity, neutron, magnetic susceptibility etc) is about 15 centimetres. This is pretty good, given the nature of the measurements, but geologists would like better. The determination or ore zone thickness from gamma ray logs in uranium or potash exploration is an example. The definition of partings in coal seams and the measurement of coal quality from density logs through thin beds is another. The logs can describe thin seam thickness to within 1 centimetre using a televiewer image but grade is problematic.
If we cross‐plot a wireline log with laboratory data, in the example on the right it is P‐wave sonic versus UCS, there will usually be some scatter. A calibration or systematic error would cause a global shift of the data. Scatter is caused by a depth mismatch, lack of precision on the part of the laboratory, and/or the shoulder effect (lack of perfect log resolution).
Empirical analysis with scatter
So, scatter on a cross‐plot does not necessarily mean that the relationship is poor. It might just result from boundary effects or poor sampling discipline. It is important to depth match the data first then select zones for laboratory analysis.
The wireline log is blocked and block‐averaged using the same sample boundaries (this can minimise the shoulder effect). The analysis should improve markedly.
All this is not an issue if the sample population is large. It is usually necessary to combine data from multiple boreholes to achieve a large population for a particular coal seam. In that case the outliers cancel each other out and the regression curve is valid.
6
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7
The micro‐res tool retains the correlation benefits of its parent, the focussed electric sonde. Its sensitivity to changes in moisture content is illustrated by the general lithology log on the right (events above the coal seam).
Micro‐resistance describing resistive formations above a coal seam.
Such a high level of sensitivity to moisture content of the formation is of value to the explorer, if only as a correlation tool.
Structure logs The micro‐resistivity measurement gave rise to the resistivity dipmeter sonde, where three or four orientated pads allow the structure intersected by a borehole to be described in detail.
The dip and direction of bedding as well as fault locations with associated drag and rotation, can be of enormous value in some exploration projects. The log is best used to measure sub horizontal features like sedimentary bedding. It does not describe near vertical events (parallel to the borehole path) or fractures well.
If multiple sense electrodes are placed on each pad, data may be combined to create an orientated image of the borehole wall, or at least a partial image (limited by pad coverage). This allows both horizontal and sub vertical events to be described. This "micro‐scanner" log is routinely used in oilfield jobs where direct contact with the borehole wall is important.
In mineral exploration logging, where drill‐bits are generally of smaller diameter and mud is rarely required, the acoustic televiewer has provided very high resolution images of rocks intersected by a borehole. Logging and processing time are longer than they are for the dipmeter but the end result describes bedding, fractures and borehole wall anomalies, such as breakout, in great detail.
The sonde is based on measuring the travel time and amplitude of a high frequency reflected sonic pulse. Harder rocks reflect more energy so amplitudes are high, softer rocks and fractures absorb energy so reflected amplitudes are lower. The measurement occurs hundreds of times per centimetre of log and high resolution orientated images are generated. The log is normally presented with low amplitudes as dark events on an otherwise lighter image.
Fractures described by a travel time image (left) and an amplitude image (right). The time image resolves
an open fracture at 1375.5 metres.
A word of appreciation for the log analyst is warranted at this point. Structures seen on the acoustic images must normally be picked and classified manually.
8
Picking images can be labourious work, particularly if the end user requires a fracture frequency log. There might be thousands of events to process. One task is to differentiate between natural and drilling‐induced fractures. Sometimes, particularly in angled boreholes, this is a difficult thing to do with any confidence.
End products include tadpole plots, polar and rose diagrams, breakout analysis and fracture frequency per prescribed depth interval. Picked events are classified as bedding, minor fractures, veins etc.
The juxtaposition of physical property logs with the televiewer images is a powerful exploration tool.
In dry holes or in clear water, there is the alternative of logging the optical televiewer sonde. This system offers very high resolution orientated images of the borehole wall based on reflected light rather than sonic energy.
In most respects the optical image is similar to the acoustic one. The image will be familiar to the geologist as it is an orientated photograph, without perspective, of his rock mass. From the logger's point of view, the devil is in the detail...there is often too much of it. Picking and classifying fractures on optical images is usually more difficult than it is on acoustic images. Picking bedding is often somewhat easier.
Bedding picks and a truncated OTV image
The optical televiewer log is more of a geological tool than a geophysical one. It is a fantastic record of the rocks intersected by a borehole and, very often, the log analyst requires the assistance of a geologist to extract all the value from a dataset.
There is a fundamental strength of the standard wireline log, a formation density or natural gamma log for instance, that should not be overlooked...simplicity.
The three basic benefits from a precise wiggly line (depth, thickness and cross‐hole correlation) are easy to harvest because the presentation is so simple...a depth‐correct description of the formation in terms of one (relevant) physical property...one wiggly line. That one log curve might be all that is required to glean 95% of the required knowledge for which the borehole was drilled.
9
There is little point in capturing irrelevant data, or data that require complex analysis, if one simple curve will do the job.
An optical televiewer image from an angled hard‐rock exploration borehole
Datasets can be very complex. It is not unusual for eight or more tools to be lowered into a borehole. The quality assurance and analysis of multiple parameters is a complicated business.
22.. MMeeaassuurreemmeenntt FFooccuuss
A review of one wireline log measurement
The Photo‐Electric density sonde
The lithological analyses, described in section 1, are based on physical property logs. If we consider the electron density measurement, we can say that its log of sedimentary formations is dominated by porosity and this tends to mask variations in chemistry.
The density log has a dual response which includes both physical properties (water or air‐filled pore volume) and chemical properties (the presence larger atoms in the matrix for instance).
Chemistry has a bigger influence in non‐porous (hard) rocks, where iron content plays an important role.
The total natural gamma measurement can be considered as a log of chemistry only and the spectral gamma version differentiates the specific contributions of potassium, uranium and thorium.
If we look at a spectral gamma ray log, we can consider what might happen to its energy spectrum if we added a man‐made radioactive source to the bottom of the sonde.
The natural gamma spectrum, shown below (next page) left, comprises source gamma ray peaks at unique energies for K, U and Th and their daughters plus all the lower energy gamma rays that result from Compton scattering. The scattered gamma rays mask the original peaks and result in a general increase in counts to the low energy end of the spectrum.
The discrete peak at 0.66 MeV is a small man‐made Cs137 locking source fitted next to the sonde's crystal detector. This allows automatic correction for lateral drift caused mainly by temperature variation.
The various sources of gamma radiation pass right next to the sonde's sodium iodide detector so many gamma rays will retain their original characteristic energy at the moment they are measured. The important thing to note is that a logging tool is capable of measuring the gamma energy spectrum at quite high resolution. That capability leads to the spectral density logging system.
10
A natural gamma spectrum (left) with man‐made Cs137 locking source and a gamma‐gamma spectrum (right)
If a 100Ci Cs137 source is added to the sonde at a distance of 15cm from the detector, overall count rates will be very high compared to the natural events, which then play little part in the measurement.
The sodium iodide detector is now a relatively large distance from the main source of gamma rays so we would expect to record no counts with an energy of 0.66MeV because all gamma rays from this source that reached the crystal would have scattered many times and lost energy. Again, we see a gradual climb to the left, due to Compton scattering, with the maximum count rate at quite a low energy.
Beyond that peak, at very low energies, count rates fall away to zero. Some sonde designs incorporate aluminium or beryllium (low density or low Z) windows so that the maximum number of low energy gamma rays reach the detector.
Note that a tiny Cs137 locking source, positioned next to the sodium iodide crystal, is still required to ensure measurement stability. The energy of its gamma rays has not yet decayed and so is recorded as 0.66 MeV on the spectrum. It is small enough to fall outside the range of most Departments of Health regulations.
The standard gamma‐gamma electron density sonde counts all gamma rays captured within the crystal detector. Count rates are inversely proportional to formation density.
Hard and soft energy windows (modified from Dr. Paul Glover,
Laval University)
The spectral density sonde allows us to window the counted gamma rays into soft (below 0.2MeV) events and hard (higher energy) events. Above 0.2MeV the count rate at the crystal detector depends entirely on Compton scattering. In denser rocks, the curve moves to the left, to lower energies, due to multiple collisions within a denser cloud of electrons. So counts in the higher energy hard window will be reduced.
11
Below 0.2MeV, the count rate depends on both Compton scattering and photo‐absorption by atoms in the formation. At such low energies, gamma rays are available for absorption by any elemental atom but some elements have higher photo‐electric absorption capacities (cross‐sections) than others. Elements of higher Z (more protons), having more tightly bound electrons, tend to have higher absorption cross‐sections than those with lower Z.
So the count rate in the soft window is partly dependent on the average Z of the logged formation. The ratio S/H, soft to hard window, will be a function of the Z effect only.
In uranium logging, the measurement of natural gamma radiation requires correction for the same phenomenon. In that case, the uranium family of relatively huge atoms in the formation have a large absorption cross‐section and, at high U grade, the formation actually absorbs a proportion of its own gamma rays (from the various daughter isotopes) before they can reach the sonde.
The natural gamma log understates daughter counts, and therefore derived U grade. Correction, based on empirical trials in assayed rocks or jigs, is required.
The photo‐electric (PE) log is calibrated in barns per electron (b/e). The barn is a unit of area. It has roughly the same cross‐sectional area of a uranium nucleus.
Barns per electron represents the probability of a gamma ray interacting with an electron in a particular environment. Each element has a unique cross‐section per electron.
Higher values indicate a higher probability of absorption.
Barite (barium Z is 56), used in drilling mud and often cited in the literature, has a huge absorption cross section of 267 b/e but is not normally a factor in mineral exploration.
The Z to Pe factor is not a linear relationship.
12
The tables above list some important elements and minerals with their photo‐electric absorption factors. Note that water has a very small value, due to the preponderance of low Z hydrogen so water in pore spaces, hence the porosity variable, has very little effect on the Pe log.
What appear to be siderite layers (dotted curve) in a complex coal sequence from Botswana.
Siderite has a Pe absorption coefficient of 14.25.
Like gamma ray and magnetic susceptibility, the Pe log is a measure of formation chemistry, in this case the average Z of the formation. It is insensitive to borehole caving and fractures. It is used in detailed lithological analysis both quantitatively (see section 3 below) and qualitatively for lithology and cross‐hole correlation.
It is good practice to combine logs of chemistry, Pe and gamma ray, with neutron and density‐based porosity logs in order to differentiate low gamma formations such as limestone, dolomite and sandstone, for example.
3. Guest Article The Photo‐Electric Density Logs
Digital Log Analysis For Potash Mineral Exploration and Beyond
This article will present a model to solve for 6 minerals/fluids common in Potash mineral exploration utilizing a set of well log curves including photoelectric factor (PE), bulk density (Pb), neutron porosity (PHIN), compressional sonic (DTC) and gamma ray (GR) to solve each foot of interval logged to estimate volumetric mineral/fluid composition.
The Prairie Evaporite zone located in both Saskatchewan, Canada, and North Dakota, USA, will be used in the example. The use of photoelectric (PE) logs in mineral exploration is not common today. I think it should be an important part of the log suites run for mineral exploration, as shown by previous authors.
Theory The PE log reading increases with increasing effective atomic number (Zeff). PE = (Zeff/10.0)^3.6 with units of barns per electron. The Pb log reading increases with increasing bulk density.
It is important to understand that two sandstones with densities of 2.62 and 2.65 will both have the same PE reading since PE is independent of bulk density, and is a function of the effective atomic number. So the PE log is an important complement to the Pb (and derived porosity) log.
The neutron log has limitations when several minerals of interest have little or no hydrogen content. The neutron log is also affected by neutron absorbers like chlorine and boron. The sonic log has limitations when there are gases present. The GR log has limitations by not differentiating between potassium, thorium and uranium unless there is also a spectral gamma ray (SGR) log. Density logs have limitations in that caliper corrections for Pb logs will be far greater than caliper corrections for PE logs.
13
PE logs can also be made more valuable by creating mathematical relationships between the PE reading and Pb reading for selected minerals which provide additional measurements of lithology. Techniques such as M‐N Lithology can be enhanced to use PE data to replace PHIN data in their equations. Similarly, MID plots can be utilized to create additional equations for various minerals and fluids. I have also developed an alternative method that currently solves for 29 minerals/fluids with a great deal of accuracy.
Example The well log data in a 6 mineral/fluid model can be solved as a set of simultaneous linear equations. Since there are 5 log measurements, plus a unity equation, then we can solve for 6 unknowns (minerals/fluids). The unity equation simply states that the sum of the individual volumetric mineral/fluids fractions is equal to one. Since reservoir rocks are complex mixtures of minerals/fluids, a rigorous solution will require including far more than 6 minerals/fluids in the analysis. However, the purpose of this article is to present a simplified technique to estimate mineral/fluid composition for a six mineral/fluid model.
A key part of a successful analysis is to first identify the most common minerals/fluids present in the zone you are analyzing. In a typical potash zone the most common minerals/fluids are NaCl, KCl, Carnallite, Anhydrite, Clay and Water/Brine. These will be used in our example.
The next step in the analysis is to set up a grid of the log tracks being used, the minerals/fluids selected, and the log readings for each log track. Table 1 shows this for a one foot interval. Of course, the complete analysis would include all depths that you want to analyze. The first column shows the log tracks selected: Pb, PE, GR, PHIN and DTC. The second column shows the unit of measurement for these log tracks. The next six columns shows the individual mineral/fluid properties for the six mineral/fluids selected for each of the log tracks selected. The last column shows the actual log readings for each of the log tracks selected for the one foot interval being used in the example.
Table 1 ‐ Log Readings and Mineral Properties
Log Track Mineral/Fluid NaCl KCl Carnallite Anhydrite Illite Fluids Log
Reading
Mineral/Fluid Abbrev/units N K C A I F
Bulk Density (Pb) gm/cm3 2.03 1.86 1.60 2.97 2.52 1.19 1.96
Convert Pb to Electron Density (ED) where ED = (Pb + 0.1883) / 1.0704 since Volumetric Photoelectric factor (U) uses ED and not Pb
Electron Density (ED) 2.07 1.92 1.67 2.95 3.18 1.05 2.01
Photo Electric (PE) barn/electron 4.63 8.47 4.07 5.03 3.47 1.05 6.24
Calculate volumetric photoelectric factor (U) where U = PE * ED since PE is not volumetric
U = PE * ED barn/volume 9.58 16.26 6.80 15.00 8.73 1.27 12.53
Gamma Ray GR API 0.00 731.00 208.00 0.00 290.00 0.00 531.70
Neutron Porosity ‐2.00 ‐1.00 28.91 ‐0.50 35.00 92.60 ‐1.00
Sonic (DTC) usec/ft 69.10 74.00 156.00 94.40 76.40 163.81 75.18
Unity Equation volume 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Since the photoelectric log is not a volumetric measurement respect to the fractional mineral composition, the PE log data must be converted to the volumetric photoelectric factor (U) where U = PE x Electron Density. This is accomplished by first converting the Bulk Density (Pb) to Electron Density (ED) where ED = (Pb + 0.1883) / 1.0704. Then the volumetric photoelectric factor (U) can then be calculated by multiplying PE x ED. All of the other log readings are volumetric measurements respect to the fractional mineral/fluid composition.
The next step in the analysis is to set up the six equations for the one foot interval we are analyzing in the example. Table 2 is simply a rearranging of the data from Table 1 expressed as linear equations. The letters and corresponding mineral/fluids are N (NaCl), K (KCl), C (Carnallite), A (Anhydrite), I (Illite) and F (Fluid of Water/Brine). The constants on the left hand side of the equations are the properties of the individual minerals/fluids. Note that the constants for U are simply PE x ED for each mineral/fluid. The right hand side values are usually the measured log readings. The only exception is that the 12.54 for U is calculated by multiplying 2.01 time 6.24 (ED * PE).
14
Table 2 ‐ Solving Simultaneous Linear Equations
Log Track Simultaneous equations to solve for volumetric mineral fractions
Bulk Density (Pb) (2.03 * N) + (1.86 * K) + (1.60 * C) + (2.97 *A) + (2.52 * I) + (1.19 * F) = 1.96
Electron Density (ED) ED = (Pb + 0.1883) / 1.0704 will be needed to calculate U = PE * ED
Photo Electric (PE) PE is not volumetric, need to use volumetric photoelectric factor U = PE * ED
U = PE * ED (9.58 * N) + (16.26 * K) + (6.80 * C) + (15.00 * A) + (8.73 * I) + (1.27 * F) = 12.53
Gamma Ray (0 * N) + (731 * K) + (208 * C) + (0 * A) + (290 *I) + (0 * F) = 531.7
Neutron Porosity (‐2.0 * N) + (‐1.0 * K) + (28.91 * C) + (‐0.50 * A) + (35.0 * I) + (92.6 * F) = ‐1.0
Sonic (DTC) (69.1 * N) + (74.0 * K) + (156.0 * C) + (94.4 * A) + (76.4 * I) + (163.81 * F) = 75.18
Unity Equation (1 * N) + (1 * K) + (1 * C) + (1 * A) + (1 * I) + (1 * F) = 1
Once the six equations are determined, then the next step is to solve the 6 linear equations and 6 unknowns. The values of N, K, C, A, I and F are the volumetric fractions of the respective minerals/fluids. When solving six equations and unknowns, it is critical that you use algorithms that guarantee that each of the values of N, K, C, A, I and F are non‐negative since you cannot have a negative fraction. I have found that linear programming techniques are a better method to solve linear equations than simultaneous linear equations or matrix algebra.
Conclusions This example demonstrates how to build a simple model to solve for 6 minerals/fluids in Potash mineral exploration. I have successfully scaled‐up this technique to work with up to 29 minerals/fluids. I have used this technique effectively for quantitative analysis of other non‐metallic mineral exploration including Trona and Coal mining. I have also used this technique effectively in Oil and Gas exploration. I have used this method to solve for 7 separate fluid/porosities including brine, bitumen, oil, NGL, natural gas, methane and nitrogen.
When liquids and gases are included, it is essential to vary each fluid’s properties with pressure, temperature, fluid composition and salinity. Another key to successfully implementing these concepts is to include minerals in their simplest form. Instead of including several different clays/micas (illite, kaolinite, smectite, biotite, chlorite, etc) as is common, a better approach is to include their building blocks instead. For example, since clays are essentially oxides and water of hydration, I include minerals such as Al2O3, AlO(OH), Al(OH)3, MgO, Mgo~H2O, K2O, TiO2, ZrO2, Fe2O3 and FeO(OH) as clay building blocks. In this way, you don’t need to know exactly which clays/micas are present. Since clays have different iron contents, this approach gives more flexibility. Caliper corrections are also an important consideration in generating accurate results. It is also important to include any key minerals that are in the drilling fluids, such as barite or bentonite, since they will be picked up by the readings of the logging tools especially in areas with the largest hole correction or drilling fluid loss.
Perhaps one of the greatest benefits of utilizing digital log analysis with PE curves in the mining industry is that many digital LAS files already exist. For example, the Prairie Evaporite zone could be more thoroughly analyzed utilizing existing LAS files in North Dakota that were logged looking for the Bakken formation. Assays can be generated from log results without the need for cores or laboratory analysis. As solution mining techniques explore for deeper minerals, a wealth of useful data is already available and ready to analyze.
I have not tested this technique in Metal exploration. As long as the metals of interest do not have excessively high PE factors, and as long as Barite is avoided as a drilling fluid additive due to its high PE value, this technique should also work successfully in Metal exploration.
Contributed by Fulton Loebel, Petrophysical Consultant, [email protected]
References : Tixier, M.P. and Alger, R.P. (1970): Log Evaluation of Non‐Metallic Mineral Deposits, Geophysics, V. 35, No 1, p 124‐142. Edmondson, H. and Raymer, L.L (1979): Radioactive Logging Parameters for Common Minerals, Log Analyst, V. 20, No 5, p 38‐47. Burke, J., Campbell, R. and Schmidt, A. (May 25‐28, 1969): The Litho‐Porosity Cross‐plot, SPWLA, Clavier, C. and Rust, D. H. (Nov‐Dec 1976): MID Plot : A New Lithology Technique, Log Analyst, p 16‐24. Yang, C. and Guoxiang, C. (2013): Quantitative Evaluation of Potash Grade and Mineralogy Based on Geophysical Well‐log Analysis: Preliminary Study of the Prairie Evaporite in Saskatchewan, V. 1, Saskatchewan Geological Survey. Glover, Paul : Petrophysics MSc Course Notes, Imperial College, London. The Litho‐Density Log, pp. 139‐149. Dr. Paul Glover, petrophysics ,MSc course notes.
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4. The Logger on Site Where there's a will...
Some fun with transporting equipment
On remote logging jobs, the best solution for short term operations is often to airfreight equipment to site or near site and, once the client signals that he has received the shipment, mobilise the logger by air. Once on site, he can build the equipment into the client's own vehicle and should be ready to log within a couple of days.
Operations in the Atlas Mountains using the client's double cab, a PVC sheet (purchased at the market in
Marrakesh), some planed timber and a drill rod.
A better option is to transport a purpose‐built logging box that fits between to wheel arches of a pickup truck. Helicopters are great for moving the box around inhospitable topography but, of course, they are expensive and not always available.
If the logger has to design his own unit, it is important to create a system that employs a boom rather than a tripod. That is a key safety factor.
This truck was used high up in the Chilean Andes. It had no boom. The tripod collapsed due to the high winch position and an expensive sonde was lost.
It is remarkable how often (otherwise very clever) logging technicians fail to appreciate the need to use a bottom pulley or at least keep the cable pull at a sensible angle when using a tripod. There are stable tripod designs out there that reduce the risk of collapse but, regardless, it is best to design out the risk completely.
Safety is a big issue. The trick is to build something that adheres to operational safety rules in a reasonable timeframe.
These days are probably gone.
Safe electrical installation, winch caging and radiation security all need to be considered.
One option is to build the logging system, with appropriate cushioning for fragile components, into a wooden packing case so when the top and, perhaps, one side is removed, the unit is operational.
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How to get the best from the logs
Objectivity in fracture frequency logs
In hard igneous rocks, fracture volume may be described qualitatively by reference to resistivity, neutron and sonic logs. In sedimentary rocks, these techniques only work well in clean (clay‐free) formations. For quantitative measurement, the only option is to pick and classify events on a televiewer image.
Acoustic televiewer log with classified picks on right (tadpole plot).
The log on the right illustrates a typical presentation of picked fractures in a very hard rock environment. Classification in this case is limited to fracture (black tadpole) and open fracture (red tadpole). The most important thing is to pick every natural event and purge all DIFs (drilling‐induced fractures). This adds a subjective element to the process that cannot be avoided. In some data sets DIFs can comprise well over half of the visible events on an image.
From a geotechnical perspective, objectivity is important. When in doubt, it is probably better to classify an uncertain DIF as a natural fracture in order to avoid understating a threat.
Picking the image shown below requires the processor to ignore three significant sub vertical events, all orientated to the south. These appear to be drilling‐induced. They are non‐symmetrical and they happen to align with the local stress regime, which is an indication that they are drilling‐induced. Nevertheless, for an important geotechnical analysis, the processor should check the drill‐core, if available, to confirm their origin.
Refer to previous issue 5, the May 2014 bulletin, at www.wirelineworkshop.com/bulletin for more on this subject.
Steeply dipping open events appear to be drilling‐induced.
Having picked all natural fractures, the generation of a fracture frequency (usually per metre or foot) log is straightforward as part of log analysis software packages such as WellCAD.
The fracture frequency log can be further processed in order to add an extra component to a rock mass strength rating.
In most rock mass ratings, parallel fractures are less of a threat than intersecting ones. An imperfect but useful and certainly objective process is to ignore orientation but group the fractures into dip angle sets. If multiple sets occur within one metre block, a
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weighting can be applied depending on number of fractures and number of sets intersecting. The weighting is prescribed by the geotechnical engineer on site.
Fracture frequency and intersection index from picked image data
In the example above, the dipping events are divided into four groups, low, medium‐low, medium‐high and high angle from horizontal. The weighted intersection log is shown on the far right.
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