Reading the Rocks from Wireline Logs

Based on a workshop presented by the Kansas Geological Survey for PTTC's North Midcontinent Region, Lawrence, Kansas on March 21, 2003

BOTTOM LINE

The science of wireline logging began in 1927, and continues to progress in the scope of equipment, greater depths achieved, higher resolution and improved steering capabilities. The information necessary for modern operators to plan, use and interpret wireline logs has become a complex development requiring specialists to implement. This volume provides information to the independent operator to better understand and evaluate the modern technologies available.

PROBLEM ADDRESSED

The Kansas Geological Survey has developed a number of software programs, interactive websites, and tools to assist the independent operator in using a wide suite of wireline logging methods to better interpret the rocks and make management decisions based on this knowledge.  

KEY WORDS:

Compositional Analysis, Dipmeter, Electrical Imaging, Gamma Ray, Lithologic Determination, Pattern Recognition, Photoelectric Index, Porosity Logs, Resistivity/SP logs, RHOmaa-Umaa Crossplot

SPEAKERS

John H. Doveton, Kansas Geological Survey 

TECHNOLOGY OVERVIEW

Electric logging history is one of rapid advances and changes in equipment and procedures. It is interesting to note that the very first electric logging experiment in 1927 in France was run for the purpose of identifying specific lithologies to use in stratigraphic correlation. The workshop presents the modern advances and technologies to refine that original goal of defining subsurface lithologies to assist in oil and gas recovery operations. Spontaneous Potential logs were added in 1931 to improve the identification of porosity zones and permeable beds. Early logging devices recorded data by hand, but in 1936 the new industry was revolutionized by photographic recording of traces and the modern wireline log came into being. The introduction of the Gamma Ray tool allowed the first measurements through casing; a process still being refined by new technologies. Combining computers with wireline logging starting in the 1960s has allowed for the development of much more sophisticated equipment working at high speed and resolution, and today's software programs allow any operator with a computer to interpret data. 

Wireline Logging Measurements of Subsurface Geology
The manual provides a short review of the background of how wireline logging functions, starting with resistivity logs. In practice, logging tools record the resistivity of the undisturbed formation and the area around the borehole that has been invaded by drilling fluids. The resistivities of sedimentary rocks are determined by the rock types and the geometry of the components. Certain minerals including quartz, calcite and dolomite, which are dominant frameworks in sedimentary rocks, serve as insulators. In the oil reservoir, hydrocarbons also act as insulators to resistivity. Measurement of resistivity is due to the properties of the formation water and brine in the reservoir. To illustrate how resistivity logging works, several examples are given on induction electric survey and Dual Induction-Laterolog resistivity techniques. 

Dipmeter
The Dipmeter is a tool which is designed to measure microresistivity, and uses resistivity to determine dip and strike characteristics of a surface. Four traces are provided by a dipmeter using the azimuth as a reference and the borehole as the deviation angle. Lithological changes are minor at this scale but can be expressed in shifts in depth between correlated features, giving the strike and dip. Patterns developed from multiple dipmeter readings can be used to interpret sedimentary boundaries between units, stratigraphic markers, and changes in mineralogy. 

Electrical Imaging
Electrical imaging was developed as an extension of dipmeter technology. Improvement to the original Formation Microscanner by Schlumberger can now give vertical resolution in order of one inch. Image processing on workstations displays the borehole traces in color bands showing 360° in the borehole. Resistivity contrasts between sedimentary layers, such as sandstones and shales, can give a clear picture of bedding.

Spontaneous Potential
Spontaneous Potential tools measure the natural electrical potentials in boreholes. When the salinities and resistivities of mud filtrate and formation water are the same, the potential is zero and the SP log will be a featureless line. By changing the mud filtrate, porous and permeable intervals can be registered and a base line established. SP logs perform best in sandstone-shale sequences because sandstones generally have higher porosities and lower resistivities. Hydrocarbon saturations cause a suppression of the response towards the shale baseline. Examples of spontaneous potential logs and interpretations are provided from Kansas.

Gamma Ray Logs
The decay of radioactive isotopes is the key to the creation of the Gamma Ray log. Gamma rays have longer penetrations than alpha or beta rays and can be measured by simple counting tools. The original Geiger Counter has been replaced by scintillation detectors for accurate, recordable counts. The gamma ray log functions to discriminate the shales from "clean" formations and to estimate the relative shale proportions in reservoir units. The higher levels of radiation in shale caused by absorption of thorium by clay minerals and the potassium of the various shales can be measured. In contrast, sandstones, limestones and dolomites have low levels of radioactivity. 

Spectral Gamma Ray Log
While the standard Gamma-Ray log is displayed as two curves from potassium-40 and isotopes of uranium and thorium, the Spectral Gamma-Ray log is displayed as three curves from all sources of thorium, uranium and potassium. The spectral gamma ray log can be used to estimate volumes and types of clay minerals, and is also useful for identifying fractures that have uranium salts precipitated in them by ground-water. Significant potassium and thorium concentrations in carbonates are also found in clay minerals and may show on porosity logs as shales. The spectral gamma-ray log helps to differentiate the radioactive carbonates from shales and clays. Several case studies of the use of spectral gamma-ray logs in Kansas show how the data can be cross-plotted using digital recording applications to identify facies. 

Porosity Logs
The three types of logs (neutron log/compensated neutron log, sonic acoustic velocity log, and density log) referred to as porosity logs do not actually measure pore volume directly, but use the physical characteristics of water and minerals to show the contrast and allow measurement of pore volume or porosity. Neutron tools use a radioactive source to penetrate the borehole, but the depth of investigation is shallow—about six inches in a radius from the borehole. Neutron logs measure hydrogen concentration, but are applied primarily to porosity evaluation. Compensated neutron logs are capable of differentiating marine shales and carbonates with a high variability in porosity. Sonic tools developed to aid the interpretation of seismic data are now widely used for estimating porosity. Vertical resolution of sonic tools is about two feet, and some long-spaced sonic tools can span 10-foot intervals. Density logs measure porosity using a simple mass balance relationship between the matrix mineral density and the mud filtrate. In some sedimentary rock sequences, density logs can be used to identify lithology.

Pattern Recognition of Lithologies from Log Overlays and Crossplots
Identification of lithology from logs is restricted to differentiating shales and non-shales, or very low density minerals like anhydrite, halite or coal. Identification of sandstones and carbonates is inferred from the logs based on cores and drill cuttings. Log overlays and crossplots of porosity logs were introduced to resolve the more complex lithologic identification problems. Overlays of density and neutron logs use the relative offset between the two curves as a diagnostic feature in lithological facies. More precise estimates of true porosity can be made using porosity log crossplots, and crossplot data can indicate quantitative composition of mineral components in mixed lithologies. A number of examples show the crossplot techniques for sedimentary successions, and the method for interpreting logs for igneous and metamorphic rock sections.

Photoelectric Index
The Photoelectric Index (Pe) uses the latest generation of density logging tools to measure the absorption of low-energy gamma rays. Logged values are a direct function of the aggregate atomic number of the elements in the formation. The values are only slightly influenced by pore volume, fluid or gas content; and thus are sensitive indicators of mineralogy. Pe can give a better indication of lithology in thin beds, where the averaging effect of adjacent thick beds may smooth the neutron and density responses.

RHOmaa-Umaa Crossplot
This crossplot was introduced to utilize measurements of the photoelectric index, neutron porosity, and bulk density for matrix mineral evaluation. The calculations are based on density of the fluids and absorption characteristics of the pore fluids and the mud filtrate. The RHOmaa-Umaa plot is a powerful mineral discriminator, and particularly useful to distinguish clay types.

Numerical Methods for Mineral Estimation from Well Logs
Compositional analysis of rocks using wireline logs provides alternative ways of representing logging data. Compositional analysis is able to account for depth information, which is lost using crossplot overlays. The logs are superimposed as continuous variables on a horizontal axis that form traces with respect to depth as the vertical axis. Compositional analysis uses both geometrical and algebraic relationships. Three end points in mineral composition can be plotted in a composition space triangle (calcite-dolomite-quartz), and areas outside the triangle identify shale, anhydrite, gas, and water. EXCEL spreadsheets are used to manipulate the data and calculate the compositions. Case studies include information on calculation of clay-mineral volumetrics. 

Summary
Case studies and examples are used to illustrate each of the logging technologies and interpretation methods summarized in the volume. The appendix includes additional useful information; such as a Table of Logging Tool Response in sedimentary minerals, analysis spreadsheets, several software programs for display and interpretation, and additional examples from Kansas

CONNECTIONS:

John Doveton
Kansas Geological Survey
1930 Constant Avenue
Lawrence, KS 66047
Phone: 785-864-2100
Email: doveton@kgs.ku.edu

For information on PTTC’s North Midcontinent Region and its activities contact:
Rodney R. Reynolds, Project Manager, Kansas University Energy Research Center
1930 Constant Ave., Lawrence, KS 66047-3726
Phone: 785-864-7398, Fax: 785-864-7399, Email: rreynolds@ku.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

Petroleum Technology Transfer Council, 16010 Barkers Point Lane, Ste 220, Houston, TX 77079
toll-free 1-888-THE-PTTC; fax 281-921-1723; Email hq@pttc.org; web www.pttc.org