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Geophysics

Well Log Lab

Load, draw and interpret well logs: lithology, porosity, saturation, fluids, layers and aquifers.

Load well logs as LAS, Excel or CSV files, type the readings, or trace a printed log from an image. The lab recognises the curves of the main service companies, draws the composite log in the standard layout and interprets it: shale volume, porosity, mineral volumes, water saturation, gas, oil and water, invaded permeable beds, bad hole, fracture candidates, source rocks, layer boundaries and net pay — or, in water wells, aquifers and their salinity. Crossplots, well correlation, a response chart of every log and a guide to all the common logs complete it.

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Well Log Lab
0 m50 m100 m150 m200 m250 mAlluviumInjana Fm. · sandstoneMudstoneFatha Fm. · gypsumLimestoneMarly limestoneDolomite

Overview

Each log sees one property of the rock: gamma ray the clay and radioactive minerals, density and neutron the porosity and the fluid, the photoelectric factor the matrix, resistivity the fluid that fills the pores, sonic the stiffness and the intergranular porosity, caliper the hole. Read together they tell the rock, how much space it has and what fills it. The lab solves them together, sample by sample, for the volumes of shale, quartz, calcite, dolomite and pore space that best rebuild the measured logs, and checks itself by the misfit.

Gas is found where density porosity exceeds neutron porosity in the matrix of the rock (crossover); fractures and vugs where density–neutron porosity exceeds sonic porosity; invasion where shallow and deep resistivities part or mudcake narrows the hole; source rock where gamma ray, uranium and resistivity rise and density falls. In groundwater mode the same logs give the aquifers, the formation-water resistivity and the dissolved solids. For every hydrocarbon layer the range of water saturation under the uncertainty of the parameters is given.

What you enter · What you get

What you enter

  • LAS 2.0 files, Excel (.xlsx), CSV or text tables, or typed readings
  • An image of a printed log (scan or photo)
  • Interval, formation type and well type (oil and gas or groundwater)
  • Rw, Rmf, temperature, a, m, n and cutoffs (defaults given)

What you get

  • Composite log with interpreted tracks (volumes, porosity and saturation, lithology and fluids)
  • Table of layers with lithology, porosity, saturation range, fluid and net pay
  • Density–neutron, Pickett and M–N crossplots
  • Correlation of several wells
  • Results as LAS and CSV, composite log as PNG, PDF report

How it works in the field

Load the logs
Check the recognised curves
Set the interval and parameters
Read the composite log, layers and crossplots

Method and equations

Shale volume

Gamma-ray index between the clean and shale lines (5th percentile and the shale line of the interval by default), converted by Larionov (Tertiary or older rocks), Stieber or kept linear; SP when there is no gamma ray.

IGR = (GR − GRcl) / (GRsh − GRcl) · Vsh = 0.083 (23.7 I − 1)

Mineral solution

Density, neutron and volumetric photoelectric cross-section are written as sums of the end-member responses of shale, minerals and water, with the unity equation, and solved by non-negative least squares (Lawson & Hanson 1974). Coal, salt and anhydrite are recognised first by their unique density–neutron pairs. The sonic enters only when density and neutron are absent.

ρb = Σ Vi ρi + φ ρf · Σ Vi + φ = 1

Water saturation

Archie for clean rock, Simandoux and Indonesia (Poupon & Leveaux 1971) for shaly rock; Rw from the lowest apparent Rwa of clean porous levels when not entered, brought to formation temperature by Arps.

Sw = (a Rw / (φm Rt))1/n

Fluids, fractures and layers

Gas where density porosity exceeds neutron porosity by more than 0.08 in the matrix of the dominant mineral; fracture or vug candidates where density–neutron porosity exceeds sonic porosity (Raymer–Hunt–Gardner) by 0.025; layers are runs of one lithology and one fluid, thin runs merged into the neighbour of nearest gamma ray.

SPI = φND − φS

Groundwater

In clean porous levels the formation-water resistivity follows from Archie with Sw = 1; it is brought to 25 °C and converted to electrical conductivity and dissolved solids (TDS ≈ 0.64 EC), classed fresh (< 1000 mg/L), brackish (1000–10 000) or saline.

Rw = Rt φm / a · EC = 104 / Rw,25

Accuracy and error limits

  • Porosity from density or neutron logs is typically ±1.5–3 porosity units; water saturation from Archie or shaly-sand equations is ±10–20 saturation units because of the uncertainty in Rw, m and n. Shale volume from gamma ray can be overestimated in radioactive sands.
  • Readings in washed-out or rugose hole are unreliable and are flagged; logs traced from images add about ±1–3 % of the track width.
  • To reduce the error: calibrate with core porosity and water analyses, measure Rw from produced water, use m and n from core tests, and depth-match all curves before interpreting.

Who uses it

Petroleum geologists and petrophysicistsHydrogeologists and water-well drillersStudents of geophysics and petroleum engineeringResearchers reading logs in reports and books

Standards and references

  1. Archie, G.E. (1942). The electrical resistivity log as an aid in determining some reservoir characteristics. Trans. AIME, 146, 54–62.
  2. Asquith, G. & Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
  3. Rider, M. & Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French Consulting.
  4. Ellis, D.V. & Singer, J.M. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.
  5. Schlumberger (1989). Log Interpretation Principles/Applications; (2009) Log Interpretation Charts.
  6. Poupon, A. & Leveaux, J. (1971). Evaluation of water saturation in shaly formations. SPWLA 12th Symposium.
  7. Simandoux, P. (1963). Dielectric measurements on porous media: application to the measurement of water saturations. Revue de l’IFP, 18, 193–215.
  8. Larionov, V.V. (1969). Borehole Radiometry. Nedra, Moscow.
  9. Raymer, L.L., Hunt, E.R. & Gardner, J.S. (1980). An improved sonic transit time-to-porosity transform. SPWLA 21st Symposium.
  10. Timur, A. (1968). An investigation of permeability, porosity and residual water saturation relationships. SPWLA 9th Symposium.
  11. Lawson, C.L. & Hanson, R.J. (1974). Solving Least Squares Problems. Prentice-Hall.
  12. Haworth, J.H., Sellens, M. & Whittaker, A. (1985). Interpretation of hydrocarbon shows using light (C1–C5) hydrocarbon gases from mud-log data. AAPG Bulletin, 69(8), 1305–1310.
  13. Keys, W.S. (1990). Borehole Geophysics Applied to Ground-Water Investigations. USGS Techniques of Water-Resources Investigations 2-E2.
  14. CWLS (1992). LAS Version 2.0: A Floppy Disk Standard for Log Data.
Results support professional judgement. They do not replace a licensed engineer’s or geologist’s sign-off.

At a glance

Category
Geophysics
Runs on
Phone, tablet and desktop browser
Export
Composite log PNG, results LAS and CSV, layers CSV, crossplots PNG, PDF report
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